Glossary

A

A bands Large, dark bands in a skeletal muscle fiber that alter­nate with lighter I bands to give a striped appearance to skeletal muscle fibers under a microscope. The A bands are composed of thick filaments of the contractile protein myosin.

abduction The joint movement whereby an extremity is moved away from the median plane.

abomasum The “true stomach” of the ruminant; secretes acids, mixes and contracts ingesta, and moves liquid chyme into the small intestine.

absorptive cell A cell commonly found in the small intestine that can absorb nutrients from the luminal surface via phagocytosis and pinocytosis. Absorptive cells have large surface areas as a result of the presence of microvilli. The expanded surface area increases the absorptive capability of the cell.

accommodation The focusing of the lens of the eye to allow close-up and distant vision. It is accomplished by the muscles of the ciliary body that apply or relieve tension on the suspen­sory ligaments that attach it to the lens.

acetabulum The socket portion of the ball-and-socket hip joint. It is formed at the junction of the ilium, ischium, and pubic bones of the pelvis.

acetylcholine A neurotransmitter associated with somatic nerves and with parasympathetic nervous system effects even though it is used in the preganglionic neuron in both the sympathetic and parasympathetic nervous systems; has a stimulatory effect on the gastrointestinal tract, it increases secretions and muscle contractions in the esophagus, stomach, ruminant forestom­achs, intestine, and colon.

acetylcholinesterase The enzyme that breaks down acetylcholine. acids Substances that dissolve in water to yield hydrogen ions and produce a solution with a pH less than 7.

acinar gland The secretory units of exocrine glands that contain one or more saclike structures.

acrodont teeth Dentition in which the teeth are fused to the biting edge of the mandible and maxillae, found in some species of lizards.

acromegaly A form of gigantism that results from an excess of growth hormone (GH).

acrosome The caplike structure that partially covers the head of a spermatozoon. It contains digestive enzymes that are activated when the sperm enter the female reproductive tract. These help the cell penetrate through the layers around the ovum to fertilize it.

ACTH See adrenocorticotropic hormone.

actin A protein that forms microfilaments. It is found in the cytoskeleton, in myofibrils of muscle fibers, and in spindle fibers during cell division.

actin filaments Formed of one of the two contractile proteins of muscle (the other is myosin), these slide over each other to produce the shortening of the muscle cell that we refer to as muscle contraction.

activation energy The minimum energy required for a chemical reaction to occur.

active immunity Activation of the immune system by either administration of a vaccine that contains a modified antigen or exposure to the antigen (e.g., by disease).

active site The specific area on an enzyme that connects with a substrate to cause a chemical reaction.

active transport The process that moves ions or molecules across the cell membrane and against the concentration gradient; requires energy.

adaptive immunity Portion of the immune system that is able to target specific antigens.

adduction The joint movement whereby an extremity is moved toward the median plane.

adenine (A) One of the nucleotides present in both DNA and RNA. It is a purine base that pairs with RNA's uracil and DNA's thymine.

adenosine diphosphate (adp) Adenosine diphosphate is the “discharged” form of adenosine triphosphate (ATP). It is a nucleotide that contains two phosphoric acid groups. When a phosphate group is split off of an ATP molecule to produce ADP, energy is released that powers the sliding of the actin and myosin filaments in muscle over each other. When the phos­phate group is reattached, which requires another energy source, ADP is converted back to ATP, and the molecule is ready to provide energy again.

adenosine monophosphate (amp) A nucleotide that contains one phosphoric acid group. AMP is produced by the hydrolysis of one high-energy phosphate bond of ADP. This releases energy that can be used by cells or phosphorylated back into ADP.

adenosine triphosphate (atp) A high-energy molecule pro­duced in the mitochondria of cells. It is a nucleotide that con­tains three phosphoric acid groups. When a phosphate group is split from an ATP molecule to produce ADP, energy is released that powers the sliding of the actin and myosin filaments in muscle over each other. When the phosphate group is reat­tached, which requires another energy source, ADP is converted back to ATP and the molecule is ready to provide energy again. The more active a cell or body part is, the more ATP it will have produced and stored. For instance, muscles have a great deal of stored ATP, whereas fat has relatively little.

ADH See antidiuretic hormone.

adipocyte A type of fixed cell in the connective tissue that stores fat (lipids) in its cytoplasm. The nucleus and other organelles are pushed to the periphery of the cell.

adipose Fat.

adipose cells See adipocyte.

adipose connective tissue A subclass of connective tissue proper, adipose connective tissue is a vascularized type of connective tissue whose general functions are to protect, insulate, and provide a major source of energy to the body. Adipose connec­tive tissue can occur as either brown or white adipose tissue. White adipose tissue, found commonly throughout the body, is a storage area for lipids. These lipids may be used for the pro­duction of energy, or ATP. Brown adipose tissue, found in neo­nates and hibernating species, has its lipids converted to heat.

ADP See adenosine diphosphate.

adrenal cortex The outer portion of the adrenal gland that pro­duces glucocorticoid, mineralocorticoid, and sex hormones.

adrenal glands Two endocrine glands located near the cranial poles of the kidneys. Each consists of an outer cortex and an inner medulla.

adrenal medulla The inner portion of the adrenal gland that produces the hormones epinephrine and norepinephrine.

adrenergic neurons Neurons that secrete catecholamines (e.g., norepinephrine) as their neurotransmitter.

adrenocorticotropic hormone (ACTH) A hormone secreted by the anterior portion of the pituitary gland, which in turn acti­vates the cortex of the adrenal gland. The adrenal cortex then releases its own hormones. ACTH is vital to the normal function and development of the adrenal cortex.

adult hemoglobin The primary type of hemoglobin found in the red blood cells of animals from a couple of weeks to a couple of months after birth.

aerobic metabolism Oxygen-consuming metabolism. The type of metabolism in muscle in which the supply of available oxygen is sufficient to keep up with the energy needs of the muscle fibers. Aerobic metabolism extracts the maximum amount of energy from glucose molecules.

aerobic respiration The cell function that produces chemical energy with the use of oxygen.

afferent A directional term meaning toward some reference point. afferent glomerular arterioles The smallest arteriole branches that carry blood into the renal glomerulus for filtration.

afferent nerve Nerve that carries impulses toward the central nervous system.

afterbirth The name given to the placenta at parturition because it is delivered after the newborn.

agglutination Precipitation or clumping of antigen-antibody complexes; one of the methods by which the immune system neutralizes antigens.

aglyphous Referring to having solid teeth, without a groove for venom (used to classify snake groups).

agranulocytes White blood cells without cytoplasmic granules. The agranulocytes are the monocytes and the lymphocytes, also known as nongranulocytes.

air sacs Nine thin, transparent membranes found in birds that are connected to the primary and secondary bronchi and act as reservoirs for air entering and leaving the lungs.

albumin Protein manufactured by the liver that plays an impor­tant role in maintaining the osmotic fluid balance between capillaries (blood) and tissues; a lack of albumin results in movement of fluid from the capillaries into the tissues, produc­ing edema and fluid accumulation in body cavities.

aldosterone A mineralocorticoid hormone secreted by the cortex of the adrenal gland. It stimulates the kidney to conserve sodium ions and water and to eliminate potassium and hydro­gen ions.

alimentary canal Also called the digestive tract, this encompasses all of the parts of the digestive system that transport food from the mouth to the anus: the mouth, esophagus, stomach, small and large intestines, and anus.

alkaline The adjective that describes substances with a pH above 7 (basic). Basic, or alkaline, solutions have fewer hydrogen ions (or more hydroxide ions) than pure water.

allantoic sac Part of the placenta. It is a fluid-filled sac formed by the allantois that surrounds the amniotic sac.

allantois Part of the placenta. It is the membrane that forms the allantoic sac.

all-or-nothing principle The principle that an individual muscle fiber either contracts completely or does not contract at all.

alopecia Loss of hair.

alpha cells Cells in the pancreas that produce glucagon.

alpha helix The coiled structure in a complex protein composed of hydrogen bonds and amino acids.

alphaι-adrenergic receptors Receptors associated with the sympathetic nervous system response; these receptors, when stimulated by catecholamines, cause vasoconstriction.

altricial Chicks that are hatched with their eyes closed and their skins bare and that are immobile.

alula Bone in birds that originates from the wrist and is compa­rable to the first digit; carries feathers that aid in steering.

alveolar ducts The smallest air passageways in the lungs. The alveolar ducts carry air to the alveolar sacs.

alveolar glands Secretory units of exocrine glands that are saclike in form; also called acinar glands.

alveolar sacs Clusters of alveoli at the ends of the alveolar ducts. The alveoli are arranged like bunches of grapes.

alveoli (singular, alveolus) Microscopic, thin-walled sacs sur­rounded by networks of capillaries. The interface between the wall of the alveoli and the wall of the capillary is where the actual exchange of gases takes place in the lungs.

amino acids The basic building blocks of peptides and proteins; those organic compounds, numbering around 80, that are have both an amino group (NH₂) and a carboxyl group (COOH). Amino acids make up proteins when joined together in peptide bonds. They occur naturally in all plants and animals.

aminopeptidase Protease secreted in an inactive form from the pancreas and activated by trypsin.

amnion Part of the placenta. It is the membrane that forms the fluid-filled amniotic sac.

amniotic sac Part of the placenta. It is the fluid-filled sac that immediately surrounds the developing fetus.

amoeboid motion Amoeba-like movement accomplished by the extension of pseudopodia to create a streaming movement of cytoplasm.

amorphous Having no defined shape.

AMP See adenosine monophosphate.

amphiarthrosis A slightly movable cartilaginous joint, such as the pelvic symphysis.

ampulla An enlargement in each semicircular canal that contains the receptor structure (the crista).

ampulla of the vas deferens An enlargement of the vas deferens just before it enters the urethra. It is present in some species and absent in others.

amylase Enzyme produced by the pancreas and, in some species, the saliva; attacks starch and breaks it into disaccharides (which contain two simple sugar molecules).

amyloid An excessive amount of a waxlike, proteinaceous sub­stance in the body's tissue.

anabolism mhe form of metabolism by which cells build complex compounds from simpler ones; the opposite of catabolism. The process by which the cell uses energy to manufacture large molecules Irom smaller ones; these molecules are used to main- teallin the c and carry out metabolic processes.

anaerobic A biochemical pathway that can function without ohxeygen. T term also may be used to describe microbes that cvaen li in the absence of oxygen.

anaerobic glycolysis An alternative expression for glycolysis, referring to the fact that the reaction does not require oxygen.

anaerobic metabolism Non-oxygen-dependent metabolism. The Ivpe ol' metabolism in muscle that occurs when the need fonoerrgey t produce muscular activity exceeds the available oxygen supply. Anaerobic metabolism is not as efficient as aerobic metabolism and results in the formation of lactic acid as a by-product. Lactic acid can cause discomfort in muscle teiqssuuireesand r oxygen for conversion back to glucose.

anaerobic respirationellThe c function that produces energy chemically without the use of free oxygen.

anagen phasectivTehe a phase of hair growth.

anal sacs Perianal sacs, containing apocrine and sebaceous glands, that are located at the 5 o'clock and 7 o'clock positions roelative t the anus. The sacs reside between the internal and external sphincters of carnivores and produce a strong-smelling fluid when expressed; they are important for fecal territorial marking and are expressed during fearful episodes.

analgesia Decreased perception of pain.

anaphasehasTehe p of mitosis when the daughter chromo­

somes begin to migrate to their respective centrioles, away from ftehnetecr o the dividing cell.

anaphylaxis A severe, potentially life-threatening, allergic response. anapsid skulls Skulls of primitive reptiles (all extinct except turtles) that have no openings in the temporal region.

anatomy The study of the form and structure of an animal body tasn. d its par Through anatomy we can describe where things acraeteldo in or on the animal body and what they look like.

anconeal processeakA-sbhaped process at the proximal end of

thc trochlear notch of the ulna. When it fails to unite with the ulna, an Iimiiiiled anconeal process can cause the elbow joint to become unstable, leading to lameness.

androgens Hormones that promote the development of male characteristics; male sex hormones. The principal androgen is testosterone.

anemia Decreased oxygen-carrying capacity of the blood caused by i usu ITicien I numbers of red blood cells, decreased hemoglobin concentration, or a combination of both of these conditions.

anesthesia Complete loss of sensation.

anestrus Tlre period when the ovary “shuts down” between the celsetsrous cy or breeding seasons of some animal species.

anglele The ang of the hoof wall as viewed from a lateral aspect owohten the f is flat on the ground.

anions A Ilegalivelv charged atom or molecule; a negatively cnh. arged io

anisodactyly Toe position in which three toes face forward and coensce fa the rear.

antagonist Something that opposes the action of something else. An antagonist muscle or muscle group directly opposes the faction o a “prime mover” muscle or muscle group that is rdoirdeucctliyngp a desired movement.

antebrachium The “forearm” region of the thoracic limb.

anterior A direcl ioιιal term meaning toward the front (oma human body).

anterior chamber The portion of the aqueous compartment of the eye in front of (rostral to) the iris.

anterior pituitary gland The aclenehypophysis; the rostral fportion o the pituitary gland that produces seven hormones, fhmicahny o w influence other endocrine glands.

antibodies Proteins produced by plasma cells (transformed B lymphocytes) in response to the presence of an antigen. A spe­cific serum antibody is generated for a specific antigen.

anticlinal vertebra The thoracic vertebra whose spinous process rpariogjhetcts st up dorsally in contrast to the caudally inclined spinous processes cranial to it and the cranially inclined spinous processes caudal to it. It acts as a landmark on radiographs of the thoracolumbar region, particularly in dogs, in which it is the 11th thoracic vertebra.

anticoagulantubAstasnce that prevents blood from clotting

dwdheedn it is a to the blood.

anticodon The tri plel pair of nucleotides in transfer RNA (tRNA) tohraret scponds to the triplet bases or codons of messenger

RNA (mRNA).

antidiuretic hormone (ADH) A hormone released by the poste- ruiiotarrpy.it It facilitates water conservation in the body by promoting water reabsorption from urine in the collecting odwucts. L levels of ADH cause diabetes insipidus, a condition tehsuatltsr in excessive water loss from the body through increased urine volume.

antigens Cells or organisms that are “not self.” An antigen also rceuanctburea st on a cell membrane that the body recognizes os foreign. The presence of an antigen initiates an immune response in a healthy animal.

antiparasitic drugugA dr that kills parasites.

antiport systemo When tw separate materials are moved across the plasma membrane in opposite directions at the same time. antitussive drug A drag that suppresses the cough reflex. antrum (1) fie fluid-filled space within an ovarian follicle. (2) uTshceumlar part of the stomach that is responsible for grind- ifonogd;o f located between the body of the stomach and the pylorus.

anucleated Having no nuclei.

anuria Condition in which no urine is being passed from tohdey.b

aorta Major artery of the systemic circulation that receives blood from the left ventricle.

aortic valvemiAlunsear valve; it separates the left ventricle and

trhtae. ao

apex (of a tooth)ifp The t o the tooth root where the blood and uneprpvley s enter the tooth; the uppermost point of a structure.

apical surface The side of an epithelial cell that faces in toward the body cavity.

apocrine gland A gland whose secretions contain some of its cellular material. Part of the secretory cell is destroyed and must regenerate before the cell can secrete again. Examples of these glands are mammary glands and some sweat glands.

apocrine sweat glands Exocrine glands that secrete substances into the hair follicle, rather than directly to the skin surface.

aponeurosis A broad sheet of fibrous connective tissue that attaches certain muscles to bones or to other muscles.

apoptosis Programmed cell death.

appendicular skeleton The bones of the limbs (appendages). apteryia Bare areas of the skin of birds where feathers do not originate.

aqueous compartment The compartment of the eye in front of (rostral to) the lens and ciliary body. It contains a watery fluid, called aqueous humor, and is subdivided by the iris into the anterior chamber and the posterior chamber.

aqueous humor The watery fluid that fills the aqueous compart­ment of the eye. It is produced in the posterior chamber and drained from the anterior chamber by the canal of Schlemm.

arachnoid The delicate, weblike layer of the meninges between the dura mater and the pia mater.

areolar connective tissue A soft, spongy connective tissue, also known as loose connective tissue. It is located throughout the body and is composed of a soft ground substance, numerous cell types (white blood cells, fibroblasts, macrophages), and all three types of fiber (elastic, reticular, and collagenous).

arrector pili muscle Smooth muscle that is attached to the base of the hair follicle. It is responsible for the involuntary “hair raising” response to cold, fear, or aggression.

arthrodial joint A gliding joint in which two flat, articular sur­faces rock on each other. This type of joint usually allows only the movements of flexion and extension.

articular cartilage The thin layer of hyaline cartilage that covers the articular surfaces of long bones in synovial joints. It forms a smooth layer over the joint surfaces of the bones, which decreases friction and allows free joint movement.

articular process The process of a vertebra that forms a synovial joint with an adjacent vertebra.

articular surface The smooth joint surface of a bone that con­tacts another bone in a synovial joint.

arytenoid cartilages Two of the cartilages of the larynx. The vocal cords attach to the arytenoid cartilages. The arytenoid cartilages and the vocal cords form the boundaries of the opening into the larynx (glottis).

ascites An abnormal condition in which an excessive amount of fluid accumulation is present in the abdominal cavity. Abdomi­nal distention or a potbellied appearance can be clinically evident.

asternal rib A rib whose costal cartilage joins the costal cartilage of the rib ahead of it instead of directly joining the sternum.

ataxia Incoordination. An ataxic animal makes jerky, spastic movements.

atlas The first cervical vertebra. It forms the atlantooccipital joint with the occipital bone of the skull and the atlantoaxial joint with the axis (the second cervical vertebra).

atom The smallest unit of an element having all the characteris­tics of that element. It consists of a positively charged nucleus surrounded by a system of negatively charged electrons.

atomic nucleus A dense region at the center of an atom consist­ing of positively charged protons and uncharged neutrons.

atomic number The number of protons found in the nucleus of an atom. In a neutrally charged atom, the atomic number is also the number of electrons.

atomic weight The average mass of an atom of an element. Gen­erally equal to the atomic mass of the protons and neutrons in tehcaeuasteom, b the electrons are so tiny they are almost irrel­evant to the mass.

atony Lack of normal muscle tone.

ATP See adenosine triphosphate.

ATP synthase The enzyme in the mitochondria of body cells that functions as a tiny “molecular machine” to regenerate ATP from ADP and detached phosphate groups. It is powered by the proton gradient present across interior mitochondrial membranes.

atrophy Shrinkage.

auricle Ear-shaped appendage of either atrium of the heart. auriculars Small contour feathers located around the external ear openings in birds.

auscultation Listening, with the ear or with a stethoscope, to sounds produced within the body.

autoimmune disease An abnormal condition in which the body starts recognizing some of its own cells as “not self” and initiates an immune response to destroy them.

autolysis The self-digestion of tissues or cells by enzymes that are released by their own lysosomes.

autonomic nervous system The part of the nervous system that controls smooth muscle, cardiac muscle, and endocrine glands automatically without conscious control; has motor and sensory branches.

autonomic reflex A reflex that results in stimulation or inhibi­tion of smooth or cardiac muscle or endocrine gland function; mechanisms of homeostasis are autonomic reflexes.

avascular Without a blood supply.

axial skeleton The bones along the central axis of the body; made up of the skull, the hyoid bone, the spinal column, the ribs, and the sternum.

axis The second cervical vertebra. It forms the atlantoaxial joint with the first cervical vertebra, the atlas.

axon Extension of the neuron that conducts the nerve impulse away from the cell body to the terminal bouton (synaptic bulb) at the end of the neuron.

azotemia A buildup of waste materials, particularly creatinine and BUN, in the blood because of insufficient removal of these substances by the kidneys.

B

B lymphocyte The type of lymphocyte that is responsible for humoral immunity through its transformation into a plasma cell and production of antibodies.

ball-and-socket joint Also called a spheroidal joint, it consists of a spherical joint surface (the ball) that fits into a closely match­ing, concave joint surface (the socket). The shoulder and hip joints are ball-and-socket joints. Ball-and-socket joints allow the greatest range of joint movement.

barbs Slender projections off the main feather shaft. Barbs make up the vanes on each side of the shaft.

barbules Microscopic projections of feather barbs that help maintain a contour feather's structure.

barrel Trunk ol' the body—formed by the rib cage and the abdomen.

basal bodies A pair ol' tubular structures. Each is composed of ontinueb umleiscr surrounding another pair of microtubules.

Basal bodies act as the base of cilia and flagella.

basal surface The side of an epithelial cell that faces a lower level of Conneetive tissue.

basement membrane A Iloiicellular, collagen-based structure tuhpaptosrts epithelial tissue.

bases Substances that dissolve in water to yield hydroxyl ions and give the solulloii a pH greater than 7.

basopenia Theoretically, a decrease in the total number of baso- phps in peripheral blood. This condition is difficult to describe because basophils normally are rarely found in peripheral blood.

basophilia An increase in the total number of basophils in lpoeordip.heral b

basophil One ol' the granulocytic white blood cells, characterized rbeysethnecep of numerous, dark blue-staining granules in its cytoplasm.

behavioral thermoregulation Carried out by cold-blooded animals that move into and out of relatively warm locations to roedgyulate b temperature.

belly The thic central portion of a muscle.

beta cells Cells in the pancreas that produce insulin.

beta₁-adrenergic receptors Receptors associated with the sym­pathetic nervous system response; these receptors, when stimu- leyactheodlabmicnaets, tend to cause an increase in the rate

and force ol' contraction of the heart.

beta₂-adrenergic receptors Receptors associated with the sym­pathetic nervous system response; these receptors, when stimu- leyactheodlbamciantes, tend to cause dilation of the bronchioles

and Vasoolilalion of some blood vessels.

beta oxidation The breakdown of fatty acid chains into two carbon fnragments i the mitochondria. These two carbon fragments can robowekbnen d further into ketone bodies and acetyl CoA.

bicarbonate Often referred to as sodium bicarbonate, it acts as a buffer in the secretions (e.g., pancreatic juice, saliva) to reduce ftchideitay o the stomach or rumen; also helps to buffer the pH loofotdh.e b

Bidder's organs Undeveloped ovaries located anterior to the testes in male toads. Environmental and physiologic influences enable the male to become female.

bifurcation of the trachea Thn division of the trachea at its caudal ehnnteod i t left and right main bronchi, which enter the lungs.

bilateral symmetry TOio- concept that the left and right halves of an animals body are mirror images of each other. Paired struc- tuucrhes, s as the kidneys, are located one on each side of the body, and single structures, such as the heart, are located near tdhiaenme plane.

bile acids Hydrophilic (water-loving) molecules with a hydro- eprh-ohbaitcin(gw)at end that are secreted by the liver into the

duodenum; these combine with fat droplets to make them more water soluble.

bilirubinel loTwhe y breakdown product of hemoglobin.

biologic value The percentage of absorbed protein that is retained by the body. Biologic value is a measure of the body's aobnivlietryt t c absorbed protein into body tissue.

biopsy The removal of a small sample of tissue for microscopic examination. This procedure is often but not exclusively used to determine whether a tumor is benign or malignant.

blastel lA c in the production stage of a particular substance. blastic transformation Through this process, B lymphocytos become plasma cells that produce antibodies.

blastocyst The stage of development of a zygote that is ready for implantation in the uterus. It is shaped like a tiny, hollow ball of cells w^,illι a “bump” on one side that eventually develops into tmhbe reyo.

bloatc uTmheulaation of gas within the rumen or monogastric

stomach that causes severe distention.

blood-brain barrier The Ianclional barrier between the capillaries irhnaeint b and the nervous tissue; composed of capillary walls, hweithout t openings found in other capillaries, and glial cells.

blood feathereveAlodping feather that contains blood in its

shaft for growth and nourishment.

body condition scoring (BCS) Use of a numerical scale to indi­cate the amount of fat in an animal's body.

body of the penis The largest portion of the penis. It contains the majority of the erectile tissue of the organ.

body of the stomach The central part of the stomach, between tuhse fund and the antrum.

bolus (of food) Amount of food swallowed at one time.

bone cortexterThe ou layer of a bone that is composed of coonme.pact b

bone marrow The soft material that fills the spaces inside bones. Two types of bone marrow are red bone marrow, which forms blood cells, and yellow bone marrow, whids consists primarily of adipose connective tissue (fat).

bones of the cranium The bones of the skull that surround the Xrain. Tlιe externally visible bones of the cranium are the occipi- toanl eb, the interparietal bones, the parietal bones, the tempo- oal bones, and the frontal bones. The internal (hidden) bones of tahneiucmr are the sphenoid bone and the ethmoid bone.

bones of the facel The skul bones that do not surround the bhreain. T externally visible bones of the face are the incisive bones, the nasal bones, the maxillary bones, the lacrimal bones, tghoemzaytic bones, and the mandible. The internal (hidden) fbones o the face are the palatine bones, the pterygoid bones, tohnmeee,vr b and the turbinates.

bovine spongiform encephalopathy (BSE) Also known as mad cow disease. TMs disease was first contracted by cows in Great Britain that had consumed the flesh of sheep thought to carry irniofencst.ious p

Bowman's capsule Part ol' the renal corpuscle. It consists of two layers: an inner, visceral layer that lies direc11y on the glomerular capillaries and an outer, parietal layer. Il IuiKlions as a plasma filter in the process of urine formation.

brachium The upper arm. The area of the thoracic limb between ltbhoewe and the shoulder.

brachycephalic Short-faced. Brachycephalic breeds of dogs ionsctolunde B Terriers, Pugs, English Bulldogs, and Pekingese.

brainstem The connection between the rest of the brain and the spinal cord, composed of the medulla oblongata, the pons, and the midbrain; heavily involved in autonomic control functions related to the heart, respiration, blood vessel diameter, swallow­ing, and vomiting.

breathing The movement of air into and out of the lungs. Also known as ventilation.

brisket Area at the base of the neck between the front legs that covers the cranial end of the sternum.

bristle Modified contour feather with a stiff rachis and few barbs at the base; found around the eyes, mouth, nostrils, and toes of some bird species, these serve a tactile function.

broad ligaments Paired sheets of connective tissue that suspend the uterus from the dorsal part of the abdominal cavity and attach it to the abdominal wall. They are often subdivided into the mesovarium, which supports the ovary; the mesosalpinx, which supports the oviduct; and the mesometrium, which sup­ports the uterus.

bronchi The largest air passageways in the lungs. The left and right main bronchi are formed by the bifurcation of the trachea. The main bronchi divide into smaller and smaller branches within the lungs.

bronchial tree The air passageways in the lungs between the main bronchi and the alveoli. The branching nature of the bronchi as they form smaller and smaller air passageways resem­bles the branching of a tree.

bronchioles Some of the smallest branches of the bronchial tree. The bronchioles subdivide down to the alveolar ducts, which are the smallest air passageways that lead directly to the alveolar sacs.

bronchitis A lower respiratory tract infection affecting the lining of the larger air passageways in the lungs. Bronchitis can be a serious disease because the inflammatory fluids and excess mucus resulting from the irritation are difficult for the animal to cough up from down deep in the lungs.

bronchoconstriction Contraction of the smooth muscle that surrounds the air passageways in the lungs. Bronchoconstric­tion narrows the air passageways. It can be physiologic, as when a mild degree of bronchoconstriction reduces the work of moving air in and out of the lungs at rest. Bronchoconstric­tion also can be pathologic, for example when inhaled irritants cause severe bronchoconstriction that makes breathing very difficult.

bronchodilation Relaxation of the smooth muscle that sur­rounds the air passageways in the lungs. Full bronchodilation causes the air passageways to dilate to their full diameters. Physi­ologic bronchodilation occurs during intense physical activity, when maximum air must be moved into and out of the lungs. A class of drugs called bronchodilators can be used to help relax severe bronchoconstriction, such as might occur in an asthma­like condition.

brood patch Area of thickened skin on the lower abdomen of birds, from which feathers are plucked to allow transfer of heat to eggs during incubation.

brown adipose tissue Brown fat; commonly found throughout the body of hibernating species and neonates. It is a specialized form of adipose tissue that releases its stored lipid reserves in the form of heat. This is accomplished because of the high degree of vascularization and concentration of mitochondria found in brown adipose tissue.

brush border Microvilli on the free surfaces of intestinal epithe­lial cells and kidney tubule cells that resemble the bristles of a brush.

buccal cavity Although this translates literally to cheek cavity, the buccal cavity usually refers to the mouth or oral cavity; pro­nounced “BHU-kal.”

buccal surface The surface of the caudal part of the upper and lower arcade teeth that face the cheeks; bucco is Latin for cheeks.

buffer A substance that minimizes the change of the acidity of a solution when an acid or base is added to the solution.

bulb of the glans An enlargement in the penis of the dog and related species. It is made up of erectile tissue that slowly engorges with blood during copulation. When muscles sur­rounding the vagina and vulva of the female clamp down on the enlarged bulb, the male cannot withdraw the penis. He typically dismounts and turns so that the two animals are tail to tail. This position is known as the tie and usually lasts 15 to 20 minutes, after which the animals can separate.

bulbar conjunctiva The transparent membrane that covers the front (rostral) portion of the eyeball.

bulbourethral glands Male accessory reproductive glands that secrete a mucus-containing fluid just before ejaculation that lubricates the urethra for the passage of semen and clears it of urine. Bulbourethral glands are present in all common domestic animals except dogs.

bumblefoot General term for pathology of the avian foot. Includes cuts, abrasions, blisters, ulcers, punctures, and infections.

bursa of Fabricius Specialized organ in birds that is necessary for B-lymphocyte development. Mammals do not have this organ, but B (bursa-derived) lymphocytes were so named before their development in mammalian lymphatic tissue was determined.

C

calcaneal tuberosity Large process of the fibular tarsal bone that projects upward and backward; commonly referred to as the point of the hock; site of attachment of the gastrocnemius (calf) muscle; equivalent to the human heel.

calcified Hardening of organic tissue by the deposit of lime and calcium salts.

calciotropic Involved in the regulation of calcium levels in body. calcitonin The hormone secreted by the thyroid gland that pre­vents the level of calcium in the blood from getting too high.

callus The healing tissue between the ends of a fractured bone that is eventually replaced by true bone as the fracture heals.

calorigenic Heat producing.

