Glossary

11 -cιs-retinal: A carotenoid constituent of visual pig­ments. It is the oxidized form of retinol, which func­tions as the active component of the visual cycle. It is bound to the protein opsin, forming the complex rhodopsin.

2,3-bis-phosphoglycerate (BPG): Athree-Carbonisomer of the metabolic intermediate 1,3-bisphosphoglycerate. It is notable because it binds to deoxygenated hemo­globin in the red blood cell. In so doing, it indirectly regulates the ability of red blood cells to release oxygen near tissues that need it most.

5-hydroxytryptamine: A monoamine neurotransmit­ter synthesized in serotonergic neurons in the central nervous system (CNS) and enterochromaffin cells in the gastrointestinal tract.

Abdominal cavity: The space bounded by the abdom­inal walls, diaphragm, and pelvis, it contains most of the organs of digestion, the spleen, the kidneys, and the adrenal glands.

Abdominopelvic cavity: The portion of the ventral body cavity that contains abdominal and pelvic sub­divisions. It also contains the peritoneal cavity.

Abduction: To move away from the midline of the body or from an adjacent part or limb.

Abomasum: The fourth and final stomach compart­ment of the stomach in ruminants. It serves pri­marily in the acid hydrolysis of microbial and dietary protein, preparing these protein sources for further digestion and absorption in the small intestine.

Absolute refractory period: The period following stimulation during which no additional action potential can be evoked.

Absorption: The active or passive uptake of gases, fluids, or solutes.

Accessory organs: Organs that assist with the func­tioning of other organs within a system.

Accessory pancreatic duct: An additional duct called the duct of Santorini, which connects directly to the duodenum.

Accommodation reflex: A reflex action of the eye, in response to focusing on a near object, and then looking at a distant object (and vice versa), compris­ing coordinated changes in convergence, lens shape, and pupil size. It is dependent on cranial nerve II (afferent limb of reflex), higher centers, and cranial nerve III.

Acetylcholine: A white crystalline derivative of choline that is released at the ends of nerve fibers in the somatic and parasympathetic nervous systems and is involved in the transmission of nerve impulses in the body.

Acetylcholinesterase: Any of various enzymes in the blood and in certain tissues that catalyze the hydro­lysis of acetylcholine.

Acetyl CoA: A small molecule that carries acetyl groups in cells. It is a critical part of energy produc­tion by cells.

Anatomy and Physiology of Domestic Animals, Second Edition. R. Michael Akers and D. Michael Denbow.

© 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

Acquired reflex: An acquired response that is under the control of (conditional on the occurrence of) a stimulus.

Acromion: Bone comprising the tip of the shoulder.

Acrosome: A saclike organelle positioned on the ante­rior end of the sperm head. It contains enzymes that are necessary for sperm penetration of the egg at fertilization.

Actin: A protein found in muscle that, together with myosin, functions in muscle contraction.

Active transport: The mediated transport of biochem­icals and other atomic/molecular substances across membranes.

Active zone: The region on the surface of functional (globular) proteins that fit and interact chemically with other molecules of complementary shape and charge.

Adduction: To move inward toward the median axis of the body or toward an adjacent part or limb.

Adenine: A purine, that is, one of the types of nitrog­enous bases found in nucleic acids such as DNA and RNA.

Adenohypophysis: The anterior lobe of the pituitary gland.

Adenosine diphosphate (ADP): A molecule of adenine with two phosphate groups attached. It is an impor­tant precursor for generation of adenosine triphos­phate (ATP).

Adenosine triphosphate (ATP): A critical cellular molecule composed of adenine with three phos­phate groups attached. The Ihirdphosphate depends on a high energy bond, which, when broken, can be used by the cell to produce energy.

Adenylate cyclase: An essential cellular enzyme localized in the cytoplasmic side of the plasma membrane. When activated it converts ATP to cyclic AMP (cAMP). cAMP serves as an intracellular regu­lator for hormone action.

Adrenal gland: Paired endocrine glands located at the superior pole of each kidney. The adrenal has a cortex and medulla. The cortex is responsible for production of two major classes of steroid hor­mones: glucocorticoids and mineralocorticoids. The adrenal medulla produces the catecholamines epi­nephrine and norepinephrine.

Adrenaline: See epinephrine.

Adrenergic fibers: Nerve fibers that release norepi­nephrine.

Adrenergic receptors: Cellular proteins that bind norepinephrine and epinephrine and similar mole­cules. They mediate neural signaling in the CNS and hormonal signaling via epinephrine in some peripheral tissues. The name was derived because epinephrine and norepinephrine were originally called adrenalin and noradrenalin.

Adrenocorticotropic hormone (ACTH): A glycopro­tein hormone made in the anterior lobe of the pitu­itary. It controls secretion of glucocorticoids (cortisol, corticosterone, etc.) from the adrenal cortex.

Adventitia: The superficial layer of connective tissue surrounding an internal organ; fibers are continu­ous with those of surrounding tissues, providing support and stabilization.

Aerobic respiration: Respiration in which molecular oxygen is consumed and carbon dioxide and water are produced.

Afferent: Carrying to or toward a center.

Afferent nerve: A nerve conveying impulses from the periphery to the central nervous system; also called the centripetal nerves.

Afferent neuron: A nerve cell that carries impulses toward the central nervous system. These neurons ini­tiate nerve impulses following receptor stimulation.

Afterload: The arrangement of a muscle so that it lifts a weight from an adjustable support or works against a constant opposing force to which it is not exposed when at rest.

Agonist: A substance that can combine with a cell receptor to produce a reaction typical for that substance.

Agranulocytes: Nongranular leukocytes (monocytes or lymphocytes).

Air sacs: One of many spherical outcroppings of the respiratory bronchioles in the mammalian lung, the primary sites of gas exchange with the blood.

Albumin: A common liver-produced protein found in many tissues, including plasma. It is soluble in water and a principal component in egg white.

Alimentary tract: The tubular portion of the digestive tract. See also gastrointestinal tract (GI tract).

Alkaline tide: A period of urinary neutrality or alka­linity after meals.

All fraπs-retinal: Retinol and derivatives of retinol that play an essential role in metabolic functioning of the retina, the growth of and differentiation of epithelial tissue, the growth of bone, reproduction, and the immune response. Dietary vitamin A is derived from a variety of carotenoids found in plants. It is enriched in the liver, egg yolks, and the fat component of dairy products.

Allantois: A sac connected to the embryo that makes respiration by the embryo possible. It also stores excretions.

Allometric growth: A growth rate of an organ or tissue that is faster than the rate of general body growth.

Allosteric protein: A protein (most often in reference to an enzyme) that changes conformation (and thereby functional capacity) when it binds to another molecule or when covalent changes alter its shape.

Alveolar ducts: Part of the respiratory passages beyond a respiratory bronchiole; from them arise alveolar sacs and alveoli.

Alveolar macrophages: The cells within the lungs that phagocytose microbes and particulate matter, for example, inhaled dust.

Alveolar sac: An air-filled chamber that supplies air to several alveoli; also, terminology used to describe the glandular structures in the mature mammary gland.

Alveolar ventilation rate: An index of respiratory efficiency; it measures volume of air wasted and flow of fresh gases in and out of alveoli.

Alveoli: Blind pockets at the end of the respiratory tree, lined by a simple squamous epithelium and surrounded by a capillary network; sites of gas exchange with the blood; a bony socket that holds the root of a tooth.

Alveoli (mammary): Multicellular, spherical, hollow units of the mammary gland that are responsible for synthesis and secretion of milk. Milk is secreted into and stored in the internal Iumenal space of the alveoli between milking or suckling episodes.

Amacrine cells: Interneurons in the retina that operate at the inner plexiform layer, the second synaptic retinal layer where bipolar cells and ganglion cells synapse.

Aminopeptidase: An enzyme that is used as a bio­marker to detect damage to the kidneys, and that may be used to help diagnose certain kidney disorders.

Amnion: The innermost of the fetal membranes, it forms the fluid-filled sac that surrounds and pro­tects the developing fetus. The amnion and amni­otic fluid protect the developing embryo from shock and permit movement.

Amoeboid action: The flowing movement of the cytoplasm of a phagocyte.

AMPA (α-amino-3-hydroxy 5-methylisoaxzole-4- propionic acid) receptor (AMPARs): Anon-NMDA- type ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system (CNS). Its name is derived from its ability to be activated by the artificial glu­tamate analog, AMPA. AMPARs are found in many parts of the brain and are the most commonly found receptor in the nervous system.

Amygdaloid body: Almond-shaped groups of neurons located deep in the medial temporal lobes of the brain in complex vertebrates.

Anaerobic glycolyis: The anaerobic metabolic break­down of a nutrient molecule, such as glucose, without net oxidation. Fermentation does not directly produce energy in cells; it merely allows glycolysis (a process that yields two ATP per glucose) to continue.

Anal canal: The distal portion of the rectum that contains the anal columns and ends at the anus.

Anaphase: A stage of mitosis during which the chromosomes separate and move away from each other.

Anastomosis: The joining of two tubes, usually refer­ring to a connection between two peripheral vessels without an intervening capillary bed.

Anatomical dead space: The volume of the conduct­ing airways from the external environment down to the terminal bronchioles.

Anatomical position: An anatomical reference posi­tion; the body viewed from the anterior surface with the palms facing forward. For domestic species, this would be four legs on the ground and head facing forward.

Anatomy: The study of the structure of the body.

Anencephaly: Congenital absence of most of the brain and spinal cord.

Anestrus: Term that can refer to the absence of estrus without ovulation, the period of sexual quiescence between two estrous cycles, or prolonged failure of estrus in mature animals.

Anisotropic: Having physical properties that differ according to the direction of measurement.

Annular cartilage: The lowermost of the laryngeal cartilages, it may be palpated just below the thyroid prominence adjacent the cricoid cartilage and the first tracheal ring is the cricothyroid mem­brane, a site used for rapid emergency airway access (Cricothyroidotomy).

Antagonist: Something, such as a muscle, disease, or physiological process that neutralizes or impedes the action or effect of another.

Anterior: On or near the front, or ventral surface, of the body.

Anterior chamber: The fluid-filled space inside the eye between the iris and the cornea's innermost surface, the endothelium.

Anterior horn: The front section of the lateral ven­tricle of the brain, extending forward from Monro's foramen; also called the ventral horn.

Anterior median fissure: The longitudinal groove in the midline of the anterior aspect of the medulla oblongata, continuous with the anterior median fissure of the spinal cord and ending at the foramen cecum medullae oblongatae.

Anterior neuropore: The anterior opening leading from the central canal of the embryonic neural tube to the exterior.

Anterior segment: The front third of the eye that includes the structures in front of the vitreous humor: the cornea, iris, ciliary body, and lens.

Anterior vagal trunks: A nerve trunk (or trunks) formed by fibers from both left and right vagus nerves.

Antibody: Proteins produced by B lymphocytes (plasma cells) that are produced in response to exposure to a specific antigen. Antibodies are mem­bers of the immunoglobulin family of proteins.

Anticodon: The sequence of three nucleotides in the transfer RNA that is complementary to the three nucleotide sequence of the messenger RNA. The anticodon corresponds to a specific amino acid also attached to the tRNA during active protein synthesis.

Antidiuretic hormone (ADH): A hormone produced in the hypothalamus but stored and released from the posterior lobe of the pituitary. Its secretion pro­motes reabsorption of water by the distal convo­luted tubules and collecting ducts of the kidney nephrons.

Antigen: Usually a foreign substance that, when it enters the body, elicits an immune response (anti­body production or cell-mediated response).

Antiport: A form of membrane carrier protein that transports two different ions or molecules across a membrane in opposite directions. The transfer can occur with both ions simultaneously or sequentially.

Antithrombin: Any substance that inhibits or pre­vents the effects of thrombin so that blood does not coagulate.

Anus: The external opening of the anal canal.

Aorta: The large, elastic artery that carries blood away from the left ventricle and into the systemic circuit.

Aortic arch: The curve between the ascending and descending portions of the aorta.

Aortic body: A receptor in the aortic arch sensitive to changing oxygen, carbon dioxide, and pH levels of the blood.

Aortic valve: A one-way valve that permits the flow of blood from the left ventricle into the aorta during ventricular emptying but prevents the backflow of blood from the aorta into the left ventricle during ventricular relaxation.

Apex: Extremity of a conical or pyramidal structure. The apex of the heart is the rounded tip directed anteriorly and slightly inferiorly.

Apical: The tip of a cell or a structure. For example, in activity secreting epithelial cells, products are packaged and ultimately released from the cell at the apical end of the cell.

Apneustic area: A respiratory center whose chronic activation would lead to apnea at full inhalation.

Aponeurosis: A sheetlike fibrous membrane resem­bling a flattened tendon that serves as a fascia to bind muscles together or to connect muscle to bone.

Aponeurotic attachments: Fibrous or membranous sheets connecting a muscle and the part it moves.

Apoptosis: One of the main types of programmed cell death (PCD). As such, it is a process of deliberate life relinquishment by a cell in a multicellular organism.

Apositional growth: The enlargement of a bone by the addition of cartilage or bony matrix at its surface.

Appendicular: Pertaining to the upper or lower limbs. Aqueous humor: The clear, watery fluid in the eye that fills the space between the back surface of the cornea and the front surface of the vitreous humor.

Arachnoid granulations: Any of numerous villuslike projections of the cranial arachnoid through the dura into the superior sagittal sinus or into its lateral venous lacunae; also called the arachnoid villus or pacchionian body.

Arachnoid mater: One of the three meninges, the membranes that cover the brain and spinal cord. It is interposed between the two other meninges, the more superficial dura mater and the deeper pia mater, and is separated from the pia mater by the subarachnoid space.

Arbor vitae: White matter of the cerebellum.

Arytenoid cartilages: A pair of small cartilages in the larynx.

Ascending aorta: Part of the aorta from which the coronary arteries arise.

Ascending segment: A part of the large intestine.

Ascites: The overproduction and accumulation of peritoneal fluid.

Aspartate: A salt or ester of aspartic acid.

Association area: An area of the cerebral cortex where motor and sensory functions are integrated.

Astigmatism: A refractive error of the eye in which there is a difference in degree of refraction in differ­ent meridians. It is typically characterized by an aspherical, nonfigure of revolution cornea in which the corneal profile slope and refractive power in one meridian are greater than that of the perpen­dicular axis.

ATP: See adenosine triphosphate (ATP).

ATP synthase: Enzyme complex located in the inner membrane of the mitochondria that catalyzes the formation of ATP from ADP and inorganic phosphate as a consequence of oxidative phosphorylation.

ATPase: General name for a large class of enzymes that catalyze reactions that hydrolyze ATP, thereby liberating energy for biochemical processes.

Atria: Thin walled chambers of the heart that receive venous blood from the pulmonary or systemic circuit atrial natriuretic peptide.

Atrioventricular (AV) node: Specialized Cardiocytes that relay the contractile stimulus to the bundle of His, the bundle branches, the Purkinje fibers, and the ventricular myocardium; located at the bound­ary between the atria and ventricles.

Auditory ossicles: The three smallest bones in the body. They are positioned within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ossicles would cause a moderate to severe hearing loss.

Auditory tube: A passageway that connects the naso­pharynx with the middle ear cavity; also called the eustachian tube or pharyngotympanic tube.

Auricle: A broad, flattened process that resembles the external ear; in the ear, the expanded, projecting portion that surrounds the external auditory canal; also called the pinna. In the heart it is the externally visible flap formed by the collapse of the outer wall of a relaxed atrium.

Auscultation: The technical term for listening to the internal sounds of the body, usually using a stethoscope.

Autocrine: Mechanism whereby ligands (growth factors, hormones) produced by a particular cell act to modify the action of that cell.

Autonomic nervous system: Efferent division of the peripheral nervous system that innervates cardiac and smooth muscles and glands; also called the involuntary or visceral motor system.

Autoregulation: Changes in activity that maintain homeostasis in direct response to changes in the local environment; does not require neural or endo­crine control.

Autorhythmic: Spontaneous and periodic; for example, in smooth muscle it implies spontaneous (without nervous or hormonal stimulation) and periodic contractions.