CAMs See cell adhesion molecules.

canal of Schlemm The structure that drains aqueous humor from the anterior chamber of the eye. It is located at the edge of the anterior chamber, where the iris and the cornea meet.

canaliculi Tiny channels through the matrix of bone. Threadlike projections from osteocytes communicate with each other and with blood vessels through the canaliculi.

cancellous bone Spongy bone. A form of bone composed of a seemingly random arrangement of spicules of bone separated by spaces filled with bone marrow. Appears spongelike to the naked eye. Found in the ends (epiphyses) of long bones and the inte­riors of short bones, flat bones, and irregular bones.

canine parvoviral enteritis Caused by the canine parvovirus, this infectious disease has an extremely high mortality rate in nuppies. Infections occurring in utero are known to cause acute myocarditis and overall poor health in the litter. Because it tends tecokllsatta c in the mitotic phase, rapidly dividing epithelial tissue is particularly affected by parvoviral infections. Vomiting, ldoioardryhea, b stool, and dehydration are clinical signs of nisease. Ai immunization is available.

canine teeth Sharp, pointed teeth between the most caudal inci­sors and the most rostral premolars.

cannon bones The large metacarpal and metatarsal bones of the horse.

canthus The corner of the eyelids where they come together. Each eye has a medial and a lateral canthus.

capacitation The process spermatozoa undergo in the female reproductive tract that increases their fertility before contact with the ovum. Part of the process exposes the digestive enzymes icnrotshoemae. This helps the cell penetrate through the layers surrounding the ovum.

capillary refill time (CRT) Te he t it takes for mucous mem- eobtruarnnes t r to their normal, pink color after being pressed to e white color. If the return to normal color is too slow, it can ionodricate p peripheral blood perfusion.

capsular spacece The spa between the visceral and parietal layers I' Bowman’s capsule.

carapaceorsTahl e d shell of turtles and tortoises.

carbohydrate metabolism Metabolic processes that store and rneelregayse e contained within carbohydrates for the purposes reoopfwagitrh,, r and normal function of the body.

carbohydrates One o f the essential nutrients necessary for all life functions; sugars. They are a quick source of energy and may be stored in the body as glycogen.

carboxypeptidase Protease secreted in an inactive form from tehase pancr and activated by trypsin.

cardiat The par of the stomach where the esophagus enters.

cardiac muscle Striated, involuntary muscle that is found exclu­sively in the heart. Cardiac muscle is influenced by the auto- rnvoomuisc ne system. It has one centrally located nucleus and intercalated discs that form special connections between the muscle branches.

cardiac output The amount of blood that leaves the heart.

cardiac sphincter The smoollι muscle sphincter around the fentrance o the esophagus into the stomach.

cardiac tamponade A condition in which the heart becomes unable to expand normally between contractions because of opmressure fr fluid in the pericardial space.

carnassial teeth In the dog, the large, first lower molar tooth and fpoeurrth up premolar tooth. The carnassial teeth have deep roots.

carnivore An animal whose diet is primarily meat.

carpal bonesonTeshe b of the carpus. Consist of two parallel

rows of short bones located between the distal ends of the radius and ulna and the proximal ends of the metacarpal bones.

carpus The joint composed of the carpal bones. Referred to as the “knee’ ol' the horse and the “wrist” of humans.

carrier proteins Any prolei n that facilitates diffusion of a specific molecule through the cell membrane.

cartilage An opaque, dense connective tissue composed of a rela- ltively smal number of cells that are contained within a nonliv- irnixg. mat Cartilage absorbs shock and protects the epiphysial fnds of bones. Cartilage is not innervated or vascularized, which emsiasktaens itt r to pain but also to healing. It is found in joints,

rbuocdtyursets such as the ears and nose, the costal cartilage of itebh, e r cag and the fetal skeleton.

cartilaginous joint A j^mt in which the bones are united by cartilage; also called an amphiarthrosis. Onty a slight rocking emrmotitotnedis p between the bones.

caruncle Numerous mushroomlike structures in the lining of the uterns of ruminant animals. They join with the cotyledons of the placenta to form placental attachment sites called placentomes.

catabolism The breaking down of nutrients into smaller and seirmiaplsler mat for use by the cell to release energy; the oppo- fsoiltiesmo. anab

catagen phase The i ra ιιsil io>ιιal phase between anagen and thealsoegsen p of the hair growth cycle.

catalase An e^ rιιe found in almost all cells that breaks down ehryodxriodgeen p into water and oxygen.

catalyst Substance that induces chemical reactions by lowering tchtievation energy needed.

catecholamines The group of neurotransmitters that includes norepinephrine, epinephrine, dopamine, and others with shiemmiliacralc properties.

cations Positively charged ions.

caudalectAiondairl term meaning toward the tail end of an

animal.

caveolaevagTiinnayt iionns of the plasma membrane of verte-

eblrlas,te c particularly endothelial and fat cells, which are obelieved t initiate endocytosis, oncogenesis, and the formation of plaque in blood vessels. The invaginations are approximately 50-100 nm in size and are composed of protein, cholesterol, and other lipids. Caveolae may also play a role in the uptake of pathogenic bacteria and certain viruses.

cecocolic orifice The opening between the cecum and the colon; most developed in the horse and other nonruminant herbivores.

cecumlinTdhe b pouch leading off from where the ileum meets

tohloenc; in nonruminant herbivores, the cecum can be well developed, with a base, body, and apex (tip).

cell The most basic structural unit of all animals and plants. Cells make up all tissues of an organism and perform all of the func- tyhiiocnhs b w life is defined, including growth, reproduction, taanbdolmisem.

cell adhesion molecules (CAMs) Glycoproteins that aid not only ionntdhinegb of cells but also in lubricating the movement of one cell past another. They also help to transport specialized ecoealsls t ar of need.

cell membranelecTtihvelyse permeable outer membrane of

the cell that is composed of a phospholipid bilayer, protein, and cholesterol; also called the plasma membrane or plasmalemma.

cell metabolism Functions that break down nutrients, produce TATP,P, use A and create complex molecules from simple ones.

cell-mediated immune response Response of the body’s cells tehgautlartes the destruction of infectious bacteria and viruses edcuirfiicng sp immune responses.

cell-mediated immunity The portiιι of the immune system that produces “killer" cells that directly attack foreign invaders.

cellular respiration The oxidalioιι of organic material to yield energy, carbon dioxide, and water.

cellulase An em^,rιιe that breaks down the sugar cellulose.

cellulose A complex carbohydrate that makes up the cell walls and fibers of plant tissue. Herbivores digest cellulose through microbial fermentation. Cellulose is not digested by carnivores and most omnivores.

cementum The hard connective tissue that covers the root of a tooth and helps secure it in its bony socket.

central canal Small diameter canal, filled with cerebrospinal fluid, in the center of the spinal cord that is continuous with the fventricles o the brain.

central nervous system (CNS) The brain and spinal cord.

central sulcus The central depression of the frog in the equine hoof.

centriole A tubular organelle composed of nine triplets of micro­tubules that aids in the process of cell division. Centrioles split ion tw and migrate to opposite poles of a dividing cell to orga­nize the spindle fibers, enabling the cell to divide in two.

centromere Also called a kinetochore; the protein disc that holds fahrpoamiraotidcs together as a chromosome and then holds

thhraotmcosome to a spindle fiber during cell division.

centrosome An area of condensed cytoplasm located near the nucleus that contains the centrioles of the cell.

cere Fleshy colored skin located at the base of the upper beak in many bird species; supplied with touch corpuscles.

cerebellar dysfunction Caused by inflammation, underdevelop­ment, or dngeneration of the cerebellum, the condition is char- aycterized b an exaggerated and awkward gait, usually resulting fyrpoemrexhtension of the legs. Cerebellar dysfunction does not cause any degree of paralysis, partial or complete.

cerebellum Second largest component of the brain; allows the vboeody t ha coordinated movement, balance, posture, and ecoflmexpesle. x r

cerebral cortex Gray matter that makes up the outer layer of the cerebrum.

cerebral hemispheres The two halves of the cerebrum. cerebrospinal fluid (CSF) Fluid that bathes and protects the brain and spinal cord from the hard inner surface of the skull aerntdebsrpaien.al v

cerebrumortTiohnat p of the brain responsible for functions most commonly associated with “higher-order” behaviors (learning, intelligence, awareness); receives and interprets sensory information, initiates conscious (voluntary) nerve iompulses t skeletal muscles, and integrates neuron activity trhmaatlilsy no associated with communication, expression of emotional responses, learning, memory and recall, and other behaviors associated with conscious activity.

cervical vertebraeonTeshe b of the neck portion of the spinal column.

cervixphTinhcetesr muscle “valve” between the uterus and the

vagina; controls access to the lumen of the uterus from the vagina. It is normally closed except during breeding and parturition.

chemical bondyorcAe f b which atoms are bound in a molecule. fyTphes t o chemical bond are covalent bonds, ionic bonds,

aydnrdohgen bonds.

chemical control systemespTirhaet orry control system that

monitors the pH of the blood and its content of O₂ and CO₂. If fany o the measured values varies outside preset limits, the cohnetmroilcal c system initiates changes in the breathing ropiantgtern t b them back into balance.

chemical digestion The breakdown of food by the action of rchemicals o enzymes.

chemical element Any ol' 116 known substances that cannot be separated into smaller substances. The smallest unit of an element is an atom.

chemical equation A symbolic representation of a chemical reaction. Arrows are used to denote in which direction the reac­tion is occurring. Chemical symbols are used to denote the reactants and products of the reaction.

chemical reactionrocAesps that results in the creation of new chemicals, involving changes in the movement of electrons in forming and breaking chemical bonds.

chemical signaling The specific interaction of hormones and neurotransmitters with cell surfaces for the purpose of changing cell artivity.

chemical symbolbreTvhiaetiaobn of the name of a chemical

element. Used to identify the element in the Periodic Table of the Elements.

chemotaxis The movement of white blood cells into an area of inflammation in response to chemical mediators released at the site by injured tissue or other white blood cells.

chestnut Believed to be the vestigial remnants of carpal and tarsal Xones Ot extra toes possessed by ancestors of the modern horse; trehney,y ar ho keratinized growths located on the medial fore­arms and hocks of horses.

chief cells Cells in the stomach that produce the enzyme precur- seoprsipnogen.

choanaeernTawl o int nares that open from the nasal chambers

ionotfo the r of the mouth of birds.

cholecystokinin (CCK) rmAohne released by the duodenum hwyhmene c enters from the stomach. It slows gastric emptying and motility while increasing intestinal motility. It also stimulates the pancreas to release digestive enzymes into the duodenum.

cholesteroleroAidst alcohol that is found in many fat-based

toiussguheosutthr the body. Cholesterol can be synthesized in

trohdeyb o obtained from the diet.

cholinergic neurons Neurons that secrete acetylcholine as their neurotransmitter.

cholinergic receptors Receptors for acetylcholine; may be mus- rcarinic o nicotinic receptors.

chondroblastsellsFixed c that form cartilage.

chondrocyte Mature cartilage cell.

chondroitin sulfate A glyceaaminoglycan found in cartilage. chondronectindhAesnive a glycosaminoglycan found in

cartilage.

chordae tendineae Fine, threadlike cords that connect the two atrioventricular valves to the appropriate papillary muscles in tehnetrvicles.

chorion Part of the placenta; the outermost layer that attaches to the ιeOTiιιe lining. The chorion is linked to the fetus by the ourmdb. ilical c

chorionic gonadotropin A hormone produced by the placenta roefganapnt animal.

choroid A portion of the uvea, or middle vascular layer, of the eye. The choroid consists mainly of pigment and blood vessels and is located between the sclera and the retina.

chromatids Strands of genetic material that, when joined together with another chromatid by a centromere, form a chromosome.

chromatin A material that is composed of DNA and proteins and makes up chromosomes.

chromatophore Any pigment cell or color-producing plastid. chromosomes Threadlike accumulations of DNA in the nuclei of cells that are particularly visible during mitosis. The DNA of the chromosomes contains the genetic material of the cell. The number of chromosomes is constant within a given species.

chyle The milky lymph from the intestines consisting primarily of small molecules of absorbed fats.

chylomicrons Microscopic particles of fat found in chyle and blood. Their numbers are highest after a meal.

chyme The semifluid, partially digested food that leaves the stomach and enters the duodenum.

chymotrypsin Protease secreted in an inactive form from the pancreas and activated by trypsin.

cilia Hairlike processes of the luminal surfaces of cells that assist in the movement of mucus, fluid, and solid material across the cell surface.

ciliary body A portion of the uvea, or middle vascular layer, of the eye. The ciliary body is a ring-shaped structure located immedi­ately behind the iris. It contains the ciliary muscles that adjust the shape of the lens and the cells that produce aqueous humor.

ciliary muscles Multiunit smooth muscles of the ciliary body that adjust the shape of the eye's lens.

circular muscle The smooth muscle in the muscle layer of the gastrointestinal tract that encircles the organ and causes mixing or segmental contractions; plays a role in peristalsis; sphincters are made of concentrated circular muscle.

circulating pool of neutrophils Neutrophils found in the periph­eral blood flowing through the center of blood vessels.

circumduction A joint motion whereby the distal end of an extremity moves in a circle.

cisterna A reservoir that stores fluid.

citric acid cycle A complicated metabolic pathway that is included in the metabolism of fats, proteins, and carbohydrates and that involves the oxidation of pyruvic acid and the release of energy. Also called the Krebs cycle and the tricarboxylic acid cycle.

claws Accessory appendages of the integumentary system, present mainly in carnivores, used for grasping prey and self-defense.

cleavage The process of very rapid cell division after an ovum has been fertilized. The cells divide so rapidly that they do not have time to grow appreciably between divisions. The number of cells increases rapidly, but the overall size of the cell mass does not increase much.

clitoris One of the structures of the vulva of the female. Homolo­gous to the penis of the male, the clitoris contains erectile tissue and is richly supplied with sensory nerve endings.

clutch The total number of eggs laid in a single nesting period. CNS See central nervous system.

coated pit Parts of the cell membrane that have a hairlike coating necessary for endocytic functions. These portions of the cell membrane pinch off to form vesicles that aid in the intracellular transport of materials.

coccygeal vertebrae The bones of the tail portion of the spinal column.

coccyx The human “tailbone.” It consists of four to five coccygeal vertebrae fused into a solid structure.

cochlea The snail shell-shaped cavity in the temporal bone of the skull that contains the hearing portion of the inner ear.

cochlear duct A long, fluid-filled tube that runs the length of the cochlea. It contains the receptor organ of hearing (the organ of Corti).

codon The genetic code of an amino acid expressed in DNA or messenger RNA as three bases.

coenzyme An organic molecule that is required by an enzyme to carry out a metabolic reaction. Coenzymes, such as nicotin­amide adenine dinucleotide (NAD), are often derived from vitamins.

cofactors Elements, such as coenzymes, that act concurrently with another element to carry out a chemical reaction.

coffin bone The distal phalanx bone of the horse.

collagenous fiber A structural protein that is commonly located in tendons and ligaments.

collateral sulcus Deep ridge on either side of the frog that sepa­rates it from the bars in the equine hoof.

collateral Located on both sides.

collecting ducts The system of tubules that collects tubular fil­trate from the distal convoluted tubules and carries it to the renal pelvis. They are not considered part of the nephron.

colloidal Referring to a gelatinous and viscous fluid.

colloids Emulsions; heterogeneous mixtures that contain much larger sized solutes than those found in solutions.

colon The last large component of the intestinal tract; responsible for absorption of water and electrolytes; extensively developed in nonruminant herbivores, such as horses.

colostrum The initial secretion of the mammary gland before milk is produced. Colostrum is rich in nutrients, has a laxative effect on the newborn, and contains antibodies to the diseases the dam has been exposed to or vaccinated against. If the newborn drinks the colostrum within the first few hours of birth, the large antibody molecules will be absorbed intact by the intestine and impart passive immunity to the young animal. columella The middle ear bone in birds.

columnar epithelial cells Tall, thin epithelial cells with nuclei located at the basal end; often ciliated.

columnar epithelium Epithelium composed of columnar cells. common bile duct The duct leading from the gallbladder to the duodenum; may be formed by the merging of the cystic duct from the gallbladder and the hepatic duct from the liver.

common vaginal tunic The outer connective tissue sac that sur­rounds the testis. It is derived from the layer of parietal perito­neum that was pushed ahead of the testis as it descended through the inguinal ring. Also called the parietal vaginal tunic. compact bone Heavy, dense bone made up of tiny, tightly com­pacted, laminated cylinders of bone called Haversian systems; makes up the shafts (diaphyses) of long bones and the outer surfaces of all bones.

complement A group of inactive enzymes (proteins) in plasma that can be activated to rupture the cell membrane of a foreign cell. Complement can also act as an opsonin.

compound A substance made up of two or more elements.

compound follicles Follicles through which more than one hair emerges.

compound gland An exocrine gland with branched ducts. concentration gradient Tlre s^^urιι between the area of highest concentration and the area of lowest concentration.

conduction of the action potential Another name for a nerve impulse.

condyle A large, rounded articular (joint) surface. Examples are fnound o the distal ends of the humerus and femur.

cones Photoreceptors in the retina of the eye that perceive color aetnadil.d

congenital Present at birth.

congestive heart failure Failure of the heart to maintain circula­tory equilibrium with resulting congestion of the venous system. It is cli;iraclerized by edema of the lungs, enlargement of the heart, increase in heart rate, and dilation of peripheral blood vessels.

conjunctiva The thin, transparent membrane that covers the front portiιι of the eyeball and lines the interior surfaces of the eyelids.

conjunctival sac The space between the bulbar and palpebral portions of the conjunctiva; the space between the eyelid and tyheebaell.

connective tissue Tissue made up of cells and extracellular sub­stances that connect and support cells and other tissues.

connective tissue papilla The base of the hair follicle that ulti­mately provides the material necessary to create hair.

connective tissue proper Includes all types of connective tissue except for bone, blood, and cartilage. Connective tissue proper is didided into two subclasses: loose connective tissue and dense connective tissue.

connexon Proteinaceous channel that aids in the intercellular ftransport o nutrients.

contact inhibition TTrat properly^, that inhibits cells from dividing when in proximity to other cells.

contact signaling Cell-to-cell recognition, which is important in iemspmonusnees r to infection.

contractile proteins Proteins that are essential to muscle contrac- tuicohns, s as myosin and actin.

contractility The inherent ability of the heart to develop a force boyntcracting, which increases chamber pressures.

contralateral reflex Reflexes that are initiated on one side of the rbaovdeyl and t to the opposite side to produce their effect.

contrast radiography Radiographs (x-rays) taken after a con­trast medium has been introduced into the patient's body to ma^e organs or structures more visible on the finished image.

conus papillaris A structure histologically similar to the pectin (a vascular structure in avian eyes). Found in the eyes of lizards laonsdelcy related reptiles.

coping The process of trimming and shaping a bird's beak. coprodeum Anterior section of the avian cloaca that receives excrement from the intestine.

copulation The act ol' breeding; consists of intromission of the oenis into the vagina, thrusting, and ejaculation.

corium The dermis of the skin.

cornea The clear “window” on the front of the eye that admits light to the interior of the eye; part of the outer, fibrous layer of tyheebaell.

cornification Formation of a layer of tough keratin on an epithe- luiarflasce.

cornual processhoTrnhe “ core” of horned animals; a process ofthe frontal bone. The hollow cavity within the cornual process is conliiiuous with the frontal sinus (the paranasal sinus of the fornoen)t.al b

corona glandisideTsthe w portion of the glans penis of the. horse.

corona radiata A thin layer of granulosa cells that surrounds the ovum as it develops in the ovarian follicle and after it is released vbuyloation.

coronary bandt The par of the hoof that articulates with the skin.

coronary coriumt The par of the corium that has differentiated trovpide nourishment to the hoof at the site of the coronary

band.

corpus callosum White fibers that connect and provide com­munication pathways between the two cerebral hemispheres.

corpus cavernosum penis The larger of the two erectile tissue structures in the body of the penis. It is located dorsal to the somrpaullser c cavernosum urethrae.

corpus cavernosum urethraelerThe smal of the two erectile triuscsutueresst in the body of the penis. It forms a “sleeve”

around the urethra and is located ventral to the larger corpus cavernosum penis.

corpus hemorrhagicum Thebfood-Alled remnant of the ovarian follicle immediately after ovulation.

corpus luteum Literally “yellow body.” The solid endocrine struc­ture that forms from the empty ovarian follicle after ovulation. Under stimulation from luteinizing hormone from the anterior lpaintudi,tary g the granulosa cells left in the empty follicle omrumltiply t f the solid corpus luteum. It produces progestin hormones, principally progesterone, that are necessary for the maintenance of pregnancy.

cortex (1) foe outer, superficial layer of an organ or structure. t2) foe portion of the kidney. It contains the renal cor- rpouxsicmleasl, p convoluted tubules, distal convoluted tubules, collecting duds, and peritubular capillaries. (3) In hair, it is the luaryreorusnding the medulla, composed of hard keratin.

costal cartilage The cartflaginous, ventral portion of a rib. cotyledon Numerous areas on the surface of the placenta of rumi­nant animals that join with the caruncles in the lining of the uterus to tom placental attachment sites called placentomes.

cotyledonary placental attachmentypeThe t of placental attachment found in common ruminant animals. It consists of numerous cotyledons on the surface of the placenta joining with caruncles in the lining of the uterus to form attachment sites called plscentomes.

cough A pvoledive response to irritation or foreign material in trhacehtea or bronchi. The sudden, forceful expiration of air is iontended t force the irritant or foreign material up and out of the iesniralry tract. Productive coughs are moist and generally help an animal to clear mucus and other matter from the aoinrpwraoyds.uNctive coughs are dry and generally not ben-

eoficial t the animal.

covalent bond Chemical bonds in which electrons are shared.

cranial nerves Set ol' 12 pairs of nerves originating directly from the brain; may be sensory or motor or may contain both sensory ond motor nerves.

cranial-sacral system Another term that defines the parasym­pathetic nervous system, based on the fact that the parasympa­thetic nerves emerge from the cranial nerves and from the sacral spinal cord segments.

cranial A directional term meaning toward the head of an animal. cranium The cranial portion of the dorsal body cavity, formed from several skull bones. It houses and protects the brain. It is the reference point for the directional term cranial.

creatine phosphate (CP) The molecule in muscle cells that splits to release the energy necessary to reattach the detached phos­phate group to an adenosine diphosphate (ADP) molecule to convert it back to the high-energy molecule adenosine triphos­phate (ATP).

cremaster muscle The bandlike muscle that raises and lowers the testes in the scrotum to help control their temperature. The testes must be maintained at a temperature slightly cooler than body temperature to produce spermatozoa.

Creutzfeldt-Jakob disease A disease in humans that was discov­ered by Hans Gerhard Creutzfeldt and Alfons Maria Jakob. The disease has no known cure and causes a fatal spongiform encephalopathy. Prions are known to be the infectious agent.

cribriform plate The sievelike area of the ethmoid bone through which the many branches of the olfactory nerve pass from the upper portion of the nasal cavity to the olfactory bulbs of the brain.

cricoid cartilage One of the cartilages of the larynx. The cricoid cartilage is ring-shaped. It helps form and support the caudal portion of the larynx.

crista The short name for the receptor structure of the semicir­cular canals.

crista ampullaris The full name of the receptor structure of the semicircular canals.

cristae The folds within the mitochondria that increase ATP output by increasing the surface area and thus the number of reaction sites.

crop Dilated portion of the esophagus in some species of bird that acts as a storage pouch for food.

cross-bridges Tiny “levers” on the myosin filaments of muscle. A muscle cell contracts by ratcheting the cross-bridges back and forth to pull the thinner actin filaments toward the center of the myosin filaments.

crossed extensor reflex Reflex initiated by a stimulation of a limb that results in extension of the limb on the other side of the body.

crown (1) Top part of the head. (2) The enamel-covered, exposed part of a tooth.

CRT See capillary refill time.

crude protein The total nitrogen content of a feed multiplied by 6.25. The crude protein gives a close approximation of the protein content in a particular feed.

crura A portion of the roots of the penis. The connective tissue bands that attach the penis to the brim of the pelvis.

cryptorchidism The condition of one or both testes failing to descend into the scrotum. Cryptorchidism in animals may be unilateral (one side only) or bilateral (both sides).

crypts Invaginations in the intestinal mucosa that produce the cells that form the villi.

cuboidal cells Cube-shaped cells with centrally located nuclei. cuboidal epithelium Epithelium composed of cuboidal cells. cumulus oophorus The small mound of granulosa cells on which the oocyte sits as it develops in the ovarian follicle.

cupula The gelatinous structure that sits on top of the receptor hairs in the crista ampullaris of the semicircular canals.

cutaneous membrane Also known as the integument. The outer layer (epidermis) is composed of keratinized stratified squa­mous epithelium. This helps to waterproof and prevent dehy­dration of the body. The inner layer or dermis is composed of dense irregular connective tissue, as well as collagenous and elastic fibers. This also helps the cutaneous membrane by giving it reinforcement and flexibility.

cutaneous muscles “Skin muscles”; thin muscles in the connec­tive tissue beneath the skin. When a cutaneous muscle contracts, it causes the skin to twitch.

cuticle The single layer of cells that make up the outermost layer of the hair shaft.

-cyte Suffix meaning cell.

cytochromes Essential to the final stage of the electron transport system, this copper-containing complex of enzymes transfers electrons to oxygen. The oxygen in turn can bond with hydro­gen, thus producing water.

cytokines Signaling molecules responsible for cell movement/ signaling.

cytokinesis The separation of the cytoplasm into two separate daughter cells during the mitotic stage of cell division called telophase.

cytologist One who studies cells.

cytoplasm The part of the cell's protoplasm that is located outside the nuclear envelope.

cytosine (C) One of the nucleotides present in both RNA and DNA. It is a pyrimidine base that pairs with RNA and DNA's guanine.

cytosis The active transport of materials into or out of the cell. The transported material is bound by a membrane.

cytoskeleton The internal structure of the cell that maintains the cell's shape and aids in some functions. The cytoskeleton is composed of three types of filament: microfilaments, microtu­bules, and intermediate filaments.

cytosol The fluid component of protoplasm that acts as its base. cytotoxic T cells Also known as killer cells or killer T lymphocytes.

They attach to antigenic cells and destroy them, but they are not themselves damaged.

D

DCT See distal convoluted tubule.

deamination The process during catabolism in which the amino group NH₂ is removed.

decomposition reaction A chemical reaction in which a complex reactant is divided into simpler molecules or elements. The opposite of a synthesis reaction.

deep A directional term meaning toward the center of the body or a body part; see internal.

defecation The expelling of feces.

defibrillation The process of sending an electrical current through the heart to restart coordinated pumping activity and restore regular rhythm.

dehydration synthesis The combination of two or more simple materials to form one or more complex materials by removing water. For example, two monosaccharides can combine to form a disaccharide:

delivery of the newborn Passage of the newborn animal through the vagina from the uterus to the outside world. Accomplished by a combination of uterine and abdominal muscle contractions.

delivery of the placenta Passage of the placenta through the vagina from the uterus to the outside world after delivery of the newborn. Accomplished by a combination of uterine and abdominal muscle contractions.

deltoid crest See pectoral crest.

dendrites The receptive sites of the nerve cell; they extend from the cell body, giving the cell a starlike shape; they receive stimuli and convey them as nerve impulses to the cell body.

dens Process on the cranial end of the second cervical vertebra (axis) that fits into the caudal end of the first cervical vertebra (atlas).

dense bodies Structures in smooth muscle cells to which the small contractile units of actin and myosin attach. The dense bodies of smooth muscle correspond to the Z lines of skeletal muscle to which the actin filaments attach.

dense connective tissue A highly fibrous connective tissue with little vascularization. It functions to reinforce and bind body structures. Dense connective tissue is of two types: dense regular and dense irregular.

dense irregular connective tissue A collagen-based fibrous con­nective tissue that is found in the dermis, spleen, and liver. It has thicker bundles of fiber than dense regular connective tissue and is designed to withstand tension from multiple directions.

dense regular connective tissue Tightly bound, minimally vas­cularized fibrous connective tissue found in ligaments, tendons, and fascia. In ligaments it binds joints, whereas in tendons it binds muscle to bone. In fascia, it helps support surrounding tissues.

dental formula The shorthand abbreviation that shows the number of incisors (I), canines (C), premolars (P), and molars (M) that a species has; the first number of the pair indicates the number of teeth in half of the upper arcade; the second number indicates the number of teeth in half of the lower arcade; lowercase letters indicate deciduous teeth; for example: I3/3 C1/1 P2/2 M3/3.

dental pad The hard, thick connective tissue pad that ruminants have in the space occupied by upper incisor teeth in other species.

dental prophylaxis The cleaning of teeth; slang is dental prophy. dentin The layer surrounding the tooth pulp; more dense than bone but not as dense as the overlying enamel.

deoxyhemoglobin Hemoglobin that is not carrying any oxygen; also known as empty hemoglobin.

deoxyribonucleic acid Referred to as DNA. The genetic material of a living thing, found in strands called chromatin within the nucleus of the cell.

depolarization A reduction in the voltage across a neuron or muscle cell membrane from its normal polarized state (resting membrane potential). This results in an action potential (nerve impulse) in the case of a neuron or the initiation of contraction in the case of a muscle cell.

dermis The deep, connective tissue portion of the skin that con­tains blood vessels, glands, and hair follicles.

desmosome A type of intercellular attachment found in epithe­lial tissue. The bond is formed from the interlocking of fila­ments that connect the plasma membranes of adjacent cells.

determinate layer Species that can lay only the number of eggs in a normal clutch size.

development The growth of an organism to full size or maturity.

dewclaw A toe that does not reach the ground, such as the first digit of dogs and cats and the rudimentary medial and lateral toes of cattle.

diabetes insipidus A disease resulting from a deficiency of antid­iuretic hormone from the posterior pituitary gland. It results in polyuria and polydipsia.

diabetes mellitus A disease resulting from a deficiency of the hormone insulin from the pancreatic islets. The lack of insulin prevents glucose from entering cells and being used as an energy source. This results in signs that include hyperglycemia, glycosuria, polyuria, polydipsia, polyphagia, weight loss, and weakness.

diapedesis The process by which white blood cells leave capillar­ies and enter tissue by squeezing through the tiny spaces between the capillary cells.

diaphragm The thin, dome-shaped sheet of muscle that forms the boundary between the thoracic and abdominal cavities. A muscle that helps produce inspiration when it contracts. The diaphragm is dome-shaped at rest, with its convex surface directed cranially. When it contracts, the dome of the diaphragm flattens out, which increases the volume of the thoracic cavity and causes air to be drawn into the lungs.

diaphragmatic flexure Where the left dorsal colon of the horse bends back on itself to form the right dorsal colon.

diaphragmaticus Diaphragm.

diaphysis The shaft portion of a long bone.

diapsid skulls Skulls of various reptiles (lizards, snakes, croco­diles) with two pairs of temporal openings behind each eye.

diarthrosis A freely movable synovial joint.

diastole The part of the cardiac cycle associated with relaxation of the atria and ventricles and the filling of the chambers with blood.

diencephalon Serves as a nervous system passageway between the primitive brainstem and the cerebrum; three major struc­tures of the diencephalon are the thalamus, the hypothalamus, and the pituitary.

diestrous Animals that have two estrous cycles per year.

diestrus The active luteal stage of the estrous cycle. During this period, the corpus luteum has reached maximum size and is producing maximum amounts of progestin hormones.

differentiation The progressive acquisition of individual charac­teristics by cells to enable them to perform different functions. diffuse placental attachment A loose form of placental attach­ment to the uterine wall; attachment sites are spread diffusively over the whole surface of the placenta and the whole lining of the uterus; found in horses, swine and camelids.

diffusion The tendency for molecules to move from an area of high concentration to an area of low concentration.

digestive enzyme An c^rικ in the digestive tract that aids the breakdown of complex food substances into simpler molecules that can be absorbed into the bloodstream.

digestive system The co∏ection of organs that take in, digest and absorb food for use by the body to maintain health and normal function. This system includes all structures of the mouth, esophagus, stomach, and the small and large intestines. Although ftnot par o the digestive tract, accessory digestive organs such as tire gallbladder and pancreas provide secretions that aid in thheemcical part of the process.

digestive tract Also called the alimentary canal, tins encompasses all of the parts of the digestive system that transport food from the moullι to the anus: mouth, esophagus, stomach, small and leasrtgineeisn, t and anus.

digitod A toe made up of two or three bones called phalanges.

dilated cardiomyopathy Enlargement of the heart resulting foom dilation of the heart's chambers and thinning of the ven­tricular walls. If left untreated, the condition will ultimately lead to heart failure.

dipeptide A molec ule that consists of two amino acids joined by laeepstiindge p bond.

diploid chromosome number The chromosome number in all of an animal's cells except for the gametes (reproductive cells). Iwtaiyssal an even number.

disaccharideug “Two sugars”; includes sucrose, maltose, isomalt­ose, and lactose.

discoid placental attachment Attachment of the placenta to the uterus in a single, disc-shaped area; found in primates, rabbits, ayonddenmtsa. n r

distal convoluted tubule (DCT) The last tubular part of the nephron before it enters the collecting duct. DCTs are found in tyh'se kidne cortex.

distal phalanx bone The phalanx that is located most distally fordoym; the b the tip of the digit.

distal sesamoid bone The navicular bone oehorses. It is located in the digital flexor tendon deep in the hoof behind the joint between the middle and distal phalanges.

distal A directional term used only for extremities of the body. It amplies a position or direction away from the body proper.

disulfide bonds Strong covalent bonds that unite amino acids in rgolotebiunlar p formations.

diuresis Producing and passing large amounts of urine.

DNA polymerase Ao ^^,rae that reestablishes the double helix yf I )NΛ by uniting the nucleotide bases with their correspond­ing base pairs. This creates a new strand for each original DNA strand.

dolichocephalic Long-faced. The Rough Collie is a dolichoce- ropegheadl.ic d b

dopamine A caleclιolamiιιe neurotransmitter.

dorsal A dnectional term meaning toward the top ol' an animal when it is standing on all four legs; toward the backbone.

dorsal body cavity The space in the skull and spinal column that contains the brain and spinal cord. The portion in the skull is called the cranium,oarntidonthe p in the spinal column is called the spinal canal.

dorsal horn The aκa of the spinal cord's gray matter “butterfly” where the neurons that forward sensory (afferent) nerve impulses trhoaeint b or other parts of the spinal cord are located.

dorsal nerve rootranTche b off each side of the spinal cord cbhetween ea set of adjacent vertebrae that conducts sensory iompulses int the spinal cord from the periphery of the body.

dorsal planeomAicn anat reference plane that divides the body

onto dorsal (upper) and ventral (lower) parts that are not neces­sarily equal.

double helix Also called the Watson-Crick helix; the double coils seen in I )NΛ that contain specific nucleotides whereby one set determines the corresponding set.

dressing forceps A tweezerlike surgical instrument with serrated tips used to grasp gauze sponges and other surgical materials. It iesnndoedt int for use on tissue.

ductubAelitke channel that provides an exit route for secretory

or exaretory products.

duodenumst The fir segment of the small intestine after the stomach. Chyme enters the duodenum from the stomach.

dura materk The thic outermost layer of the meninges that rcaoivners the b and spinal cord; it is considered to be the tough­

fest o the meninges.

dysfunction Abnormal functioning of an organ or body part.

dystocia A difficult birth. Dystocias usually result from a fetus tohoat is t large for the birth canal or one that is oriented inap- aropriately for delivery.

E

eardrum Common name for the tympanic membrane. The paper-thin, connective tissue membrane that is tightly stretched apcernoisnsgthe o of the external ear canal into the middle ear. eccrine glands Exocrine glands that secrete substances directly onto the skin without the loss of cellular material. They contain soiimlepd,le, c tube structures for excretory purposes.

eccrine sweat glands Sweat glands that cover the entire surface of the body of some animal species for the purpose of thermoregulation.

ecdysis Shedding of the skin of reptiles.

eclampsia A condition seen in lactating dogs and cats that results fyrpoomcalhcemia. Early signs of eclampsia include muscle tremors and spasms.

ectothermic “Cold-blooded,” not internally regulated. Body tem- apreireasture v with the environment. Includes all animals except mammals and birds.

edema An abnormal accumulation of fluid, either localized or generalized, within the tissues or cavities of the body.

effector celleulclhA c s as a muscle or gland cell that carries out scotimone a when stimulated by a nerve impulse.

efferent A directional term meaning away from some reference point.

efferent ducts of the testes The passageways that allow sperma- toozoa t move from the rete testis to the head of the epididymis.

efferent glomerular arterioles Arterioles that leave the glom- feruli o the kidney. They are carrying blood that has been fil- tyered b the glomeruli, so it contains less water. Blood in the leoffmereernutlagr arterioles has a higher concentration of

blood cells and plasma proteins than blood in the afferent glo- tmereiroulelasr. ar

efferent nerve Nerve that carries impulses away from the central n∣ervous system.

effusion Excess fluid that has escaped into a body cavity to the detriment ol' normal body function.

egg binding The: condition resulting when an egg gets stuck in tvhideuoct of a bird.

eicosanoids Any of a group of substances derived from 20-carbon unsaturated fatty acids, such as arachidonic acid. Includes prostaglandins, leukotrienes, and thromboxanes. They are the principal mediators of inflammation.

ejaculation The rellex expulsion of semen from the penis.

elastase Protease secreted in an inactive form from the pancreas catnivdaated by trypsin.

elastic cartilage Also called yellow cartilage; similar to hyaline ccrtilage, except that it is more opaque and contains many elastic fibers. It is found in the external ear and in the epiglottis.

elastic connective tissue Connective tissue composed of large numbers ol' elastic fibers; found in tissues that expand and con- turcahct, s as the lungs and vocal cords.

elastic fibers Fibers composed of elastin. Elastic fibers form a delicate mesh in tissues.

electrocardiogram A recording of the electrical activity of the heart.

electrolyte A sulastance that conducts an electric current in solution.

electronhtwAeliigght subatomic particle that carries a negative

cherge. Together with the nuclei, they make up the atom; they tairceletshe par responsible for chemical bonding.

electron microscope A powerful microscope that magnifies a syample b using an electron beam for illumination.

electron shell Thegrouping of electrons around the nucleus of aohnme a. t T electron shell is determined by the energy level of ltehcet reon. The electrons in the outer electron shell are the eosnpeosnrsible for chemical reactions.

electron transport system The final and most productive stage eolfl ulacr respiration, which takes place inside the mitochondria.

electrostatic attraction The a∏raclive force between two parti- fcplpeos soiteo electric charge. The force is proportional to the

magnitude of the charge and inversely proportional to the dis- teatwncee nb the particles.

element Any ol' 116 known substances that cannot be separated lienrto smal substances. The smallest unit of an element is an atom.

embryo The name generally given to the developing offspring during the first trimester of pregnancy. During this period, the newly implanted zygote and its placenta are getting themselves organized and the body tissues, organs, and systems begin to form.

embryonic hemoglobin Hemoglobin found in red blood cells ldyuring ear fetal life.

emulsification The mixing of fat or oil and water by agitation or shaking.

enamel Outer coating layer of the crown of a tooth; toughest substance in the body.

endocardiumrmTohste inne layer of the heart.

endochondral bone formation The type of bone formation ownhe reby b grows into and replaces a cartilage model. This is

the method by which most bones form in a developing fetus, starting with cartilage “prototypes” that are gradually replaced tobyneb. I is also the means by which long bones increase in length at the epiphyseal (growth) plates. New cartilage is created tosnidethe ou surfaces of the plates, and bone replaces old cnartilage o the inside surfaces. This allows the bones to increase inngtlhe as the animal grows.

endocrine glands Glands or cells that release their regulatory products (hormones) directly into the bloodstream. Endocrine golnatnrdols c most metabolic functions. Examples of endo- lcarnindes g include the pituitary, parathyroid, and pancreas.

endocrine system The system of glands that controls and regu- loadteys b functions through the internal secretion of hor­mones. The hormones are released directly into the bloodstream, lwohwisch al them to exert their actions on target cells through- odt the body.

endocrinology The study of the endocrine system.

endocytosis The taking in of a material from the outside of the ceyealtlinbg cr a “mouth” with the plasma membrane. The membrane engulfs the material and pinches off at the ends to form a vesicle. Endocytosis includes the processes of pinocytosis ahnagdopcytosis.

endolymph The fluid in the receptor structures of the inner ear. endolymphatic sacs Sacs that are located in the inner ear and cnodnotlayimn peh.

endomysium The thin, delicate layer of connective tissue that surrounds each individual skeletal muscle fiber.

endoplasmic reticulum (ER) A system of channels within the cell touhmnat r fr the nucleus to the exterior cell membrane. The two forms ol' ER have their own functions. Rough ER is the site for protein synthesis, and smooth ER is the site for lipid synthesis.

endosteum The fibrous membrane that lines the hollow interiors ofnebs.

endothelium Derived from mesothelium, the endothelium is fcomposed o simple squamous epithelium. It lines the heart, ebslsoeolsd, v and serous cavities of the body.

endotracheal (ET) intubation The placement of a soft rubber or uplbaestic ET t through the larynx and into the trachea to estab- lpisehn an o airway.

endotracheal (ET) tubefutbAberso r or plastic tube that is opuasgshed thr the larynx and into the trachea to establish an owpaeyn; air often used to administer inhalation anesthetics.

energy of activation The amount of energy necessary to initiate eaarction.

enteric Referring to the intestine.

enteritis Inflammation of the intestines.

enterogastric reflex The rellex in which the presence of food in tohme ascth stimulates motility and digestive secretions in the

intestine.

enzyme A specialized globular protein that carries out and/or shpeemedicsalup c reactions in the body by acting as a catalyst awnedrinlog the temperature necessary for the reaction to take epclaificec. Sp enzymes are exclusive to specific reactions, and although they may change the rate of reaction, they are never rchanged o used up themselves in the process.

eosinopenia A decrease in the total number of eosinophils in peripheral blood.

eosinophil The granulocytic white blood cell characterized by the presence of numerous red-staining granules in its cytoplasm.

eosinophilia An increase in the total number of eosinophils in peripheral blood.

epicardium Outermost layer of the heart. epidermal Referring to the epidermis.

epidermal orifice The opening of the hair follicle through which the hair emerges.

epidermis Composed of keratinized stratified squamous epithe­lium, it is the outermost layer of the skin.

epididymis The ribbonlike structure that lies along the surface of the testis. It is actually one long, convoluted tube that links the efferent ducts with the vas deferens. Spermatozoa are stored in the epididymis as they await ejaculation.

epidural anesthesia The administration of anesthetic agents into the space between the dura mater and the surrounding bone of the vertebrae.

epiglottis The most rostral of the laryngeal cartilages. It projects forward from the ventral portion of the larynx, and its bluntly pointed tip usually tucks up behind the caudal rim of the soft palate when the animal is breathing. When the animal swallows, the epiglottis is pulled back to cover the opening of the larynx, like a trapdoor.

epimysium The tough, connective tissue layer that covers and delineates individual muscles. It surrounds groups of skeletal muscle fascicles.

epinephrine Commonly called adrenaline. A hormone secreted by the medulla of the adrenal gland under stimulation by the sympathetic portion of the autonomic nervous system. It pro­duces part of the fight-or-flight response that results when an animal feels threatened.

epiphyseal plate The growth plate of a long bone. Epiphyseal plates are located at the junction of the proximal and distal epiphyses with the diaphysis. They are areas where long bones increase in length by the process of endochondral bone formation. When an animal reaches its full size, the epiphyseal plates of its bones completely ossify and the bones cease their growth.

epiphysis The end of a long bone. Each long bone has a proximal and a distal epiphysis.

epithelial tissues A collection of tissues that are made up of layers of cells that line and cover body surfaces. These cells may be in single layers or multilayered and can regenerate quickly.

epithelialization The rapid division of epithelial cells around a wound edge. Epithelialization attempts to cover the opening of a wound. This process is assisted by the contraction of collagen fibers, which bring the edges of the epithelial layer into close opposition.

equator The center of the spindle apparatus where the chromo­somes line up during metaphase in cell division; also known as the equatorial plate.

equilibrium (1) The balance of solutes and solution between the inside and outside of the cell. For example, in the case of salts and water, the concentrations on either side of the cell mem­brane will be in a state of equilibrium. (2) The sense that helps an animal maintain its balance by keeping track of the position and movements of its head.

erectile tissue A spongy network of fibrous connective tissue and blood sinuses. When more blood flows into erectile tissue than leaves it, the sinuses engorge with blood and create hydraulic pressure that enlarges and stiffens the organ in which the erectile tissue is located.

erection Enlargement and stiffening of an organ that contains erectile tissue, such as the penis, clitoris, or nipple.

ergots Believed to be the vestigial remnants of metacarpal and metatarsal pads, these are the horny, keratinized growths located behind the fetlocks of all equids.

eructation The expulsion of gases orally; burping or belching. erythrocytes Also called red blood cells or red corpuscles, these cells are anucleated and biconcave in shape. They are respon­sible for carrying oxygen from the lungs to tissues. Erythrocytes are formed in the red bone marrow of adults and in the liver, spleen, and marrow of a fetus. They are also manufactured in the spleens of anemic and ill adults.

erythropoiesis Production of erythrocytes.

erythropoietin The hormone produced by the kidney that stimu­lates the red bone marrow to increase its production of red blood cells.

esophageal groove See reticular groove.

essential amino acid An amino acid that cannot be produced in sufficient amounts in the body; therefore, it must be obtained in the diet. Different species have different essential amino acid needs.

essential fatty acids Unsaturated fatty acids such as linolenic, arachidonic, and linoleic fatty acids that are necessary for normal body functions yet are not synthesized by the body in sufficient amounts; therefore, they must be supplemented in the diet.

essential nutrients A select group of nutrients that cannot be manufactured in the body from the “building-block” molecules amino acids, monosaccharides, glycerol, and fatty acids. Animals must obtain essential nutrients from their diet.

estrogens Hormones that promote the development of female characteristics; female sex hormones.

estrous An adjective used with the noun “cycle” to refer to the entire reproductive cycle in females.

estrous cycle The period from the beginning of one heat period to the beginning of the next. It includes the stages of proestrus, estrus, metestrus, and diestrus.

estrus The heat period; the stage of the estrous cycle when the female is sexually receptive to the male and will allow breeding to take place.

ethmoid bone A skull bone; an internal bone of the cranium. The single ethmoid bone is located just rostral (ahead of) the sphenoid bone. It contains the cribriform plate, which transmits the many branches of the olfactory nerve to the olfactory bulb of the brain.

ethmoidal sinus The paranasal sinus in the ethmoid bone of a horse or human.

eukaryotes This classification of cells is found in all living things, such as plants and mammals, except for prokaryotes. Eukaryotes have a true nucleus that contains chromosomes and has a nuclear envelope. They also have membrane-bound organelles.

Eustachian tube The tube that connects the middle ear cavity with the pharynx. It allows equalization of the air pressure on the two sides of the tympanic membrane.

euthanize To end an animal's life humanely.

exchange reaction A chemical reaction in which chemical sub­stances exchange molecules or elements to form different chem­ical s ubsta ιιces. A combination of decomposition and synthesis reactions.

excising Removing all or part of a tissue or organ by cutting it out sary.cally.

excitatory neurotransmitters Chemicals released by neurons at the synapse that tend to cause excitation or depolarization of outrhoenrs ne or target tissues.

excretionlimTinheatieon of waste materials from the cell or

body.

excretory ducts Ducts that transport waste products or secre­tions out of an organ or gland.

exocrine glands Glands that release their secretions through ducts that lead directly to the location intended to be controlled. Some examples include sweat glands and salivary glands.

exocytosis Tlie passage of materials too large to diffuse though the cell membrane by packaging them in vesicles, transporting them to the cell membrane, and then pressing them out of the cell.

exons Parts ol' a gene's DNA sequence that are coded. Exons are separated by noncoding portions, called introns, which are spliced out to join the exons together to form messenger RNA. expiration Exhalation; the process of pushing air out of the lungs. expiratory muscle A muscle whose action is to decrease the size of the thoracic cavity; this squeezes air out of the lungs, thereby xroducing expiration (exhalation).

extension The joint movement that increases the angle between townoesb.

external An alternative directional term for superficial. Estnrnal aolwsoarmd eans t the surface of the body or a body part.

external acoustic meatus The bony canal in the temporal bone that leads into the middle and inner ear cavities of the bone. In tvhinegli animal, it contains the external ear canal.

external auditory canal The tube that begins at the base of the pinna and carries sound waves to the tympanic membrane. In most domestic animal species, it is L shaped, with a vertical dpionrgtion lea down to a horizontal portion.

external ear The outer portion of the ear. It consists of the struc­tures that colled and transmit sound waves to the middle ear: the pinna, Ilie external auditory canal, add the tympanic membrane.

external respiration The process of respiration that occurs in the lungs. Oxygen and carbon dioxide are exchanged between the air inhaled into the alveoli of the lungs and the blood in the capillaries that surround them.

extracellular fibers The fibers of connective tissue located eolulstside c that perform a variety of functions depending on ftehgerede o their elasticity or concentration.

extracellular fluid The fluid located outside the cell.

extracellular matrixnliTvihneg no substance found between rcoelvlisdtehsat p support and nourishment.

extraocular musclesl The smal skeletal muscles that move and position the eyeballs.

extravascular hemolysis Destruction of red blood cells outside leaosbsoedl. v

exudate Theaccumdation of fluid, pus, or serum in a cavity or tissue. FOid or serum has often leaked through vessel walls or capillaries into the adjoining space.

eyelids d'lιe conj Unctdva-Iined folds of skin that protect and cover tyheebaell.

F

fabella One of two small sesamoid bones located in the proximal gastrocnemius (calf) muscle tendon just above and behind the foenmdoylreasl c of dogs and cats.

facetticAulaflrat ar surface, for example those between carpal

ebtowneeesnand b the radius and ulna.

facilitated diffusion The diffusion of molecules across the cell membrane with the aid of carrier proteins. This reaction is eounable t b performed as simple diffusion, and it requires no reTnPe. rgy o A

false vocal cordsestTibhuelavr folds; connective tissue bands in

the Ixmix of nonruminant animals in addition to the vocal cords. aoTlchsael f v cords are not involved in voice production.

fascia An arr∣uιgemeιιt of dense regular connective tissue that lies ouvscelre.m This layer helps to support, separate, and connect omuscles t other structures.

fasciclerouAp g of skeletal muscle fibers bound together by a lfraoyuesr o fib connective tissue called the perimysium.

fat solubleculAe,mole such as a vitamin, that can dissolve in fat.

fatty acidsrgaTnhice o compounds of hydrogen, oxygen, and carbon that, when mixed with glycerol, form fat. There are several types of fatty acid. Saturated fatty acids are solid at room temperature and have no double bonds in their carbon chain. Unsaturated fatty acids, such as those in olive oil, are liquid at room temperat ure and have one or more double bonds. Poly­unsaturated fatty acids cont⅛ι two or more double bonds. Vola­tile fatty acids are created in the rumen and reticulum of ruminant animals, such as cows, through the process of cellulose fermentation and are essential for energy production. All fatty aecids ar insoluble in water.

fault barn Area o a feather vane that lacks barbules, also called a stress bar; caused by an interruption of the feather's blood suurpinpgly d development.

faveoli Minute pits or depressions.

feak The act of rubbing the beak on a rough surface to clean it and maintain its shape.

feather follicle Depression in the skin that gives rise to and sta- bwilizes a ne feather.

feedback system A system by which the level of a hormone in ltohoedb affects the gland that produced it. Negative feedback decreases further hormone production and positive feedback increases further hormone production.

feline panleukopenia Caused by the feline parvovirus, this infectious disease has an extremely high mortality rate in kittens. Infections in intrauterine cases are known to cause miscarriages, as well as fetal and newborn deaths. Because the virus tends to ealtltsack c in the mitotic phase, epithelial tissue is at high risk cokf atta as a result of its constant cell division. Therefore char­facteristics o feline panleukopenia include vomiting, diarrhea, and dcby'dral iιι. Vaccines are available.

female pronucleus The name for the nucleus of an ovum after the head of a soc^^^^^iZooii has penetrated into the cell (fertilization S the ovum) but before the nuclei of the sperm (the male pro- ndcleus) and ovum have come together. When the female and rmoanlueclpei combine, the diploid chromosome number is rnedstored a the genetic makeup of the offspring is determined.

femur The long bone of the thigh region. It forms the hip joint with the pelvis at its proximal end and the stifle joint with the tibia at its distal end.

fenestrations Small openings or holes (literally, windows); in the walls of the glomerular capillaries, fenestrations allow certain molecules to leave that would normally be too large to escape.

fermentation Anaerobic oxidative decomposition of cellulose irnto simple compounds, such as volatile fatty acids. The enzymes that break down the cellulose are produced by micro­organisms, which are contained in the rumen or cecum of herbivores.

fermentative digestion Digestive process in which food is broken yndzoywmnebs e produced by microbes.

fertilization ysTichael p entry of the head of a spermatozoon ivnutmo,an o which is then called a zygote. The nuclei of the

ovum and sperm merge, establishing the genetic makeup of the future offspring.

fetal development The sraιd stage (trimester) of pregnancy hdiucrhing w all the cells, tissues, organs, and systems of the feevteulos pd from primitive, undifferentiated stem cells.

fetal growth The third stage (trimester) of pregnancy during lwhich al parts of the fetus grow in size and complexity to prepare for the transition to a free-living, independent existence after parturition (the birth process).

fetal hemoglobin The predomnant hemoglobin in red blood cells during foe later part of gestation. It is gradually replaced by adult hemoglobin during the first few weeks to months after birth.

fetlock joint mThe la t for the most proximal joint of the equine digit, which is the joint between the large metacarpal or metatarsal and the proximal phalanx. The proximal sesamoid bones are located on the palmar (front leg) or plantar (rear leg) fsurface o this joint.

fetus The name given to the developing offspring, beginning about the second trimester of pregnancy. The body tissues, organs, and systems develop in the early fetal period, and then ftfhseproing grows to its full birth size.

fibrin A protein created when thrombin acts on fibrinogen. Fibrin instieaslse to the coagulation of blood. It forms a lattice of interwoven fibers around blood cells and platelets that solidify to form a blood clot.

fibrinogen A protein formed in the liver and released into the bloodstream, especially in the presence of inflammatory pro- rcienssoegse.nF, ib when acted on by thrombin, forms fibrin,

ewahteicsh cr the meshwork of a blood clot.

fibrinolysis Destruction of the fibrin strands that make up the matrix ol' a clot; part of the process of the breakdown of a clot. fibroblastel lFixed c involved in the development of connective

triosbsulaes.tsFib can differentiate into chondroblasts and

osteoblasts to create substances specific to their cell type.

fibrocartilageetwFeoeunnd b the vertebrae of the spine, fibro-

cartilage is different from other types of cartilage in that it has enroicphondrium. In addition, it possesses dense bundles of collagenous fibers but few chondrocytes. It is usually found intermingled with hyaline cartilage and has an excellent ability teoosmisrpt ressicon.

fibrocyte Mature, fiber-forming cell.

fibrous adhesions Fibrous connections that are generated during the healing process; often seen in the abdominal and tvhitoiersacic ca after surgical procedures.

fibrous joint An immovable joint; also known as a synarthrosis. The bones ol' a fibrous joint are firmly united by fibrous tissue. The sutures that unite most of the skull bones are fibrous joints. fibrous tunic Outer layer of tissue of the eye, composed of the sclera at the back and the cornea in front.

fibulao nAe thin b located beside the tibia in the lower region of

tehlveicp limb. It is a complete bone in the dog and cat, but roonxliymtahle p and distal ends are present in horses and

chaettle. T fibula does not support any appreciable weight. It cmtsainly a as a muscle attachment site.

fight-or-flight response A wlιole-body response resulting from eaenlianngimal f threatened that prepares the body for intense physical activity. It results from a combination of direct sympa- trhvetic ne stimulation and the release of epinephrine and

norepinephrine into the bloodstream from the medulla of the adrenal gland. Effects in the body include increased heart rate tapnudt,ou increased blood pressure, dilated air passageways in tuhnegls, and decreased gastrointestinal function.

filtrationa ssTaghe p of a fluid, in response to pressure, through a

semipermeable membrane that allows the liquid portion to pass tuhtrough b not cells and large molecules such as proteins.

fimbriae The muscular, fingerlike projections that form the fringe o the infundibulum of the oviduct. The fimbriae ifreel the way across the surface of the ovary to where feollicles ar developing. They help position the infundibulum so tvhuamt the o or ova will be guided into the oviduct when

ovulation occurs.

first-intention healing Healing that occurs in tissues in which the dwgoeusnd e are held in close apposition to one another, as in the case ol' a sutured wound. Little to no granulation tissue is fhoerrme ed. T is generally minimal scarring.

fission The asexual division of an organism or cell into two indi­vidual “daughter” cells.

fissuresrooDveesp g found in the cerebral cortex.

fixator A muscle that stabilizes a joint so that other muscles can fpfreocdtiuvece e movements of other joints.

fixed cells One ol the two subdivisions of connective tissue cells. eFlilxsed c are stationary within the connective tissue and perform functions such as matrix production and regulation.

flagellum (plural, flagella) The primary means of motility for spermatozoa and unicellular organisms. This threadlike tail propels the organism by means of a whiplike movement.

flanku Lateral surface of the abdomen between the last rib and the ghsin. d le

flat bone Bones that are relatively thin and flat. They consist of two thin plates of compact bone separated by a thin layer of coannec. ellous b Many of the skull bones are flat bones.

flexionintThe jo movement that decreases the angle between two bones.

floating rib The most caudal one or two ribs in the rib cage. A rohisobtsawel c cartilage does not unite with anything but ends iunsctlhee m of the thoracic wall.

fluid mosaic ^rThe constantly changing pattern of proteins and efltuwideebn the two sides of the liquid bilayer.

folliclele A Iluid-Iilled structure or cavity. An ovarian follicle con- fsists o an oocyte surrounded by fluid and the follicular cells trhoadtupced it. Thyroid follicles are microscopic and consist of small globules of thyroid hormone precursor surrounded by soiimdaplle cub cells.

follicle-stimulating hormone (FSH) The anterior pituitary hormone that stimulates the growth and development of folli­cles in the ovaries of the female. In the male, it stimulates sper­matogenesis in the seminiferous tubules of the testes.

follicular atresia The shrinkage of ovarian follicles that began developing but stopped.

follicular cells The cells that surround oocytes in ovarian folli­cles. Also known as granulosa cells. They produce estrogenic hormones in developing follicles.

foramen (plural, foramina) A hole in a bone.

foramen magnum The large hole in the occipital bone through which the spinal cord exits the skull.

foramen of Panizza A shunt connecting the left and right aorta in the crocodile heart; important in switching from terrestrial to aquatic circulatory patterns.

forestomachs Prestomach chambers in a ruminant animal. Includes the reticulum, rumen, and omasum.

fossa A depressed or sunken area on the surface of a bone. Fossae are usually occupied by muscles or tendons.

fossa glandis A shallow depression at the tip of horse's penis from which the urethral process protrudes.

fovea Depression in the retina of the eye containing a high con­centration of cone photoreceptor cells.

free radicals Molecules that contain an odd number of electrons, making them highly reactive with other molecules. They bond to other molecules in the body, creating a new free radical that often causes a chain reaction of free radical formation.

freely permeable Those structures that allow the passage of fluids. frog The thick triangular pad located on the plantar and palmar surfaces of the horse's hooves. It is one of the important struc­tures of the “circulatory pump” in the equine foot.

frontal bones Skull bones; external bones of the cranium. The two frontal bones make up the “forehead” region of the skull. They contain the large frontal sinuses. The cornual process (horn core) in horned animals is an extension of the frontal bone.

frontal plane A human anatomic reference plane that divides the body into anterior and posterior parts. Equivalent to the dorsal plane of four-legged animals.

frontal sinus The large paranasal sinus in the frontal bone of the skull.

functional group Unique, specific groups of atoms within mol­ecules that are responsible for the characteristic chemical reac­tions of those molecules.

functional proteins Complex proteins bearing a spherical shape. They are highly biochemically active. Also called globular proteins.

fundus of eye The caudal interior surface of the eye. It includes the retina and the optic disc.

fundus of stomach The blind pouch of the stomach that relaxes and distends with food.