Autorhythmic fibers: Self-excitable and repeatedly generated action potentials that trigger heart con­tractions. They continue to stimulate a heart to beat even after it is removed from the body.

Axial skeleton: The bones constituting the head and trunk of a vertebrate body.

Axon: The elongate extension of a neuron that con­ducts an action potential.

Axon hillock: In a multipolar neuron, the portion of the cell body adjacent to the initial segment.

Axon terminals: The branched endings of a neuronal axon, which release a neurotransmitter that influ­ences target cells in close association with the axon terminals.

Baroreceptor reflex: A reflexive change in cardiac activity in response to changes in blood pressure.

Baroreceptors: The receptors responsible for baroreception.

Basal cells: The innermost layer of the epidermis; also called the stratum germinativum or stratum basale. Cells produced here in the germinal layer form the prickle cells in the above layer (the stratum spinosum).

Basal lamina: The thin layer of extracellular matrix that creates a boundary between the epithelial cells and the surrounding stromal tissue and between other cell types; also called the basement membrane.

Basophils: Circulating granulocytes (white blood cells) similar in size and function to tissue mast cells.

Bicarbonate ions: HCO₃ ; anion components of the carbonic acid-bicarbonate buffer system.

Bicuspid valve: The left atrioventricular (AV) valve; also called the mitral valve.

Bile duct: Any of a number of long tubelike structures that carry bile.

Bilirubin: A pigment that is the by-product of hemo­globin catabolism.

Biliverdin: Green bile pigment formed from the oxi­dation of bilirubin.

Biogenic amines: Class of neurotransmitters, includ­ing catecholamines and indolamines.

Biopsy: To remove a small piece of tissue or cells (biopsy of embryo) from a living animal for analysis or study.

Bipolar cell: As a part of the retina, the bipolar cell exists between photoreceptors (rod cells and cone cells) and ganglion cells.

Bipolar neuron: Neuron with an axon and a dendrite extending from opposite sides of the cell body.

Blastocoele: The cavity in the center of the blastocyst. Blastocyst: An early embryo composed of an inner cell mass, the blastocoele, and trophoblast.

Blastomere: A cell created by the cleavage divisions of the early embryo.

Blind spot: The specific region of the retina where the optic nerve and blood vessels pass through to connect to the back of the eye.

Blood-brain barrier: The isolation of the central nervous system from the general circulation; pri­marily the result of astrocyte regulation of capillary permeability.

Blood pressure: A force exerted against vessel walls by the blood in the vessels, due to the push exerted by cardiac contraction and the elasticity of the vessel walls; usually measured along one of the muscular arteries, with systolic pressure measured during ventricular systole and diastolic pressure during ventricular diastole.

Bohr effect: The increased oxygen release by hemo­globin in the presence of elevated carbon dioxide levels.

Bolus: Any kind of ball-shaped organic structure of an organism or of its discharged substances.

Bone remodeling: The continuous turnover of bone matrix and mineral that involves first, an increase in resorption (osteoclastic activity), and later, reac­tive bone formation (osteoblastic activity).

Bony collar: A bone collar that forms concurrently with the primary ossification center. Cells of the perichondrium begin to form bone. The bone collar holds together the shaft, which has been weakened by the disintegration of the cartilage. The connective tissue about the bone collar, previously a perichon­drium, is now called periosteum.

Boyle's law: The principle that, in a gas, pressure and volume are inversely related.

Brachiocephalic: Relating to the arm and the head.

Brachiocephalic trunk: An artery that arises from the arch of the aorta and divides into the right subcla­vian and right carotid arteries.

Brain stem: The portion of the brain—consisting of the medulla oblongata, pons Varolii, and midbrain— that connects the spinal cord to the forebrain and cerebrum.

Bronchi: A branch of the bronchial tree between the trachea and bronchioles.

Bronchial tree: The trachea, bronchi, and bronchioles.

Bronchioles: The branching air passageways inside the lungs.

Brush-border: The epithelial surface consisting of microvilli.

bST or bovine somatotropin: A protein hormone, also called the growth hormone (GH), that is pro­duced in the anterior pituitary gland. A common terminology is to use a lowercase letter to indicate the species of origin (e.g., pST for porcine somato­tropin) and a lowercase "r" to indicate that the protein is derived by recombinant DNA technology. For example, rbST stands for recombinant bovine somatotropin or growth hormone.

Buccal: Pertaining to the cheeks.

Bulk flow: The movement of large particles and mac­romolecules across a plasma membrane.

Calcification: Impregnation with calcium or calcium salts; also called calcareous infiltration.

Calcitonin: A 32-amino acid polypeptide hormone that is produced primarily by the C cells of the thyroid and, in many animals, in the ultimobran- chial body.

Calmodulin: Aubiquitous, eukaryotic, calcium-binding protein that regulates cellular processes by modify­ing the activity of specific calcium-sensitive enzymes.

Calsequestrin: The principal calcium-binding protein present in the sarcoplasmic reticulum of cardiac and skeletal muscle.

Calvaria: The skullcap, consisting of the superior portions of the frontal, parietal, and occipital bones.

Canaliculi: Microscopic passageways between cells; bile canaliculi carry bile to bile ducts in the liver; in bone, canaliculi permit the diffusion of nutrients and wastes to and from osteocytes.

Cancellous bone: Structure composed of a network of bony struts or spikes; also called spongy bone.

Canine: Referring to the cuspid tooth.

Capacitance of vessels: The capacitance of a vessel is a measure of how easily it stretches.

Capacitation: Activation of spermatozoa that occurs after deposition into the female reproductive tract. It involves release of enzymes from the acrosome of the sperm cells and is necessary for fertilization.

Capillary: A small blood vessel, located between an arteriole and a venule, whose thin wall permits the diffusion of gases, nutrients, and wastes between plasma and interstitial fluids.

Capillary bed: The network of capillaries supplying an organ.

Capillary exchange: A dynamic process that has the role of supplying extravascular cells with the sub­stances essential for survival.

Capillary hydrostatic pressure (HP_c): In capillaries, hydrostatic pressure is exerted by blood. Thus, cap­illary hydrostatic pressure (HP_c) is equivalent to the blood pressure in the capillaries.

Carbamino compounds: The combination of CO₂ with terminal amine groups in blood protein of which the most important is the globin of hemoglobin.

Carbaminohemoglobin: Hemoglobin bound to carbon dioxide molecules.

Carbohydrate: Organic compound composed of carbon, hydrogen, and oxygen in a 1:2:1 ration. Examples include glucose, lactose, starch, and cellulose. Many are critical for ATP production in cells.

Carboxyl group: A carbon atom linked to an oxygen atom by a double bond and to a hydroxyl group. This combination is common in fatty acids and amino acids.

Carboxyl terminal (C terminal): The end of a protein chain that has a free carbonyl group.

Carcinoma: Refers to a cancer of the epithelial cells or the epithelium. It is the most common type of cancer.

Cardia: The area of the stomach surrounding its con­nection with the esophagus.

Cardiac accelerator nerves: Sympathetic nerves respon­sible for increasing cardiac rate.

Cardiac center: Part of the medulla oblongata respon­sible for controlling the heart rate.

Cardiac cycle: One complete heartbeat, including atrial and ventricular systole and diastole.

Cardiac muscle: The muscle of the heart, consisting of anastomosing transversely striated muscle fibers formed of cells united at intercalated discs; also called the myocardium or, simply, heart muscle.

Cardiac notch: The lateral deflection of the anterior border of the left lung. It is produced to accommo­date the space taken up by the heart.

Cardiac output: The amount of blood ejected by the left ventricle each minute; normally about 51 mL.

Cardiac reserve: The potential percentage increase in cardiac output above resting levels.

Cardiocytes or cardiac myocytes: A type of involun­tary mononucleated striated muscle found exclu­sively within the heart. Its function is to "pump" blood through the circulatory system by contracting.

Cardiogenic shock: Essentially the failure to pump, a condition wherein the heart is so inefficient that it cannot sustain adequate circulation.

Cardiovascular center: Poorly localized centers in the reticular formation of the medulla oblongata of the brain. It includes Cardioacceleratory, Cardioinhibi- tory, and vasomotor centers.

Carotid bodies: A small cluster of chemoreceptors and supporting cells located near the bifurcation of the carotid artery. It measures changes in blood pressure and the composition of arterial blood flowing past it, including the partial pressures of oxygen and carbon dioxide, and is also sensitive to changes in pH and temperature.

Carotid sinus: A dilated segment at the base of the internal carotid artery whose walls contain barore­ceptors sensitive to changes in blood pressure.

Caruncle: In ungulates, this is the buttonlike area of the uterine endometrium that creates the maternal side of the cotyledonary placenta.

Caseins: Proteins that constitute the largest group of specific proteins found in milk. They are empiri­cally defined by their precipitation from milk at pH 4.6 and their capacity to produce micelles contain­ing calcium. Major subtypes include a, β, and γ caseins.

Catecholamines: Epinephrine, norepinephrine, dopa­mine, and related compounds.

Caudate nucleus: An elongated, curved mass of gray matter consisting of three portions: an anterior, thick portion that projects into the anterior horn of the lateral ventricle; a portion extending along the floor of the body of the lateral ventricle; and an elongated, thin portion that curves downward and backward in the temporal lobes to the wall of the lateral ventricle; also called the caudatum.

Caudodorsal blind sac: A region of the rumen.

Caveoli: A small vesicle or recess, especially one com­municating with the outside of a cell and extending inward, indenting the cytoplasm and the cell membrane.

Cecum: An expanded pouch at the start of the large intestine.

Cell body: The body of a neuron; also called the soma.

Cellular immunity: That part of the immune system that depends on the actions of specialized cells to provide protection. Protection is most closely asso­ciated with activation of a class of lymphocytes, the T cells.

Cellular physiology: The study of how cells work. Cementum: Bony material that covers the root of a tooth and is not shielded by a layer of enamel.

Central chemoreceptors: Receptors located in the medulla near the respiratory center that respond to changes in extracellular fluid H⁺ concentration resulting from changes in arterial PCO₂ and adjust respiration accordingly.

Central nervous system: Brain and spinal cord.

Centriole: A small organelle located near the nucleus of the cell. It is involved in synthesis of microtubules and in cell division.

Cephalic: Pertaining to the head.

Cephalization: An evolutionary trend in the animal kingdom toward centralization of neural and sen­sory organs in the head or anterior region of the body.

Cerebral aquaduct: The slender cavity of the mid­brain that connects the third and fourth ventricles; also called the aqueduct of Sylvius.

Cerebral peduncles: The massive bundle of corticof- ugal nerve fibers passing longitudinally over the ventral surface of the midbrain on each side of the midline.

Cerebroventricles: Fluid-filled spaces in the brain.

Cerebrum: The largest portion of the brain, including practically all the parts within the skull except the medulla, pons, and cerebellum, and now usually referring only to the parts derived from the telen­cephalon and including mainly the cerebral hemi­spheres that are joined at the bottom by the corpus callosum. It controls and integrates motor, sensory, and higher mental functions, such as thought, reason, emotion, and memory.

Cervical thoracic air sacs: A pair of air sacs located on each side in the neck region of birds; one sac usually extends from each lung, but sometimes a series of cervical sacs are located along the neck, as in geese.

Cervix: The proximal portion of the uterus, it forms the boundary between the vagina and uterus.

Chaperone: An intracellular protein that allows other proteins to avoid alterations in folding or conforma­tion so that the supported protein maintains its function.

Chemical synapses: Specialized junctions through which cells of the nervous system signal to one another and to nonneuronal cells such as muscles or glands.

Chemically gated channels: Channels in the plasma membrane that open or close in response to the binding of a specific chemical messenger with a membrane receptor site that is in close association with the channel.

Chemoreceptor: A cell or group of cells that trans­duce a chemical signal into an action potential.

Chemoreceptor reflex: Chemoreceptors that detect decrease in blood oxygen, increase in carbon dioxide, or decrease in pH, and produce an increased rate and depth of respiration, and, by means of the vasomotor center, vasoconstriction.

Chemotaxis: The attraction of phagocytic cells to the source of abnormal chemicals in tissue fluids.

Chloride shift: The movement of plasma chloride ions into red blood cells in exchange for bicarbonate ions generated by the intracellular dissociation of carbonic acid.

Choanae: Openings from the nasal cavity into the nasopharynx. They are also known as internal nares. Cholecystokinin (CCK): A duodenal hormone that stimulates the contraction of the gallbladder and the secretion of enzymes by the exocrine pancreas; also called pancreozymin.

Cholesterol: A common lipid with a characteristic four-ring structure; it is important for membrane fluidity and as the precursor for a myriad of steroids.

Cholinergic neurons: Nerve endings that, upon stim­ulation, release acetylcholine.

Chondroblasts: Cells of growing cartilage tissue responsible for increasing tissue mass.

Chondrocyte: The primary cell type found in devel­oped cartilage.

Chondroitin sulfate: A major proteoglycan found in cartilage.

Chordae tendineae: Fibrous cords that stabilize the position of the AV valves in the heart, preventing backflow during ventricular systole.

Chordamesoderm: The middle germ layer that devel­ops into muscle, bone, cartilage, blood, and connec­tive tissue.

Chorion: The outer extraembyronic membrane that is derived from the trophoblastic ectoderm. It develops villi that make the fetal sites of placental attachment.

Choroid plexus: A vascular proliferation of the cere­bral ventricles that serves to regulate intraven­tricular pressure by secretion or absorption of cerebrospinal fluid.

Choroids: The vascular layer of the eye lying between the retina and the sclera. The choroid provides oxygen and nourishment to the outer layers of the retina.

Chylomicrons: Relatively large droplets that may contain triglycerides, phospholipids, and choles­terol in association with proteins. They are synthe­sized and released by intestinal cells and transported to the venous blood by the lymphatic system.

Chyme: A semifluid, acidic mixture of ingested food and digestive secretions that forms in the stomach during the early phases of digestion.

Cilia: Extensions of the plasma membrane containing doublets of parallel microtubules. They are approxi­mately 5 to 10 micrometers in length. There are two types of cilia: (1) motile cilium, which constantly beats in one direction, and (2) nonmotile cilium, which cannot beat and usually serves as a sensor.

Ciliary body: The part of the eye containing the ciliary muscle and ciliary processes.

Ciliary muscles: A smooth muscle that affects zonules in the eye (fibers that suspend the lens in position during accommodation), enabling changes in lens shape for light focusing.

Ciliary processes: Processes formed by the inward folding of the various layers of the choroid, that is, the choroid proper and the lamina basalis, and are received between corresponding folds of the sus­pensory ligament of the lens.

Cingulate: A gyrus in the medial part of the brain. It partially wraps around the corpus callosum and is limited above by the cingulate sulcus.

Circulatory shock: When mean arterial blood pres­sure falls so low that adequate blood flow to the tissues can no longer be maintained.

Circumduction: The circular movement of a limb such that the distal end of the limb delineates an arc.

Circumvallate papilla: one of the large, dome-shaped papillae on the superior surface of the tongue that form a V, separating the body of the tongue from the root.

Circumventricular organs: Sites in the neuroendo­crine system that allow factors to "circumvent" the brain-blood barrier. These organs secrete or are sites of action of different hormones, neurotransmitters, and cytokines.

Citric acid cycle: This fundamental biochemical pathway (also called the TCA, tricarboxylic acid cycle, or Krebs cycle) is central to aerobic respira­tion. Acetyl groups derived from nutrient sources are area oxidized to CO₂ and H₂O. In the process, the coenzymes NAD and FAD become oxidized and are ultimately utilized in the electron transport chain of the mitochondria to produce ATP.

Clavicular air sac: A single, median air sac between the clavicles and surrounding the bifurcation of the trachea of birds.

Cloaca: The common opening in birds through which the intestinal, urinary, and reproductive tracts empty.

Clostridium botulinum: A bacterium that occurs widely in nature and is a cause of botulism; its six main types, A through F, are characterized by dis­tinct but pharmacologically similar, very potent neurotoxins.

Clot: A network of fibrin fibers and trapped blood cells; also called a thrombus if it occurs within the circulatory system.

Clot retraction: Condensation of the clot into a denser, compact structure; caused by the elastic nature of fibrin.