G

G cells Cells in the antrum of the stomach that produce gastrin. GABA Gamma-aminobutyric acid; inhibitory neurotransmitter. GAGs See glycosaminoglycans.

gallbladder The muscular, blind sac underneath the liver that stores bile until it is needed; cholecystokinin stimulates the gall­bladder to contract, forcing the bile into the cystic duct and common bile duct and then into the duodenum.

gamete A reproductive cell—spermatozoon in the male or ovum in the female. Gametes have half the number of chromosomes (the haploid chromosome number) present in all other body cells (the diploid chromosome number).

ganglion (plural, ganglia) Cluster of neuron cell bodies outside the CNS.

gap junctions Proteinaceous pores that exist in the intestinal epithelial cells of most animals. These pores allow the passage of nutrients, as well as providing a channel for intercellular communication.

gastric Referring to or pertaining to the stomach.

gastric atony The state in which the stomach is very relaxed or has little or no muscle tone.

gastrin A hormone produced in the lining of the stomach when food arrives. It stimulates the gastric glands to secrete hydro­chloric acid and digestive enzymes to start the digestive process and causes the fundus to relax.

gastritis Inflammation of the stomach.

gastrointestinal tract The part of the digestive tube composed of the stomach, small intestine, and large intestine.

general anesthesia Complete loss of sensory perception accom­panied by loss of consciousness.

general senses The senses that are distributed throughout the body. Their receptors are fairly simple, and they keep the central nervous system informed about general conditions inside and outside the body.

genes Specific sites on chromosomes that dictate heredity. Some genes may control one specific phenotypic trait, whereas other traits require many genes for proper expression. Pairs of genes that control the same trait and are located on the same part of paired chromosomes are called alleles.

genetic code The unique order of pyrimidine- and purine-based nucleotides that govern the arrangement and transmission of genetic information in all living things, with the exception of RNA-based viruses.

genetic material Materials, such as DNA, that perpetuate the genetic code through the function of reproduction.

gestation The period of pregnancy.

GFR See glomerular filtration rate.

GI tract Abbreviation for gastrointestinal tract.

gingiva The epithelial tissue that composes the “gums.” ginglymus joint A hinge joint in which one articular surface swivels around another. The only movements possible are flexion and extension.

gizzard Muscular stomach in birds that grinds food into a digest­ible form.

gland sinus The large space in the mammary gland into which the large milk ducts empty. It is located just dorsal to the teat.

glandular epithelium Epithelial tissue composed of one cell (goblet cell) or groups of cells that produce and secrete sub­stances into the lumen.

glans of the penis The distal, free end of the penis. It is richly supplied with sensory nerve endings.

glenoid cavity The concave articular surface of the scapula; the socket portion of the ball-and-socket shoulder joint. In birds, the wing is attached to the body by forming a joint in the depression.

gliding joint An arthrodial joint in which two flat articular sur­faces rock on each other. The carpus is an example of a gliding joint.

globular proteins Complex proteins bearing a spherical shape. glomerular capillaries Part of the renal corpuscle. Urine pro­duction begins here when plasma is filtered out of the glomeru­lar capillaries and into the capsular space of Bowman's capsule. glomerular filtrate The fluid that has been filtered out of the glomerular capillaries and into the capsular space.

glomerular filtration rate (GFR) The rate at which plasma is filtered into the capsular space. It is expressed in milliliters per minute.

glomerulus The tuft of capillaries found in the renal corpuscle; also called glomerular capillaries.

glottis The opening into the larynx. The arytenoid cartilages and the vocal cords form the boundaries of the glottis.

glucagon A hormone produced by the pancreas that raises blood glucose.

glucocorticoid hormones A group of hormones with similar actions, secreted by the cortex of the adrenal glands. The most prominent effect of these hormones is to raise the level of glucose in the bloodstream.

gluconeogenesis The production of glucose from amino acids that occurs in the liver.

Glucose + Galactose = Lactose + Water

glucose Monosaccharide (simple sugar) that is used by the body for energy.

glucosuria See glycosuria.

glycerol The main component of triglycerides present in all fats. Triglycerides are soluble in water and alcohol.

glycine Inhibitory neurotransmitter.

glycocalyx Composed of glycoproteins, the outer covering of the cell that not only aids in cell adhesion but also allows the cell to be identified by other cells.

glycogenesis The creation of glycogen from glucose in the liver. glycogenolysis The breaking apart of glycogen into glucose molecules.

glycolipid A compound composed of a carbohydrate, usually in the form of sugar, and a fatty acid together in a compound.

glycolysis The first step of cellular respiration that converts glucose into lactate or pyruvate and releases a small amount of ATP.

glycoprotein A compound composed of a carbohydrate, usually in the form of sugar, and a protein.

glycosaminoglycans (GAGs) Carbohydrates composed of amino sugars, which are found in proteoglycans.

glycosuria The presence of glucose in the urine.

goblet cell A type of cell, located in the respiratory and intestinal tracts, that secretes mucus.

Golgi apparatus An organelle located near the nucleus that looks like sacs that are stacked and flattened at the ends. It is believed to be involved in the synthesis of glycoproteins, lipo­proteins, and enzymes.

gonad The organ that produces the reproductive cells; the testis in the male and the ovary in the female.

gonadotropin A hormone that stimulates the growth and devel­opment of the gonads (ovaries and testes). Usually refers to follicle-stimulating hormone (FSH) or luteinizing hormone (LH). granulation tissue The new vascular and cellular tissue formed during the restoration of wounded tissue. It mostly consists of connective tissue and new blood vessels.

granulocytes White blood cells that are characterized by the presence of granules in their cytoplasm. The granulocytes are neutrophils, eosinophils, and basophils.

granulopoiesis A general term for the production of any or all of the granulocytes.

granulosa cells The follicular cells of the developing follicle. They produce estrogenic hormones.

gray matter That part of the CNS made up of neuron cell bodies. greater curvature of the stomach The larger, outer curve of the stomach.

gristle Lay term for cartilage.

gross anatomy The study of body structures that are visible without additional aid to the naked eye.

ground substance The shapeless, viscous matrix present in con­nective tissue in which cells receive nutrients and void waste products. It also helps to protect the body from infectious agents by acting as a barrier.

growth hormone (GH) The anterior pituitary hormone that pro­motes body growth in young animals and helps regulate the metabolism of proteins, carbohydrates, and lipids in all of the body's cells.

growth one phase (G1) Part of interphase, during cell division. The cell enlarges and organelles replicate over time, which varies between cell types.

growth plate The epiphyseal plate of a long bone. Located at the junction of the proximal and distal epiphyses with the diaphysis. Growth plates are areas where long bones increase in length by the process of endochondral bone formation. When an animal reaches its full size, the growth plates of its bones completely ossify and the bones cease their growth.

growth two phase (G2) Part of interphase, during cell division. During this phase, enzymes and proteins are synthesized and the centrioles complete their replication.

guanine (G) One of the nucleotides present in both RNA and DNA. It is a purine base that pairs with DNA and RNA's cytosine.

gubernaculum The short, inelastic band of connective tissue that attaches the testes to the scrotum. Growth of the embryo, while the gubernaculum stays the same length, results in movement of the testes caudally and ventrally. Eventually, they descend through the inguinal rings into the scrotum.

gular fluttering Rapid vibrations of the upper throat patch in many species of owls, herons, quail, pigeons, and doves. Used to increase cooling by the evaporative loss of heat from air passed over the oral cavity.

gustatory sense The sense of taste.

gut Also called the gastrointestinal tract. Includes the stomach, small intestine, and large intestine.

gut-associated lymph tissue (GALT) Lymphoid tissue scattered throughout the lining of the intestine. It is often compared with the bursa of Fabricius in birds, where B lymphocytes are processed.

gyrus (plural, gyri) The folds that provide the wrinkled appear­ance of the surface of the cerebral hemispheres.

H

hair bulb The bulbous portion of the hair follicle, located within the dermis, that provides the material for hair production.

hair follicle Tubelike invaginations of the epidermis that traverse tehrme dis and pass into the connective tissue, where the hair is

rhoeoted. T arrector pili and sebaceous glands are located in close proximity to the hair follicle.

hamuli Microscopic hooks that link the barbules of feathers together.

haploid chromosome number The cheemosome number in the gametes (reproductive cells). It is half the diploid chromosome number that is present in all other bodo cells.

haptoglobin A fransport plasma protein that carries free hemo- golmobin fr intravascular hemolosis to the macrophages of tnhoenumcloear phagocote sostem in the liver for further

breakdown.

hard palate The bony roof of the mouth; the division between the moullι and the nasal cavity The soft palate is immediatelo caudal to the hard palate. The hard palate is made up of portions of the maxillary and palatine bones.

hardware disease A disease in ruminant animals caused bo irri­tation ol' the lining of the reticulum bo swallowed metal objects. haustra Sacculations of the colon and cecum.

Haversian canal The central canal that runs the length of a Haversian sostem. The Haversian canal contains the blood ovemsspehls, l vessels, and nerves that supplo and nourish the osteocotes.

Haversian system The microscopic, laminated cylinders of bone that mafe up compact bone. Oriented lengthwise in a long abvoenres,iaHn sostems consist of a central Haversian canal surrounded bo concentric laoers of bone. Osteocotes in their leacunae ar present at the junctions of the bono laoers of the Haversian sostem.

head A spheroidal articular surface on the proximal end of a long bone; present on the proximal ends of the humerus, femur, and rhiebs. T head of a bone is joined to the shaft bo an area that is often narrowed, called the neck.

health A state ol' normal anatomy and physiology that allows the boodo t function normallo.

hearing The auditory sense; the mechanical sense that converts the sound wave vibrations of air molecules to nerve impulses that are interpreted by the brain as sounds.

heart ratefteHn ow o the heart contracts, usuallo stated in beats upteer. min

heatstroke A dangerous body reaction to prolonged heat expo­sure. A the animal’s core temperature rapidly rises, the animal ebaekcomes w and confused and may lapse into unconscious- nhiecshs, w can lead to convulsions and death. Heatstroke often occurs in animals that are locked in automobiles in the hot summer mil or confined in sunny areas without water or access to shade. Heatstroke victims must be cooled rapidly to prevent brain damage and death.

heel The most posterior region of the hoof.

helper T cells The most numerous of the T lymphocytes. They help the immune response by secreting substances known as Iymphokines into Ilie surrounding tissue, which increases activa- timn ol' B lymphocytes and cytotoxic T cells.

hematocrit The percentage of a total blood sample volume made lfeuodpood r b cells; the laboratory test performed to determine the percentage of red blood cells in a blood sample; also known as the packed cell volume (PCV).

hematopoiesis Blood cell production.

hematopoietic tissue Tissue that produces blood cells. Red bone marrow is hematopoietic tissue.

hemidesmosomes The half-units of desmosomes.

hemipenes Saclike projections from the posterior cloaca of male snakes and lizards, which together act as a penis and are inserted lionatocathe c of the female.

hemoconcentrationonAditcion resulting from a loss of plasma

from bleaiwraypsa,ssag blood, lymph vessels, and nerves enter and leave.

hindgut The location of the microbial fermentation chambers in simple-stomached herbivore species, such as the horse and the rabbit; the ^ιrιι and colon.

hinge joint A jteιt where one surface swivels around another like o door hinge; also called a ginglymus joint. Tte only movements possible in a hinge joint are flexion and extension. The elbow joint is an example of a hinge joint.

hip dysplasia A disorder of the hip joint in which the normally tight-fitting ball-and-socket hip joint is abnormally loose. The looseness (laxity) of the dysplastic joint allows the head of the froeamttluer t “ around” in the acetabulum, resulting in damage to the j∞ιt surfaces and osteoarthritis development. Movement of the damaged hip causes pain.

histamine Produced by mast cells from histidine during tissue injury, this biochemical increases blood flow and heart rate during the inflammatory response.

histiocytes Macrophages located in loose connective tissue. histology The microscopic study of the structure of tissues and organs; microanatomy.

histone A globular protein found in the cell nucleus that connects with nucleic acid to form nucleoproteins. Histones form a complex with DNA m chromatin and act as regulators of gene activity.

hock Ankle joint:, or tarsus; joins the tibiotarsus and the tarso- metatarsus of birds.

holocrine gland A gb∣ιl whose granular secretions contain not corneltyorthye se product but also the cells themselves. Holo- lcarninde g cells are destroyed in the process of secretion. The lsaenbdaceous g is an example of a holocrine gland.

homeostasis A state ol' equilibrium maintained in the body by feedback and regulatory processes in response to internal and ehxatnegrensa;l c the maintenance of balance in the body. The fconcept o homeostasis includes the many mechanisms that imtiocanlitor cr levels and functions in the body and stimulate corrective actions when things stray from normal. By keeping icmtivpiotiretsant a within relatively narrow ranges, the process of homcosl asis helps maintain normal body structure, function, raenfdortehe health.

homogeneous Having a uniform composition.

homologous Similar in basic structure and from the same embryological origin. The penis of the male and the clitoris of tehmeafle are homologous structures.

hoof wall The external, cornified portion of the hoof. It grows coownnswtaanrtdly d from the coronary band to the sole. The

deepest layer articulates with the corium by way of the laminae.

Hooke, Robert Robert Hooke, in 1665, was the first person to identify the structure of the cell under a microscope.

hormones Chemical messengers of the body that are produced and excreted by endocrine glands for the purpose of regulating srpgaencisfic o or cells.

horn tubes Minute lines that traverse the hoof wall vertically erom the germinating layer of the coronary band to the sole.

horn A horny, keratinized extension of the frontal bone in rumi­nate ungulates. At its root, it arises from the corium. Epidermal irnigion, horns vary in shape and size depending on age, sex, aecnidess.p

humerusngThe lo bone of the brachium or upper arm.

humoral immunityfypAe t o defensive immune response regu­lated by B lymphocytes. When B lymphocytes are activated by the presence of an antigen, they transform into plasma cells that produce antibodies against the antigen.

hyaline cartilage Abtesh, translucent cartilage present in the tciolasgtael, car trachea, and embryonic skeleton. Hyaline carti­lage is composed of densely packed collagen fibers and is covered by Ilie pdrictenteium, except when present as articular cartilage in joints.

hyaluronic acid A small protein containing no sulfate that acts ar an intercellular material present in the zonula adherens. It is important in the formation of tight junctions.

hyaluronidase An ^^,πιe contained within white blood cells or infectious bacteria that hydrolyzes hyaluronic acid.

hydraulic pressure Pressure exerted by confined fluids. During erection ol' the penis, more blood enters the erectile tissue of the penis than leaves it. The resulting hydraulic pressure produces enlargement and stiffening of the penis.

hydrogen bondsonWdseak b that unite hydrogen with nitrogen

xoyrgoen.

hydrolysis One ol' the most basic and prevalent life processes. Hydrolysis breaks down more complex materials into simpler yodndiensgb a water. Water breaks down or dissociates into a hydrogen atom (H) and a hydroxyl group (OH), which cling to individual parts of the material, thus separating it into two seirmiaplsl.er mat

hydrophilic The tendency of a tissue to absorb or be attracted to water.

hydrophobic The tendency of a tissue to be repelled by water or teuoblbe. insol

hydrostatic pressure The force that propels a liquid.

hyoid apparatus The hyoid bone.

hyoid bone The bone in the neck region that supports the base ofngthue,t the pharynx, and the larynx and aids the process

loofwsiwnagl. It is usually referred to as a single bone, but it is

fcomposed o several portions. The hyoid bone is attached to the temporal bone by two small rods of cartilage.

hyperadrenocorticism Excessive secretion of hormones from the cortex ol' the adrenal gland; also called Cushing’s syndrome. hyperbilirubinemia An excess amount of bilirubin in plasma. hypercalcemia An excess level of calcium in the blood. hyperemia The reddish tinge of mucous membranes caused by an excessive flow of blood to the extremities.

hyperglycemia Too high a level of glucose in the blood. hypermetria A cond i I ion in which voluntary movements become jerky and exaggerated.

hyperplasia Excessive development of a body part as a result of anno ramb al proliferation of cells.

hyperreflexive Reflex response that is more pronounced than normal.

hypersegmented neutrophil A iieaili-opliil that has more than five nuclear lobes when seen in peripheral blood.

hyperthermia An abnormally high body temperature. hypertonic When the concentration of particles in solution is thsiigdheer ou the cell. This may cause water to move from the inside to the outside of the cell to attain equilibrium. In this way, the cell shrivels and becomes crenated.

hypoadrenocorticism Deficient secretion of hormones from the cortex of the adrenal gland; also called Addison’s syndrome.

hypocalcemia Too low a level of calcium in the blood. hypoglycemiaw Too lo a level of glucose in the blood. hypophysis Pituitary gland.

hyporeflexive Reflex response is less than normal. hypothalamusortAiopn of the diencephalon that has extensive

loinks t the brain and to the pituitary gland. It functions as an important bridge between the nervous and the endocrine systems.

hypothermia An abnormally low body temperature that slows down all metabolic processes. Hypothermia can result from xrolonged exposure to cold environmental temperatures, or it eumgay b dr induced, such as with many general anesthetic drugs. Efforts should always be made to keep anesthetized animals and animals recovering from general anesthesia warm. hypotonic When the concentration of a solution outside the cell iwselro than it is on the inside of the cell, water will tend to oflow int the cell toward the higher concentration. This causes swelling and possible rupture of the cell.

hypoxia Oxygen deficiency; causes bluish tinge of mucous mem- bhrearenes. T are many causes of hypoxia, ranging from anemia teospriratory blockage.

I

I bands Large, li^ht bands in a skeletal muscle fiber that alternate kweirth the dar A bands to give a striped appearance to skeletal emrsuscle fib under a microscope. The I bands are composed of tnhtisn filame of the contractile protein actin.

iatrogeniconAditcion caused by medical treatment given to an

animal.

icteruselloTwhieshy color given to tissues, membranes, and

secretions by the presence of bile pigments; may indicate ele­vated level.s of bilirubin, possibly resulting from liver failure; also known as jaundice.

IgA Immunoglobulin A; it can leave blood and enter tissue, where it plays an important role in preventing diseases caused by anti- gnetenrs that e the body through mucosal surfaces.

IgD Immunoglobulin D; its function is unknown.

IgE Immunoglobulin E; it is associated with allergies.

IgG Immunoglobulin G; it is produced during the first exposure itgoeann; ant it is also made by newborn animals.

IgM Immunoglobulin M; it is produced after an animal has been eoxposed t an antigen for an extended time or when an animal ixospeosed t an antigen for the second time.

ileocecal sphincterircuTlhaer c smooth muscle that regulates the

fmovement o intestinal contents from the ileum to the cecum.

ileum The last ol' three segments of the small intestine; it empties ionlotonthe c or cecum.

ileusf Lack o movement of the bowel.

ilium The most cranial of the three pairs of bones that make up the pelvis. It forms the sacroiliac joint with the sacrum.

immunization The process of creating immunity within an animal, usually by introducing a killed or modified culture of tehcteioinufs agent to the body to allow it to create antibodies; also called vaccination.

immunoglobulins Created by B lymphocytes; also called antibod­ies; these ∣na>le^^^^⅛ιsed molecules are produced by exposure to an infectious agent’s antigen. In future encounters with the same antigen, the antibodies will identify and fight it.

immunosuppressedo Refers t an immune system that cannot remlicailt a no immune response.

impermeableo Refers t structures that do not allow the passage of fluid.

implantation Embedding of a developing blastocyst in the lining toefruthse. u

implantation angle The degree of angulation of a shaft of hair. incisive bones Skull bones that are part of the external bones of the lace. De two incisive bones are the most rostral of the skull lbnones. I al common domestic animals, except ruminants, the mcisive bones house the upper incisor teeth.

incisors The most rostral teeth of the mouth that are narrowed ipnto a shar ridge at their tip.

inclusion A lem∣ιorary component of a cell that is lifeless, having rboeuenghbt into the cell via phagocytosis.

incus One ol' the three ossicles, the tiny bones that transmit sound iwbarvaetiovns across the middle ear. The incus, or anvil, is the fmiddle o the three ossicles.

indeterminate layer Species that can produce more eggs than trhmeairl no clutch size.

infection Invasion and replication of microorganisms causing detrimental effects on the body.

inferior A direct ioιιal term meaning toward the lower portion ol' a human body.

inflammation The first step in the healing process when the body is injured. Hs purpose is to “clean up” the damaged area through various inflammatory processes so healing can begin.

inflammatory process The series of cellular and metabolic events that take place primarily in connective tissue as the result ol' injury or loss of tissue. The inflammatory process acts to isolate and destroy infectious agents, as well as to prepare the toirssue f healing.

infraorbital pouch A pouch of cutaneous tissue found rostral to the medial canthus of the eye in sheep and other ungulates; also called the lacrimal pouch.

infundibulum The funnel-shaped ovarian end of the oviduct. Cilia lining the infundibulum beat rhythmically to ensure fpassage o the ovum into the oviduct.

inguinal pouch Also known as the mammary pouch; a pouch of cutaneous tissue found within the inguinal area of sheep.

inguinal rings Slitlike openings in the abdominal muscles located in the groin (inguinal) region. The spermatic cords of the male pass through the inguinal rings from the scrotum to the interior of the abdominal cavity.

inhibitory neurotransmitters Chemicals released by neurons at the synapse that tend to depress or decrease depolarization of other neurons or target tissues.

innate immunity Portion of the immune system that is present at birth, and which is nonspecific.

inner ear The most internal portion of the ear. It is contained in the temporal bone and contains both hearing and equilibrium structures.

innervated Having a nerve supply.

inorganic compound A chemical that does not contain hydro­carbon groups.

insertion of a muscle The more movable of the attachment sites of a muscle. When a muscle contracts, it exerts traction on its insertion site, usually producing movement of a bone or other structure.

inspiration The process of drawing air into the lungs; inhalation.

inspiratory muscle A muscle whose action is to increase the size of the thoracic cavity; this causes air to be drawn into the lungs, thereby producing inspiration (inhalation).

insulin A hormone produced by the beta cells of the pancreatic islets. Its main action is to allow glucose to be absorbed into body cells and used for energy; this decreases the level of glucose in the blood.

integral proteins The proteins located within the lipid bilayer that create channels that aid the selective permeability of the cell membrane.

integument The outer covering of the body, consisting of the skin and all of its related components, such as nails, hair, hooves, and horns.

integumentary system The skin and all of its related compo­nents, such as nails, hair, hooves, and horns.

intercalated discs End-to-end attachment sites between adjacent cardiac muscle cells. The intercalated discs securely fasten the cells together and also transmit impulses from cell to cell. This allows large groups of cardiac muscle cells to function as a single unit.

intercostal space The space between two ribs.

interdigital pouch A pouch of cutaneous tissue that exudes a waxy substance; found between the toes of sheep and other cloven-hoofed animals.

interferon A substance produced by a cell after a virus has invaded it. Interferon prevents further development or spread of the virus.

intermediate fibers Fibers that are specialized to the cell in which they are contained. They are composed of tough protein fibers that help to reinforce the cell.

intermediate filaments See tonofilaments.

internal respiration The exchange of oxygen and carbon dioxide between the blood in the capillaries all over the body and the cells and tissues of the body.

internal An alternative directional term for deep. Internal also means toward the center of the body or a body part.

interneuron Typically a short neuron that connects two other neurons; usually mentioned in the context of the reflex arc.

interparietal bones Skull bones that are part of the external bones of the cranium. The two interparietal bones are located on the dorsal midline just rostral to the occipital bone. The interparietal bones are usually distinct in young animals, but in older animals they may fuse into one bone and may even fuse to the parietal bones and become indistinguishable.

interphase The period between cell divisions during which all normal growth and functions occur.

interpleural space The space between the pleural covering of the right lung and the pleural covering of the left lung. The mediastinum.

interstitial cell Endocrine cells located between the seminiferous tubules of the testes. They produce androgens, the male sex hormones.

interstitial cell-stimulating hormone (ICSH) The anterior pitu­itary hormone that stimulates the interstitial cells of the testes to produce androgens, the male sex hormones; also known as Iuteinizing hormone (LH).

interstitial fluid Tissue fluid; fluid contained within the tissues, except for the fluid found within lymph and blood vessels.

intervertebral disc The cartilaginous disc located between the bodies of adjacent vertebrae. It acts as a shock absorber for the vertebrae.

intracellular fluid The fluid that is contained within cells.

intramembranous bone formation Membrane bone formation.

The type of bone formation that only occurs in certain skull bones when bone forms in the fibrous tissue membranes that cover the brain in a developing fetus.

intramuscular injection A route of drug administration that involves injecting a drug through a hypodermic needle that has been inserted into the belly of a muscle. The large blood supply of the muscle leads to rapid absorption of the drug into the bloodstream for distribution to the rest of the body.

intravascular hemolysis Destruction of red blood cells within blood vessels.

intrinsic factor A protein produced by the stomach and required for absorption of vitamin B12.

intromission Insertion of the penis into the vagina.

introns Spaces between coded exons of a gene's DNA sequence that do not contain codes.

involuntary muscle An old name for smooth muscle. involuntary striated muscle An old name for cardiac muscle. involution of the mammary gland “Drying up” of the mammary gland when the stimuli for lactation cease; the cessation of milk production with shrinkage of the mammary gland back to near its prelactation size.

involution of the uterus Shrinkage of the uterus after parturition back to near its prepregnant size.

ion An electrically charged atom or molecule. Cations are posi­tively charged ions, and anions are negatively charged ions.

ionic bond A type of chemical bond formed by the electrostatic attraction between two oppositely charged atoms or molecules (ions).

ipsilateral reflex The reflex stimulus and response are on the same side of the body.

iris A portion of the uvea, or middle vascular layer, of the eye. The iris is a pigmented, muscular diaphragm that controls the amount ol' light that enters the posterior part of the eyeball.

irregular bone A bone whose shape does not fit into the long bone, short bone, or flat bone categories. Irregular bones either have characteristics of more than one of the other three shape categories or have a truly irregular shape. Examples include vertebrae, some strangely shaped skull bones (such as the sphe­noid bone), and sesamoid bones.

ischium The most caudal of the three pairs of bones that make up the pelvis.

islets of Langerhans Clusters of cells in the pancreas that uroduce insulin, glucagon, and other endocrine products of the pancreas. Also called the pancreatic islets.

isotonic Equal osmotic pressure present on either side of the cell membrane.

isotope One ol' two or more atoms having the same atomic number (same number of protons) but different atomic masses (different number of neutrons).

ivermectin A commonly used antiparasitic drug that works by interfering with parasites’ nervous and muscular system func­tions, which paralyzes and kills them.

J

jaundice An abnormal condition characterized by the yellowing of tire mucous membranes, skin, and sclerae as a result of excess bilirubin moving from the blood into tissues. This may be cvyaerused b li failure, excessive destruction of red blood cells, or blockage of the bile ducts. Also known as icterus.

jejunum The second of the three segments of the small intestine, usually the longest segment.

jointu The junction between two bones. Joints can be completely immovable (fibrous joints), slightly movable (cartilaginous joints), or freely movable (synovial joints).

joint capsule The membrane that encloses the ends of the bones in a synovial joint; consists of an outer fibrous membrane and ran inne synovial membrane that produces viscous synovial hfliucihd w lubricates the joint surfaces.

joint cavity The fluid-filled potential space between the joint surfaces ol' a synovial joint; the joint cavity is normally filled by the uiscous, lubricating fluid (synovial fluid) produced by the smynbroavniael me lining of the joint capsule; also known as the joint space.

joint space Alternate name for the joint cavity.

junctional complex The point at which epithelial cells join to one earnyother in v close proximity.

K

keel Bony ridge on the sternum of a bird to which the flight cmhu. scles atta

keratinouAght, waterproof protein that makes up scales, the

outer sheaths of beaks and claws, and feathers; a major compo- fnent o the epidermis, nails, hair, horns, and hooves.

keratin fibers The strong strands of the fibrous protein keratin tehat ar insoluble in water.

keratinization The normal formation of keratin (a tough, water- proon prolein) inside epithelial cells of the skin. As the epithelial cuerlel,s mat they fill with granules containing keratin.

keratinized stratified squamous epithelium The epithelial clas- fsification o the epidermis. It is highly regenerative and water- rperboyof, the helping the body to retain moisture and thermoregulate.

keratinocytes Cells that synthesize keratin. They have three dis­tinct stages visible in the epidermis: the basal, prickle, and granu­lar cell stages. As keratinocytes travel progressively away from the basemen I membrane toward the superficial epithelium, they lose ttleu^ organelles to make way for more keratin. As a result of this process, the cells die by the time they reach the surface.

ketone bodies Products of lipid metabolism (lipolysis), including beta-hydroxybutyric acid, acetoacetic acid, and acetone. They are ruosduualcleydp from acetyl CoA from fatty acids within the liver. kneepuTs he car of hoofed animals.

Krebs cycle The metabolism of sugars, fatty acids, and amino aocuigdhs thr oxidation to produce carbon dioxide, water, and energy.

Kupffer cells Macrophages present in the liver. Fifty percent of macrophages are Kupffer cells.

L

labia Literally means lips. The external boundary of the vulva of tehmeafle.

labialr Liplike o pertaining to the lips.

labial surfaceurfTahce s of the rostral part of the upper and lcoawder ar of teeth that face the lips.

lacrimal apparatusearT-phreotducing and draining structures yoefst.he e

lacrimal bones Skull bones; external bones of the face. The two scmrimallalla bones form part of the medial portion of the forbit o the eye and house the lacrimal sacs, which are part of tehare-tdrainage system of the eye.

lacrimal gland The main tear-producing gland. It is located dorsal and lateral to the eyeball inside the bony orbit.

lacrimal puncta Openings in the upper and lower eyelid margins located near the medial canthus of each eye. They drain tears aowmay fr the surface of the eyes.

lacrimal sacl A smal sac that receives tears from the lacrimal puncta and sends them down into the nasolacrimal duct.

lactationrodMuicltkiopn by the mammary gland.

lactic acidreodAucwtast p of anaerobic metabolism in skeletal nmduscle; an e product of the metabolism of carbohydrates. It Is crealed by the conversion of pyruvate into lactic acid after the fermentation of cellulose. The buildup of lactic acid in a muscle tehenat has b forced into anaerobic mode by overstrenuous activity can cause discomfort.

lactic acidosis In the rumina nt., this is the same as grain overload and rumen acidosis; a condilioιι in which too much carbohy- vdarialtaebilse a to the rumen microbes, resulting in production

eof larg amounts of lactic acid and subsequently causing severe changes in the rumen microflora and systemic acidosis.

lacunaevitSiems all ca within the matrix of some connective tissues, ^mlι as cartilage and bone, within which cells (chondro­cytes and osteocytes) are contained.

lamina propria The areolar connective tissue located in the mburcaonuessm. e

laminae The intendigitations between the corium and hoof that serve as the attachment sites between the hoof and coffin bone.

Iaminitis Manifests itself as extreme pain and heat in the equine hoof because of swelling and inflammation of the sensitive laminae. fl most often affects equids in their front feet, but in severe cases, it may involve all four feet. There are many causes, such as ingesting large quantities of grains and carbohydrates, drug read ions, retained placenta, and trauma. In severe cases, the col'lin bone may penetrate the sole of the hoof; also called founder.

Langerhans cells Tlie macrophages of the epidermis that phago­cytize invading microorganisms and serve as antigen-presenting cells.

lanolin Fat-based secretion of the sheep's sebaceous glands; a by-product of the wool industry, it is used for lotions and ointments.

laryngeal hemiplegia The medical name for what is commonly called roaring, a d>ιιdθ>ιι often seen in horses. It is produced by parflysis of the muscles that tighten the arytenoid cartilages and vocal cord on one side, usually the left, of the larynx. The result is that the paralyzed vocal cord partially obstructs the glottis and causes difficulty breathing, particularly when the animal tries to breathe heavily when exercising. Vibrations of tahlyezpedar vocal cord produce a characteristic roaring sound as the animal breathes.

laryngoscope An instrument used to aid the passage of an endo- turbaecheal t through the glottis and down into the trachea. The laryngoscope consists of a handle that contains batteries and a long, narrow blade with a small light source near the end of it. The blade is used gently to press the tip of the epiglottis ven- txrpaollsyi,neg the opening of the glottis.

laryngospasm A spasmodic closure of the opening of the glottis; cocftuerns o in cats during endotracheal intubation because of the sensitivitv of a cat's larynx.

larynx The “voice box”; a short, irregular tube of cartilage and muscle that connects the pharynx with the trachea. Its functions raooricedeuvction,p preventing foreign material from being

inhaled, and controlling airflow to and from the lungs.

lateral cartilagese Two larg bands of cartilage contained within the eιιal term meaning away from the median plane (center line) of the body.

lesser curvature of the stomach The smaller, inner curve of the stomach.

leukemia “White blood”; a cancer or malignancy of one type of white ‰'d cells. Production of that type of cell is abnormal ond uncontrolled.

leukocytes Also called white blood cells, leiikoceles come in many different varieties, including basophils, eosinophils, neutrophils, lymphocytes, and monocytes. They may be granulated or non­granulated and are capable of amoeboid motion. The main function of leukocytes is body defense.

leukocytosis An increase in the total number of white blood cells in perird^'al blood.

leukopenia A decrease in the total number of white blood cells m peeira^^'al blood; also called leukocytopenia.

leukopoiesis A general term for production of white blood cells.

leukotrienes Eicosanoids formed from the activation of white blood cells. They act to sustain inflammation in asthmatic and ealalcetrigoincsr.

ligamentsf Bands o fibrous connective tissue that are present in aonudndar many synovial joints. When present, ligaments connect the bones of the joint to each other.

ligands Small molecules that bond to larger chemical groups or molecules.

ligated Surgically tied off, as with blood vessels; usually done uwtiuthres material.

limbus The junction of the cornea and sclera of the eye.

linea alba The sheet of fibrous connective tissue (aponeurosis) that connects the abdominal muscles from each side on the lviennet.ral mid

lingual surface The surface of the lower arcade of teeth that faces the tongue; lingua ns Latin for tongue.

lipase Enzyme produced by the pancreas and, in some species, the saliva; breaks apart fats into their components.

lipid bilayer A doable-layered membrane made of phospholip- iudcsh, s as the cell membrane and nuclear envelope.

lipidsro uTphe g of fatty or fatlike substances that are insoluble

liecnro. whoalt, A ether, chloroform, and other nonpolar sub­stances can, however, dissolve them.

lipolysis The liinakdowii of fats.

lipoproteinculAe mole that contains both a lipid and a protein.

fTtehney o function as transmembrane proteins to transport cmroslescules a cell membranes or transport proteins for the

movement of fat molecules in the blood.

lissencephalic Associated with a malformation of the gyri in the coerrtexb.ral c In dogs, it is associated with behavioral abnor-

imsuaalilties, v deficits, ataxia, and seizures.

lobeereEbarcahl c hemisphere is divided by sulci into lobes; dif-

fbeersent lo of the cerebral hemispheres specialize in certain functions.

lobes (of the lung) Subdivisions of the lungs. Lung lobes are defined by major branches of bronchi entering them, rather tryhoassnlyb g visible grooves and clefts.

local anesthesia Loss ol' sensation from a localized area of the body.

long bonehatBones t are longer than they are wide. Most of the

louimnchebs,b s as the humerus, femur, and radius, are long bones.

longitudinal fissure Prominent groove that divides the cerebrum into right and left cerebral hemispheres.

longitudinal muscle The muscle in the intestinal tract layer that iesspronsible for shortening segments of the intestine when it contracts; is involved with peristaltic movement.

loop of Henle The middle part of the tubular portion of a tnephron. I has a descending part that travels from the cortex tdoultlhae me and an ascending part that travels back to the cortex. fie loop of Henle is located between the proximal con­voluted tubule and the distal convoluted tubule.

loose connective tissue A subclass of connective tissue proper, loose cinnective tissue is a vascularized type of connective tissue ewnheorsael g function is to support the structures it surrounds. foTsyepes o lo connective tissue include areolar, adipose, and oeticular connective tissues.

lower arcade In reference to teeth, it means the teeth in the rmandible, o the lower set of teeth in the mouth.

lower respiratory tract All of the respiratory structures within the lrugs; includes all the respiratory passages from the bronchi down to the alveoli.

lumbar vertebrae The group of vertebrae located dorsal to the aegbidoonm. inal r

lumen The opening in the middle of the intestinal tract or any rhgoalnlo.w o

luteinizing hormone (LH) The anterior pituitary hormone that svtuimlatuiolantes o in most species and then causes the empty

feoovlellioclpe t d into the corpus luteum.

luteolysis Destruction of the corpus luteum.

lymph Excess I issue fluid that is picked up by lymph vessels and roeturned t peripheral blood.

lymphocytes Nongranulocytic white blood cells that are involved in the immune response. The two types of lymphocyte are T lymphocytes, which are involved with cell-mediated immunity, and B lymphocytes, Vvhtoli are involved with humoral immunity.

lymphocytosis An increase in the number of lymphocytes in lpoeordip. heral b

lymphokines Proteins that develop on the surface of helper T lymphocytes. The lymphokines activate killer T lymphocytes, which are responsible for cell-mediated immunity.

lymphopeniaecrAeasde in the number of lymphocytes in lpoeordip. heral b

lysosome An organelle that fights pathogens, repairs damaged tissues, and aids in intracellular digestion by engulfing materials within its membrane-bound vesicle bodies. It contains the nigestive enzymes that help destroy microorganisms that have been pltagocyl ized by the neutrophil.

M

macrominerals Minerals, such as calcium, phosphorus, magne­sium, sodium, potassium, chlorine, and sulfur, that are required en larger quantities than other minerals.

macromoleculee A larg molecule consisting of smaller units loignektehde rt, such as a polysaccharide or polypeptide.

macrophages Phagocytic cells that can engulf relatively large ircteslls o b of debris. They may be fixed in place, or they may toruanvedl ar in the tissues. Mature macrophages may become bmiloere mo during times of infection and inflammation.

macroscopic anatomy Also called gross anatomy; the study of tbsody par large enough to be seen without magnification, such agus,nag,l le or brain.

maculachA pat of sensory epithelium in the vestibule of the

.itnner ear I consists of hair cells covered by a gelatinous mass containing otoliths. The macula senses the position of the head. male pronucleus The name for the nucleus of a spermatozoon after its head has penetrated into the ovum (fertilization of the ovum) but before the nuclei of the ovum (the female pronucleus) and the sperm have come together. When the female and male pronuclei combine, the diploid chromosome ensutomrbeder is r and the genetic makeup of the offspring is determined.

malleus One of the three ossicles, which are the tiny bones that transmit sound wave vibrations across the middle ear. The rmalleus, o hammer, is the outermost of the three ossicles and icoshaetdta t the tympanic membrane.

mandible A skull bone; one of the external bones of the face. The mandible is the lower jaw, the only movable skull bone. The mandible houses all of the lower teeth. It is usually referred to laoesnea, sing b but in dogs, cats, and cattle, the two halves of the mandible are separate bones joined by a cartilaginous man- dmibpuhlyasrissy at the rostral end.

mandibular symphysis The Giuilagiiious joint (amphiarthrosis) that ?les the two sides of the mandible at the rostral (front) end.

manubrium The Iirsl, most cranial sternebra. Its full name is manubrium sterni.

marbling The common name for fat deposits in the connective tissue layers of meat (skeletal muscle).

marginal pool of neutrophils Neutrophils found lining the walls of small ‰'d vessels, mainly in the spleen, lungs, and abdomi- rngaalnos. These neutrophils are not circulating but are moving snlgowly alo the walls of the vessels.

mast cell A transient cell of connective tissue containing heparin and histamine used in the inflammatory response. Mast cells oecognize foreign invaders and release granules of histamine apnardinhe to increase blood flow. They resemble basophils, yobut the d not circulate in blood.

mastication Chewing.

mastitis Infection of a mammary gland.

matrix The intercellular material of connective tissue.

matter Anything that has mass and exists as a solid, liquid, or gas. mature follicle An ovarian follicle that is fully developed and ready Inr ovtoation; also known as a vesicular follicle or a Graaf­ian follicle.

maxilla The bone of the upper jaw.

maxillary bones Skull bones; external bones of the face. The two maxillary bones make up most of the upper jaw and house the upper canine teeth, if present, and all of the upper cheek teeth (premolars and molars).

maxillary sinus The paranasal sinus in the maxillary bones. mechanical control system The respiratory control system that sets inspiration and expiration limits for normal resting breath­ing; operates on the basis of stretch receptors in the lungs that communicate with the respiratory center in the brainstem.

mechanical digestion The physical breakdown of food into tsimclaelsl. par

meconium Dark, tarry material in the intestine of a newborn ahneimal. T first feces passed by the newborn.

medial A directional term meaning toward the median plane (center line) of the body.

median plane An anatomic reference plane. A median plane is a sagittal plane that runs down the center of the body and divides ot into equal left and right halves. It is also called a midsagittal plane.

mediastinum The area of the thorax between the lungs. It con­tains the heart and most of the other thoracic structures, such as the trachea, esophagus, blood vessels, nerves, and lymphatic structures.

medulla (1) toe inner, deep layer of an organ or structure. (2) The inner part of the kidney. It contains the loop of Henle, peritubular capillaries, and collecting ducts. (3) The innermost lfayer o a hair strand, made of two or three layers of flexible, soft keratin.

medulla oblongata The part of the brainstem just above (cranial to) the spinal cord.

megaesophagus A condition in which the esophagus loses muscle tone and dilates into a flaccid, saclike structure.

megakaryocytes Large, Inrfltinucleated cells in red bone marrow that are the parent cells of platelets. Platelets are formed when chunks of cytoplasm break off a megakaryocyte and enter the circulation.

meibomian glands The Inrsal glands of the eyelid margins. They produce a waxy substance that helps prevent tears from over­flowing onio an animal's face.

meiosis The reduction division that reproductive cells undergo during their development. It results in a reduction of the chro- ummosboemr e n from the normal diploid number to the

haplord number (half of the diploid number).

Meissner's corpuscles The oval, tactile nerve endings, both myelinated and unmyelinated, found within the dermal papillae of the epidermis. Sensitive to light touch, they are especially common in body parts with no hair follicles, such as the soles eoeftt.he f

melanomacrophage A Iiislocyte laden with phagocytosed melanin. Also called melanophages.

melanin That snl lnrons pigment produced by melanocytes, espe­cially when stimulated by sunlight. Melanin is present in the skin, hair, choroid of the eye, and abnormally in melanomas.

melanocyteeclaltAedc lo within the lower epidermis that pro- cyersosseisnaste and melanin. Melanin, contained within

melanosomes, is transferred to keratinocytes by way of the melanocyte's long projections.

melanocyte-stimulating hormone (MSH) The ∣∣∣∣∣erior pituitary hormone that apparently influences the pigment cells of the skin (the melanocytes). Its precise role in the body of mammals is not well understood.

melanosomes Granules filled with melanin that are transferred from the melanocytes to the keratinocytes.

melatonin A Irormonelike substance produced by the pineal body. lt apparently affects moods and wake-sleep cycles and may affect the timing of seasonal estrous cycles in some species. membrane potential The dinereiice in voltage that exists on either side ol' a cell membrane caused by the different concen- ftrations o positive and negative charges.

membrane proteins Proteins imbedded in the cell's membrane that function as cell receptors and membrane transport molecules.

membrane receptors The integral proteins and glycoproteins that fgrm binding sites; may aid in contact signaling.

memory cells After an initial immune response, lymphocytes that are programmed to remember the antigen that caused the iemspmonusnee r and to produce a more rapid immune

roesponse t that antigen the second time the body is exposed to it.

meninges Set of connective tissues that surround the brain and spinal cord; the three layers of the meninges, from outside to innermost layer, are the dura mater, the arachnoid, and the pia mater.

meniscus One ol' two concave, half moon-shaped, cartilaginous structures on the proximal surface of the tibia that help support tohnedcyles of the femur.

Merkel's cells Thought to aid in tactile sensory function, these eells are located in small numbers within the epidermal-dermal junction.

Merkel's disc The junction formed by Merkel's cells and sensory nerves.

merocrine glandlanAd g whose secretions contain none of its cells, thus leaving the gland cells intact. Examples include sali- evatry and sw glands.

mesobronchi In the avian respiratory system, bronchi that have entered the lung and lost their reinforcing cartilaginous rings.

mesoderm Tlie middle layer of fetal body tissues. Located between the oιιler layer (ectoderm) and the inner layer (endoderm), the imves oderm g rise to all connective and muscle tissues.

mesothelium A layer ol' cells that lines the body cavities of the fetus and that covers the serous membranes in adult animals.

messenger RNA (mRNA) One ol' the main components of protein synthesis; mRNA transfers the specific amino acid sequence ol' the genetic code of DNA to the cytoplasm, where protein is synthesized.

metabolic turnover The cιιl iιιnons breakdown of old cells and ebrody matt to be replaced with new cells and body matter.

metabolism All of the complex, interrelated chemical processes tehat mak life possible. Its two fundamental components are anabolism and catabolism.

metabolites Substances that are produced during metabolism. metacarpal bones Bones ol' the forelimb that lie between the coanrepsal b and phalanges.

metaphasehasTehe p of mitosis when the newly formed chro­

mosomes align on a medial plane or “equator” between the two centrioles located at each end of the dividing cell.

metaphase plate During metaphase, the site where chromo­somes line up and are evenly distributed.

metatarsal bones The bones of the pelvic limbs located between tshues tar and the phalanges.

metatarsal padottTohme b of a bird's foot; the large rear footpad of animals such as cats and dogs located over the ventral fsurface o the junction between the metatarsal bones and the nprdoximal e of the phalanges.

metestrus The stage of the estrous cycle after ovulation, when the cuoterupmus l develops. It occurs between estrus and diestrus. micelle A very small droplet of fat surrounded by hydrophilic (water-loving) molecules that allow it to move readily in the lniqviuriodnement of the small intestine.

microbe A microscopic organism that may or may not cause disease.

microfilament Closely associated with microtubules, these sub- microscopic structures are found in most cells and are com- yposed mostl of actin.

microglial cells Macrophages located in brain tissue.

microminerals A group of minerals called the trace elements, soupcphe ra,s c iron, boron, molybdenum, and cobalt, which are

ryequired b the body in minute amounts.

microscopic anatomy The study of anatomic parts too small to be seen with the unaided eye, such as cells and tissues.

microtome A cutting instrument used to make extremely thin ifssliscuees o t for microscopic study.

microtrabeculae A cmponeιιl of the cytoskeleton thought to aoudrpmdpo,frt, s and substance to the cell's inner anatomy.

microtubules Tiny, hollow, tubelike structures that aid certain cells with rigidity and transportation. They also form the spindle fibers in the process of mitosis.

microvilli Fingerlike protrusions of the luminal surfaces of some epithelial cell membranes that increase a cell's exposed surface area.

micturition The process of expelling urine from the body; also called urination or uresis.

midbrain The mesencephalon.

middle ear The middle portion of the ear; an air-filled cavity in the temporal bone of the skull. It contains the ossicles and the opening of the Eustachian tube. The middle ear amplifies and transmits sound wave vibrations from the tympanic membrane to the cochlea of the inner ear.

milk fever A disease seen in lactating cattle that results from hypocalcemia. The signs of milk fever include muscle weakness and an inability to stand.

milk let-down The immediate effect of nursing or milking. The movement of milk from the alveoli and small ducts down into the larger ducts and sinuses, where it is accessible for nursing or milking. It results from the release of oxytocin from the poste­rior pituitary gland. The oxytocin causes myoepithelial cells surrounding the alveoli and small ducts of the mammary gland to contract, squeezing the milk into the lower parts of the gland. mineralocorticoid hormones A group of hormones secreted by the cortex of the adrenal glands that regulate the levels of some important electrolytes in the body. The principal mineralocor­ticoid hormone is aldosterone.

minute volume The volume of air that an animal breathes in and out during 1 minute. It is calculated by multiplying the animal's tidal volume by its respiratory rate, which is the number of breaths taken per minute.

mitochondria Intracellular organelles, which are the primary sources of ATP formation for aerobic cell respiration. Contain DNA and RNA, making the mitochondria capable of their own protein synthesis and replication.

mitochondrial matrix The enzyme-rich liquid in the mitochon­dria that surrounds the cristae and provides them with the enzymes necessary for the proper function of the Krebs cycle.

mitosis Cell division of somatic cells for growth and to replace old or dead cells; the type of cell division that occurs in all body cells except the reproductive cells. When cells divide by mitosis, the chromosomes first duplicate themselves and then pull apart into two daughter cells. This preserves the diploid chromosome number.

mitotic phase The period during which cell division occurs. It is divided into the main phases of interphase, metaphase, ana­phase, and telophase.

mitral valve Also called the left atrioventricular valve; separates the left atrium and ventricle and protects the pulmonary venous system from the high pressures in the left ventricle during systole.

mixed exocrine glands Exocrine glands, such as salivary glands, that can produce both mucous and serous secretions.

mixed nerve A nerve made up of both sensory nerve fibers and motor nerve fibers.

mixture A mixture is a combination of two or more substances. There are three types of mixture: solutions, colloids, and suspensions.

modulation Regulating or changing something. In the process of nociception, dorsal horn neurons in the spinal cord can modu­late pain impulses before they reach the brain. This can take the form of amplification or suppression of the pain impulses. See wind-up.

molars The grinding teeth located at the back of the mouth. molecule The smallest particle of a substance, composed of two or more atoms, that retains the properties of the substance.

molting Process of feather or hair replacement; occurs one to several times a year, depending on the species.

monoamine hormones Hormones derived from amino acids such as tyrosine and tryptophan. Their name comes from the fact that they retain one amino group.

monocyte A large, phagocytic white blood cell. It is the largest white blood cell normally found in peripheral blood; it is an agranulocyte.

monocytopenia A decrease in the number of monocytes in peripheral blood.

monocytosis An increase in the number of monocytes in periph­eral blood.

monoestrous An animal that has only one estrous cycle each year. monogastric Having one stomach; usually refers to nonrumi­nant animals that do not rely on gastric fermentative processes to any great extent.

monoglyceride A product of triglyceride breakdown; made from one fatty acid plus a glycerol molecule.

mononuclear phagocyte system A collective term for mono­cytes and tissue macrophages found throughout the body.

monosaccharides Simple sugars; single sugar molecules, includ­ing glucose, galactose, and fructose.

morphologic The unique structures and forms of each individual organ, tissue, cell, or the organism as a whole.

morula The solid mass of cells into which the zygote has devel­oped a few days after fertilization of the ovum. It resembles a tiny raspberry.

motor nerve Nerve that carries efferent impulses to muscles, although motor function may be used to describe any nerve that carries an efferent impulse, including those that supply endo­crine glands and tissues that are not muscle.

motor neuron A neuron carrying impulses from the CNS to a peripheral effector organ such as a muscle or gland.

motor unit One nerve fiber and all the skeletal muscle fibers it innervates. Motor units with small numbers of muscle fibers per nerve fiber are capable of fine, delicate movements. Motor units with large numbers of muscle fibers per nerve fiber are capable of large, powerful movements.

mRNA See messenger RNA.

mucin The main constituent of mucus, produced by goblet cells in the respiratory and intestinal tracts. It is composed of proteoglycans.

mucosa The inner layer of the intestinal tract; this layer usually contains glands that secrete into the lumen of the gastrointesti­nal tract.

mucous An adjective to describe something pertaining to mucus or structures that either secrete or are covered by mucus.

mucous cells Cells in the lining of the stomach that produce mucus, which helps prevent autodigestion of the stomach.

mucous membrane The mucus-producing layer of stratified squamous or columnar epithelium found over the lamina propria. It is present in organs that have contact with the outside of the body. Mucus production in mucous membranes is com­monly found in the digestive, respiratory, and reproductive tracts, with the exception only of the urinary tract. The mucus secreted in them helps to fight infection because it is loaded with antibodies. Mucus also helps to absorb nutrients and lubricate surfaces.

mucous secretions Viscous secretions composed mostly of glycoproteins.

mucus Complex, gel-like substance that is secreted by goblet cells or other mucous glands; acts as a lubricant and protective barrier.

multicameral lung A multichambered lung. multicellular Composed of many cells. multinucleated Having more than one nucleus.

multiparous An animal that normally gives birth to more than one offspring at parturition.

multiunit smooth muscle The type of smooth muscle composed of individual smooth muscle cells or small groups of cells. Found where small, delicate involuntary contractions are needed. Multiunit smooth muscle requires nerve impulses to stimulate its contractions.

muscarinic receptor A type of cholinergic receptor that binds with acetylcholine.

muscle spindle A sensory organ located within muscle that detects stretch of the muscle.

muscle tissue A collection of tissues that support the body and enable it to move, thermoregulate, and transport materials. Some muscles may be controlled voluntarily, whereas others act involuntarily. Examples of involuntary muscle include cardiac and smooth muscle; voluntary muscle includes all of the skeletal muscles.

mutagen An agent that can create a transmissible genetic change in an organism’s DNA.

mutation A sudden and irreversible genetic change that causes a difference between offspring and their parents.

mute Organized waste product of birds that is ejected from the cloaca by the muscular anus; normally consists of a dark fecal center surrounded by a ring of white urates.

myelin Fatty substance that covers some axons. When fixed for microscopic examination, it appears white, hence myelinated neurons make up the “white matter” of the brain and spinal cord.

myelin sheath Cell membrane of glial cells (e.g., oligodendro­cytes, Schwann cells) wrapped around an axon; increases speed of impulse conduction along the axon.

myelography The radiographic (x-ray) procedure of injecting contrast media into the subarachnoid space to visualize better the outline of the spinal cord.

myocardium The middle layer of the heart and the main muscle layer responsible for contraction during systole.

myoepithelial cells Cells in the mammary glands that have char­acteristics of both muscle cells and epithelial cells. They sur­round the alveoli and small ducts of the glands. When stimulated by the hormone oxytocin, they contract, squeezing milk down into the large ducts and sinuses; see milk let-down.

myofibrils Microscopic, fiberlike structures that occupy most of the cytoplasm (sarcoplasm) in skeletal muscle cells. Myofibrils are composed of filaments of the contractile proteins actin and myosin and are packed together longitudinally in the muscle cells.

myoglobin A protein in muscle cells that has properties similar to hemoglobin. It can store and release large quantities of oxygen to fuel aerobic metabolic processes in the muscle cells.

myometrium The muscle layer of the uterus.

myosin filaments One of the two contractile proteins of muscle (actin is the other one) that slide over each other to produce the shortening of the muscle cell that we refer to as muscle contraction.

myosin A protein present in muscle fibers that aids in contraction and makes up the majority of muscle protein.

myositis Inflammation of a voluntary muscle that causes pain and stiffness.

N

NAD⁺ See nicotinamide adenine dinucleotide.

NADH See nicotinamide adenine dinucleotide.

nape Back of the neck.

nares The nostrils.

nasal bones Skull bones that are part of the external bones of the face. The nasal bones form the bridge of the nose, or the dorsal part of the nasal cavity.

nasal conchae Skull bones that are part of the internal bones of the face; also known as the turbinates. The nasal conchae are four thin, scroll-like bones that fill most of the space in the nasal cavity. The turbinates are covered by the moist, vascular lining of the nasal passages. Their scroll-like shape helps the nasal lining warm and humidify the inhaled air and trap tiny particles of inhaled foreign material.

nasal meatus Any of the main passageways in the nasal cavity. The nasal cavity is subdivided by the dorsal and ventral nasal turbinates into three main passageways: the dorsal, middle, and ventral nasal meatuses. A small fourth passageway, the common nasal meatus, is continuous with the three main passageways.

nasal passages The convoluted air passageways in the nose that conduct air between the nostrils and the pharynx.

nasal septum The midline barrier that separates the left and right nasal passages.

nasolacrimal duct The tube that carries tears from the lacrimal sac to the nasal cavity.

natural killer (NK) lymphocytes Lymphocytes that are neither T lymphocytes nor B lymphocytes but have the ability to kill some types of tumor cell and cells infected with various viruses. navicular bone The distal sesamoid bone of the horse. The navicular bone is located deep in the hoof behind the joint between the middle and distal phalanges.

neck The area of a bone that joins the head with the main portion of the bone.

necrotic Refers to the death of a particular portion of tissue. negative (nitrogen) balance When the rate of protein loss in feces and urine exceeds the amount of protein ingested.

neonatal period The first few weeks and months after birth.

nephron The basic functional unit of the kidney. It is composed of the renal corpuscle and the tubule system, which is made up of the proximal convoluted tubule, loop of Henle, and distal convoluted tubule.

nephrosis An abnormal condition of the kidney involving degen­erative changes, particularly in the renal tubules; may be associ­ated with protein loss in the urine, which leads to a systemic hypoproteinemia.

nerve A bundle of myelinated nerve fibers in the peripheral nervous system that conduct sensory and/or motor impulses to and from the central nervous system and the periphery of the body.

nerve fiber An axon.

nerve impulse A wave of cell membrane depolarization that travels from the point of stimulus down the length of a nerve cell process.

nervous tissue A collection of tissues that collect, process, and convey information. Nervous tissue includes the brain, spinal cord, and nerves. Sensory (afferent) nerves convey information about the body's surroundings to the brain, whereas motor (efferent) nerves send instructions from the brain to the body. Some nervous tissues, called mixed nerves, can perform both functions.

neural tunic Inner layer of the eye, consisting of the retina.

neuroglia Auxiliary cells in the nervous system that support and protect the neurons. Examples include oligodendrocytes, microglia, and astrocytes.

neurology The study of the nervous system.

neuromuscular junction The “connection” between the end bulb of a motor nerve fiber and a skeletal muscle cell. There is actually a tiny space, called the synaptic space, between the end of the nerve fiber and the sarcolemma of the muscle fiber.

neurons Cells of the nervous system that are structurally com­posed of a cell body (perikaryon), dendrites, and an axon. They not only initiate nerve impulses but also conduct them.

neurotransmitter A chemical released by the presynaptic neuron that diffuses across the synaptic cleft, binds with the receptor on the postsynaptic membrane, and stimulates (excitatory neu­rotransmitter) or inhibits (inhibitory neurotransmitter) the postsynaptic neuron. Neurotransmitters also stimulate effector cells such as muscle or gland cells.

neutral fats A lipid composed of three fatty acids and a glycerol. Also known as triglycerides.

neutralize To cause the pH of a solution to approach 7 (neutral). neutron A subatomic particle with no electrical charge that joins with the protons to make up the entire mass of the nucleus.

neutropenia A decrease in the number of neutrophils in periph­eral blood.

neutrophil A granulocytic white blood cell. Neutrophils are phagocytes and are known as the first line of defense against invading microorganisms because of their fast response time.

neutrophilia An increase in the number of neutrophils in periph­eral blood.

nicotinamide adenine dinucleotide (NAD⁺ or NADH) The coenzyme used in many oxidation-reduction functions. When oxidized, it is written as NAD⁺. In its reduced form, having given up an electron, it is written as NADH.

nicotinic receptor A type of cholinergic receptor that binds with acetylcholine.

nictitating membrane Thin, translucent third eyelid that moves across the eye from the medial canthus laterally to moisten and protect the eye. A T-shaped plate of cartilage covered by con­junctiva. It has lymph nodules and a gland that contributes to the tear film on its ocular surface. No muscles attach to the third eyelid. Its movements are entirely passive.

nitrogen balance When the rate of protein synthesis equals the rate of protein breakdown in the healthy animal. In other words, the amount of nitrogen ingested in the form of protein equals the amount excreted in urine and feces.

nociception Pain perception.

nociceptors Pain receptors.

nodes of Ranvier Myelin gaps in the covering of the axon between adjacent Schwann cells that are involved in rapid con­duction of nerve impulses along the axon.

nonessential amino acid An amino acid that is produced in the body in sufficient amounts; it does not have to be supplemented by diet. Different species produce different nonessential amino acids; therefore, what may be nonessential in one species may be essential in another.

nonspecific immunity Protects an animal against anything the body recognizes as foreign. It involves inflammation, phagocy­tosis, and the protective barrier of the skin and mucous mem­branes, which prevent antigens from entering the body. The immune system is not activated during nonspecific immunity.

nonsteroidal anti-inflammatory drug A drug that relieves pain (analgesia) and reduces inflammation but that is not related to the glucocorticoid hormones from the adrenal cortex. Glucocorticoid-like drugs are commonly referred to clinically as corticosteroids. Carprofen and meloxicam are examples of nonsteroidal anti-inflammatory drugs used in veterinary medicine.

nonstriated involuntary muscle An old name for smooth muscle. norepinephrine A hormone secreted by the medulla of the adrenal gland under stimulation by the sympathetic portion of the autonomic nervous system. It produces part of the fight-or- flight response that results when an animal feels threatened.

nuclear envelope A double-layered membrane made of lipids that surrounds the nucleus and separates the inner nucleoplasm from the outer cytoplasm; also called nuclear membrane.

nuclear membrane See nuclear envelope.

nuclear pores Pores that traverse both layers of the nuclear enve­lope, allowing the inflow of protein molecules and the outflow of RNA molecules.

nuclei Clusters of neuron cell bodies within the CNS.

nucleic acids The class of substances that include RNA and DNA, located within the cells of all living things. They are extremely dense and are composed of complex patterns of pentose sugars, phosphoric acids, and the nitrogen bases (purines and pyrimidines).

nucleolus (plural, nucleoli) The dark, spherical object contained within the nucleus that is the site of ribosomal RNA synthesis. Nucleoli are composed of DNA, RNA, and protein. Each cell's nucleus may have more than one nucleolus.

nucleoplasm The gelatinous substance that is the protoplasm of the nucleus.

nucleotides The combinations of phosphoric acid, pentose sugars, and pyrimidine or purine bases that make up nucleic acids.

nucleus The part of the cell that contains DNA and aids in several body functions, including reproduction, metabolism, and growth.

nuptial pads Dark, rough calluses on the undersides of the fore- lfimbs o male frogs that give males a better grip on the female during amplexus (external fertilization of eggs by the male).

nutrient foramen A large channel through the cortex of a large boounghe thr which large blood vessels pass carrying blood to aonmd fr the bone marrow.