Cochlea: A coiled, tapered tube containing the audi­tory branch of the mammalian inner ear. Its core component is the organ of Corti, the sensory organ of hearing.

Cochlear duct: An endolymph-filled cavity inside the cochlea, located between the scala tympani and the scala vestibuli, separated by the basilar membrane and Reissner's membrane (the vestibular mem­brane), respectively.

Codon: Sequence of three bases in DNA or a strand of mRNA that corresponds with the instructions for incorporating a specific amino acid into a growing protein chain.

Coenzymes: Organic molecules that serve as cofac­tors required for the action of certain enzymes. Most coenzymes are derived from vitamins.

Cofactor: A companion molecule necessary for func­tion of many enzymes. Most often this is either a metal ion or organic molecule.

Collagen: The most abundant class of proteins in the body, collagens are predominant in the extracel­lular matrix surrounding cells and in connective tissues.

Collaterals: A branch of a nerve axon or blood vessel.

Colloid osmotic pressure (OP_c): Pressure created in a fluid.

Colon: The large intestine.

Colostrum: Mammary gland secretion that accumu­lates prior to the onset of normal milk production. It is rich in antibodies.

Common hepatic duct: The duct formed by the junc­tion of the right hepatic duct (which drains bile from the right functional lobe of the liver) and the left hepatic duct (which drains bile from the left func­tional lobe of the liver).

Compact bone: Dense bone that contains parallel osteons.

Complementarity of structure and function: An essential concept to understanding of how an animal works and its limitations. In short, the principle that function and structure are interdependent.

Complete fracture: Break involving the entire width of the bone.

Complete tetanus: An acute, often fatal disease that is characterized by spasmodic contraction of volun­tary muscles, especially one occurring in the neck and jaw, and that is caused by the bacterium Clos­tridium tetani, which usually enters the body through an infected wound and produces a neurotoxin; also called lockjaw.

Compound fracture: A fracture in which broken bone fragments lacerate soft tissue and protrude through an open wound in the skin.

Concentration gradient: The difference in concentra­tion of a material between tissue regions or loca­tions within cells. For example, the concentration of sodium is typically higher in the extracellular fluid than the cytoplasm. Movement of molecules down their concentration gradients is often utilized to "drive" various transport functions.

Concentric: Having a common center or center point, as of circles.

Concentric lamellae: One of the tubular layers of bone surrounding the central canal in an osteon; also called Haversian lamellae.

Conchae: Three pairs of thin, scroll-like bones that project into the nasal cavities; the superior and medial conchae are part of the ethmoid, and the inferior conchae are separate bones.

Cones: Cells in the retina of the eye that function only in relatively bright light. They gradually become less concentrated toward the periphery of the retina.

Conjunctiva: A membrane that covers the sclera (white part of the eye) and lines the inside of the eyelids. It helps lubricate the eye by producing mucus and tears, although a smaller volume of tears than the lacrimal gland.

Connexon: Hollow cylinders made of transmem­brane proteins that connect adjacent cells at gap junctions, allowing chemical substances to pass through.

Continuous capillary: A capillary in which pores are absent; it is less permeable to large molecules than other types of capillaries.

Contractility: The ability to contract; possessed by skeletal, smooth, and cardiac muscle cells.

Control center: One of three interdependent compo­nents of homeostatic control mechanisms; it deter­mines the set point.

Conus medullaris: The terminal end of the spinal cord. It occurs near lumbar nerves 1 (Ll) and 2 (L2). After the spinal cord terminates, the spinal nerves continue as dangling nerves called the cauda equina. Convergence: The converging of many presynaptic terminals from thousands of other neurons on a single neuronal cell body and its dendrites so that activity in the single neuron is influenced by the activity in many other neurons.

Convergent: The coordinated turning of the eyes inward to focus on an object at close range.

Coracoid process: A long curved projection from the neck of the scapula, overhanging the glenoid cavity and giving attachment to the short head of the biceps, the coracobrachial muscle, the smaller pec­toral muscle, and the Coracoacromial ligament.

Cori cycle: A metabolic pathway that allows lactic acid produced by muscle action to be to be con­verted to glucose by the liver. This action typically occurs during muscle rest.

Cornea: The transparent front part of the eye that covers the iris, pupil, and anterior chamber, provid­ing most of an eye's optical power.

Corpora quadrigemina: The four colliculi—two infe­rior, two superior—located on the posterior aspect of the midbrain.

Corpus callosum: The commissural plate of nerve fibers connecting the two cerebral hemispheres except for most of the temporal lobes; also called the commissure of cerebral hemispheres.

Corpus Iuteum (CL): Yellow to orange structures formed on the surface of the ovary after ovulation from granulosal and thecal cells from the ruptured follicle. The CL produces progesterone and oxytocin.

Corpus striatum: Either of two gray and white, stri­ated bodies of nerve fibers located in the lower lateral wall of each cerebral hemisphere.

Corticoids: General name for the class of steroid hor­mones secreted by the adrenal gland.

Costal cartilages: The cartilage forming the anterior continuation of a rib.

Cotransport: A membrane carrier system in which the transfer of one molecule depends on the simul­taneous transfer of another molecule.

Cotyledons: In ruminants, the points of attachment between the fetal and maternal placenta. It is com­posed of the maternal cotyledon (the caruncle of the uterus) + the fetal cotyledon (from the chorion of the conceptus).

Covalent bond: A common chemical bond in which electrons are shared between atoms.

Cranial: Pertaining to the head.

Cranial base: The structure that forms the inferior aspect of the cranium and is divided by bony ridges into three distinct fossae.

Cranial nerves: Nerves that emerge from the brain stem instead of the spinal cord.

Cranial vault: Eight skull bones that surround and protect the brain; braincase.

Cranium: The braincase; the skull bones that sur­round and protect the brain.

Creatine kinase: The enzyme that catalyzes the trans­fer of phosphate from phosphocreatine to ADP, forming creatine and ATP; important in muscle contraction.

Creatine phosphate: An organic compound found in muscle tissue and capable of storing and pro­viding energy for muscular contraction; also called phosphocreatine.

Cricoid cartilage: A ring-shaped cartilage that forms the inferior margin of the larynx.

Crop: A thin-walled, expanded portion of the alimen­tary tract that is found in many animals and is used for the storage of food prior to digestion.

Crossed-extensor reflex: A withdrawal reflex. When the reflex occurs, the flexors in the withdrawing limb contract and the extensors relax, while in the other limb the opposite occurs.

Crossmatching: Matching blood types.

Cryptorchidism: Failure of the testicles to descend into the scrotum during fetal development. The undescended testis remains in the abdominal cavity.

Crystallins: A water-soluble structural protein found in the lens of the eye accounting for the transpar­ency of the structure. Crystallins from a vertebrate eye lens are classified into three types: alpha, beta, and gamma.

Curare: A purified preparation or alkaloid obtained from the Chondrodendron tomentosum, it is used to relax skeletal muscles.

Cyclic AMP: An important intracellular signaling molecule, it is formed by the action of the enzyme adenylate cyclase on ATP. Its appearance is typically induced by binding of a particular hormone to its surface receptor.

Cyclicity: The condition during which the female dis­plays estrus cycles with predictable duration.

Cystic duct: A duct that carries bile between the gall­bladder and the common bile duct.

Cytokines: Small growth factor-like proteins that serve to regulate the activity of the various immune system cells.

Cytokinesis: The dividing of cytoplasm between two cells at the time of mitosis.

Cytology: The study of the structure, organelles, and function of cells.

Cytoplasm: The portion of the cell interior not occu­pied by the nucleus.

Cytosine: A pyrimidine that is one of the two classes of nitrogenous bases found in nucleic acids such as DNA and RNA.

Cytoskeleton: The network of microtubules and microfilaments found in the cytoplasm of the cell.

Cytosol: The fluid portion of a cytoplasm, it is effec­tively prepared by breaking the cells apart and saving the supernatant from a 100,000 ? g centrifu­gation minus any lipid.

Cytoxic T cell: An activated lymphocyte, part of cel­lular immunity, also called a killer T cell; it acts to destroy cells that have been infected by viruses by releasing enzymes.

Dale's Principle: A principle postulated by the English neuroscientist Henry Hallett Dale, which states that, although different neurotransmitters can be produced at different synapses within the brain, individual neurons are capable of releasing only one neurotransmitter from its axonal terminal. Dale's Principle has been shown to be false because many nerve terminals release neuropeptides as well as amino acids or amines.

Dalton's Law: Named for the English chemist John Dalton. In a mixture of gases, the portion of the total pressure resulting from each type of gas is deter­mined by the percentage of the total volume repre­sented by each gas type.

Dark adaptation: The ability of the eye to adjust to various levels of darkness and light.

Dark cells: Cells that line the endolymphatic space of the ear.

De novo: Refers to a synthesis process occurring within a tissue or cell.

Deamination: The removal of an amine group from a molecule often in relation to the metabolism of the amino acids from proteins.

Deciduous: An adjective meaning "temporary" or "tending to fall off" (derived from the Latin word decidere, to fall off).

Decomposition reaction: Description of a chemical reaction that results in the breaking down of a larger molecule into its component parts, that is, a protein into amino acids or starch into glucose monomers.

Defensins: Locally produced tissue proteins that can protect cells from infection by bacteria.

Deglutition: Swallowing.

Dehydration synthesis: A chemical reaction wherein a larger molecule is created by covalently bonding smaller molecules together. In the process, water is often produced.

Deltoid tuberosity: A bump or raised area on the outside of the humerus where the deltoid muscle attaches.

Denaturation: Dramatic change in the tertiary struc­ture of a protein or other macromolecule that leaves it nonfunctional. This can be caused by heating (egg albumin after frying or poaching) or exposure to chemicals.

Dendrite: A branching process of a neuron that receives stimuli and conducts potentials toward the nerve cell body.

Dentate gyrus: One of the two interlocking gyri com­posing the hippocampus.

Dentin: The bonelike material that forms the body of a tooth; it differs from bone in that it lacks osteo- cytes and osteons.

Deoxygenated blood: Blood whose red blood cells carry very little oxygen, as found in all veins except the pulmonary vein; also called oxygen-poor blood.

Deoxyhemoglobin: Hemoglobin without oxygen bound to it.

Depolarization: A reduction in membrane potential from resting potential; movement of the potential from resting toward 0 mV.

Depression: An inward displacement of a body part.

Dermis: A deep layer of the skin, it is primarily com­posed of dense irregular connective tissue. A hypo­dermic injection, for example, would be placement of the material just under the skin into the adjacent tissue.

Descending segment: The portion of the large intes­tine that leads to the rectum.

Desmin: Acomponentofthecytoskeleton—important structural components of living cells. Their size is intermediate between that of microfilaments and microtubules.

Desmosomes: A structure that forms the site of adhe­sion between two cells, consisting of a dense plate in each adjacent cell separated by a thin layer of extra­cellular material; also called the macula adherens.

Diabetes insipidus: A disease characterized by the production of large amounts of dilute urine and intense thirst. It is named because the symptoms are similar to those common in diabetes mellitus, but the cause is insufficient production of antidiuretic hormone (ADH) rather than insulin insufficiency or failure of insulin action.

Diabetes mellitus: A disease caused by either failure of the endocrine pancreas to produce sufficient amounts of insulin or failure of insulin function.

Diacylglycerol: A lipid produced in the plasma mem­brane after the cleavage of inositol phospholipids. This usually occurs following hormone signaling. It is two fatty acid chains linked to glycerol.

Diapedesis: The movement of white blood cells through the walls of blood vessels by migration between adjacent endothelial cells.

Diaphysis: The shaft of a long bone.

Diastole: Period of the cardiac cycle when either the ventricles or the atria are relaxing.

Diastolic blood pressure: Pressure measured in the walls of a muscular artery when the left ventricle is in diastole.

Diencephalon: The posterior part of the prosenceph­alon, composed of the epithalamus, the dorsal thala­mus, the subthalamus, and the hypothalamus.

Diestrus: The stage of the estrus cycle characterized by major secretion of progesterone from the corpus Iuteum and periods of minimal or no reproductive behavior.

Differential blood count: A common laboratory pro­cedure often using a stained blood smear to tabulate the relative proportions of classes of white blood cells in a sample. Variations from normal propor­tions for a given species or stage of development is used diagnostically.

Diffuse placenta: The placental type characterized by the distribution of chorionic villi across the surface of the chorion (e.g., pigs, mares).

Diffusion: Passive molecular movement from an area of higher concentration to an area of lower concentration.

Digestion: The chemical breakdown of ingested materials into simple molecules that can be absorbed by the cells of the digestive tract.

Digestive tract: An internal passageway that begins at the mouth, ends at the anus, and is lined by a mucous membrane; also called the gastrointes­tinal tract.

Dipeptidase: An enzyme that catalyzes the hydroly­sis of dipeptides into their constituent amino acids. Diploe: The diploic veins are found in the skull and drain the diploic space. This is found in the bones of the vault of the skull, and is the marrow­containing area of cancellous bone between the inner and outer layers of compact bone. The diploic veins drain this area to the outside of the skull and are usually four in number: one frontal, two pari­etal, and one occipital.

Directly gated: Also called a transmitter or ligand­gated channel, in which the ligand receptor and ion channel are one and the same.

Disaccharide: A sugar molecule composed of two simple sugars, for example, glucose + galactose, to yield milk sugar or lactose.

Displaced fractures: A break or fracture in which the two ends of the broken bone are separated from one another.

Distal: Away from the origin. For example, the wrist is distal from the elbow, or the distal convoluted tubules of the kidney nephrons are distant from the proximal (near the site of filtration) convoluted tubules.

Distributing arteries: Medium-sized artery with a tunica media composed principally of smooth muscle; regulates blood flow to different regions of the body.

Divergence: The diverging, or branching, of a neu­ron's axon terminals so that activity in this single neuron influences the many other cells with which its terminals synapse.

Diverticulum: A sac or pouch in the wall of the colon or other organ.

DNA (deoxyribonucleic acid): The critical polynucle­otide created by covalent bonding of nucleotides, phosphate groups, and deoxyribose in repeating chains. It is the basis for transference of genetic information.

Dolichephalic: Having a disproportionately long head. Dominant follicle: The final maturation stage of fol­licle development in cattle, that is, the follicle likely destined to ovulate.

Dopamine: An important neurotransmitter in the central nervous system.

Dorsal: Toward the back, posterior.

Dorsal root: The afferent sensory root of a spinal nerve. At the distal end of the dorsal root is the dorsal root ganglion, which contains the neu­ron cell bodies of the nerve fibers conveyed by the root.

Dorsal root ganglion: A nodule on a dorsal root that contains cell bodies of neurons in afferent spinal nerves. All of the axons in the dorsal root convey somatosensory information, bringing sensory infor­mation into the brain and spinal cord.

Dorsal sac: Toward the back; posterior.

Downregulation: A term frequently used to describe a reduced response to hormone stimulation over time. It often reflects a decrease in the number of available hormone receptors.

Downstroke or falling phase: The repolarization phase of an action potential in which the cell or neuron membrane potential moves toward a more neg­ative value.

Duct: A tubular canal or passageway most often associated with secretion of products from a glan­dular organ.

Ductus arteriosus: A vascular connection between the pulmonary trunk and the aorta that functions throughout fetal life; it normally closes at birth or shortly thereafter and persists as the Iigamentum arteriosum.

Duodenal gland: Small gland that opens into the base of intestinal glands. It secretes a mucuslike alkaline substance.

Duodenal papilla: A conical projection from the inner surface of the duodenum that contains the opening of the duodenal ampulla.

Duodenum: The proximal region of the small in­testine that contains short villi and submucosal glands.

Dura mater: The tough fibrous membrane covering the brain and the spinal cord and lining the inner surface of the skull; also called the dura pachymeninx.

Dural sinuses: Venous channels found between layers of dura mater in the brain. They receive blood from internal and external veins of the brain and ultimately empty into the internal jugular vein. Damage to the walls of the dural sinuses may result in dural sinus thrombosis.

Dystocia: Difficult birth, a term often used in animal management-recording schemes to indicate that an animal required assistance with giving birth.

Dystophin: A structural protein found in small amounts in normal muscle but absent or present in abnormal amounts in individuals with muscular dystrophy.

Eccentric: Departing from a recognized, conventional, or established norm or pattern.

Eccrine glands: A common type of sweat gland that produces primarily a watery secretion containing dissolved salts.

Ectoderm: One of three germinal cell layers (ecto­derm, mesoderm, and endoderm) in the developing embryo. The mammary gland, for example, is derived from the ectoderm.

Edema: Abnormal accumulation of fluid in body parts or tissues; causes swelling. It is common sur­rounding the udder of first-calf heifers.

Effector: Aperipheral gland or muscle cell innervated by a motor neuron.

Effector organ: The muscles or glands that are inner­vated by the nervous system and that carry out the nervous system's orders to bring about a desired effect, such as a particular movement or secretion.

Efferent: Away from.

Efferent nerve: A nerve conveying impulses from the central nervous system to the periphery; also called a centrifugal nerve.

Elastic cartilage: A yellowish flexible cartilage in which the matrix is infiltrated by a network of elastic fibers; it occurs primarily in the external ear, eusta­chian tube, and some cartilages of the larynx and epiglottis; also called yellow cartilage.

Elastin: A relatively common extracellular matrix protein, which gives strength and flexibility to tissues. For example, it allows the expansion and recovery of the aorta or the spring in the fibroelastic cartilage of the external ear.

Electrical synapses: A mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction.

Electrocardiogram (ECG or EKG): A graphic record of the electrical activities of the heart, as monitored at specific locations on the body surface.

Electrochemical gradient: The simultaneous exis­tence of an electrical gradient and concentration (chemical) gradient for a particular ion.

Electron acceptor: An atom or molecule that easily takes up an electron, thereby becoming reduced in the process.

Electron carrier: A molecule that transfers an electron from a donor molecule to an acceptor molecule, for example, cytochrome c in the mitochondria in con­version of NADH to NAD.

Electron transport: The movement of electrons from a higher to lower energy via movement along a series of carrier molecules—for example, the oxida­tive phosphorylation process inside mitochondria.

Elevation: Movement in a superior, or upward, direction.

Embryology: The study of embryonic development, focusing on the first 2 months after fertilization.

Emesis: Vomiting.

Enamel: Crystalline material similar in mineral com­position to bone, but harder and without osteocytes, which covers the exposed surfaces of the teeth.

End buds: The swollen terminal ends of mammary ducts in the gland of peripubertal rodents. These structures are responsible for the rapid growth and elongation of the ductular tree. Although alveolar budlike structures in the mammary glands of peri­pubertal ruminants serve as sites of focus for rapid growth of mammary ducts, ruminants apparently do not have morphologically similar end buds.

End diastolic volume (EDV): The volume of blood in the ventricle at the end of diastole, when filling is complete.

End systolic volume (ESV): The volume of blood in the ventricle at the end of systole, when emptying is complete.

Endocardium: The simple squamous epithelium that lines the heart and is continuous with the endothe­lium of the great vessels.

Endochondral ossification: The conversion of a car­tilaginous model to bone; the characteristic mode of formation for skeletal elements other than the bones of the cranium, the clavicles, and sesamoid bones.

Endocrine system: A control system of ductless glands that secrete chemical "instant messengers" called hor­mones, which circulate within the body via the blood­stream to affect distant cells within specific organs.

Endocrinology: Study of the endocrine system.

Endocytosis: The uptake of material into a cell by the invagination of the plasma membrane followed by internalization of membrane-surrounded vesicles.

Endolymph: The fluid contained in the membranous labyrinth of the inner ear. The main cation of this unique extracellular fluid is potassium.

Endoneurium: The innermost layer of connective tissue in a peripheral nerve, forming an interstitial layer around each individual fiber outside the neu­rolemma. In some circumstances, also called the epilemma, sheath of Henle, or sheath of Key and Retzius.

Endoplasmic Reticulum (ER): A cellular organelle composed of a network of membranous sacs or tubules whose intercompartment is continuous with the Golgi apparatus. The outer surface is fre­quently studded with ribosomes. In this situation, newly made proteins are often vectored into the cisteral space of the ER to transport to the Golgi. Without ribosomes it is called smooth endoplasmic reticulum (SER); with ribosomes it is called rough endoplasmic reticulum or RER.

Endosteum: An incomplete cellular lining on the inner (medullary) surfaces of bones.

Endothelium: A single layer of simple squamous cells that line the walls of the heart, blood vessels, and lymphatic vessels.

End-product inhibition: A process whereby the accu­mulation of the last product in a biochemical pathway acts to inhibit further production of the product.

Enterendocrine cells: Endocrine cells scattered among the epithelial cells that line the digestive tract.

Enteric nervous system: An interdependent part of the autonomic nervous system. Despite its many interactions with other parts of the ANS, it can be regarded as a nerve body of its own.

Enterogastric reflex: The reflexive inhibition of gastric secretion; it is initiated by the arrival of chyme in the small intestine.

Enterokinase: An enzyme in the lumen of the small intestine that activates the proenzymes secreted by the pancreas.

Eosinophil: A microphage (white blood cell) with a Iobed nucleus and red-staining granules; it partici­pates in the immune response and is especially important during allergic reactions.

Ependymal cells: A type of neuroglia cell lining the central canal of the spinal cord or the brain.

Epicardium: A serous membrane covering the outer surface of the heart; also called the visceral pericardium.

Epidermal growth factor (EFG): A protein that along with its related receptors (EGFR or ERBBl), makes up a family of related growth factors and receptors that are believed to be involved in regulation of mammary growth and mammary cancer and other tissue development in many species. Receptors for this family include EGRR as well as for several ligands such as amphriegulin, TGF-α, and ERBB2 (called HER in humans and Neu in rodents).

Epidermis: The outer or superficial layer of the skin. It is composed of stratified squamous epithelial cells with selected specialized immune cells and associ­ated sensory elements.

Epidural space: The space between the walls and the dura mater of the vertebral canal.

Epiglottic: Pertaining to, or connected with, the epiglottis.

Epinephrine: A catecholamine hormone secreted by the adrenal medulla and a neurotransmitter, released by certain neurons and active in the central nervous system. It is stored in the chromaffin granules and is released in response to hypoglycemia, stress, and other stimuli. It is a potent stimulator of the adren­ergic receptors of the sympathetic nervous system and a powerful cardiac stimulant that accelerates the heart rate and increases cardiac output. It also promotes glycogenolysis and exerts other meta­bolic effects.

Epineurium: The outermost layer of connective tissue of a peripheral nerve, surrounding the entire nerve and containing its supplying blood vessels and lymphatics.

Epiphyseal line: The line of junction of the epiphysis and diaphysis of a long bone where growth in length occurs.

Epiphyseal plate: The plate of cartilage between the shaft and the epiphysis of a long bone during its growth; also called the epiphyseal cartilage.

Epiphysis: The head of a long bone.

Epiploic appendage: One of a number of small pro­cesses of peritoneum projecting from the serous coat of the large intestine except the rectum; it is gener­ally distended with fat.

Epithelium: A tissue composed of a layer of cells. It is one of four primary body tissues. Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. Epithelial cells are also the primary functional (parenchymal) parts of organs and glands.

Equilibrium: The sense of balance, which maintains physical balance in animals.

Equilibrium potential: The potential that exists when the concentration gradient and opposing electrical gradient for a given ion exactly counterbalance each other so there is no net movement of the ion.

Eructation sequence: The coordinated neural and mechanical steps that occur when a ruminate animal regurgitates material to "chew its cud."

Erythrocytes: A red blood cell; it has no nucleus and contains large quantities of hemoglobin.

Erythropoiesis: The process of erythrocyte formation.

Erythropoietin (EPO): Ahormone released by tissues, especially the kidneys, exposed to low oxygen con­centrations; stimulates erythropoiesis (red blood cell formation) in bone marrow.

Esophageal hiatus: The defect in the diaphragm through which the esophagus passes from the thorax into the abdomen.

Esophagus: A muscular tube that connects the pharynx to the stomach.

Estrogen: A steroid hormone produced predomi­nately in female reproductive tissues (ovary, pla­centa) associated with development of sexual receptivity and secondary sex characteristics (mam­mary development, characteristic female body development).

Estrous cycle: The reproductive cycle of nonprimate females defined as the period from one estrus (heat) to the next. Consecutive ovulations can also be used to signal cycles. Each cycle consists of a follicular and luteal phase.

Estrus: Period during which the female is sexually receptive to the male.

Ethmoid bone: A light spongy bone located between the eye sockets, forming part of the walls and septum of the superior nasal cavity, and containing perforations for the passage of olfactory nerve fibers.

Ethmoidal: Of, relating to, or being a light spongy bone located between the orbits, forming part of the walls and septum of the superior nasal cavity and containing numerous perforations for the passage of the fibers of the olfactory nerves.

Eutherian mammal: Those that produce a placenta. Eversion: A turning outward, as of the eyelid. Ewe: An adult female sheep.

Excitable membrane: Membranes that propagate action potentials, a characteristic of muscle cells and nerve cells.

Excitation-contraction coupling: The process whereby the spreading depolarization is converted into force production by muscle fibers.

Excitatory postsynaptic potential (EPSP): A small depolarization of the postsynaptic membrane in response to neurotransmitter binding, bringing the membrane closer to threshold.

Excretion: A removal from body fluids.

Excretory lacrimal ducts: Tear ducts.

Exocrine: Glands that secrete their products and tem­porarily store their secretions in a duct.

Exocytosis: Fusion of a membrane-enclosed intracel­lular vesicle with the plasma membrane, followed by the opening of the vesicle and the emptying of its contents to the outside.

Expiratory reserve volume: The amount of additional air that can be voluntarily moved out of the respira­tory tract after one normal exhalation.

Extension: A pulling or dragging force exerted on a limb in a distal direction.

External nares: The entrance from the exterior to the nasal cavity.

External respiration: The diffusion of gases between the alveolar air and the alveolar capillaries and between the systemic capillaries and peripheral tissues.

Exteroceptor: A sense organ, such as the ear, that receives and responds to stimuli originating from outside the body.

Extracellular matrix: A complex network or mesh of proteins (collagen, elastin, etc.) and polysaccharides (glycosaminoglycans, etc.) secreted by cells. It serves as a structure scaffold for tissue and organ develop­ment and tissue function.

Extrafusal muscle fibers: A class of muscle fiber innervated by alpha motor neurons. Contraction of these fibers allows for movement. Extrafusal muscle fibers and associated alpha motor neurons are called a motor unit. The connection between alpha motor neurons and extrafusal muscle fiber is a neuromus­cular junction.

Extraocular muscles: The six muscles that control the movements of the eye. The actions of the extraocu­lar muscles depend on the position of the eye at the time of muscle contraction.

Extrapyramidal system: A neural network located in the brain that is part of the motor system involved in the coordination of movement. Extrapyramidal neurons, like related gamma system neurons, excite or inhibit anterior horn cells.

Extrinsic muscles: The six skeletal muscles that attach to and move each eye.

Eyebrows: A bony ridge above the eye that protects the eye and bears a tuft of facial hair in most mammals. The main function of the eyebrows is to prevent moisture (mostly sweat and rain) from dripping into the eye.

Eyelashes: The hairs that grow at the edge of the eyelid. Eyelashes protect the eye from debris and perform some of the same function as whiskers do on a cat or a mouse, in the sense that they are sensitive to being touched, thus providing a warning that an object (such as an insect or dust mote) is near the eye (which is then closed reflexively).

Eyelid: A thin fold of skin and muscle that covers and protects an eye.

F actin: One of the major protein components found in muscle, existing as F actin or G actin.

Facial: Pertaining to the face.

Facilitated diffusion: The passive movement of a substance across a cell membrane by means of a protein carrier.

Falciform ligament: A sheet of mesentery that con­tains the Iigamentum teres, the fibrous remains of the umbilical vein of the fetus.

Far point of vision: Distance from the eye where accommodation is not needed to have the image focused on the retina.

Fascia: A sheet or band of fibrous connective tissue enveloping, separating, or binding together muscles, organs, and other soft structures of the body.

Fast glycolytic fibers: Muscle fibers with low myo­globin content, few mitochondria, few blood capil­laries, and a large amount of glycogen. Also called fast twitch B or fatigable fibers, they hydrolyze ATP very quickly, fatigue easily, and are needed for sports such as sprinting.

Fast oxidative fibers: Muscle fibers that contain large amounts of myoglobin, many mitochondria, and many blood capillaries. Also called fast twitch A or fatigue-resistant fibers, they generate ATP by the aerobic system, split ATP at a slow rate, have slow contraction velocity, are resistant to fatigue, are found in large numbers in postural muscles, and are needed for aerobic activities such as long distance running.

Fatty acid: A common lipid composed of a carboxylic acid typically attached to a long hydrocarbon chain. Examples are palmitic or oleic acid.

Fenestrated: Pierced with one or more small openings. Fenestrated capillaries: Capillaries with openings that allow larger molecules to diffuse.

Ferritin: A protein that stores iron in the body. The serum ferritin level—the amount of ferritin in one's blood—is directly proportional to the amount of iron stored in the body.

Fibrinolysis: The breakdown of the fibrin strands of a blood clot by a proteolytic enzyme.

Fibroblast growth factor (FGF): One of a number of ~18,000 dalton proteins from a family with at least 23 members. FGFs were first isolated from bovine pituitary, but they are known to be widely distrib­uted and are involved in tissue growth and devel­opment, including embryonic development.

Fibroblasts: Connective tissue cells responsible for the production of extracellular fibers and the secre­tion of the organic compounds of the extracellular matrix.

Fibrocartilage: Cartilage containing an abundance of collagen fibers; it is located around the edges of joints, in the intervertebral discs, the menisci of the knee, and so on.

Fibrous tunic: Outer layer of the eye; composed of the sclera and the cornea.

Filiform papillae: Thin, longer papillae that don't contain taste buds but that are the most numerous. These papillae are mechanical and not involved in gustation.

Filtration: The movement of a fluid across a mem­brane whose pores restrict the passage of solutes on the basis of size.

Filum terminale: A fibrous extension of the spinal cord, from the conus medullaris of the neural tube.

First-class lever: An anatomical structure that acts as a hinge or point of support.

Fissures: A normal groove or furrow, as in the liver or brain, that divides an organ into lobes or parts.

Fixed macrophages: Macrophages that are resident cells in a particular tissue or organ, as opposed to circulating monocytes that are recruited to a tissue because of chemotaxis.

Flat bones: A bone having a thin, flattened shape, as the scapula.

Flexion: The act of bending a joint or limb in the body by the action of flexors.

Flexor reflex: Reflex initiated by a painful stimulus (actual or perceived); causes automatic with­drawal of the threatened body part from the stimulus.

Folia: A broad, thin, Ieaflike structure, as of the cere­bellar cortex.

Foliate papillae: The leaf-shaped ridges on the lateral borders of the tongue.

Follicle-stimulating hormone (FSH): A glycoprotein hormone produced in the anterior pituitary gland in response to secretion of gonadotropic releasing hormone (GnRH) from the hypothalamus. FSH pro­motes follicular development in females and Sertoli cell function in males.

Follicular phase: The period of the estrous cycle when the dominant follicle produces estradiol. Females dis­play behavioral estrus and ovulation during this time.

Foramen of Magendie: An opening in the hollow nerve tube connecting the fourth ventricle of the brain with the subarachnoid space.

Foramen ovale: Hole or opening in a bone or between body cavities.

Foramina of Luschka: Along with the median aper­ture, they comprise the three openings in the roof of the fourth ventricle. They are found at the extremi­ties of the lateral recesses.

Fornix: An archlike anatomical structure or fold, such as the arched band of white matter located beneath the corpus callosum of the brain.

Fornix vagina: The cranial region of the vagina that creates a crypt extending to the cervix.

Fourth ventricle: A cerebrospinal fluid-filled space within the hindbrain bordered dorsally by the cer­ebellum and ventrally by the tegmentum.

Fovea: A part of the eye, a spot located in the center of the macula. The fovea is responsible for sharp central vision.