O

obturator foramina A pair ol' large holes in the pelvis located on either side of the pelvic symphysis. The role of the obturator feomrasmina se to be to lighten the pelvis because no large rensesrevless o v pass through them.

occipital bone A skull bone that is one of the external bones of the cranium. The occipital bone is the caudal-most bone of the skull. It forms the atlantooccipital joint with the first cervical ivaertebra v the occipital condyles. The large foramen magnum icncitphiteaol bone is where the spinal cord exits the skull.

occipital condyle One of two articular surfaces on the occipital bone. W.¹ occipital condyles are located on either side of the foramen magnum and form the atlantooccipital joint with the first cervical vertebra (the atlas).

occlusal surfaceurfTahce s of the tooth that meets the surface of another tooth in the opposite arcade (e.g., the surfaces between the upper and lower premolars).

olecranon process The large process on the proximal end of the ourlnmasthat f the point of the elbow. The olecranon process ies the sit where the tendon of the powerful triceps brachii muscle attaches.

olfactory sense The sme of smell. The receptors for smell are located in the nasal passages.

oligodendrocytes Glial cells in the brain and spinal cord whose cmelblrualanre me forms the myelin sheath for axons in the

CNS.

oligopeptides Chains of 2-20 amino acids; also called polypeptides. oliguria Passing small amounts of urine.

omasumhaAmbcer of the ruminant forestomach that absorbs nutrients and water.

omentum Tlie supportive mesenteries which arise from the greater and lesser curvatures of the stomach.

omnivore An animal whose diet is a mixture of plants and meat. oncogeneeneA, g often carried by tumor viruses, that causes

malignancy in cells.

oncotic pressure The difference between the osmotic pressure of blood and the osmotic pressure of interstitial fluid or lymph.

oocyte The immature form of the female reproductive cell. oogenesis The production of female reproductive cells (ova) in olvlaicrlieasn. f

operculum Fleshy flap of skin at the external ear openings of some species that aids in funneling sound into the ear; flap of skin covering the ear of many owl species.

opisthoglyphous Referring to snakes with grooved rear fangs for evleinveormy. d

opsonin Plasma protein, usually an immunoglobulin, that coats iagne na,nt usually a microorganism, making it more attractive

thoagpocytes.

opsonization The process by which opsonins coat an antigen to rmeake it mo susceptible to phagocytosis.

optic disc The area of the retina where nerve fibers on its surface coonverge t form the beginning of the optic nerve. It contains nhotporeceptors and is the blind spot of the eye.

orchiectomy A surgical procedure by which the testes are removed Irorn a male animal; castration.

organ of Corti The receptor organ of hearing located in the cochlea. H consists of hair cells with a gelatinous structure (the tmecbtroarniael) me resting on the hairs surrounded by fluid.

Vibrations ol' the fluid cause distortion of the hairs, which gen- revraetes ne impulses that are interpreted by the brain as sound. organrouAp g of tissues that work together for common

purposes.

organelles Specialized structures within a cell that carry out spe­cific functions for that cell. Examples of organelles include mitochondria, Golgi apparatus, lysosomes, and ribosomes.

organic compounds A compound containing hydrocarbon groups.

organismvinAg li individual, animal, or plant, unicellular or multicellular, that is capable of independent existence.

origin of a musclere The mo stable of the attachment sites of a muscle. ^^Γen a muscle contracts, its origin attachment sites do not move much. This provides stability so that the insertion of tuhseclem can move bones or other structures.

oropharynx The portion of the pharynx caudal to the mouth thhicrohugh w food and air both pass.

os cordis The visceral bone in the heart of cattle that helps support the valves of the heart.

os penis The visceral bone in the penis of dogs that partially surrounds the penile portion of the urethra.

os rostri The visceral bone in the snout of swine that strengthens iotroftingthe r behavior of pigs.

osmoregulators Proteins that regulate and adjust the osmotic eptrweseseunre b the cell and extracellular fluids. Examples iunmcliunde alb and enzymes.

osmosis The passive movement of water through a semiperme- ambbler amne into a solution where the water concentration

iwselro.

osmotic diuresis Diuresis produced by excess dissolved substances iuhniedt fl circulating through the tubules of the nephrons. The excess gucose in the tubular fluid of animals with diabetes mel- lriotudsucpes an osmotic diuresis that results in polyuria.

osmotic pressureorcTehe f of fluid moving from one side of a semipermeable membrane to the other side because of differ­ences in solute (dissolved substance) concentrations on the two sides of the membrane. Under osmotic pressure, fluid moves from the side of the membrane with lower solute concentration teo the sid with higher solute concentration in an attempt to equalize the concentrations on both sides of the membrane.

ossicles Skull bones that are the bones of the ear. The ossicles are isoninxyets, b three on each side, in the middle ear that transmit svoeund wa vibrations from the tympanic membrane to the.irnonmer ear F the outside in, they are the malleus, the incus, aesn.d the stap

ossification The miIieralizalion or hardening of bone. osteoarthritis Inflammation of a joint characterized by progres- seitveeridoration of the articular cartilage.

osteoblasts The cells that produce bone.

osteoclasts Large, Inulliiiuclear cells of the bone that absorb bone and reshape and remodel damaged bones.

osteocyte Mature bone cell. The osteocytes are located in spaces in the ossified matrix called lacunae.

osteomyelitis A inflammatory bacterial infection of bone, often resulting in local bone death.

otoliths The literal meaning of otolith is ear stone. Ololillis are tyisntyalscr of calcium carbonate that lie in the gelatinous matrix that covers the hair cells of the macula (the sensory epithelium of the vestibule). The otoliths help the macula keep track of the position of the head.

oval window The membrade-covered opening into the cochlea that the stapes lies against. Vibrations of the oval window mem­brane sei the fluid in the cochlea in motion; this stimulates the sensory structures in the organ of Corti and produces the nerve impulses that are interpreted by the brain as sound.

ovarian follicle The fluid-filled structure in the ovary in which iomcymteasture o develop into mature ova. The lining cells of tohlleicfle, the granulosa cells, produce estrogen hormones.

ovaries The female gonads; they produce the female reproduc­tive cell, the ovum, as well as estrogen and progestin hormones; homologous to the testes of the male.

oviduct Also called the Fallopian tubes or uterine tubes; the yoviduct, b way of ciliary movement, transports ova from the ovary via the infundibulum to the uterus; the site of fertilization ieynciems;an sp tubular extension of the uterine horn.

oviparous Producing eggs in which the embryo develops outside the female's body, as in birds.

oviposition The act of laying eggs.

ovulation The traumatic rupture of a mature ovarian follicle that releases the ovum.

ovum (plural, ova) The ma⅛re female reproductive cell. The eolveuasmedis r from the mature follicle into the oviduct for fertilization by spermatozoa. The female gamete.

oxidation When a substance is combined with oxygen, or when aonmat loses electrons and thereby becomes more positively charged.

oxidative phosphorylation A process that allows free energy within the mitochondria to be used in the form of ATP.

oxyhemoglobin Hemoglobin that is carrying oxygen attached to icruolnesm. ole

oxyntic cells Slightly older term for parietal cells; gasinc gland rcoeldlsuctheat p hydrochloric acid.

oxytocin One ol' the posterior pituitary hormones. It is produced iynp otthhealhamus and then stored and released from the

posterior pituitary gland. It stimulates contraction of the myo- fmetrium o the uterus at breeding and parturition and contrac- ftion o the myoepithelial cells of the lactating mammary gland.

P

Pacinian corpuscle Tactile nerve endings located within the sub- icsustuaeneous t of the skin. These nerves can sense deep and heavy pressure, as well as stretch.

packed cell volume (PCV) The percentage of red blood cells in a blood sample; also known as the hematocrit.

pain An unpleasant sensory response caused by the stimulation of pain receptors in the body.

palatal surface Tlre surface of the upper arcade of teeth that faces the inside of the mouth; palatalemetehans the t are facing

trdhe har o soft palate.

palatine bones Skull bones that are part of the internal bones of tchhe.e fa T two palatine bones make up the caudal portion of

tdhe har palate.

palmar The caudal surface of the forelimb from the carpus distally.

palpatexamToinee parts of the body by touching and feeling

them.

palpebral conjunctiva The transparent membrane that lines the ionrntieornp of the eyelid.

palpebral reflex Reflex closure of the eyelids when the medial fcanthus o the eyelids is touched; used in anesthesia monitoring teop tahssess d of anesthesia.

pampiniform plexus The network of veins in the spermatic cord of the male. It is derived from the testicular vein and surrounds the testicular artery. It functions as a heat-exchange mechanism that helps keep the testes slightly cooler than the rest of the body owoitlihnogut c the body as a whole. Warm blood coming down tehsteictular artery is cooled by the blood in the pampiniform

plexus; at the same time, the blood in the plexus, which is roeturning t the systemic blood supply, is warmed by the blood itnertyh. e ar

pancreas Endocrine and exocrine gland that produces and seestcirveetes dig enzymes into the intestine and produces hor- lmudoinnegs, inc insulin and glucagon.

pancreatic islets The endocrine portion of the pancreas; com- fposed o thousands of microscopic clumps of cells scattered throughout the organ; also called the islets of Langerhans.

papilla (1) A nipplelike protuberance. (2) In birds, depressions in the Ieallier tracts that give rise to new feathers.

papillary layerf Layer o loose connective tissue that is intimately adjoined to the epidermis.

papillary muscles Muscular, nipplelike projections in the heart thhoart anc the chordae tendineae. When contracted, papillary cmt uscles a to open the atrioventricular heart valves.

parabronchi Subdivisions of the secondary bronchi in birds; coonnected t air capillaries with which gas exchange occurs.

paraffin A fine, purified hydrocarbon wax that is used to preserve tissue samples and support them so that they can be sliced into thin Seclioiis for microscopic examination.

paralyzed Referring to the condition of temporary or perma- fnent loss o muscle power or sensation.

paramecium A single-celled, ciliated protozoan.

paranasal sinus A space within a skull bone that is an outpouch­ing ol' the nasal cavity. Depending on the species, paranasal seinuses ar found within the frontal bones, maxillary bones, sopnhees,noid b and ethmoid bones; commonly referred to by teuhrsme. t sin

parasympathetic nervous system Part of the autonomic nervous system that is responsible for the “rest-and-restore response”; also called the craniosacral system Iiauoi of the lfocation o the parasympathetic nerves emerging from the brainstem and sacral vertebral segments.

parathyroid glands Endocrine glands consisting of several small cnaotdedules lo in, on, or near the thyroid gland. They produce parathyroid hormone.

parathyroid hormone (PTH) The hormone secreted by the para­thyroid gland that prevents the level of calcium in the blood from getting too low; also called parathormone.

paresis Partial or incomplete paralysis.

paretic Referring to a part of the body that suffers from paresis. parietal Pertaining to the wall of an organ or cavity.

parietal bones Skull bones that are among the external bones of the cranium. The two parietal bones form the dorsolateral walls of the cranium. They are large and well developed in the dog and cat but relatively small in horses and cattle.

parietal cells Cells in the gastric glands that secrete hydrogen and chloride ions to form the hydrochloric acid in the stomach.

parietal eye Part of the epithalamus; usually associated with the pineal gland; may be photoreceptive in some species and is associated with the endocrine system. Found in lizards and snakes. Also referred to the parietal organ or third eye.

parietal layer The layer of pleura or peritoneum that lines the thorax or abdomen, respectively.

parietal vaginal tunic The thick outer connective tissue layer around the testis and spermatic cord. Derived from parietal peritoneum. Also called the common vaginal tunic.

partial pressure In a mixture of gases, the partial pressure is the portion of the overall pressure each gas exerts. It can be calcu­lated for a particular gas by multiplying the total pressure of the gas mixture by the percentage content of that particular gas.

parturition The birth process.

parvovirus A virus of the family Parvoviridae that is highly con­tagious and often deadly.

passive immunity The transmission of intact, preformed anti­bodies from one animal to another. The antibody molecules can help protect the recipient animal from disease-causing agents. An important source of passive immunity is colostrum, the first secretion of the mammary gland after parturition. If it is drunk by the newborn within the first few hours after birth, the anti­body molecules will be absorbed into the bloodstream intact and help protect the newborn; however, after that time, the antibody molecules will be broken down by digestion and not absorbed intact.

pastern Area of the proximal phalanx of hoofed animals.

patagial ligament Ligament that runs along the leading edge of the patagium of birds, allowing extension and flexion of the wing.

patagium Lightly vascularized web of skin in birds extending from the shoulder to the wrist; provides elasticity to the wing during flight.

patella The kneecap; the largest sesamoid bone in the body. The patella is located on the front surface of the stifle joint in the tendon of the large quadriceps femoris muscle. It rides in the trochlea of the femur.

patellar ligament The name given to the quadriceps femoris tendon distal to the patella. It helps provide stability to the stifle joint.

patent ductus arteriosus Persistent fetal connection between the aorta and the pulmonary artery that can result in congestive heart failure if not corrected.

pathogen A microorganism that can cause a disease. pathogenicity Disease-causing potential of a microorganism.

Virulence.

paucicamerial lung A lung with four chambers.

PCT See proximal convoluted tubule.

PCV See packed cell volume.

pecten (1) Dark, ribbonlike structure attached to the retina and extending into the vitreous humor in the eye of birds; thought to provide nourishment to the eye.

pectin Sugar polymer commonly found in fruits.

pectoral crest Thin, widened area on the proximal humerus where the wing muscles of a bird attach. Also called the deltoid crest.

pectoralis Large flight muscle originating from the keel and inserting on the humerus of birds; contraction results in the downstroke.

pellet Tight bundle of indigestible food components (small bones, fur, feathers) that is eliminated by regurgitation by some species of birds.

pelvic flexure Where the left ventral colon of the horse bends back on itself to begin the left dorsal colon.

pelvic limb The hindlimb.

pelvic symphysis The cartilaginous joint (amphiarthrosis) that unites the two halves of the pelvis ventrally; the pubic symphysis.

pelvis The most proximal bony structure of the pelvic limb. Also known as the os coxae. The pelvis attaches to the sacrum dorsally at the sacroiliac joints and forms the hip joints with the heads of the femurs.

penis The male copulatory organ; homologous to the clitoris of the female.

pentose sugar A simple sugar that has five carbon atoms per molecule. An important component of riboflavin and ribonu­cleic acid (RNA).

pepsin A protease activated from pepsinogen in the stomach. pepsinogen The enzyme precursor secreted by the chief cells in the stomach; pepsinogen is activated to pepsin.

peptidase Enzyme that breaks peptides into amino acids, dipep­tides, and tripeptides.

peptide A molecule containing two or more amino acids joined together. Peptides combine to form proteins.

peptide bond Covalent joining of one amino acid to another to form a peptide.

peptide hormone Hormone consisting of chains of amino acids. perception The conscious recognition of something; for example, in the process of nociception, pain nerve impulses are perceived in the brain.

pericardial fluid The transudate fluid secreted into the pericar­dial sac around the heart. It helps to lubricate the surface of the heart as it beats.

pericardium Tissue that forms a sac around the heart to protect it and to control the movement of the heart within the thorax. perichondrium The fibrous connective tissue surrounding the external surface of cartilage. The perichondrium is vascularized and provides a limited amount of nutrition to the cartilage.

periderm Epidermal covering of a new feather as it emerges from the skin.

perikaryon The cell body of the neuron; soma.

perilymph The fluid that surrounds the membranous portion of the inner ear, where the sensory receptors of the inner ear are found.

perimysium The fibrous connective tissue layer in skeletal muscle that snrrounds groups of muscle fibers and binds them into groups oιlled fascicles.

perinuclear cisterna The space between the internal and external lfayers o the nuclear envelope.

periosteum The fibrous membrane that covers the outsides of Xones except for their articular (joint) surfaces.

peripheral blood Blood outside bone marrow that is flowing around the body in blood vessels.

peripheral nervous system (PNS) Nerves outside the central nervous system (i.e., outside the brain and spinal cord).

peripheral proteins Proteins located on the inside of the cell's lipid Iiilaver that have enzymatic capabilities. These proteins are lbeislse mo than integral proteins because they are attached odirectly t the cytoskeleton.

peristalsis Arhythmic, wavelike motion that progressively moves through a tubular organ, such as the small intestine. Peristalsis assists in the movement of food through the alimentary canal. It is aιι IiivoIuiiIary movement that is stimulated when the tube is distended.

peritoneal fluid A fluid secreted by the peritoneum in the tabdomen. I lubricates the internal organs as they move over ro.ne anothe

peritoneum The thin membrane in the abdominal cavity that covers the abdominal organs (the visceral layer of peritoneum) and lines the abdominal cavity (the parietal layer of perito­neum). A potential space between the two layers contains a small amount of lubricating fluid that allows abdominal struc- teoures t slid smoothly over each other.

peritonitis Inflammation of the peritoneum.

peritubular capillaries Capillaries that branch off from the effer- leonmt egrular arterioles. They flow through the kidney closely

iatshsociated w the tubules of the nephron and eventually con- evnaedrvgee a l the kidney as the renal vein. Tubular secretion and reabsorption are associated with the peritubular capillaries.

peroxidase A catalyst (enzyme) that converts free radicals into ehryodxriodgee, n p a function that is essential for cellular

respiration.

peroxisome Found in high numbers in the kidney and liver cells eorftembroastte v animals, this single-membraned vesicle

detoxifies the body by releasing catalase and other enzymes.

petechiae Small hemorrhages found on the skin, mucous mem­branes, and serosal surfaces anywhere in the body.

pH A chemical concept defined as the negative logarithm of the hnydrogen io concentration. It is a number that indicates rela­tive acidity or alkalinity. The pH ranges from 0 to 14. The lower the pH, the more acidic the environment, and the higher the pH, the more alkaline the environment. A pH of 7 is neutral (cnideiicther a nor alkaline).

phagocytize The verb describing ingestion of solid material eblyl.a c

phagocytosis Ingestion of microorganisms or other substances by phagocytic cells (neutrophils, monocytes, and macrophages).

phagosomeesiTclhee v formed by phagocytosis, which contains

omaterial t be digested.

phalangeal bonesonTeshe b that compose the digits. phalanx (plural, phalanges) A bone ol' a digit (toe or finger). pharynx The throat; a common passageway for the respiratory easntdivedig systems.

pheomelanin A sulfin'-based, yellow-brown pigment that pro- eddudciessha r color in hair.

phonation Voice peoduction.

phospholipid A molec ule composed of three parts: phosphorus, fatty acids, and a nitrogenous base. Any lipid that contains phos­phorus. Phospholipids are the main components of the cell membrane.

phosphorylation Tfie addition of phosphorus to an organic compound.

photoreceptorsnsoTrhye se receptors that convert photons of

lnigerhgtye to nerve impulses that are interpreted by the brain as vision; the rods and cones.

physiology The study of the functions of the animal body and ithtsrs.opuagrh T physiology, we can describe how parts of the obrokdy w and what their functions are.

pia mater The innermost layer of the meninges.

pigmentation The degree of coloration, dependent on the con­centration of melanin. Higher concentrations of melanin (pigment) will create a darker color.

pillars Muscular folds that divide the sacs of the rumen.

pineal body A structure in the brain located at the caudal end eoefpthe d cleft that separates the two hemispheres of the cere­brum, just rostral to the cerebellum. It produces the hormone- like snbetance melatonin that appears to influence the body's lboicoklo. gical c

pinna The externally visible part of the ear that collects sound waves and funnels them down into the external ear canal; the.ear flap

pinocytosis The iιιgeslioιι by a cell of liquid material through endocytosis (“cell drinking”).

pitting edema The condition in which dents are left behind in dmeomisatt,oeus tissue when pressed firmly.

pituitary fossa A depression in the dorsal surface of the sphenoid bone that houses the pituitary gland in the living animal.

pituitary gland The master endocrine gland. A pea-sized endo- lcarninde g located at the base of the brain; made up of the aunittaerryior pit gland, which produces seven known hor­mones, and the posterior pituitary gland, which stores and roeleases tw hormones from the hypothalamus; also called the hypophysis.

pivot joint A j^mt that allows only a rotary motion. The only true pnvot joint in most animal bodies is the atlantoaxial joint between the first and second cervical vertebrae; also called a trochoid joint.

placenta Alifo-support system for a developing fetus; a multilay­ered, fluid-filled, membranous sac that surrounds the fetus and loinks it t the blood supply of the uterus. There is normally no direct mixing of fetal and maternal blood, but the blood vessels lanoroseuecgh e to each other that nutrients, wastes, and respi­ratory gases are easily exchanged between the fetal and maternal bloodstreams. It is also an important endocrine organ. At par­turition, it is delivered last; so it is sometimes referred to as the afterbirth.

placentomechmTheentatta site with the uterine lining for coty­ledonary placentas. It is formed from the tight connection of a cnotyledon o the surface of the placenta with a caruncle in the lfining o the uterus. Normally, large numbers of placentomes lciennktathe pla with the uterus.

plane of reference Any of four basic imaginary slices through an animal body oriented at right angles to each other. They provide points or areas of reference for descriptions of direction or location. The four anatomic planes of reference are the sagit­tal plane, median plane, transverse plane, and dorsal plane.

plantar The caudal surface of the hindlimb from the tarsus distally.

planum nasale The topmost plane of the muzzle in such species as cats, dogs, pigs, and sheep.

planum nasolabiale The topmost plane of the muzzle including the upper lip in such species as horses and cows.

plaque A flat, thickened site present in the desmosomes of the epithelial tissue.

plasma The liquid matrix of blood, which contains proteins and suspended cells. Plasma also contains diffused gasses, electro­lytes, and a variety of biochemicals.

plasma membrane See cell membrane. plasmalemma See cell membrane.

plastron The ventral shell of turtles and tortoises.

platelets Also known as thrombocytes. Platelets are small pieces of cytoplasm that break off cells in the bone marrow (mega­karyocytes) and enter peripheral blood. They are involved in hemostasis by helping plug leaks in blood vessels and initiating the blood-clotting process. The majority (two-thirds) of the body's platelets are found in the spleen; the rest are circulating through the blood.

pleomorphic Varying shapes; used to describe a monocyte nucleus that can take on many shapes without dividing into distinct segments.

pleura The thin membrane in the thoracic cavity that covers the thoracic organs (the visceral layer of pleura) and lines the tho­racic cavity (the parietal layer of pleura). A potential space between the two layers contains a small amount of lubricating fluid that allows the thoracic structures to slide smoothly over each other as they and the thorax itself move.

pleural fluid The transudate fluid secreted by the serous mem­branes of the pleural cavity. It helps to lubricate the lungs during respiration.

pleurodont teeth Teeth fused with the inner surface of the jaw (alveolar ridge), not in sockets.

pluripotent stem cell Primitive cell type found in red bone marrow. It is the cell type from which all blood cells are formed. pneumonia A lower respiratory tract infection affecting the tiny bronchioles and alveoli in the lungs. Pneumonia can be a serious disease because the inflammatory fluids and excess mucus that are produced by the irritation are difficult for the animal to cough up from deep in the lungs.

pneumothorax Air in the chest; an abnormal condition resulting from air leaking into the thoracic cavity from the lung or the outside world. It can result in collapse of the lung in that area because of loss of the normal partial vacuum in the thorax.

PNS See peripheral nervous system.

podocytes Meaning “foot cells.” Podocytes make up the visceral layer of Bowman's capsule in the kidney. Podocytes form a per­meable covering on the outside of the glomerular capillaries.

point The portion of the frog that points towards the toe of the horse's hoof.

polar Having the quality of defined direction.

polar body A “garbage can” for excess chromosomes. The polar body is a by-product of ovum development in the ovarian fol­licle; it will not develop into a mature ovum.

polar molecule A molecule with oppositely charged ends.

poll Top of the head between the bases of the ears.

polled breeds Hornless animals from species that normally produce horns. The breeder achieves this by consistently select­ing polled offspring in their breeding lines. An example of a common polled beef breed is the Hereford.

polychromasia “Many colors”; this term is used to describe immature red blood cell cytoplasm when it is still metabolically active and has started producing hemoglobin. This condition results in both acidic (red) and alkaline (blue) stain being taken up by the cytoplasm, giving it a lavender color.

polycythemia An abnormal increase in the number of red blood cells in the circulation.

polydipsia Excessive thirst.

polyestrous An animal that has continual estrous cycles if she is not pregnant. As soon as one cycle ends, another begins.

polymorphonuclear A term that describes a nucleus that can have many shapes.

polypeptide Chains of more than 10 amino acids; also called oligopeptides.

polyphagia Excessive appetite.

polysaccharide “Many sugars”; a carbohydrate containing many monosaccharides that may be released in the process of hydro­lysis. Polysaccharides are divided into two groups: cellulose and starch.

polyunsaturated Describes a fatty acid in which more than one double bond is present in the carbon chain.

polyuria Production of an excessive volume of urine.

pons A part of the brainstem located just rostral to the medulla oblongata.

pore A minute opening or space.

portal system An arrangement of blood vessels that carries blood from one organ or tissue directly to another organ or tissue before returning it to the heart.

positive nitrogen balance When the amount of protein ingested is greater than the amount excreted.

posterior chamber The portion of the aqueous compartment of the eye behind (caudal to) the iris.

posterior pituitary gland The neurohypophysis; the caudal portion of the pituitary gland that stores and releases two hor­mones (antidiuretic hormone and oxytocin) that are produced in the hypothalamus.

posterior A directional term meaning toward the back of a human body.

postganglionic neuron In reference to the autonomic nervous system, it is the second of two neurons that typically make up the nerves of the autonomic nervous system; so called because this neuron originates from a ganglion and carries the impulse from it to the target organ or tissue.

postprandial lipemia A condition of cloudy plasma that results from small fat particles found in blood soon after eating.

postsynaptic neuron The neuron that contains the receptors to which a released neurotransmitter binds, causing depolarization or inhibition of depolarization of the neuron.

precocial Chicks that are hatched with downy feathers and their eyes open and that are mobile.

precursor Something that precedes or develops into something else.

preganglionic neuron In reference to the autonomic nervous system, it is the first of two neurons that typically make up the nerves of the autonomic nervous system; so called because the first neuron is located “before” the ganglion.

prehend To grasp.

prehensile Capable of grasping.

prehension Grasping.

premolars The teetIi rostral to the molars.

prepuce The skin-covered sheath around the free end of the penis.

pressure The sense of something pressing on the body surface; often combined with the tactile sense, which is the sense of touch.

pressure gradient Tlre spcdrurιι between an area of highest pressure and an area of lowest pressure.

presynaptic neuron The neuron that is stimulated by the depo­larization wave to release a neurotransmitter into the synapse. primary growth center The main growth area of a bone devel­oping by the endochondral (cartilage) method. Primary growth centers are the areas of bone development that are located in the main portions of the cartilage rod bone templates in a develop- ientugsf.

primary hairs The large, straight hairs predominant in complex hair follicles.

primary structure A long chain of amino acids held together with peptide bonds.

prime mover A muscle or muscle group that directly produces a desired movement.

prion A smaU, protein-based particle that is both infectious and reetsilient y not a living pathogen.

process A general name for a lump, bump, or other projection on a bone. Processes can be either articular processes, which contribute to joint formation, or nonarticular processes, which auraellyus sites where tendons attach.

proctodeum Posterior section of the cloaca of birds that is ayccessed b the coprodeum and urodeum and stores the excre- iml ent unt it is eliminated.

productw The ne substance created by the interaction of two or hmeomreiccal substances.

product moleculesrodTuhcet p of an enzyme reaction caused by the o^iiA ability to weaken the bonds of the substrate molecule.

proestrus The stage of the estrous cycle when follicles are actively developing and growing in the ovary. As they grow, the follicles ipsriondguce r levels of estrogens, which gradually induce the physical and behavioral changes that prepare the animal for breeding.

progesteronerinTcihpeapl progestin hormone produced by the cuoterupmus l of the ovary. It helps prepare the uterus for implantation of the fertilized ovum and helps maintain preg- ncaency o it begins.

progestins Hormones produced by the corpus luteum of the ohveyary. T are necessary for the maintenance of pregnancy, particularly during the early gestational period. The principal progestin hormone is progesterone.