Frank-Starling law of the heart: Intrinsic control of the heart such that increasing the end diastolic volume, that is, increased venous return, results in a greater stroke volume.

Freemartin: A sterile heifer born twin to a bull. It has incomplete development of the reproductive tract and malelike behavior.

Frontal lobe: The largest portion of each cerebral hemisphere, anterior to the central sulcus.

Frontal plane: A sectional plane that divides the body into an anterior portion and a posterior portion; also called the coronal plane.

Frontonasal suture: Separates the frontal bones from the nasal bones.

Frontoparietal suture: A suture extending across the skull between the parietal and frontal bones.

Fulcrum: An anatomical structure that acts as a hinge or point of support.

Fully saturated: A fatty acid molecule with no double bonds between any of its carbon atoms.

Functional syncytium: A group of smooth or cardiac muscle cells that are interconnected by gap junc­tions and function electrically and mechanically as a single unit.

Fundus: The base of an organ.

Fungiform papillae: Mushroom-shaped papillae (projections) on the tongue. They are located on the top surface of the tongue, toward the back. They have taste buds on their surface that can distinguish the four tastes: sweet, sour, bitter, and salty.

Funiculi: One of three major divisions of white matter in the spinal cord, consisting of fasciculi.

G actin: Globular protein molecules that, when bound together, form fibrous actin (F actin).

G protein: A generic term for a member of a large family of guanine triphosphate (GTP)-binding pro­teins that are important elements in cell signaling. The binding of a hormone or other signaling ligand in this family activates these cellular proteins, many of which act as kinases.

Galactopoiesis: This term refers to the maintenance of continuation of an established lactation. For example, secretion of growth hormone and prolac­tin are believed to be essential for maintaining lacta­tion in most species, so these hormones would be classified as galactopoietic hormones.

Gamma-aminobutryic acid (GABA): A neurotrans­mitter of the central nervous system whose effects are generally inhibitory.

Gamma motor neurons: Regulates the gain of the stretch reflex by adjusting the level of tension in the intrafusal muscle fibers of the muscle spindle. This mechanism sets the baseline level of activity in a motor neurons and helps to regulate muscle length and tone.

Ganglia: A tissue mass that contains the dendrites and cell bodies (or "somata") of nerve cells—in most cases, ones belonging to the peripheral nervous system. Within the central nervous system such a mass is often called a nucleus. An interconnected group of ganglia is called a plexus.

Ganglion cell: A type of neuron located in the retina of the eye that receives visual information from pho­toreceptors via various intermediate cells such as bipolar cells, amacrine cells, and horizontal cells.

Gap junction: A gap between adjacent cell mem­branes containing very fine, Iatticelike connections that allows physiologic components to pass directly from cell to cell; also called a nexus.

Gastric glands: The tubular glands of the stomach whose cells produce acid, enzymes, intrinsic factor, and hormones.

Gastric inhibitory peptide (GIP): A duodenal hormone released when the arriving chyme con­tains large quantities of carbohydrates; triggers the secretion of insulin and a slowdown in gastric activity.

Gastric pits: Small pit in the mucous membrane of the stomach at the bottom of which are the mouths of the gastric glands that secrete mucus, hydrochloric acid, intrinsic factor, pepsinogen, and hormones.

Gastrin: A hormone produced by enteroendocrine cells of the stomach, after exposure to mechanical stimuli or stimulation of the vagus nerve and of the duodenum, after exposure to chyme that contains undigested proteins.

Gastroileal reflex: Peristaltic movements that shift materials from the ileum to the colon; triggered by the arrival of food in the stomach.

Gastrointestinal tract: Also referred to as the GI tract, the alimentary canal, (nourishment canal) or the gut, it is the system of organs within multicel­lular animals. It takes in food, digests it to extract energy and nutrients, and expels the remaining waste.

Generator potential: A small depolarization pro­duced by neurotransmitter binding or activation of a sensory receptor in the nervous system.

Germ layers: The ectoderm, mesoderm, and endo­derm, which are the earliest recognizable tissues in the developing embryo.

Gestation: The act or faculty of tasting.

Gingivae: The gums.

Gizzard: An adapted stomach that is found in birds, earthworms, and other animals. It has a thick, mus­cular wall, enabling powerful grinding action.

Gland of the third eyelid: A gland also known as a nictitating membrane, which can move across the eyeball to give the sensitive eye structures addi­tional protection in particular circumstances.

Glaucoma: A group of diseases of the optic nerve involving loss of retinal ganglion cells in a charac­teristic pattern of optic neuropathy.

Glenoid cavity: The hollow in the head of the scapula into which the head of the humerus sits to make the shoulder joint; also called the glenoid fossa.

Globus pallidus: The inner and lighter gray portion of the Ientiform nucleus of the brain; also called the pallidum.

Glottis: The passageway from the pharynx to the larynx. Glucose: A six-carbon (hexose) sugar that is funda­mental to the metabolism of cells. It is stored as a polymer in animal cells as glycogen.

Glucuronic acid: A carboxylic acid that has the struc­ture of a glucose molecule that has had its sixth carbon atom (of six total) oxidized. Its formula is CeHioO?-

Glutamate: A salt of glutamic acid.

Glycerol: A simple three-carbon polyalcohol that is often derived from the catabolism of glucose. Among other functions, it serves as a precursor in the formation of mono-, di-, or triglycerides.

Glycine: A nonessential amino acid derived from the alkaline hydrolysis of gelatin and used as a nutrient and dietary supplement; also used in biochemical research and in the treatment of certain myopathies.

Glycogen: A storage form of glucose; a polysaccha­ride composed of repeating glucose units. Granules of glycogen are found in liver and muscle cells.

Glycolytic: The metabolic breakdown of glucose and other sugars that releases energy in the form of ATP.

Glycoprotein: A protein that has one or more cova­lently linked oligosaccharide chains. These proteins are common as integral proteins of the outer surface of the plasma membrane of cells.

Goblet cell: A goblet-shaped, mucus-producing, uni­cellular gland in certain epithelia of the digestive and respiratory tracts.

Golgi or Golgi apparatus: The cellular organelle linked with packaging and processing (phosphory­lation, glycosylation, etc.) of proteins destined to be secreted from the cell. Stacks of Golgi membranes and associated secretory vesicles are abundant in the apical cytoplasm of fully differentiated secre­tory epithelial cells, for example, mammary, pancre­atic, etc.

Golgi tendon receptors: Proprioceptors located in tendons, close to the point of skeletal muscle inser­tion; important to smooth onset and termination of muscle contraction.

Golgi type I neurons: A nerve cell having a long axon that leaves the gray matter of the central nervous system, of which it forms a part.

Golgi type II neurons: A nerve cell having a short axon that ramifies in the gray matter of the central nervous system.

Gonadotropin-releasing hormone (GnRH): A deca­peptide hormone produced by hypothalamic neurons. Release into the portal blood supplying the anterior pituitary causes secretion of gonatotropins (follicle-stimulating hormone, FSH, and luteinizing hormone, LH).

Granulocytes: White blood cells containing granules that are visible with a light microscope; includes eosinophils, basophils, and neutrophils; also called granular leukocytes.

Gray matter: Brownish-gray nerve tissue, especially of the brain and spinal cord, composed of nerve cell bodies and their dendrites and some support­ive tissue; also called gray substance or substantia grisea.

Gray ramus: A bundle of postganglionic sympathetic nerve fibers that are distributed to effectors in the body wall, skin, and limbs by way of a spinal nerve.

Greater curvature: The larger, longer outside dimen­sion of the stomach.

Greater omentum: A large fold of the dorsal mes­entery of the stomach; hangs anterior to the intestines.

Growth factors: Small peptides that act by an auto­crine, a paracrine, or a classic endocrine loop to stimulate or inhibit growth and development of tissues or cells.

GTP (guanosine triphosphate): A critical nucleoside involved in reactions for synthesis of RNA, protein synthesis, and cell signaling.

Gyri: Any of the prominent, rounded, elevated con­volutions on the surfaces of the cerebral hemi­spheres (singular = gyrus).

H zone: Area in the center of the A band in which there are no actin myofilaments; contains only myosin.

Habenula: A circumscriptive cell mass embedded in the posterior end of the medullary stria of the thala­mus, from which it receives most of its afferent fibers; also called the habena, pedunculus of pineal body (or gland).

Hard palate: The bony roof of the oral cavity, formed by the maxillary and palatine bones.

Haustra: Saclike pouches along the length of the large intestine that result from tension in the tenia coli.

Haversian system: A central canal and the concentric osseous lamellae encircling it, occurring in compact bone.

Hearing: The sense by which sound is perceived.

Heat shock proteins: Also called stress-response pro­teins, these are produced by cells in response to elevated temperatures or other stressors and are believed to help the cell survive.

Helicotrema: The part of the cochlear labyrinth where the scala tympani and the scala vestibuli meet. It is also known as the cochlear apex.

Hematocrit: The percentage of the volume of whole blood contributed by cells; also called the volume of packed red cells (VPRC) or packed cell volume (PCV).

Hematoma: A tumor or swelling filled with blood.

Hematopoiesis: Blood cell formation; also called hemopoiesis.

Hematopoietic stem cells: Undifferentiated cells, usually located in the bone marrow, that are capable of dividing and producing daughter cells, which become various blood system components.

Hemopoiesis: Blood cell formation and differentiation. Henry's Law: Named for the English chemist William Henry. The concentration of a gas dissolved in a liquid is equal to the partial pressure of the gas over the liquid multiplied by the solubility coefficient of the gas.

Hepatic artery: An artery that distributes blood to the liver, pancreas, and gallbladder, as well as to the stomach and duodenal portion of the small intestine.

Hepatic growth factor (HGF): A heparin-binding protein that has also been referred to as hematopoi­etin or scatter factor. It has proliferation and differ­entiation effects on multiple cell types, including liver, muscle, and mammary cells.

Hepatic portal vein: The vessel that carries blood between the intestinal capillaries and the sinusoids of the liver.

Hepatocyte: A liver cell.

Hepatoduodenal ligament: The portion of the lesser omentum extending between the liver and duodenum.

Hepatopancreatic ampulla: Commonly called the Ampulla of Vater, it is formed by the union of the pancreatic duct and the bile duct.

Hepatopancreatic sphincter: Controls secretions from the liver, pancreas, and gallbladder into the duode­num of the small intestine. It is a sphincter muscle located at the surface of the duodenum.

Heterotopic: Ectopic, positioned outside the normal location.

Hiatal hernia: The protrusion (or hernia) of the upper part of the stomach into the thorax through a tear or weakness in the diaphragm.

Hilum/hilus: A localized region where blood vessels, lymphatic vessels, nerves, and/or other anatomical structures are attached to an organ.

Hippocampus: The complex, internally convoluted structure that forms the medial margin of the corti­cal mantle of the cerebral hemisphere, borders the choroid fissure of the lateral ventricle, is composed of two gyri with their white matter, and forms part of the limbic system.

Histamine: The chemical released by stimulated mast cells or basophils to initiate or enhance an inflam­matory response.

Histology: The study of tissues.

Histones: A group of proteins, high in arginine and lysine, that are closely associated with DNA in the chromosomes.

Holocrine glands: Glands that contain secretory cells that accumulate their products; then the cells are sloughed and disrupted to create the secretion from the glands—for example, the crop sac glands of some birds.

Homeorrhresis: This indicates a state of adjusted or altered physiology to support a particular activity or function (e.g., lactation or reproduction).

Homeostasis: The maintenance of a relatively con­stant internal environment.

Horizontal cells: The laterally interconnecting neurons in the outer plexiform layer of the retina. There are three basic types—HI, HII, and HIII—and all three are multipolar cells.

Hyaline cartilage: Semitransparent opalescent carti­lage that forms most of the fetal skeleton and that consists of cells that synthesize a surrounding matrix of hyaluronic acid, collagen, and protein; in the adult, it is found in the trachea, larynx, and joint surfaces.

Hyaline cartilage rings: C-shaped bands of cartilage that provide the rigid structure apparent in the trachea.

Hyaloid canal: A branch of the ophthalmic artery, which is itself a branch of the carotid artery. It is contained within the optic stalk of the eye and extends through the vitreous humor to the lens.

Hydrocephalus: A usually congenital condition in which an abnormal accumulation of fluid in the cerebral ventricles causes enlargement of the skull and compression of the brain, destroying much of the neural tissue.

Hydrochloric acid (HCl): The aqueous (water-based) solution of hydrogen chloride (HCl) gas. It is a strong acid, the major component of gastric acid, and of wide industrial use.

Hydrophilic: Refers to molecules that readily associ­ate or dissolve in water—for example, ethanol.

Hydrophobic: Refers to molecules that do not readily dissolve in water—for example, oil and water.

Hyperalgesia: An extreme sensitivity to pain, which in one form is caused by damage to nociceptors in the body's soft tissues.

Hypercapnia: High plasma carbon dioxide concen­trations, commonly as a result of hypoventilation or inadequate tissue perfusion.

Hyperopia: Also known as hypermetropia or collo­quially as farsightedness or longsightedness, it is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short or when the lens cannot become round enough), causing inability to focus on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance.

Hyperpolarization: An increase in membrane poten­tial from resting potential; potential becomes even more negative than at resting potential.

Hypertonia: Extreme tension of the muscles or arteries.

Hypertonic: Refers to any fluid with a high enough concentration of solutes to cause water to move out of the cell (cell shrinkage) because of osmosis.

Hypocapnia: A low plasma PCO₂ concentration com­monly as a result of hyperventilation.

Hypothalamo-hypophyseal portal system: A unique circulatory arrangement that allows small quanti­ties of hypothalamic releasing hormones to flow directly down the pituitary stalk to the anterior pituitary without being diluted in the general circu­lation, thus making these hormones much more potent.

Hypotonic: Refers to any fluid with a low enough concentration of solutes to cause water to move into the cell (cell swelling and possible rupture) because of osmosis.

Hypovolemic shock: The most common form of shock; results from extreme blood loss.

I band: A pale band of actin on each side of the Z line of a striated muscle fiber.

Ileocecal valve: A fold of mucous membrane that guards the connection between the ileum and the cecum.

Ileum: The distal 2.5 m of the small intestine.

Immunity: Resistance to injuries and diseases caused by foreign compounds, toxins, or pathogens.

Incisive: Having the power to cut.

Incisors: The first kind of tooth in heterodont mammals. Incomplete break: The break goes only partway through the bone. An incomplete fracture is also known as a greenstick fracture.

Incomplete tetanus: Myograph recording, or situation in which individual muscle twitches are apparent.

Incus: The anvil-shaped small bone or ossicle in the middle ear. It connects the malleus to the stapes. The incus exists only in mammals, and is derived from a reptilian upper jawbone, the quadrate bone.

Inferior colliculus: The structure in the brain that lies caudal to its counterpart, the superior colliculus, above the trochlear nerve and at the base of the projection of the medial geniculate nucleus (MGN) and the lateral geniculate nucleus (LGN).

Inferior oblique muscle: A thin, narrow muscle, placed near the anterior margin of the floor of the orbit.

Inferior rectus muscles: A muscle in the orbit that depresses, adducts, and rotates the eye laterally.

Inferior sagittal sinus: An unpaired dural sinus in the lower margin of the falx cerebri.

Inferior vena cava: The vein that carries blood from the parts of the body inferior to the heart to the right atrium.

Infraspinous fossa: The hollow on the dorsal aspect of the scapula inferior to the spine, giving attach­ment chiefly to the infraspinatus muscle.

Infundibulum: The funnel-shaped, unpaired promi­nence of the base of the hypothalamus behind the optic chiasm, continuous below the stalk of the pitu­itary gland. The funnellike opening of the oviduct is also called the infundibulum.

Ingestion: The introduction of materials into the digestive tract by way of the mouth.

Inhibin: A hormone, produced by Sertoli cells in the male and granulosa cells in the female, that acts to inhibit secretion of FSH.

Inhibitory postsynaptic potential (IPSP): Hyperpo­larization in the postsynaptic membrane that causes the membrane potential to move away from the threshold.

Innate reflexes: An automatic, instinctive, unlearned reaction to a stimulus.