prohormone A hormone precursor that has minimal hormonal eyffect b itself.

prokaryotes Included in the kingdom Monera, these unicellular organisms have no true nucleus, nuclear envelope, or membrane-bound organelles. Bacteria and cyanobacteria are included in this classification.

prolactineriTorhe ant pituitary hormone that helps trigger and

maintain lactation.

promoters Codes within the DNA sequence that indicate where RNA synthesis should begin.

proper vaginal tunic The inner connective tissue sac that sur- eounds the testis. It is derived from the visceral peritoneum that covered the testis during its early development in the abdominal cavity. Also known as the visceral vaginal tunic.

prophasehasTehe p during mitotic division when chromatin

becomes visible and organizes into chromosomes by joining two isatrands v a centromere. The nuclear envelope and the nucleoli aplesaor,disap and the centrioles divide and replicate, traveling itptoohleer” “ of the cell.

propionic acid One ol' the key volatile fatty acids produced by earnmaeernotbaitci,vef metabolism in the rumen.

proprioception The sme of body position and movement. Steetch receptors in skeletal muscles, tendons, ligaments, and juolienst caps send impulses to the nervous system to keep it ifnformed o the positions and movements of the various body parts.

prostaglandins (PGs) Hormonelike substances that are pro- xdeurcted and e many effects locally in a variety of body tissues. Sometimes called tissue hormones bacaese they regulate bio­chemical activities in the tissues where they are formed.

prostate gland The male accessory reproductive gland that gen- eurarlolyunsds the urethra; it lies just distal to the urinary

bhleadder. T prostate gland is the only accessory reproductive gland in the dog, so it is quite large in that species.

proteases Enzymes that break down the basic structure of a yprotein b hydrolyzing the peptide bonds between the amino acids.

proteasomeirngyaAnetlle o found in both the nucleus and the cytoplasm that breaks down individual proteins no longer needed by the cell. The proteasome is a complex of proteins with laoorhweol c containing proteases. Individual proteins, tagged for dcstouction by the protein ubiquitin, are drawn into the core and dcgrndcscl. Short chains of amino acids are expelled from trhoetepasome.

protective proteins Proteins that are a component of the immune ruecshponse, s as antibodies.

protein Large organic compounds that are composed of amino ladcids he in peptide bonds to form polypeptides. They are synthesized by all living things and are essential for the basic maintenance of animal tissue.

proteoglycaniscTouhse v intercellular material located in the egtawpseebn the adjacent cells in the zonula adherens. It is fcomposed o polysaccharide chains and small proteins.

proteroglypous Associated with the Proteroglyphae group of istnhakes w hollow fangs that are venomous but not aggressive.

proton A subctomic particle with a positive charge that, along with the neutrons, makes up the entire mass of the nucleus. The fnumber o protons defines the atom as a specific element.

protoplasm The viscous fluid found within the cell.

proud flesh The condition occurring when excess amounts of new tissue, known as granulation tissue, develop in an otherwise nonhealing wound; also known as exuberant granulation tissue. proventriculus Anterior glandular stomach of birds in which chemical digestion of proteins begins.

provitamin A molecule, such as beta carotene, from which an animal can manufacture a vitamin.

proximal A directional term used only for extremities of the body. It implies a position or direction toward the body proper.

proximal convoluted tubule (PCT) The first part of the tubular portion of a nephron. Its lumen is a continuation of the capsular space of Bowman's capsule in the renal corpuscle. The majority of tubular reabsorption takes place from the PCT.

proximal sesamoid bones Paired sesamoid bones in the legs of horses. They are located in the large digital flexor tendons behind the fetlock joints (the joints between the large metacar­pal and metatarsal bones and the proximal phalanges).

pruritus Itching; often caused by parasitic infestations or allergic reactions.

pseudocyesis Pseudopregnancy; an abnormally prolonged and exaggerated diestrus period that results in an animal acting and looking pregnant when it is not. Most cases resolve spontaneously.

pseudopod (plural, pseudopodia) “False foot”; the temporary extension of the cell's membrane and cytoplasm either for loco­motion or to engulf nourishment.

pseudopregnancy See pseudocyesis.

pseudostratified columnar epithelium Single-layered columnar cells that appear to be stratified because of the positioning of their nuclei. These epithelia are often ciliated and are good at moving material across their surfaces.

pterygoid bones Skull bones that are part of the internal bones of the face. The two pterygoid bones support part of the lateral walls of the pharynx (throat).

pterylae Seven tracts of skin where feathers originate.

pubis The smallest and most medial of the three pairs of bones that make up the pelvis. The pubis forms the cranial portion of the floor of the pelvis.

pulmonary An adjective referring to the lungs.

pulmonary artery Artery arising from the right ventricle that delivers blood into the pulmonary circulation.

pulmonary circulation The part of the circulatory system that delivers unoxygenated blood to the lungs and oxygenated blood to the left side of the heart.

pulmonary valve A semilunar valve; it separates the right ven­tricle and the pulmonary artery during ventricular diastole.

pulp The latticelike material in the center of the tooth, which contains the nerve and blood supply.

pupil The opening in the center of the iris.

pupillary light reflex (PLR) Reflex in which light is shone into one eye, and the pupil constricts in both eyes.

pustules Small, pus-filled elevations of the skin that are easily expressed.

pygostyle Bony plate in birds formed by the fusion of several coccygeal vertebrae; supports the tail feathers.

pyknotic The term used to describe a nucleus that has died. The chromatin has become densely compacted, so no pattern is visible.

pyloric antrum The portion of the stomach between the body and the pylorus. The pyloric antrum grinds up swallowed food and helps regulate hydrochloric acid production.

pyloric sphincter Ring of circular smooth muscle between the duodenum (small intestine) and the stomach; regulates the movement of the liquid chyme from the stomach to the intestine.

Pylorus The portion of the stomach that connects with the duo­denum (small intestine).

Q

quarters (1) The common name for the mammary glands that make up the udder of the cow. (2) The medial and lateral regions of the hoof.

queen A sexually mature, intact female cat.

R

radioactive isotope An unstable isotope of an element that decomposes spontaneously by emission of subatomic particles and radiation.

radius One of the two bones (the ulna is the other) that form the antebrachium, or forearm. The radius is usually the main weight-bearing bone.

rafts Free-floating, specialized regions in the plasmalemma, and also in the membranes of the Golgi and lysosomes. Called lipid rafts, they serve as a construction site for signalling molecules. They regulate protein trafficking, the fluidity of the membrane and neurotransmission.

ramus of the mandible The vertical portion of the mandible located at its caudal end. The ramus is where the powerful jaw muscles attach to the mandible.

reabsorption The process by which some constituents of plasma that were filtered out of the plasma by the glomerulus are returned to the bloodstream. Water, glucose, amino acids, and sodium are some of the substances that are reabsorbed.

reactants Substances initially involved in a chemical reaction.

receptor In the context of the nervous system, a specialized protein to which neurotransmitters bind.

receptor-mediated endocytosis A very specialized form of endocytosis that only allows the cell to incorporate those mate­rials that have protein receptor sites specifically for that material on the cell.

rectum The last portion of the large intestine, located between the colon and the anus. The rectum stores feces until defecation.

red bone marrow The hematopoietic (blood cell-forming) type of bone marrow.

red pulp The area of the spleen that is filled with blood sinusoids and macrophages.

reflex A rapid, automatic response to a stimulus, intended to protect the body and maintain homeostasis.

reflex arc The reflex arc is composed of the sensory receptor, sensory neuron, interneuron(s), motor neuron, and target tissue or organ involved with a stimulus and reflex response.

refractory period The period in the depolarization- repolarization cycle when the neuron cannot be stimulated to depolarize (absolute refractory period) or can only be depolar­ized with a greater than normal stimulation (relative refractory period).

regional anatomy A method of studying anatomy that examines all the component structures that make up each region of the body. For example, the regional approach to abdominal anatomy would examine all the cells, tissues, organs, muscles, blood vessels, and nerves that are present in the abdomen.

regulatory proteins Those proteins essential to maintaining normal body function, including insulin and many other hormones.

regurgitation The movement of food back up the esophagus toward the mouth; in contrast to vomiting, regurgitation usually involves undigested food and occurs without the violent abdominal contractions associated with vomiting. Regurgitated material usually does not contain stomach acid.

relaxin A hormone produced late in pregnancy that helps relax ligaments between the bones around the birth canal in prepara­tion for parturition. Depending on the species it may be pro­duced by the corpus luteum of the ovary, the uterus, or the placenta.

remiges The primary and secondary flight feathers in the wing. renal artery The major arterial blood supply to the kidney. It is a branch of the aorta and enters the kidney at the hilus. After entering the kidney, it begins branching into progressively smaller vessels that eventually become the glomerular vessels.

renal corpuscle The first part of the nephron. It is composed of the glomerular capillaries and Bowman's capsule. The capsular space of the renal corpuscle continues as the proximal convo­luted tubule.

renal pelvis The collection point for tubular filtrate as it leaves the collecting ducts. When the fluid enters the renal pelvis, it is in the form of urine that must be eliminated from the body. The renal pelvis continues as the ureter that carries urine to the urinary bladder.

renal portal system In birds, the network of veins that transport blood from the extremities to the capillaries of the kidneys before returning it to the heart.

renal vein The major vein that drains the kidney. It is formed from venules that are formed by convergence of the peritubular capillaries. It leaves the kidney at the hilus.

renin A hormone released in the kidney (by the juxtaglomerular cells of the afferent glomerular arterioles) in response to low blood pressure.

renin-angiotensin-aldosterone system A sequence of hor­monal reactions, initiated by the kidneys in response to low blood pressure, which result in vasoconstriction and increased blood volume.

repolarization The process following depolarization wherein potassium ions diffuse rapidly out of the neuron.

reproductive cells Cells found in the ovary and testis that carry the genetic code. Each cell (called an ovum and sperm for female and male, respectively) contains one half of the genetic code, which is expressed as a haploid number of chromosomes. The gametes.

reproductive system The group of organs that function to produce offspring. The organs of the reproductive system include not only those of the genital tract, such as the ovaries and testes, but also all the organs of the endocrine system that regulate reproductive hormones.

residual volume The volume of air remaining in the lungs after the maximum amount of air has been forced out by expiration.

respiration The movement of oxygen from the outside air to the cells in an animal's body, and the movement of carbon dioxide in the opposite direction.

respiratory center The area in the brainstem that controls the breathing process.

respiratory system The group of organs that function to fortify blood with oxygen and remove carbon dioxide. The respiratory system includes the lungs, the trachea, and the respiratory center in the medulla oblongata. Supportive thoracic muscles and the diaphragm also play important roles in respiration.

resting membrane potential The electric charge of some cells at rest, caused by differing concentrations of ions inside and outside the cell membrane.

resting state In reference to neurons, more sodium ions are outside the cell membrane than inside, and more potassium ions are inside the cell; it is the state of the neuron before stimulation.

rete testis A complex of ducts in the testis that conduct sperma­tozoa from the seminiferous tubules to the efferent ducts.

reticular cell Phagocytic cells of reticular connective tissue. Retic­ular cells are particularly important in lymphatic and hemato­poietic tissue.

reticular connective tissue Connective tissue composed of net­works of reticular fibers and cells; found principally in bone marrow, lymph nodes, blood vessels, liver, and kidney.

reticular fibers Extremely fine fibers in reticular connective tissue. reticular groove A muscular groove found in young ruminants that conveys milk and liquids from the cardia directly to the omasum; also called the esophageal groove.

reticular layer Layer of irregular connective tissue that composes the majority of the dermis. It is intimately associated with the papillary layer by collagen fibers.

reticuloperitonitis Also called hardware disease; inflammation and infection of the reticulum and abdominal cavity that can occur in ruminants due to irritation from sharp objects that have been swallowed.

reticulorumen Pertaining to both the reticulum and rumen; often describes the contractions of the rumen and reticulum together. reticulum The most cranial part of the forestomach; it has a hon­eycomb appearance inside.

retina The inner nervous layer of the eye where the photorecep­tors are located. The refractive structures of the eye form an image on the retina that is converted to nerve impulses by the photoreceptors (the rods and cones). The nerve impulses from the retina are transmitted by the optic nerve to the brain, where they are converted to visual images.

retinal degeneration The progressive degeneration of the rods and cones, eventually leading to blindness. It occurs as an inher­ited disease in dogs and as a result of taurine deficiency in cats. retractor penis muscle An elastic, bandlike muscle that pulls the nonerect penis of animals with a sigmoid flexure back into its S-shaped configuration.

retrices Tail feathers.

retroperitoneal Behind the parietal layer of peritoneum that lines the abdominal cavity, outside the abdominal cavity proper.

ribonucleic acid See RNA.

ribosomal RNA (rRNA) One of the main components of ribo­somes; aids in protein synthesis and the combination of amino acids to create protein molecules.

ribosome An organelle composed of ribonucleic acid, located on the rough endoplasmic reticulum or suspended in the cyto­plasm, where protein synthesis takes place.

ribs Long bones of the axial skeleton that form the lateral walls of the thorax. Their dorsal portions are made of bone and form synovial joints with the thoracic vertebrae. Their ventral por­tions are made of cartilage (the costal cartilages).

rigor mortis Literally, stiffness of death. The stiffness of skeletal muscles that usually occurs shortly after death. It results from insufficient sources of energy in muscle fibers to allow them to relax.

RNA (ribonucleic acid) The nucleic acid used in protein synthe­sis. It differs from DNA, because it uses ribose instead of deoxy­ribose and also because RNA's pyrimidine, uracil, replaces DNA's thymidine. The three types of RNA are transfer RNA (tRNA), messenger RNA (mRNA), and ribosomal RNA (rRNA).

RNA polymerase An enzyme that aids in transcription by con­verting DNA base sequences into RNA base sequences.

roaring See laryngeal hemiplegia.

rods Photoreceptors in the retina of the eye that perceive dim light images in shades of gray.

root The anchor of the hair that attaches it to the connective tissue layer.

root hair plexus The arrangement of sensory nerves located at the root of the hair follicle that enable it to sense touch.

roots of the penis The structures that attach the penis to the brim of the pelvis. They consist primarily of the two connective tissue crura covered by the ischiocavernosus muscles.

rostral A directional term meaning toward the tip of the nose. Rostral is generally used to describe positions and directions only on the head, where the term cranial loses its meaning.

rotation A joint movement that consists of a twisting motion of a part on its own axis.

rough endoplasmic reticulum (rough ER) The portion of the ER that is studded with ribosomes and is involved in protein synthesis.

round ligament of the uterus A cord of fibrous tissue and smooth muscle contained in the free edge of a lateral fold of the broad ligament in the female. Extends from the tip of the uterine horn caudally and ventrally to the area of the inguinal ring.

round window The membrane-covered opening into the cochlea that functions as a pressure-relief device. When the stapes pushes on the membrane covering the oval window, the fluid in the cochlea pushes on the membrane covering the round window, making it bulge outward. When the stapes pulls on the oval window membrane, the round window membrane bulges inward. This allows the fluid in the cochlea to vibrate freely with the movements of the ossicles and stimulate the hearing recep­tors in the cochlea.

rRNA See ribosomal RNA.

rugae Folds of the inner lining of the stomach.

rumen A large, fermentative section of the forestomachs of rumi­nants; the rumen is responsible for production of volatile fatty acids, microbial protein, and other essential nutrients needed by the ruminant.

ruminant An herbivore that has a large fermentative section of forestomach called a rumen; cattle, sheep, and goats are exam­ples of ruminants.

rumination The process of regurgitating coarse food from the reticulorumen, rechewing the food, and swallowing it again; also called “chewing the cud.”

S

SA node See sinoatrial node.

saccule One of two saclike spaces (the utricle is the other one) in the vestibule that contain sensory structures that monitor the position of the head.

sacral vertebrae The vertebrae of the pelvic region. The sacral vertebrae fuse into a solid structure called the sacrum, which forms a joint with the ilium of the pelvis on each side called the sacroiliac joint.

sacroiliac joint The joint between the pelvis and the sacrum that joins the pelvic limb to the axial skeleton.

sacrum The solid structure formed by the fusion of the sacral vertebrae.

sacs Blind pouches.

sagittal plane An anatomic reference plane; the sagittal plane runs lengthwise, dividing the body into left and right parts that are not necessarily equal halves; see median plane.

saliva The liquid secretion of the salivary glands. Saliva is secreted into the mouth in response to food, stimulation of the mouth, or thinking about eating; in most species, it contains enzymes and buffers.

salivary glands Glands located in and around the mouth that produce saliva; include the parotid, mandibular, and lingual salivary glands.

salt Any ionic compound composed of positively charged cations and negatively charged anions so that the product is electrically neutral.

saltatory conduction The skipping of the depolarization wave (action potential) in a myelinated axon from one node of Ranvier to the next node of Ranvier; means “leaping” conduction.

sarcolemma The cell membrane of a muscle cell.

sarcomere The basic contracting unit of skeletal muscle. It con­sists of the actin and myosin filaments between Z lines in a muscle cell. Myofibrils are composed of many sarcomeres stacked end to end.

sarcoplasm The cytoplasm of a muscle cell.

sarcoplasmic reticulum The organelle in a muscle cell that is equivalent to the endoplasmic reticulum of other cells. It stores calcium ions (Ca²⁺) necessary to initiate the muscle contraction process. Release of Ca²⁺ from the sarcoplasmic reticulum is stimulated by a nerve impulse.

saturated fatty acids Such as those found in animal fats; have no double bonds in their carbon chains and can therefore accommodate the maximum number of hydrogen atoms; are solid at room temperature.

scapula The shoulder blade; the most proximal bone of the tho­racic limb. In domestic animals, no bony connection exists between the scapula and the axial skeleton.

Schleiden, Mathias Mathias Schleidens work with plant tissue helped him to develop the cell theory, which states that all living things are composed of cells.

Schwann cells Glial cells associated with peripheral nerves whose cellular membrane forms the myelin sheath for axons in the PNS.

Schwann, Theodor German anatomist whose work with animal tissue helped to develop the cell theory, which states that all living things are composed of cells.

sclera The white portion of the eye; part of the outer, fibrous layer of the eyeball.

sclerotic ring Bony plates that act as a protective border of the eye sockets.

scrapie A contagious and fatal disease caused by prions, occur­ring in both sheep and goats; the incubation period is around 2 years before symptoms, such as a disabled gait, appear. This eventually leads to prolonged illness, resulting in death.

scrotum The sac of skin that houses the testes and, by raising or lowering them, helps control their temperature.

seasonally polyestrous An animal that has continuous estrous cycles during a certain portion, or portions, of the year and no estrous cycles at other times.

sebaceous glands Simple holocrine glands that secrete an oily substance called sebum through the hair follicle.

sebum Secretion of the sebaceous gland containing oils and epi­thelial cells. It is released into the hair follicle to lubricate the hair and skin.

secondary growth center Secondary areas of growth in bones developing by the endochondral (cartilage) method. Secondary growth centers are areas of bone development located outside the main portions of the cartilaginous bone templates in a developing fetus.

secondary hair The smaller, yet most numerous hairs in an ani­mal's undercoat.

second-intention healing Healing of wounds that are not sutured and that form granulation tissue. Epithelialization, fibrosis, and contraction of the wound are part of the healing process.

secretin A hormone produced in the lining of the duodenum, when chyme enters from the stomach. It stimulates the pancreas to release a fluid rich in sodium bicarbonate to help neutralize the acidic chyme. It inhibits gastric motility and delays gastric emptying.

secretion The process by which a cell or gland produces and expels some useful product; also used to refer to the product itself.

secretory granules Materials that have been brought into the cell and are separated from the cytoplasm by a single membranous boundary.

secretory unit The portion of a multicellular exocrine gland that is composed of secretory cells and produces a secretion.

segmental contractions Alternating contractions of the circular muscles in the intestine; they mix the intestinal contents and slow their transit time.

selectively permeable Structures that allow some things to pass through but not others.

semialtricial Chicks that are hatched with downy feathers, are immobile, and may or may not have their eyes open.

semicircular canals Three semicircular canals in each inner ear that are part of the vestibular system. They sense rotary motion of the head.

seminal vesicles Accessory reproductive glands that contribute various materials to semen. Seminal vesicles are present in all common domestic animals except the dog and cat.

seminiferous tubule The site where spermatogenesis (spermato­zoa production) takes place in the testis.

semiprecocial Chicks that are hatched with downy feathers and open eyes but that are not mobile.

senescence The process of aging.

sensory nerves Nerves that carry afferent impulses from sensory receptors toward the central nervous system.

sensory neuron Neuron carrying impulses toward the CNS.

sensory receptor A modified nerve ending that converts mechanical, thermal, chemical, or electromagnetic stimuli into nerve impulses that travel to the CNS and are interpreted as the appropriate sensation.

septic pericarditis Infection, and resulting inflammation, of the pericardium of the heart. In ruminant animals often caused by the penetration of sharp foreign objects that have been swal­lowed through the wall of the reticulum and diaphragm, and into the pericardium. See hardware disease.

septum (plural, septa) A partition or dividing structure in an organ or area.

serosa The outermost layer of the intestinal tract.

serous membrane A membrane that lines a serous cavity, such as the thorax or abdomen. Serous fluid produced by serous membranes helps lubricate organs.

serous secretions Thin, watery secretions; transudates.

Sertoli cells Large “nurse” cells to which spermatozoa are attached during their development. Sertoli cells normally produce small amounts of estrogen hormones. If they grow abnormally and form a Sertoli cell tumor, the unusually high amount of estro­gens produced by the tumor can cause feminization of the affected male animal.

serum The fluid portion of blood that has had the clotting factors removed. Serum is produced by letting a blood sample clot before the fluid is removed.

sesamoid bones Bones present in some tendons where they change direction markedly over joints. Sesamoid bones act as bearings over the joint surfaces, allowing powerful muscles to move the joints without the tendons wearing out as they move over the joints.

sex chromosomes Two of the normal diploid chromosome complement. The sex chromosomes determine the genetic sex of the individual. If there are two X sex chromosomes, the indi­vidual is genetically female. If there is an X and a T sex chromo­some, the individual is genetically male.

sex hormones Hormones that target the reproductive tissues. The male sex hormones are the androgens, and the female sex hormones are the estrogens.

sexual dimorphism The male and female of the species look distinctly different from one another. Sexual dimorphism occurs in many avian species.

shaft of the mandible The horizontal portion of the mandible that houses all the lower teeth.

shaft The keratinized, visible portion of hair that extends above the surface of the epithelium.

shell gland Muscular section of the oviduct in birds where the egg shell is applied; also called the uterus.

short bone A small bone shaped like a small cube or marshmal­low. The bones of the carpus are examples of short bones.

short-chain fatty acids See volatile fatty acids.

sigh A deeper than normal breath. A sigh may correct a minor oxygen or carbon dioxide imbalance in the blood, or it may expand the lungs a little more than the normal breaths do. Sighs can have an emotional basis also.

sigmoid flexure The S-shaped bend in the nonerect penis of the bull, ram, and boar.

simple ciliated columnar epithelium Single-layered, columnar­shaped epithelium containing cilia; found in the oviduct.

simple columnar epithelium Single-layered columnar epithe­lium found in the stomach and intestines because of its ability to absorb and secrete.

simple cuboidal epithelium Single-layered, cube-shaped epithe­lium found in the ovaries and in many of the ducts of the body because of its ability to aid in secretion.

simple diffusion The ability of some molecules, such as oxygen, water, and carbon dioxide, to pass through the cell membrane without the aid of carrier proteins.

simple epithelium Epithelium composed of a single layer of cells. simple gland An exocrine gland with unbranched ducts.

simple squamous epithelium Delicate, single-layered, flat-celled epithelium found in the alveoli of the lungs and lining blood vessels.

simple sugars Monosaccharides, such as glucose and fructose. sinoatrial (SA) node A group of specialized cardiac muscle cells in the wall of the right atrium of the heart that act as the heart's pacemaker. The impulse that starts each heartbeat is initiated in the SA node.

sinus hairs See tactile hairs.

sinuses Also known as the paranasal sinuses; outpouchings of the nasal passages that are housed within spaces in areas of the skull bones.

sinusitis Inflammation and swelling of the lining of a paranasal sinus.

skeletal muscle Multinucleated, striated, voluntary muscle that enables conscious movement of an animal; the type of muscle that moves the bones of the skeleton and is under conscious control.

skeletal muscle fiber A skeletal muscle cell. Because of their long, thin fiberlike appearance, skeletal muscle cells are often called skeletal muscle fibers.

skull The collective name of the 37 or 38 bones of the head. The skull is the most complex part of the skeleton. It houses the brain and all the special sense organs.

small ribonucleoproteins RNA-protein complexes that remove noncoded introns from messenger RNA and splice together the coded exons to create a complete and identical copy of the DNA gene.

smell The olfactory sense; a chemical sense that detects odor molecules in the inhaled air. The olfactory epithelium is located high in the nasal passages.

smooth endoplasmic reticulum (smooth ER) The portion of the ER that is without ribosomes and is involved in the synthesis of lipids. smooth muscle Nonstriated, involuntary muscle having only one nucleus per cell; the type of muscle found in soft internal organs and structures. Smooth muscle gets its name because its cells do not have a striped appearance under the microscope, in contrast to skeletal and cardiac muscle cells. Smooth muscle is not under con­scious control. Smooth muscle is found in the digestive tract, where it assists with the movement of food through the gut (peristalsis).

sneeze A protective reflex stimulated by irritation or foreign matter in the nasal passages. Consists of a sudden, forceful expi­ration of air that is directed through the nose and mouth in an effort to eliminate the irritant.

sodium cotransport The process by which glucose and amino acids are passively reabsorbed back into the circulation in the proximal convoluted tubule. These two substances bind to the transport protein to which sodium is also attached. When the sodium is actively transported into the tubular epithelial cells, the glucose and amino acids follow.

sodium-potassium pump Active transport molecule that moves sodium molecules out of the neuron and potassium molecules into the neuron to maintain the resting state.

soft palate The soft mucosal flap that extends caudally from the hard palate on the roof of the mouth.

sole of hoof The concave plantar or palmar portion of the hoof. The sole is avascular and lacks innervation.

solenoglyphous Having hinged, hollow fangs that fold back when the mouth is closed. Refers to snakes with mobile fangs that can be moved to give the best position for envenomation of the prey. Includes vipers, puff adders, and rattlesnakes.

solute Substance that is dissolved in another substance. The com­ponent of a solution that is present in the lesser amount.

solutions Homogeneous mixtures of two or more substances. The components can be gases, liquids, and/or solids.

solvent A substance in which another substance is dissolved. soma The cell body of a neuron; perikaryon.

somatic cells Nonreproductive cells found throughout the body, containing a diploid number of chromosomes and replicating themselves through the cell division process of mitosis.

somatic nervous system Conscious or voluntary nervous system controlling skeletal muscles; somatic motor function is the effer­ent branch, and somatic sensory function is the afferent branch.

somatic reflex A reflex resulting in the stimulation or inhibition of skeletal muscle contraction.

special senses Taste, smell, hearing, equilibrium, and vision. The organs of special sense are organized into complex structures that are all located in the head.

specialization The ability of an organism to differentiate to acquire new characteristics.

specialized connective tissue Tissue including bone, blood, and cartilage. Specialized connective tissue may be subdivided into supportive connective tissue, which includes bone and cartilage, and vascular connective tissue, including blood.

specific immunity Reactions of the immune system aimed at destroying specific antigens.

spermatic cords Cordlike connective tissue structures that enclose blood vessels, nerves, lymphatic vessels, and the vas deferens as they pass between the testes and the abdominal cavity through the inguinal rings.

spermatogenesis The production of spermatozoa in the testis.

spermatozoon (plural, spermatozoa) The male reproductive cell. The male gamete.

sphenoid bone A skull bone that is one of the internal bones of the cranium. The sphenoid bone forms the floor of the cranium and contains a depression, the pituitary fossa, that houses the pituitary gland.

sphenoidal sinus The paranasal sinus in the sphenoid bone.

spheroidal joint A ball-and-socket joint, such as the shoulder or hip joint.

sphincter muscle A circular muscle that surrounds and controls flow through an orifice.

spinal canal The long, flexible, caudal portion of the dorsal body cavity formed by the adjacent arches of the vertebrae of the spine. It houses and protects the spinal cord.

spinal column Also known as the vertebral column; the collective name for the cervical, thoracic, lumbar, sacral, and coccygeal vertebrae.

spinal nerves Nerves of the peripheral nervous system that origi­nate from the spinal cord.

spindle apparatus The structure that during mitosis connects to the centromeres of the chromosomes for the purpose of division.

spindle fibers Visible during the metaphase stage of cell division, the fusiform structure is made up from the microtubules that extend from the centrosomes. These fibers aid in mitosis by connecting to the cell's chromosomes at their centromeres, cre­ating the pull necessary to divide them.

spinous process The single, dorsally projecting process of a vertebra.

splenopancreas Pertaining to the spleen and pancreas.

spliceosomes Specialized areas in the nucleus created by small ribonucleic proteins that remove noncoded introns from mes­senger RNA.

splint bones The vestigial metacarpal and metatarsal bones of a horse's leg. There are two splint bones in each leg: one on each side of the cannon bone (the large metacarpal or metatarsal bone).

squamous cell carcinoma Composed of squamous epithelium, these cancerous masses are locally invasive and also may metas­tasize. They occur in a variety of species, most commonly on the conjunctiva, skin surface, urogenital tissue, mouth, and stomach. squamous cells Flat cells that make up squamous epithelia. squamous epithelium Epithelium composed of squamous cells. stages of labor The three main parts of the parturition (birth) process. Stage 1 consists of uterine contractions that force the membrane-covered fetus against the cervix, causing it to dilate. Stage 2 is the delivery of the newborn animal by a combination of uterine and abdominal muscle contraction. Stage 3 is delivery of the placenta by a combination of uterine and abdominal contractions.

stapes One of the three ossicles, which are the tiny bones that transmit sound wave vibrations across the middle ear. The stapes, or stirrup, is attached to the membrane that covers the oval window of the cochlea. It is the innermost of the three ossicles.

steatorrhea Fatty, greasy stools.