Inner ear: Structure comprising both the organ of hearing (the cochlea) and the labyrinth or vestibular apparatus, which is the organ of balance located in the inner ear consisting of three semicircular canals and the vestibule.

Inner hair cells: Bulbous cells that are medially placed in one row in the organ of Corti. In contrast to the outer hair cells, the inner hair cells are fewer in number, have fewer sensory hairs, and are less differentiated.

Inner synaptic layers: Where inner nuclear layer cells contact ganglion cells.

Insertion: The point or mode of attachment of a skel­etal muscle to the bone or other body part that it moves.

Inspiratory areas: Nuclei located in the respiratory center of the brain stem that are involved in control of inspiration.

Inspiratory reserve volume: The maximum amount of air that can be drawn into the lungs over and above the normal tidal volume.

Insula: An oval region of the cerebral cortex, lateral to the Ientiform nucleus, and buried in the fissure of Sylvius.

Insulin: The protein hormone produced by the β cells of the endocrine pancreas (Islets of Langerhans). When blood glucose is elevated it is secreted, which (in normal circumstances) stimulates the uptake of glucose to maintain normal blood glucose concentration.

Insulin receptor substrate (IRS): These proteins are intracellular mediators of IGF-I and insulin action. Binding insulin or IGF-I to their cell surface recep­tors causes autophosphorylation or the receptor and creation of docking sites for IRS family members. When IRS docks, this allows further interactions, a cascade of other signaling molecules, including the p85 substrate of PI3K.

Insulin-Iike growth factor I (IGF-I): A small growth factor (~7.4 kD) that appears in circulation largely in response to growth hormone stimulation of the liver. However, it is also locally produced in the stromal tissue of a number of organs. It is a potent stimulator of cell proliferation and is involved in prevention of apoptosis.

Insulin-Iike growth factor binding proteins (IGFBPs): These proteins comprise a family of at least six well-characterized members and several relatives that can bind IGF-L They are believed to modulate the biological effectiveness of IGF-L IGFBP-2, 3, and 4 are evident in serum, but IGFBPs are also produced within various tissues.

Integration: The process by which the nervous system processes and interprets sensory input and makes decisions about what should be done at each moment.

Integrins: Family of transmembrane proteins that are important in adhesion of cells to the extracellular matrix and in cell signaling.

Interatrial septum: Wall between the atria of the heart.

Intercalated discs: Regions where adjacent cardio- cytes interlock and where gap junctions permit elec­trical coupling between the cells.

Interleukins: Group of growth factor-like proteins produced primarily by immune tissues that regu­late growth and activation of various immune cells.

Intermediate mass: A type of mesoderm that is lo­cated between the paraxial mesoderm and the lateral plate.

Internal capsule: A layer of white matter separating the caudate nucleus and thalamus from the Ienti- form nucleus and serving as the major route by which the cerebral cortex is connected with the brain stem and the spinal cord.

Internal nares: The entrance from the nasal cavity to the nasopharynx.

Internal respiration: The diffusion of gases between interstitial fluid and cytoplasm.

Internasal suture: The line of union between the two nasal bones.

Interneurons: Association neurons; central nervous system neurons that are between sensory and motor neurons.

Interoceptors: A specialized sensory nerve receptor that receives and responds to stimuli originating from within the body.

Interphase: The usually prolonged period of a cell cycle between the end of one mitosis and the next; this includes G₁, S, and G₂ phases.

Interstitial growth: A form of cartilage growth through the growth, mitosis, and secretion of chon­drocytes in the matrix.

Interventricular foramen of Monroe: Channels that connect the paired lateral ventricles with the third ventricle at the midline of the brain. As channels, they allow cerebrospinal fluid (CSF) pro­duced in the lateral ventricles to reach the third ventricle and then the rest of the brain's ventricular system.

Interventricular septum: The wall between the ven­tricles of the heart.

Intervertebral foramen: Any of the openings into the vertebral canal bounded by the pedicles of adjacent vertebrae above and below, the vertebral bodies in front, and the articular processes behind.

Intestinal glands or crypts of Lieberkiihn: Glands found in the epithelial lining of the small intestine. Named for the 18th-century German anatomist Johann Nathanael Lieberkuhn, the crypts secrete various enzymes, including sucrase and maltase.

Intestinal villi: Multicellular projections from the wall of the small intestine that protrude into the lumen of the intestine. They are covered with absorptive epithelial cells.

Intrafusal muscle fibers: Muscle fibers that comprise the muscle spindle. They are fibers walled off from the rest of the muscle by a collagen sheath. This sheath has a spindle or fusiform. Although the intra­fusal fibers are wrapped with sensor receptors, their counterparts, extrafusal muscle fibers, are respon­sible for the power-generating component of muscle and are innervated by motor neurons.

Intramembranous ossification: The formation of bone within a connective tissue without the prior devel­opment of a cartilaginous model.

Intraocular pressure: The fluid pressure inside the eye. It may become elevated due to anatomical prob­lems, inflammation of the eye, genetic factors, or as a side effect from medication.

Intrapulmonary primary bronchus: The section between the pulmonary hilus and the ostium of the abdominal air sac.

Intrinsic factor: A glycoprotein, secreted by the pari­etal cells of the stomach, that facilitates the intes­tinal absorption of vitamin B₁₂.

Intrinsic muscles: Muscles located within the struc­ture being moved.

Intron: The noncoding segments of DNA that are transcribed in the production of mRNA but then excised by RNA-splicing enzymes before the mature mRNA exits the nucleus for protein synthesis.

Inversion: A chromosomal defect in which a segment of the chromosome breaks off and reattaches in the reverse direction.

Involuntary striated muscle: Also called cardiac muscle. Ionotropic receptors: A group of intrinsic transmem­brane ion channels that are opened in response to binding of a chemical messenger, as opposed to voltage-gated ion channels or stretch-activated ion channels.

IP₃: Inositol trisphosphate, a small molecule produced by the breakdown of the inositol phospholipid PIP₂ following stimulation by hormone binding. IP₃ acts as a second messenger by releasing Ca⁺⁺ from storage in RER.

Iris: The most visible part of the eye of vertebrates.

Irregular bones: Any of a group of bones having peculiar or complex forms, such as the vertebrae.

Isometric: Of or involving muscular contraction against resistance in which the length of the muscle remains the same.

Isotonic: Of or involving muscular contraction in which the muscle remains under relatively constant tension while its length changes.

Isotropic: Identical in all directions; invariant with respect to direction.

Isovolumetric contraction phase: The period of cardiac contraction before the pressure inside the heart chamber is sufficiently elevated to open the heart valves and induce blood flow.

Janus protein tyrosine kinases (JAKS): Cellular kinases that become activated when certain cyto­kine family members (e.g., prolactin and growth hormone) bind to their surface receptors on target cells. They are involved in signal transduction.

Jejunum: The middle part of the small intestine.

K-complex: An EEG waveform that occurs during stage 2 sleep. It consists of a brief high-voltage peak, usually greater than IOO μV, and lasts for longer than 0.5 second. K-complexes occur randomly through­out stage 2 sleep, but may also occur in response to auditory stimuli.

Kinocilium: A special structure connected to the hair cells of the inner ear's cochlea of amphibia.

Kupffer cells: Stellate reticular cells of the liver; phagocytic cells of the liver sinusoids.

Labyrinth: A system of fluid passages in the inner ear. Lacrimal canals: The small channels in each eyelid that commence at minute orifices, puncta lacrimalia, on the summits of the papillae lacrimales, seen on the margins of the lids at the lateral extremity of the lacus lacrimalis.

Lacrimal glands: Paired glands, one for each eye, that secrete lacrimal fluid. Each gland is about the size of an almond (2 cm) and sits alongside the eyeball within the orbit, nestled in the lacrimal fossa of the frontal bone.

Lacrimal sac: The upper dilated end of the naso­lacrimal duct, lodged in a deep groove formed by the lacrimal bone and frontal process of the maxilla. OC-Lactalbumin: A specific milk protein synthesized and secreted by the alveolar epithelial cells. The protein is also part of the lactose synthetase enzyme. It appears in the whey fraction of milk.

Lactase: A member of the β-galactosidase family of enzymes, it is involved in the hydrolysis of the disaccharide lactose into constituent galactose and glucose monomers.

Lacteal: A terminal lymphatic vessel within an intes­tinal villus.

Lactogenesis: The onset of lactation that occurs near the time of parturition. It occurs in two phases, with limited structural and functional differentia­tion of the mammary alveolar cells after Iobulo- alveolar development during gestation, followed by dramatic differentiation and copious milk secre­tion within hours or days of parturition. Hormones (e.g., prolactin or glucocorticoids) that promote this process are called lactogenic hormones.

Lacuna: A small pit or cavity.

Lamina propria: The reticular tissue that underlies a mucous epithelium and forms part of a mucous membrane.

Laryngopharynx: The division of the pharynx that is inferior to the epiglottis and superior to the esophagus.

Lateral geniculate: The part of the brain that is the primary processor of visual information, received from the retina, in the central nervous system.

Lateral rectus muscle: A muscle in the orbit. It is one of six extraocular muscles that control the move­ments of the eye and the only muscle innervated by the abducens nerve, cranial nerve VI.

Lateral sulcus (lateral fissure, Sylvian fissure): The deepest and most prominent of the cortical fissures of the brain, extending between frontal and tempo­ral lobes and then back and slightly upward over the lateral aspect of the cerebral hemisphere.

Leak channels: Resting channels that allow ions to cross the membrane down their electrochemical gradient, whether or not the cell is depolarized.

Left colic flexure: A bend in the colon, known as the "splenic flexure," that is near the spleen.

Lens fiber: Epithelial cell that makes up the lens of the eye.

Lentiform nucleus: The large, cone-shaped mass of gray matter that forms the central core of the cere­bral hemisphere, whose convex base is formed by the putamen and whose apical part consists of the globus pallidus; also called the lenticular nucleus.

Lesser curvature: The inside curve of the stomach, found opposite the greater curve located on the outer edge.

Lesser omentum: A small pocket in the mesentery that connects the lesser curvature of the stomach to the liver.

Leukocyte: A white blood cell.

Leukopoiesis (or Ieucopoiesis): The production of white blood cells.

Leydig cells: Cells in the interstitial tissue of the testis that secrete testosterone.

Ligamentum arteriosum: The fibrous strand in adults that is the remnant of the ductus arteriosus of the fetal stage.

Ligand: Any molecule that binds to a specific site on a protein; term often used in reference to hormones or growth factors binding to their receptors.

Ligand-gated ion channel: Also referred to as LGICs, or ionotropic receptors, a group of intrinsic trans­membrane ion channels that are opened in response to binding of a chemical messenger, as opposed to voltage-gated ion channels or stretch-activated ion channels.

Light adaptation: Ability of the eye to adjust to various levels of darkness and light.

Linear fracture: A fracture that runs parallel to the long axis of a bone; also called a fissured fracture.

Lingual frenulum: An epithelial fold that attaches the inferior surface of the tongue to the floor of the mouth.

Lobar arteries: Arteries that pass between lobes of the kidney.

Lobar (secondary) bronchi: Branch from a primary bronchus that conducts air to each lobe of the lungs. There are two branches in the left lung and three branches from the primary bronchus in the right lung.

Lobulo-alveolar: A developmental term that indi­cates a structural grouping of several alveoli, their terminal ducts and related common ducts, and surrounding supporting connective tissue.

Long bone: One of the elongated bones of the ex­tremities, consisting of a tubular shaft, which is composed of compact bone surrounding a central marrow-filled cavity and two expanded portions that usually serve as articulation points.

Longitudinal pillars: Supporting layers in the rumen. They provide some compartmentalization as well as mixing.

Long-term potentiation: The long-lasting enhance­ment in efficacy of the synapse between two neurons.

Lower respiratory system: A system composed of the larynx, trachea, and lungs.

Lung compliance: A static measure of lung and chest recoil, expressed as a change in lung volume per unit change in airway pressure, for example, L/ cm H₂O.

Luteal phase: The phase of the estrus cycle character­ized by major production of progesterone and the presence of a functional corpus luteum.

Luteinization: The process by which granulosal and thecal cells are transformed into luteal cells follow­ing ovulation.

Luteinizing hormone (LH): A hormone produced in the anterior pituitary gland. It causes ovulation and development of the CL in females and stimulates Leydig cells to secrete testosterone in males.

Lymphocyte: A cell of the lymphatic system that par­ticipates in the immune response.

Lysosome: A membrane-bound organelle that con­tains hydrolytic enzymes. These organelles are activated to destroy damaged cells (apoptotic actions) and are important in the action of neutrophils.

M-line: A fine dark band in the center of the H band in the myofibrils of striated muscle fibers; also called the M band.

M phase: Period of the cell cycle during which the nucleus and cytoplasm of the cell divide.

Macrophage: A protective cell type common in con­nective tissue, lymphatic tissue, and certain body organs that phagocytizes tissue cells, bacteria, and other foreign debris. It is important as an antigen presenter to T cells and B cells in the immune response.

Macula lutea: An oval yellow spot near the center of the retina. It has a diameter of about 1.5 mm and is often histologically defined as having two or more layers of ganglion cells.

Magnocellular: Cells in the brain concerned pri­marily with visual perception. In particular, these cells are responsible for resolving motion and coarse outlines.

Malleus: A hammer-shaped small bone or ossicle of the middle ear that connects with the incus and is attached to the inner surface of the eardrum.

Maltase: An enzyme produced by the cells lining the small intestine to break down disaccharides.

Mammillary bodies: A pair of small, round bodies, resembling two breasts, located in the brain and forming part of the limbic system. They are located at the ends of the anterior arches of the fornix.

Mammogenic: Substances that stimulate mammary growth and development. For example, the ovarian hormone estrogen is a classic example of a mam- mogenic hormone.

Manubrium: The upper segment of the sternum with which the clavicle and the first two pairs of ribs articulate.

Margination: The arrangement of neutrophils along an endothelial cell border prior to diapedesis into a tissue area.

Mass peristalsis: A powerful peristaltic contraction that moves fecal materials along the colon and into the rectum.

Maxillary: Of or relating to a jaw or jawbone, espe­cially the upper one.

Maxillary sinus: One of the paranasal sinuses. It is an air-filled chamber lined by a respiratory epithelium that is located in a maxillary bone and opens into the nasal cavity.

Mean arterial pressure (MAP): The average pressure responsible for driving blood forward through the arteries into the tissues throughout the cardiac cycle; it equals cardiac output times total peripheral resistance.

Meatus: An opening or entrance into a passageway.

Mechanical nociceptors: Receptors that respond to excess pressure or mechanical deformation.

Mechanoreceptors: A specialized sensory end organ that responds to mechanical stimuli such as tension or pressure.

Medial forebrain bundle: A fiber system running longitudinally through the lateral zone of the hypothalamus, connecting it with the midbrain teg­mentum and various components of the limbic system.

Medial geniculate: A nucleus of the thalamus that acts as a relay for auditory information. It receives its input from the inferior colliculus and sends infor­mation out to the auditory cortex.

Medial rectus muscle: A muscle in the orbit. As with most of the muscles of the orbit, it is innervated by the inferior division of the oculomotor nerve (cranial nerve III).

Median: Of, relating to, or situated in or near the plane that divides a bilaterally symmetrical animal into right and left halves; mesial.

Mediastinum: The central tissue mass that divides the thoracic cavity into two pleural cavities; it includes the aorta and other great vessels, the esophagus, trachea, thymus, pericardial cavity, and heart, and a host of nerves, small vessels, and lym­phatic vessels. In males, the area of connective tissue attaching a testis to the epididymis, proximal portion of ductus deferens, and associated vessels.

Medullary cavity: The space within a bone that con­tains the marrow.

Medullary rhythmicity area: The center in the medulla oblongata that sets the background pace of respiration; it includes inspiratory and expiratory centers.

Meibomian glands: A special kind of sebaceous glands at the rim of the eyelids, responsible for the supply of sebum, an oily substance that prevents evaporation of the eye's tear film; also called the tarsal glands.