sternal flexure Where the right ventral colon of the horse bends to form the left ventral colon on the ventral floor of the perito­neal cavity.

sternal rib Rib whose costal cartilage directly joins the sternum. sternebra A bone of the sternum.

sternum The breastbone; the series of rodlike bones called ster- nebrae that form the floor of the thorax.

steroid hormone Hormone whose structure is derived from cholesterol.

steroids Lipids characterized by a carbon skeleton that contains four fused rings. Cholesterol and the steroid hormones are examples of steroids in the animal body.

stifle joint The joint between the femur and the tibia. In humans, it is called the knee joint.

storage proteins Proteins that are stored for later use, such as those found in egg whites.

stratified cuboidal epithelium Multilayered, cube-shaped epi­thelium found in excretory tracts of the body.

stratified epithelium Epithelium composed of layers of cells.

stratified squamous epithelium Epithelial tissue composed of multiple layers of flat squamous cells; found in the epidermis of the skin, vagina, mouth, and anus; possesses the ability to regen­erate rapidly.

stratum basale See stratum germinativum.

stratum corneum The horny layer of the epidermis lying most superficially on the skin's surface. The cells of this layer are anucleated and keratinized, being the dead remnants of kerati- nocytes. This is the predominant layer of the epidermis.

stratum germinativum The base layer of the epidermis, com­posed of a single layer of cuboidal cells that divide to replenish the constantly eroding superficial layer of the epidermis.

stratum granulosum The granular layer of epidermis containing keratohyaline and lamellated granules, located between the stratum germinativum and the stratum lucidum. This layer of the epidermis aids in waterproofing the skin.

stratum lucidum The clear layer of epidermis present only in very thick skin, such as that of the paw pads; located beneath the stratum corneum.

stratum spinosum The prickle cell layer; the weblike layer of epidermis dense with intercellular attachments; located between the stratum granulosum and the stratum basale.

streak canal The passageway at the tip of the teat of the cow that carries milk from the teat sinus outside the body.

stretch reflex Reflex initiated by stretch receptors within a muscle that results in contraction of the muscle to compensate for the stretching.

striated Striped.

striated muscle Muscle that looks striped because of alternating light and dark bands. Examples of striated muscle are skeletal and cardiac muscle.

stroke volume The amount of blood ejected with each cardiac contraction.

stroma The foundation-supporting tissues of organs.

structural proteins Proteins that form body structures, such as hair and collagen.

subcutaneous layer The layer of adipose tissue located beneath the epidermis and dermis that insulates and protects the body.

submucosa The second innermost layer of the intestinal tract; lies between the mucosa and the muscle layer.

substrate Substance acted on by an enzyme. May also be used to describe nesting material.

sulcus (plural, sulci) Groove, especially shallow grooves in the cerebral cortex.

superficial A directional term meaning toward the surface of the body or a body part; see external.

superior A directional term meaning toward the head of a human.

superovulation Production of an abnormally high number of ova in the ovaries induced by the administration of drugs with follicle-stimulating hormone activity. Usually done as the first step in transfer of embryos to other animals.

suppressor T cells Lymphocytes that inhibit helper T cells and cytotoxic T cells by negative feedback. They also prevent B lym­phocytes from transforming into plasma cells. These cells provide the means by which the immune response can be shut down.

supracoracoideus Small, deep flight muscle originating on the keel and inserting on the top of the humerus; contraction results in the upstroke.

surfactant A component of the fluid that lines the alveoli in the lungs. Surfactant helps reduce the surface tension of the fluid, which helps prevent the alveoli from collapsing shut as air moves in and out during breathing.

suspensions Heterogeneous mixtures containing large solutes that readily separate from the solution when there is no move­ment of the suspension.

suspensory ligament of the ovary The cranial edge of the broad ligament in the female that extends cranially and dorsally from the ovary and attaches to the body wall in the area of the last rib. suspensory ligaments The tiny ligaments that attach to the periphery of the lens of the eye and connect it to the ciliary body. They are the means by which the ciliary muscles exert and relieve tension on the lens to adjust its shape for near and far vision.

sutures The immovable fibrous joints that unite most of the skull bones; also known as synarthroses.

sweat glands Coiled glands, either merocrine or apocrine, located in the corium of all or some of the body's subcutaneous flesh depending on the species. Where present, the ducts of these glands pass through the dermis to the surface of the epidermis, where the sweat evaporates, aiding in thermoregulation.

sympathetic ganglion chain Series of ganglia located outside the thoracolumbar area of the spinal column and associated with the sympathetic nervous system.

sympathetic nervous system Part of the autonomic nervous system that is responsible for the “fight or flight” response; also called the thoracolumbar system because of the location of the sympathetic nerves emerging from the thoracic and lumbar vertebral segments.

symport system A system in which all of the substances are moved in the same direction.

synapse The junction between two neurons or between a neuron and another target cell.

synaptic cleft Physical gap between two communicating neurons or between a neuron and its target cell.

synaptic end bulb Button at the end of the axon that releases the neurotransmitter; also called synaptic knob or terminal bouton.

synaptic knob See synaptic end bulb.

synaptic transmission The continuation of the nerve impulse across the synapse from one neuron to another or from one neuron to its target cell.

synarthrosis An immovable fibrous joint; for example, the sutures that unite most of the skull bones.

synergist Something that aids the action of something else. A synergistic muscle contracts at the same time as a prime mover and assists it in carrying out its action.

synovial fluid Viscous fluid formed by the lining layer of the joint capsule of a synovial joint; lubricates the joint surfaces.

synovial joint A freely movable joint; also known as a diarthrosis.

synovial membrane The membrane that lines joint capsules. It is composed of connective tissue and produces synovial fluid, which helps reduce friction in the joint.

synsacrum Strong bony plate created by fusion of the distal lumbar vertebrae, the sacral vertebrae, and the first few coccy­geal vertebrae of birds. It fuses with the pelvis and provides a stiff framework to support the legs.

synthesis reaction A chemical reaction in which elements or simple molecular reactants are combined into a more complex product. The opposite of a decomposition reaction.

synthetic phase (S phase) The period spent by the cell in prepa­ration for cell division, in which the cell begins to replicate and synthesize DNA.

syrinx Enlargement of the trachea above the sternum of birds. Contains muscles, air sacs, and vibrating membranes that col­lectively form the voice box.

system Groups of organs that are involved in a common set of activities.

systematic anatomy A method of studying anatomy that exam­ines each system of the body (e.g., skeletal system, reproductive system) as a separate topic.

systemic circulation The part of the circulatory system that pro­vides blood flow to and away from all of the body tissues except the lungs.

systole The part of the cardiac cycle associated with contraction of the ventricles and atria and ejection of blood into the arterial systems.

T

T lymphocyte The type of lymphocyte that is responsible for cell-mediated immunity.

T tubules Transverse tubules; a system of tubules in a skeletal muscle cell that extend from the sarcolemma (cell membrane) into the depths of the cell. They help carry an impulse caused by nerve stimulation of the muscle cell into its interior. tachycardia A rapid heart rate.

tactile elevations Small elevations located throughout the surface of the epidermis, usually containing a tactile hair; important in the perception of touch.

tactile hairs Hairs sensitive to touch.

tactile sense The sense of touch.

tail glands An oval region filled with many sebaceous and apo­crine glands, located on the dorsal surface of the tail. It aids in scent marking and in personal identification in certain mammals including the domestic dog and cat.

tailhead Dorsal part of the base of the tail. talons The claws of birds of prey.

tapetum A shortened name for the tapetum lucidum.

tapetum lucidum A highly reflective area of the choroid in the back of the eye of most domestic animals (except swine). It acts as a light amplifier to aid low-light vision by reflecting light back through the photoreceptors after it has passed through them once.

target An organ or tissue that responds to a particular hormone.

tarsal bones The bones of the tarsus, consisting of two rows of short bones located between the distal ends of the tibia and fibula and the proximal ends of the metatarsal bones.

tarsal glands The meibomian glands of the eyelid margins. They produce a waxy substance that helps prevent tears from over­flowing onto an animal's face.

tarsus The joint composed of the tarsal bones; referred to as the hock in most animals and the ankle in humans.

taste The gustatory sense; a chemical sense that detects taste mol­ecules dissolved in the saliva in the mouth.

teat sinus The large space within the teat of the cow that fills with milk when milk let-down occurs.

tectorial membrane The gelatinous sheet that lies on the hair cells of the organ of Corti in the cochlea. Movements of the cochlear fluid produced by sound wave vibrations cause the tectorial membrane to distort the sensory hairs, producing nerve impulses that the brain interprets as sound.

telodendron The branched end of an axon; each branch termi­nates in a neurotransmitter-filled synaptic end bulb.

telogen effluvium “Blowing the coat”; the overall hair loss that occurs from the hair follicles being in a synchronized telogenic phase. It may be brought on by many factors, such as stress, medication, malnutrition, and postpartum hormonal changes in dogs.

telogen phase The resting phase of hair growth.

telophase The phase of mitosis when the daughter chromosomes return to being long-fiber chromatids, the nuclear envelope and nucleoli reappear, and the cell has completed its formation of two completely independent daughter cells.

temperature sense The sense of hot and cold.

temporal bones Skull bones that are part of the external bones of the cranium. The two temporal bones form the lateral walls of the cranium, contain the middle and inner ear structures, and are the skull bones that form the temporomandibular joints with the mandible (lower jaw).

temporomandibular joint (TMJ) The hinge joint on each side of the lower jaw (mandible) that connects it with the rest of the skull. Each joint is formed by the convex condyle of the man­dible articulating with the concave articular surface on the ventral portion of each temporal bone.

tendons Fibrous connective tissue bands that connect skeletal muscles to bones.

terminal bouton Button at the end of the axon that releases the neurotransmitter; also called the synaptic end bulb or synaptic knob.

terminators Codes within the DNA sequence that indicate where RNA synthesis should end.

testes The male gonads. They produce the male reproductive cells, spermatozoa, as well as androgen hormones.

testosterone The principal male sex hormone. tetraiodothyronine (T₄) Thyroxine; thyroid hormone that is largely converted to T₃ before exerting an effect on target cells.

thalamus Part of the diencephalon that acts as a relay station for regulating sensory impulses to the cerebrum.

thecodont teeth Teeth held in sockets.

thermolabile enzymes Enzymes that are affected by changes in temperature and are therefore changed in structure themselves.

third eyelid The nictitating membrane. A T-shaped plate of car­tilage covered by conjunctiva. It has lymph nodules and a gland that contributes to the tear film on its ocular surface. No muscles attach to the third eyelid. Its movements are entirely passive.

thoracic cavity The chest cavity. It is separated from the abdomi­nal cavity by the thin, sheetlike diaphragm.

thoracic duct A large lymph vessel found in the thorax. It empties its contents of lymph into large blood vessels in the thorax.

thoracic limb The front limb.

thoracic vertebrae The group of vertebrae located dorsal to the thoracic region.

thoracolumbar system Sympathetic nervous system; so named because most of the nerves of the sympathetic nervous system emerge from the thoracic and lumbar segments of the spinal cord.

thorax Another name for the thoracic, or chest, cavity.

threshold The required level of stimulation, or degree of change in a cell's electrical potential, necessary to initiate an action potential.

threshold stimulus The minimum stimulus that will generate a nerve impulse.

thrombocytes Also known as platelets. Platelets are small pieces of cytoplasm that break off megakaryocytes in the bone marrow and enter peripheral blood. They are involved in hemostasis and the blood-clotting process.

thrombopoiesis The production of platelets.

thromboxane An eicosanoid produced by platelets that causes vasoconstriction and promotes the clumping of platelets.

thymine (T) Part of one of the nucleotides present only in DNA. It is a pyrimidine base that pairs only with adenine.

thymopoietin A hormonelike substance produced by the thymus. It influences the development of T cells, important components of an animal's cell-mediated immunity.

thymosin A hormonelike substance produced by the thymus. It influences the development of T cells, important components of an animal's cell-mediated immunity.

thymus An organ that is important in the development of a young animal's immune system. It produces hormonelike sub­stances, such as thymopoietin and thymosin.

thyroid cartilage One of the cartilages of the larynx. It is shaped as a V that forms and supports the ventral portion of the larynx. The human thyroid cartilage is commonly referred to as the “Adam's apple.”

thyroid gland An endocrine gland made up of two parts, located on either side of the larynx, in the neck region. It produces thyroid hormone and calcitonin.

thyroid hormones The collective name given to two hormones produced by the thyroid gland—T₃ and T₄. They help an animal generate body heat; influence the metabolism of proteins, car­bohydrates, and lipids; and encourage the growth and develop­ment of young animals.

thyroid-stimulating hormone (TSH) The anterior pituitary hormone that stimulates the growth and development of the thyroid gland and causes it to produce its hormones.

thyroxine See tetraiodothyronine.

tibia The main weight-bearing bone of the lower leg. It forms the stifle joint with the femur proximal to it and the hock with the tarsus distal to it.

tibial crest A longitudinal ridge on the front of the proximal end of the tibia.

tidal volume The volume of air breathed in and out in one breath. It varies according to the body's needs. The tidal volume is lowest at rest and higher during physical activity.

tie The process by which male and female dogs become tempo­rarily “attached” to each other as a part of the breeding process. Muscles in the female clamp down around the enlarged bulb of the glans of the penis, making it impossible for the male to withdraw the penis for a time. The male dismounts and turns so that the two animals stand tail to tail. The tie lasts about 15 to 20 minutes and does not seem uncomfortable to either animal.

tight junction A type of intercellular connection that is imperme­able to leaks. Passage of extracellular substances can occur only through the cell itself. Tight junctions are formed by the fusion of one cell's plasma membrane to another cell's plasma membrane.

tissue forceps A tweezerlike surgical instrument with delicate tips; used for grasping and moving tissue. The delicate tip is designed to be gentle on the tissues it grasps.

tissue A group of cells that are similar in structure and perform the same function. The four basic tissues in the animal body are epithelial tissue, connective tissue, nervous tissue, and muscle tissue.

toe The most anterior region of the hoof.

tonofilaments Thin filaments that provide the structural support for certain membrane junctions. Tonofilaments are especially important in tissue that needs to flex.

touch The tactile sense; the sensation of something being in contact with the surface of the body.

trace elements Microminerals; minerals required in minute amounts relative to the macrominerals.

trachea The windpipe; the tube that extends from the larynx down into the thorax, where it divides into the left and right main bronchi that enter the lungs. The trachea is held open by incomplete hyaline cartilage rings that are spaced along its length.

transamination The process of amino group transfer to other amino acids or within the same compound.

transcription The process of transcribing the genetic code from DNA and RNA through protein synthesis using messenger RNA.

transduction Conversion of a sensory stimulus to a nerve impulse.

transfer RNA (tRNA) The type of RNA that transfers amino acids to the ribosome for protein synthesis. Each of the 20 amino acids has its own specific type of tRNA, which places it in the appropriate order for the specific type of protein being synthe­sized. The nucleotides or anticodons of tRNA base-pair with the codon triplets of mRNA to accomplish the process.

transient cells One of the two subdivisions of connective tissue cells, transient cells have a mobile existence, traveling in and out of the connective tissue as needed.

transitional epithelium Epithelium that can stretch and contract without damage. Transitional epithelium is found lining the urinary bladder and ureters. The appearance of the epithelial cells varies depending on the level of tension on the tissue. Transitional epithelium therefore may appear columnar, cuboi- dal, or squamous.

translation Referring to the process of protein synthesis using messenger RNA to transfer genetic information in the form of nucleotides into amino acid form. This process occurs in the cytoplasm on ribosomes.

transmission Conduction of a nerve impulse along a nerve fiber. transport proteins Proteins that transport substances, such as hormones, from their origin to where they are needed.

transudate A thin fluid containing small amounts of protein or no protein that has been passed through a membrane. Normal serous fluid is a transudate.

transverse plane An anatomic reference plane across the body that divides it into cranial (head-end) and caudal (tail-end) parts that are not necessarily equal.

transverse process A lateral-projecting process of a vertebra. transverse tubules See T tubules.

triacylglycerols Neutral fats; triglycerides.

tricuspid valve Also called the right atrioventricular valve; sepa­rates the right atrium and ventricle.

triglyceride A glycerol composed of three fatty acids; the main storage form of water-insoluble lipids.

trigone The triangular body of the urinary bladder located between the openings of the ureters and the urethra.

triiodothyronine (T₃) The main active thyroid hormone.

trimesters The three main divisions of pregnancy. They are not equal thirds, but they give a convenient way to describe the events occurring during pregnancy.

tripeptide A peptide that consists of three amino acids.

tRNA See transfer RNA.

trochanter Greater and lesser; areas on the femur where leg muscles attach.

trochoid joint Also known as a pivot joint; one bone pivots on another in a rotary motion. The only true pivot joint in the bodies of most domestic animals is the joint between the first and second cervical vertebrae.

trypsin A protolytic enzyme formed in the intestines by the cleav­age of trypsinogen by enterokinase.

trypsinlike immunoreactivity (TLI) test Test that identifies the presence of pancreatic trypsin in the blood; a positive test result is suggestive of pancreatic disease.

trypsinogen Precursor for trypsin secreted from the pancreas.

tubular filtrate The glomerular filtrate after it has passed into the proximal convoluted tubule. It will be called the tubular filtrate throughout its entire journey through the nephron tubules even though its chemical composition will change many times before it enters the collecting duct.

tubular gland Secretory unit of exocrine glands either contain­ing or composed of tubules.

tubulin A protein present in the microtubules that gives support to the microtubule and also aids in the motility of the cell.

tubuloacinar Referring to secretory units of exocrine glands that possess both tubular and acinar (or alveolar) parts.

tunica albuginea The tough, fibrous connective tissue capsule of the testis.

turbinates SkuU bones that are part of the internal bones of the face; also known as the nasal conchae. The turbinates are four thin, scroll-like bones that fill most of the space in the nasal cavity. The turbinates are covered by the moist, vascular lining of the nasal passages. Their scroll-like shape helps the nasal lining warm and humidify the inhaled air and trap tiny particles of inhaled foreign material.

twitch contraction A single skeletal muscle fiber contraction. It can be divided into three phases: a brief, latent phase; a longer, contracting phase; and an even longer relaxation phase.

tylotrich hairs Tactile hairs used to aid in the perception of touch. They are located in close association with tactile elevation, or tylotrich, pads.

tympanic membrane The eardrum. The paper-thin, connective tissue membrane that is tightly stretched across the opening of the external ear canal into the middle ear.

tyrosine melanin The pigment that produces brown-black colors in hair.

U

udder The collected inguinal mammary glands. As of cattle, horses, sheep, goats and camelids.

ulna One of the two bones (the radius is the other) that form the antebrachium, or forearm. The ulna forms a major portion of the elbow joint with the distal end of the humerus.

umbilical arteries Arteries that carry carbon dioxide-rich, waste- filled blood from the fetus to the placenta through the umbilical cord.

umbilical cord The link between the fetus and the placenta. It is a cordlike structure that contains blood vessels and a drainage tube from the urinary bladder of the fetus (the urachus).

umbilical vein The vein in the umbilical cord that carries oxygen and nutrient-rich blood from the placenta to the fetus.

uncinate process Projection on a complete rib of birds that over­laps the adjoining rear rib to strengthen the rib cage.

ungual process The process on the distal end of the distal phalanx of dogs and cats that is surrounded by the claw in the living animal.

ungula An alternate name for the hoof.

ungulate The taxonomic classification of animals that includes all hoofed animals, both wild and domestic.

unicameral lung A one-chambered lung.

unicellular exocrine gland The only known example is the goblet cell. It is a ductless exocrine gland that secretes mucus. Goblet cells are located in the respiratory and intestinal tracts.

uniparous An animal that normally gives birth to only one off­spring at parturition.

unsaturated fatty acids Breakdown products of fat metabolism. Unsaturated refers to the fact that not all the chemical binding sites of the molecules are filled. They have one or more double bonds in their carbon chains and are liquid at room tempera­ture. If one double bond is present, the fatty acid is monoun­saturated. If more than one double bond is present, the fatty acid is polyunsaturated. Examples are found in plant oils and include arachidonic, linolenic, and oleic acids.

upper arcade In reference to teeth, it means the teeth in the maxil­lary and incisive bones; the upper set of teeth in the mouth.

upper respiratory tract The respiratory structures outside the lungs. Includes the nose, pharynx, larynx, and trachea.

urachus A tube in the umbilical cord that drains urine from the fetus's urinary bladder into the allantoic sac of the placenta.

uracil (U) Part of one of the nucleotides present only in RNA. It is a pyrimidine base that pairs only with RNA's adenine.

urea A nonprotein source of nitrogen produced in the liver that is used by rumen microbes. One of the final products of protein metabolism.

uremia Urine in the blood. Refers to a buildup of waste materials, especially urea, in the blood because of insufficient removal by the kidneys.

uresis Expelling urine from the body; also called micturition or urination.

ureters The muscular tubes that leave the kidney at the hilus and connect to the urinary bladder. They move urine to the bladder by peristaltic smooth muscle contractions.

urethra The tube that connects the urinary bladder with the outside world. In the female, it only conducts urine. In the male, it conducts urine and semen.

urethral process The distal end of the urethra of the male horse that extends from a shallow depression, the fossa glandis, at the tip of the penis.

urinary calculi (singular, calculus) Uroliths or urinary tract stones. urinary stones See uroliths.

urination See uresis.

urodeum Section of the cloaca of a bird that receives waste prod­ucts from the kidneys and genital ducts.

urolithiasis An abnormal condition characterized by the pres­ence of urinary tract stones.

uroliths Urinary tract stones; precipitated aggregates of mineral crystals that form macroscopic stones or sand anywhere in the urinary tract.

uropygial gland Also called the preen gland; secretes an oily substance that cleans and waterproofs feathers.

uterine contractions Contractions of the myometrium, the smooth muscle layer of the uterine wall.

uterus The womb; where the fertilized ovum implants and lives while it grows and develops into a new animal.

utricle One of two saclike spaces (the saccule is the other one) in the vestibule of the inner ear that contain sensory structures that monitor the position of the head.

uvea The middle, vascular layer of the eye. It includes the choroid, the ciliary body, and the iris.

uveal tunic Vascular, pigmented, middle tissue layer of the eye, consisting of the choroid, iris, and ciliary muscles.

V

vacuole A clear space in the cytoplasm of a cell; it is surrounded by cell membrane. Phagocytized microorganisms are found in vacuoles.

vagina The tube that connects the cervix with the vulva. It receives the erect penis at breeding and serves as the birth canal at parturition.

vaginal tunics Two connective tissue layers that surround the testes in the scrotum. They are derived from layers of perito­neum that were pushed ahead of the testes when they descended through the inguinal rings into the scrotum.

vagus nerve The nerve of the parasympathetic nervous system that is involved in regulating gastrointestinal motility and secre­tion; acetylcholine is the neurotransmitter of the vagus nerve. The 10^th cranial nerve.

vas deferens The muscular tube that carries spermatozoa and the fluid they are suspended in from the epididymis to the urethra for emission as a component of semen.

vascularized Region of the body supplied with a blood source via blood vessels.

vaults Small transport structures that shuttle molecules to and from the nucleus in the cell.

velvet skin The soft skin on the antlers of deer that provides a vascular source necessary during the early seasonal growth of the antler. Later in the season, the buck will scrape the velvet off, allowing the antler to harden. These hardened antlers are used for sparring with competing males during the breeding season. The velvet is often eaten by the buck because of its high nutritive value.

vent Opening of the cloaca to the outside in birds.

ventilation The movement of air into and out of the lungs. Also known as breathing.

ventral A directional term meaning toward the bottom surface of an animal when it is standing on all four legs; toward the belly.

ventral body cavity The large space in the body that is divided by the thin, sheetlike diaphragm muscle into the cranial thoracic cavity (chest) and caudal abdominal cavity (belly).

ventral horn The area of the spinal cord's gray matter “butterfly” where the neurons that transmit motor (efferent) nerve impulses to the spinal nerves are located.

ventral nerve root The branch off each side of the spinal cord between each pair of adjacent vertebrae that conducts motor impulses from the spinal cord to effectors in the periphery of the body.

vertebra One of the bones of the spinal column.

vertebral column Also known as the spinal column; the collective name for the cervical, thoracic, lumbar, sacral, and coccygeal vertebrae.

vesicle A small sac that contains fluid.

vestibular folds The false vocal cords; connective tissue bands in the larynx of a nonruminant animal, in addition to the vocal cords. The vestibular folds are not involved in voice production.

vestibule of the ear The portion of the inner ear that senses the position of the head. Its sensory epithelium is contained in two saclike spaces: the utricle and the saccule.

vestibule of the vulva The entrance into the vulva; the short space between the labia and the entrance into the vagina. The urethra of the female opens into the vestibule.

VFAs See volatile fatty acids.

villi Fingerlike projections on the surface of the small intestine that increase the surface area for absorption; contain the micro­villi brush border that digests nutrients.

virulence A specific pathogen's ability to invade a host's tissues. Determined by virulence factors. Pathogenicity.

virulent Capable of causing disease.

virus The simplest organism, composed of DNA or RNA, sur­rounded by a protein sheath and sometimes an additional enve­lope. Viruses cannot live or reproduce independently; therefore, they must establish a parasitic relationship with another cell to perform these functions. Individual viruses are identifiable by their antigens.

viscera (singular, viscus) Refers to the soft, internal organs enclosed within a body cavity, such as the lungs, kidneys, and intestines. The term is used to describe the organs of the abdom­inal and thoracic cavities.

visceral Pertaining to the soft internal organs.

visceral layer The layer of pleura or peritoneum that lies directly on the surface of organs in the thorax or abdomen.

visceral sensations Miscellaneous interior body sensations. They include hunger and thirst and stretching sensations from hollow internal organs.

visceral skeleton Bones formed in soft organs (viscera). Exam­ples include the os penis in the penis of a dog, the os cordis in the hearts of cattle, and the os rostri in the snouts of swine.

visceral smooth muscle The type of smooth muscle found in the walls of many soft internal organs, such as the intestine, urinary bladder, and uterus. Its cells are linked to form large sheets that show rhythmic waves of contraction without external nerve stimulation.

visceral vaginal tunic The thin, inner connective tissue layer that is tightly adherent to the surface of the testis and the structures of the spermatic cord. Derived from visceral peritoneum.

vitamin D A fat-soluble vitamin essential for life in most organ­isms. Vitamin D enables the body to use calcium and phospho­rus and is necessary for the formation of healthy teeth and bones. Vitamin D may be found in many common food sources, as well as being produced in the skin when it is exposed to ultraviolet rays in sunlight. An insufficient amount of vitamin D can cause rickets or osteomalacia.

vitellogenesis The formation of yolk in the vitellarium (gland) in egg-laying species.

vitreous compartment The compartment of the eye behind (caudal to) the lens and ciliary body. It contains a soft, gelati­nous fluid called vitreous humor.

vitreous humor The soft, gelatinous fluid that fills the vitreous compartment of the eye.

viviparous Giving birth to living young that develop within the maternal body. Occurs in placental mammals but also in some reptiles and amphibians.

vocal cords Two fibrous connective tissue bands attached to the arytenoid cartilages that stretch across the lumen of the larynx and vibrate as air passes over them; also known as the vocal folds.

vocal folds See vocal cords.

volatile fatty acids (VFAs) Also called short-chain fatty acids, these are the main energy source of ruminants. They are created by the microbial fermentation of cellulose in the rumen and in the colon of nonruminant herbivores. Common VFAs include acetic acid and propionate.

Volkmann's canal One of countless tiny channels through the matrix of bone that bring blood in from the periosteum to the Haversian canals in the centers of the Haversian systems. The Haversian systems run lengthwise in long bones. Volkmann's canals come in at right angles to the Haversian systems and join with the Haversian canals.

voltage The potential electrical energy created by separation of opposite electric charges on either side of the cell membrane.

voluntary muscle Also known as skeletal muscle; muscle involved in the colons movements of the body.

voluntary striated muscle An old name for skeletal Inuecle.

vomer bone A skull bone that is one of the internal bones of the face. Tie vomer bone forms part of the nasal septum.

vomeronasal organ Also called Jacobson’s organ. It consists of two long, thin sacs situated on either side of the nasal septum, which Is accessible via a pit in the roof of the mouth. It is part of the animal's olfactory system.

vulva The external portion of the female reproductive system. It fconsists o the vestibule, the clitoris, and the labia.

W

water soluble Refers to a molecule that is hydrophilic and soluble ienr. wat

wave of depolarization The opening of sodium channels, start- ioningtat the p of stimulus and continuing down the length of the Iieiiroii to the end of the axon; also called the nerve impulse and the propagation of the action potential along the neuron.

white adipose tissue White fat; foudo commonly throughout the body. It is used for thermoregulation, protection, and fsupport o the body and its organs. It is also a storage compart­ment for lipids. White adipose tissue is highly vascularized so that the lipids contained within can be readily converted to ireaingelyrgceyrvide t metabolism.

white line The white or light-colored region that marks where the wall and sole of the hoof adjoin.

white matter Myelinated axons in the CNS.

white pulp The area of the spleen that contains lymphocytes. whole blood Blood that contains plasma and all the formed ele­ments (cells and platelets).

wind-up An axaggerated pain response that can be produced when the spinal cord is bombarded with severe or chronic pain itmpulses. I can result in severely painful responses to stimuli tohualtdw normally not be distressful.

withdrawal reflex Reflex arc in which a painful stimulus on the skin causes contraction of the affected limb; also called the flexor reflex.

withers Area dorsal to the scapulas.

wool-type hairs Hair coat composed primarily of secondary ehnairs, as se in sheep.

X

xiphoid, xiphoid process The last, most caudal sternebra.

Y

yawnewe,pA slo d breath taken through a wide-open mouth. It may bforward or to the rear.

zygomatic archescheBsony ar below and behind the eyes of coommemstiocn d animals. In dogs and cats, they form the

twidest par of the skull. The zygomatic arches are made up of the rofl∣∙anIaeing zygomatic process of the temporal bone joined with the caudal-facing temporal process of the zygomatic bone.

zygomatic bones Skull bones that are part of the external bones of the face. The zygomatic bones form a portion of the orbit of tyhee e and the rostral portion of the zygomatic arch.

zygoteertTilihze df ovum.