Meissner's corpuscles: A type of mechanoreceptor and, more specifically, a tactile corpuscle (corpuscu­lum tactus). They are distributed throughout the skin, but concentrated in areas especially sensitive to light touch, such as the fingertips, palms, soles, lips, tongue, face, nipples, and the external skin of the male and female genitals. They are primarily Iocatedjustbeneath the epidermis within the dermal papillae.

Meissner's plexus (submucosa): A sensory network formed by nerve branches that have perforated cir­cular muscular fibers of the small intestine. This plexus lies in the submucosa of the intestine. It also contains ganglia from which nerve fibers pass to the muscularis mucosae and to the mucous membrane.

Melatonin: A hormone derived from serotonin and produced by the pineal gland that stimulates color change in the epidermis of amphibians and reptiles and that is believed to influence estrus in mammals.

Membranous labyrinth: A system of fluid passages in the inner ear, comprising the vestibular system and the auditory system, which provides the sense of balance.

Meningeal layers: The layers of protective tissue sur­rounding the central nervous system.

Meninges: A membrane, especially one of the three membranes enclosing the brain and spinal cord.

Merkel's discs: Mechanoreceptors found in the skin and mucosa of vertebrates that provide touch infor­mation to the brain. Each ending consists of a Merkel cell in close apposition with an enlarged nerve terminal.

Mesaticephalic: The ratio of the maximum width of the head to its maximum length, multiplied by 100.

Mesencephalon: The portion of the vertebrate brain that develops from the middle section of the embry­onic brain; also called the midbrain.

Mesenchyme: Refers to the tissue or cells derived from the embryonic mesoderm. In developing glands—mammary, for example—the stromal tissue that surrounds the developing epithelial ducts con­tains precursor cells capable of being induced to dif­ferentiate into one of several different stromal tissue cell types (i.e., endothelial, fibroblast, adipocyte).

Mesentery: A double layer of serous membrane that supports and stabilizes the position of an organ in the abdominopelvic cavity and provides a route for the associated blood vessels, nerves, and lymphatic vessels.

Mesothelium: A simple squamous epithelium that lines one of the divisions of the ventral body cavity.

Metabotropic receptors: Receptors indirectly linked with ion channels on the plasma membrane of the cell through signal transduction mechanisms.

Metaphysis: The zone of growth between the epiphy­sis and diaphysis during development of a bone.

Metarteriole: A vessel that connects an arteriole to a venule and that provides blood to a capillary plexus.

Metestrus: A stage of the estrous cycle between ovu­lation and formation of the corpus luteum.

Microtubules: Hollow tubes composed of tubulin, measuring approximately 25 nm in diameter and usually several micrometers long. They help provide support to the cytoplasm of the cell and are a com­ponent of certain cell organelles, such as centrioles, spindle fibers, cilia, and flagella.

Middle ear: The portion of the ear internal to the eardrum and external to the oval window of the cochlea. The middle ear contains three ossicles, which amplify vibration of the eardrum into pres­sure waves in the fluid in the inner ear.

Midsagittal plane: A plane passing through the midline of the body that divides it into left and right halves.

Minor duodenal papilla: Site of the opening of the accessory pancreatic duct into the duodenum.

Mitogen-activated protein kinase (MAPK): Aprotein kinase that performs an essential step in relaying signals for the plasma membrane to the cell nucleus. It is activated by a variety of proliferation or dif­ferentiation signals from outside target cells.

Mitral valve: See bicuspid valve.

Mixed nerve: A nerve that contains both sensory and motor fibers.

Modiolus: The central bony pillar of the cochlea, but also the muscle of facial expression found near the risorus muscle, between the lateral edge of the orbi­cularis oris, and the insertion of the zygomatic major muscle.

Molars: The rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food, hence the name, which means "millstone."

Monoamine oxidase (MAO): An enzyme in the cells of most tissues that catalyzes the oxidative deami­nation of monoamines such as serotonin.

Monocytes: Phagocytic agranulocytes (white blood cells) in the circulating blood.

Monoglyceride: A lipid molecule consisting of a single fatty acid bound to a molecule of glycerol.

Monosaccharide: A simple sugar with the general formula (CH₂O)_x, where x = 3 to 7; examples are glucose and ribose.

Monosynaptic reflex: A reflex that provides auto­matic regulation of skeletal muscle length.

Monotocous: Animals that typically give birth to single offspring.

Morula: A stage of early embryonic development while the egg is still within the confines of the zona pellucida. It is characterized by the appearance of blastomeres from initial cleavage divisions.

Motor areas: The cortical area that influences motor movements.

Motor end plate: The flattened end of a motor neuron that transmits neural impulses to a muscle.

Motor nerve: A nerve that passes toward or to muscles or glands.

Motor neurons: Neurons that innervate skeletal, smooth, or cardiac muscle fibers.

Motor unit: A single somatic motor neuron and the group of muscle fibers innervated by it.

mRNA: Messenger ribonucleic acid (mRNA) speci­fies the amino acid sequence of a protein. In eukary­otes, it is derived from a larger precursor immature mRNA produced by RNA polymerase in the cell nucleus from a complementary strand of DNA. Pro­cessing of this immature RNA stand to remove sections corresponding with the noncoding introns yields the mRNA used for protein synthesis.

Mucosa: A mucous membrane; the epithelium plus the lamina propria.

Mucosa-associated lymphatic system (MALT): The extensive collection of lymphoid tissues linked with the digestive system.

Multiparous: A term indicating an animal that has had more than one pregnancy and birth.

Multipennate: A muscle whose internal fibers are organized around several tendons.

Multiple motor unit summation: Increased force of contraction of a muscle due to recruitment of motor units.

Multipolarneuron: One of three categories of neurons consisting of a neuron cell body, an axon, and two or more dendrites.

Multiunit smooth muscle: A smooth muscle mass that consists of multiple discrete units that func­tion independently of one another and that must be separately stimulated by autonomic nerves to contract.

Muscarinic: A highly toxic alkaloid, C₉H₂₀NO₂, related to the cholines, derived from the red form of the mushroom Amanita muscaria and found in decaying animal tissue.

Muscle fatigue: The decline in the ability of a muscle to create force; it can be caused by barriers or inter­ferences at many of the differing stages of muscle contraction, and it is primarily regulated by the reduction in the release of Ca2⁺ (calcium) ions from the sarcoplasmic reticulum, along with falling ATP levels.

Muscle fiber: A cylindrical multinucleate cell com­posed of myofibrils that contract when stimulated.

Muscle spindles: A stretch receptor in vertebrate muscle.

Muscle tone: The continuous and passive partial con­traction of muscles. It helps maintain posture.

Muscularis externa: Concentric layers of smooth muscle responsible for peristalsis.

Muscularis mucosae: The layer of smooth muscle beneath the lamina propria; it is responsible for moving the mucosal surface.

Myelin: An insulating sheath around an axon; it con­sists of multiple layers of neuroglial membrane. It significantly increases the impulse propagation rate along the axon.

Myelocele: Protrusion of the spinal cord in cases of spina bifida.

Myelomeningocele: Protrusion of the spinal mem­branes and spinal cord through a defect in the ver­tebral column; also called the meningomyelocele.

Myenteric (or Auerbach's) plexus: Parasympathetic motor neurons and sympathetic postganglionic fibers located between the circular and longitudinal layers of the muscularis externa located in the esophagus, stomach, and intestines.

Myoblasts: A primitive muscle cell having the poten­tial to develop into a muscle fiber; also called a sarcoblast.

Myocardium: The cardiac muscle tissue of the heart.

Myoepithelial cells: Specialized cells that form a network surrounding the mammary alveoli. In response to oxytocin, these cells contract to cause milk ejection.

Myofibrils: One of the threadlike longitudinal fibrils occurring in a skeletal or cardiac muscle fiber; also called a sarcostyle.

Myofilaments: Any of the Ultramicroscopic filaments, made up of actin and myosin, that are the structural units of a myofibril.

Myoglobin: An oxygen-binding pigment that is espe­cially common in slow skeletal muscle fibers and cardiac muscle cells.

Myogram: The tracing of muscular contractions made by a myograph.

Myomesin: A 185-kDa protein located in the M band of striated muscle where it interacts with myosin and titin, possibly connecting thick filaments with the third filament system.

Myoneural junction: The synaptic connection of the axon of a motor neuron with a muscle fiber.

Myopia: Sometimes called short-sightedness or near­sightedness, it is a refractive defect of the eye in which collimated light produces image focus in front of the retina when accommodation is relaxed.

Myosin: The most common protein in muscle cells, a globulin responsible for the elastic and contractile properties of muscle and combining with actin to form actomyosin.

Myosin light-chain kinase (MLCK): This protein is important in the mechanism of contraction in smooth muscle. Once there is an influx of calcium into the smooth muscle, either from the sarco­plasmic reticulum or, more importantly, from the extracellular space, contraction of smooth muscle fibers may begin. First, the calcium will bind to calmodulin. This binding will activate the MLCK, which will go on to phosphorylate the myosin light chains. This will enable the myosin light chains to bind to the actin filament so that contraction may start.

Myotatic reflex: Tonic contraction of the muscles in response to a stretching force, due to stimulation of muscle proprioceptors; also called the stretch reflex.

Nasal bones: Two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face, and they form, by their junction, the bridge of the nose. Each has two surfaces and four borders.

Nasal cavity: A chamber in the skull that is bounded by the internal and external nares.

Nasolacrimal duct: A duct that carries tears from the lacrimal sac into the nasal cavity.

Nasomaxillary suture: The suture uniting the nasal bone and the maxilla.

Nasopharynx: A region that is posterior to the inter­nal nares and superior to the soft palate and ends at the oropharynx.

Near point of vision: Closest point from the eye at which an object can be held without appearing blurred.

Nebulin: An actin-binding molecule, which is local­ized to the I band in skeletal muscle.

Necrosis: Death of cells or tissues through injury or disease, especially in a localized area of the body.

Negative chronotropic factors: Factors that act to slow normal heart rhythm, and therefore heart rate.

Negative feedback: A corrective mechanism that opposes or negates a variation from normal limits.

Neostigmine: A drug that inhibits acetylcholinester­ase, used in its bromide form orally and its methyl­sulfate form parenterally to treat myasthenia gravis. Nerve: Any of the cordlike bundles of nervous tissue made up of myelinated or unmyelinated nerve fibers and held together by a connective tissue sheath through which sensory stimuli and motor impulses pass between the brain or other parts of the central nervous system and the eyes, glands, muscles, and other parts of the body.

Nerve growth factor (NGF): First identified because of effects on the salivary gland, there are multiple members: NGF, neurotropin 1-6, etc. These peptides are needed for development of the sympathetic nervous system and play a role in follicular devel­opment in the ovary.

Nervous system: Fast-acting control system that trig­gers muscle contraction or gland secretion.

Net filtration pressure (NFP): The net difference in the hydrostatic and osmotic forces acting across the glomerular membrane that favors the filtration of protein-free plasma into Bowman's capsule.

Neural crest: A component of the ectoderm, this is one of several ridgelike clusters of cells found on either side of the neural tube in vertebrate embryos. It has been referred to as the fourth germ layer, due to its great importance.

Neural tube: A dorsal tubular structure in the verte­brate embryo formed by longitudinal folding of the neural plate and differentiating into the brain and spinal cord.

Neurocoel: The central canal and ventricles of the spinal cord and brain; the myelencephalic cavity.

Neuroglia: Cells in the nervous system other than the neurons; it includes astrocytes, ependymal cells, microglia, oligodendrocytes, satellite cells, and Schwann cells.

Neuromuscular junction: The junction between a nerve fiber and the muscle it supplies.

Neurons: A cell in neural tissue that is specialized for intercellular communication through (1) changes in membrane potential and (2) synaptic connections.

Neurotransmitter: A chemical compound released by one neuron to affect the transmembrane potential of another.

Neurotransmitter-gated channels: Cell membrane channels of neurons that are activated by the binding of neurotransmitters.

Neurotrophic factors: Secreted by cells in a neuron's target field, these act by prohibiting the neuron from apoptosis.

Neutrophil: A microphage that is very numerous and normally the first of the mobile phagocytic cells to arrive at an area of injury or infection.

Nicotinamide adenine dinucleotide (NAD): A mol­ecule that serves as a coenzyme for oxidative path­ways (glycolysis, Krebs cycle, electron transport chain). It serves to transfer electrons in oxidativer­eduction reactions and is derived from niacin.

Nicotinic: Of or relating to nicotine.

Nicotinic receptor: Ionotropic receptors that form ion channels in plasma membranes.

Nictitating membrane: A membrane that can move across the eyeball to give the sensitive eye struc­tures additional protection. It is often called a third eyelid or haw and may be referred to as the plica semilunaris.

Nitric oxide (NO): A recently identified local chemi­cal mediator released from endothelial cells and other tissues. It exerts multiple effects, ranging from local vasodilation to acting as a toxic agent against foreign invaders or as a neurotransmitter.

NMDA receptor: A brain receptor activated by the amino acid glutamate, which, when excessively stimulated, may cause cognitive defects in Alzheim­er's disease; also called the N-methyl-D-aspartate receptor.

Nociceptor: A sensory receptor that responds to pain.

Node of Ranvier: The area between adjacent neuroglia where the myelin covering of an axon is incomplete.

Noggin: A slang term that means "head."

Nondisplaced fracture: A simple crack in the bone that has not caused the bone to move from its normal anatomic position; also called a hairline fracture.

Nonstriated involuntary muscle: Another name for smooth muscle.

Noradrenaline: See norepinephrine.

Norepinephrine: A catecholamine neurotransmitter in the peripheral nervous system and central nervous system, it is released at most sympathetic neuromuscular and neuroglandular junctions, and a hormone secreted by the adrenal medulla; also called noradrenaline.

Nuclear bag fibers: Fibers that lie in the center of each intrafusal muscle fiber of a muscle spindle. Each has a large number of nuclei concentrated in bags, which cause excitation of both the primary and sec­ondary nerve fibers.

Nuclear chain fibers: Fibers numbering 3-9 per muscle spindle, which are half the size of the nuclear bag fibers. Their nuclei are aligned in a chain and they excite the secondary nerve. Nuclear chain fibers are static; nuclear bag fibers are dynamic.

Nucleosidases: Hydrolytic enzymes that catalyze the hydrolysis of a nucleotide into a nucleoside and a phosphate.

Nucleus cuneatus: A wedge-shaped nucleus in the closed part of the medulla oblongata. It contains cells that give rise to the cuneate tubercle, visible on the posterior aspect of the medulla.

Nucleus gracilis: The medial of the three nuclei of the dorsal spinal column, receiving dorsal root fibers conveying sensory innervation of the leg.

Nulliparous: A female that has not become pregnant.

Occipital bone: A bone at the lower and posterior part of the skull, consisting of basilar, condylar, and squamous sections. It encloses the foramen magnum.

Occipital lobe: The posterior lobe of each cerebral hemisphere, having the shape of a three-sided pyramid and containing the visual center of the brain.

Oestrous: British spelling of estrous.

Oestrus: British spelling of estrus (sexual receptivity or heat in the female).

Olfaction: The sense of smell.

Olfactory: Of, relating to, or connected with the sense of smell.

Olfactory (Bowman's) glands: Any of the tubular and often branched glands occurring beneath the olfactory epithelium of the nose.

Olfactory bulb: A structure of the vertebrate forebrain involved in olfaction, the perception of odors.

Olfactory cortex: The sensory system used for olfaction.

Olfactory receptors: A type of G protein-coupled receptor in olfactory receptor neurons. In verte­brates, the olfactory receptors are located in the olfactory epithelium.

Oligodendrocyte: Central nervous system neuroglia cell type that maintains cellular organization within the gray matter and provides a myelin sheath in areas of white matter.

Olivary nuclei: A smooth oval prominence of the ventrolateral surface of the medulla oblongata lateral to the pyramidal tract, corresponding to the olivary nucleus; also called the oliva or olive.

Omasoabomasal orifice: The opening between the omasum and abomasum of the ruminant stomach compartments.

Omasum: Also known as the "manyplies" because of its appearance like pages of a wet book, it is the third compartment of the stomach in ruminants. Though its functions have not been well studied, it appears to primarily aid in the absorption of water, magnesium, and fermentation acids.

Oncotic pressure: Pressure exerted by the vitreous humor of the eye.

Oocyte: The developing egg.

Opsin: A group of light-sensitive 35-55 kDa mem­brane-bound G protein-coupled receptors found in photoreceptor cells of the retina. They are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade.

Optic chiasm: A flattened quadrangular body that is the point of crossing of the fibers of the optic nerves; also called the optic decussation.

Optic chiasma: The part of the brain where the optic nerves partially cross. Specifically, the nerves connected to the right eye that associate the right visual field of the left eye and vice versa for the left eye.

Optic disc: The point in the eye where the optic nerve fibers leave the retina.

Optic radiation: A collection of axons from relay neurons in the lateral geniculate nucleus of the thal­amus carrying visual information to the visual cortex (also called the striate cortex) along the cal­carine fissure.

Optic vesicles: An evagination on either side of the embryonic forebrain from which the optic nerve and retina develop.

Ora serrata: The serrated junction between the retina and the ciliary body. This junction marks the transi­tion from the simple nonphotosensitive area of the retina to the complex, multilayered photosensitive region.

Oral: Pertaining to the mouth.

Organ physiology: The study of specific organs, for example, cardiac or ovarian.

Origin: The point of attachment of a muscle that remains relatively fixed during contraction of the muscle.

Oropharynx: The middle portion of the pharynx, bounded superiorly by the nasopharynx, ante­riorly by the oral cavity, and inferiorly by the Iaryngopharynx.

Osmosis: The movement of water across a selectively permeable membrane from one solution to another solution that contains a higher solute concentration.

Ossification: The formation of bone.

Ossification center: The site where bone begins to form in a specific bone or part of bone as a result of the accumulation of osteoblasts in the connective tissue.

Osteoblast: A cell that produces the fibers and matrix of bone.

Osteocyte: A bone cell responsible for the mainte­nance and turnover of the mineral content of the surrounding bone.

Osteogenesis: Formation and development of bony tissue; also called osteogeny.

Osteoid: The bone matrix, especially before calcification. Osteomalacia: A disease occurring primarily in adults that results from a deficiency in vitamin D or calcium and that is characterized by a softening of the bones with accompanying pain and weakness; also called adult rickets or late rickets.

Osteon: The basic histological unit of compact bone, consisting of Osteocytes organized around a central canal and separated by concentric lamellae.

Osteonal canal: Located at the center of osteons, this canal contains blood vessels, a nerve, and bone fluid.

Osteoprogenitor cells: A mesenchymal cell that differ­entiates into an osteoblast; also called a preosteoblast.

Otolithic membrane: A gelatinous membrane located in the vestibular apparatus of the inner ear, which plays an essential role in the brain's interpretation of equilibrium. Both the saccular macula and utricu­lar macula are covered by an otolithic membrane.

Outer ear: The external portion of the ear, which includes the eardrum.

Outer hair cells: Acoustical preamplifiers.

Outer synaptic layers: The location within the retina where connections are made between photorecep­tors and cells of the inner nuclear layer, and the nuclear layer cells contact ganglion cells.

Oxidative: A reaction in which the atoms in an element lose electrons and the valence of the element is correspondingly increased.

Oxidative phosphorylation: The process of ATP synthesis during which an inorganic phosphate group becomes attached to ADP. It occurs via the electron transport chain in the mitochondria and to a lesser extent as a result of substrate-level phos­phorylation.

Oxygen debt: The amount of extra oxygen required by muscle tissue to oxidize lactic acid and replenish depleted ATP and phosphocreatine following vigor­ous exercise.

Oxygen-hemoglobin dissociation curve: A graph describing the relationship between the percentages of hemoglobin saturated with oxygen and a range of oxygen partial pressures.

Oxyhemoglobin: An oxygen-bound form of hemo­globin.

Oxytocin: A hormone produced by neurons in the hypothalamus and released by nerve terminals in the posterior pituitary gland. Primary systemic effects are to elicit milk ejection and uterine contrac­tions. However, oxytocin is also produced locally by the corpus luteum.

P wave: A deflection of the ECG corresponding to atrial depolarization.

Pacemaker cells: A device by which the contractions of the heart are controlled by electrical impulses; these impulses occur at a rate that controls the beat of the heart. The cells that create these rhythmical impulses are called pacemaker cells.

Pacinian corpuscle: An encapsulated receptor found in deep layers of the skin that senses vibratory pres­sure and touch.

Palate: The horizontal partition separating the oral cavity from the nasal cavity and nasopharynx; it is divided into an anterior bony (hard) palate and a posterior fleshy (soft) palate.

Palatine bones: Pertaining to the palate.

Palatopharyngeal arches: Either of two ridges or folds of mucous membrane passing from the soft palate to the wall of the pharynx and enclosing the palatopharyngeal muscle.

Palpebral fissure: A fissure that separates the upper and lower eyelids.

Pancreatic duct: A tubular duct that carries pancreatic juice from the pancreas to the duodenum.

Pancreatic islets: Aggregations of endocrine cells embedded within the exocrine tissue of the pan­creas; also called the islets of Langerhans.

Paneth cells: Cells that provide host defense against microbes in the small intestine. They are func­tionally similar to neutrophils. When exposed to bacteria or bacterial antigens, these cells secrete antimicrobial molecules into the lumen of the crypt, thereby contributing to maintenance of the gastro­intestinal barrier.

Papillae: Any of the small projections on the top of the tongue, in particular vallate and fungiform papillae, that contain taste buds.

Papillary muscle: A nipplelike conical projection of myocardium within the ventricle; the chordae tendineae are attached to the apex of the papillary muscle.

Parahippocampal gyrus: A long convolution located on the medial surface of the temporal lobe of the brain and forming the lower part of the gyrus for­nicatus; also called the hippocampal gyrus.

Paranasal sinuses: Bony chambers, lined by respira­tory epithelium, that open into the nasal cavity; the frontal, ethmoidal, sphenoidal, and maxillary sinuses.

Parasympathetic divisions: One of the two divisions of the autonomic nervous system; also called the craniosacral division; generally responsible for activities that conserve energy and lower the meta­bolic rate.

Parasympathetic subdivision: One of three divisions of the autonomic nervous system. Sometimes called the "rest and digest" system, the parasympathetic system conserves energy as it slows the heart rate, increases intestinal and gland activity, and relaxes sphincter muscles in the gastrointestinal tract.

Parenchyma: The functional portion of a tissue or organ. For example, in the exocrine pancreas, the acini that produce and secrete enzymes are parenchyma.

Parietal cells: Cells of the gastric gland that secrete hydrochloric acid and intrinsic factor.

Parietal lobe: The middle portion of each cerebral hemisphere, separated from the frontal lobe by the central sulcus, from the temporal lobe by the lateral sulcus, and from the occipital lobe only partially by the parieto-occipital sulcus on its medial aspect.

Parietal serosa: The part of the double-layered mem­brane that lines the walls of the ventral body cavity.

Parieto-occipital sulcus: A deep fissure on the me­dial surface of the cerebral cortex marking the bor­der between the parietal lobe and the cuneus of the occipital lobe; also called the parieto-occipital fissure.

Parotid salivary gland: Large salivary glands that secrete saliva that contain high concentrations of salivary (alpha) amylase.

Partial pressure: The pressure exerted by a single component of a mixture of gases.

Parvocellular (P cells): Slow-conducting neurons that transmit information about color vision, texture, pattern, and visual acuity. The cells transmit the information to the lateral geniculate nucleus.

Passive immunity: Immunity that is derived from transfer rather than activation of an animal's own immune system. Examples include antibodies passed to the fetus in utero or across the gut of the newborn via colostrum ingestion.

Pelvic cavity: The inferior subdivision of the abdomi- nopelvic cavity, it encloses the urinary bladder, the sigmoid colon and rectum, and male or female reproductive organs.

Pelvic symphysis: The midline cartilaginous joint uniting the superior rami of the left and right pubic bones.

Perception: Recognition and interpretation of sensory stimuli based chiefly on memory.

Pericardial cavity: The space between the parietal pericardium and the epicardium (visceral pericar­dium) at the outer surface of the heart.

Pericarditis: An inflammation of the pericardium.

Pericardium: The fibrous sac that surrounds the heart; its inner, serous lining is continuous with the epicardium.

Perichondrium: The dense irregular fibrous mem­brane of connective tissue covering the surface of cartilage except at the endings of joints.

Perikaryon: The cytoplasm that surrounds the nucleus in the cell body of a neuron.

Perilymph: Extracellular fluid located within the cochlea (part of the ear) in two of its three compart­ments; the scala tympani and scala vestibuli.

Perineurium: The sheath of connective tissue enclos­ing a bundle of nerve fibers.

Periosteum: The thick fibrous membrane covering the entire surface of a bone, except its articular car­tilage, and serving as an attachment for muscles and tendons.

Peripheral nervous system: The portion of the nervous system consisting of nerves and ganglia that lie outside the brain and spinal cord.

Peristalsis: A wave of smooth muscle contractions that propels materials along the axis of a tube such as the digestive tract, the ureters, or the ductus deferens.

Peritoneal cavity: See abdominopelvic cavity.

Peritoneum: The serous membrane that lines the peritoneal cavity.

Phagocytosis: The amebalike engulfment of extracel­lular material by one of the immune cells, most often neutrophils or macrophages.

Pharyngotympanic: See auditory tube.

Pharynx: The throat; a muscular passageway shared by the digestive and respiratory tracts.

Philtrum: The midline groove in the upper lip that runs from the top of the lip to the nose.

Phosphatase: An enzyme that hydrolyses phosphoric acid monoesters into a phosphate ion and a mole­cule with a free hydroxyl group.

Phosphatidyinositol-S-Rinase (PI3K): An enzyme involved in the synthesis of the phosphoinositide family of lipid second messengers. Members of this family of intracellular signaling molecules are thought to be critical to suppress signals that can cause apoptosis or programmed cell death.

Phosphodiesterease: Enzymes that split phosphodi­ester bonds—for example, in the conversion of cyclic AMP to AMP.

Phospholipid: A class of essential lipids needed for creation of cellular membranes. They are usually composed of two fatty acids attached to glycerol and an additional polar group, for example, choline. Phosphorylation: The addition of a phosphate group to a protein, most often by the action of an enzyme called a kinase.

Photopigments: Located in photoreceptor outer segment disc membranes, they change their con­formation on the perception of photons. The con­formational change allows the photopigment to interact with transducin and to start the visual cascade.

Photoreceptor: A specialized type of neuron found in the eye's retina that is capable of phototransduction. More specifically, the photoreceptor sends signals to other neurons by a change in its membrane poten­tial when it absorbs photons.

Physiology: The study of function, which deals with the ways organisms perform vital activities.

Pia mater: The fine vascular membrane that closely envelops the brain and spinal cord under the arach­noid and the dura mater.

Pigmented layer: The layer of the retina that consists of a single stratum of cells. In the eyes of albinos, the cells of this layer contain no pigment.

Pineal body: A small, unpaired, flattened glandular structure lying in the depression between the two superior colliculi of the brain and secreting the hormone melatonin; also called the conarium, epiphysis, or pineal gland.

Pitch: The tone of a sound, determined by the fre­quency of vibrations (i.e., whether a sound is a C or G note).

Placenta: The reproduction structure that allows met­abolic exchanges between fetus and mother. It is composed of embryonic tissue (the chorion) and maternal tissue (endometrium). The placenta also functions as an endocrine organ during gestation.

Plasmin: A proteolytic enzyme important in dissolu­tion of blood clots. It converts fibrin to soluble components.

Plasminogen: An inactive form of plasmin found in blood and tissue fluids.

Pleura: The serous membrane that lines the pleural cavities.

Pleural cavities: Subdivisions of the thoracic cavity that contain the lungs.

Pleural membrane: The membrane lining the lung and the chest cavity.

Pleurisy: An inflammation of the pleura; also called pleuritis.

Plica: A permanent transverse fold in the wall of the small intestine.

Pluripotent stem cells: Precursor cells—for example, those that reside in the bone marrow and continu­ously divide and differentiate to give rise to each of the types of blood cells.

Pneumotaxic area: A center in the reticular forma­tion of the pons that regulates the activities of the apneustic and respiratory rhythmicity centers to adjust the pace of respiration; also called pneumo- taxic center.

Pneumothorax: The introduction of air into the pleural cavity.

Polycythemia: An excessive or abnormal increase in the number of erythrocytes.

Polyestrous: Exhibiting multiple estrous cycles dis­tributed throughout the year, that is, no seasonal anestrous.

Polyestrus: Exhibiting multiple episodes of estrus distributed throughout the year.

Polymorphonuclear leukocytes: Another name for a neutrophil, the name describes the fact that the cell has a Iobed nucleus, which makes it appear as if it has multiple (poly) nuclei.

Polysynaptic reflex: A reflex in which interneurons are interposed between the sensory fiber and the motor neuron(s).

Polytocous: Mammals that give birth to multiple off­spring (litters).

Pontine reticulospinal tract: Any of several fiber tracts descending to the spinal cord from the reticu­lar formation of the pons and medulla oblongata. Some fibers conduct impulses from the neural mechanisms regulating cardiovascular and respira­tory functions to the spinal cord; others form links in extrapyramidal motor mechanisms affecting muscle tonus and somatic movement.

Portal triad: Branches of the portal vein, hepatic artery, and hepatic duct bound together in the con­nective tissue that divides the liver into lobules.

Positive chronotropic factors: Agents that increase heart rate.

Postcentral gyrus: The anterior convolution of the parietal lobe, bounded in front by the central sulcus and in back by the interparietal sulcus.

Posterior chamber: A narrow chink behind the periph­eral part of the iris of the eye and in front of the suspensory ligament of the lens and the ciliary processes.

Posterior cranial fossa: Part of the intracranial cavity, located between the foramen magnum and tento­rium cerebelli. It contains the brain stem and cerebellum.

Posterior (dorsal) horns: The occipital division of the lateral ventricle of the brain, extending back­ward into the occipital lobe; also called the dorsal horn.

Posterior median sulcus: The longitudinal groove marking the posterior midline of the medulla ob­longata and continuous below with the posterior median sulcus of the spinal cord; also called the posterior median fissure.

Posterior neuropore: The posterior opening leading from the central canal of the embryonic neural tube to the exterior.

Posterior segment: The back two-thirds of the eye, which includes the anterior hyaloid membrane and all structures behind it: the vitreous humor, retina, choroid, and optic nerve.

Postganglionic: Located posterior or distal to a ganglion.

Postsynaptic membrane: The portion of the cell mem­brane of a postsynaptic cell that is part of a synapse.

Postsynaptic potential: Changes in the membrane potential of the neuron that receives information at a synapse.

Posttranslational modification: Enzyme-mediated changes to proteins made after initial synthesis. This most often occurs in the Golgi apparatus and can include phosphorylation, glycosylation, or methylation.

Potential difference: The separation of opposite charges; requires a barrier that prevents ion migration.

P-Q interval: Time elapsing between the beginning of the P wave and the beginning of the QRS com­plex in the electrocardiogram; also called the PR interval.

Precapillary sphincter: A smooth muscle sphincter that regulates blood flow through a capillary.

Precentral gyrus: The posterior convolution of the frontal lobe, bounded in back by the central sulcus and in front by the precentral sulcus.

Preganglionic: Situated proximal to or preceding a ganglion, especially a ganglion of the autonomic nervous system.

Premolars: Transitional teeth located between the canine and molar teeth.

Presbyopia: The eye's diminished power of accom­modation, which occurs with aging.

Presynaptic inhibition: A reduction in the release of a neurotransmitter from a presynaptic axon termi­nal as a result of excitation of another neuron that terminates on the axon terminal.

Presynaptic membrane: The synaptic surface where neurotransmitter release occurs.

Pretectal nuclei: A structure located in the midbrain. It receives binocular input from the eyes and is involved with the pupillary light reflex.

Prevertebral ganglia: Any of the sympathetic ganglia lying in front of the vertebral column, including the celiac and the superior and inferior mesenteric ganglions.

Primary fissure: The trilobed structure of the brain, lying posterior to the pons and medulla oblon­gata and inferior to the occipital lobes of the cere­bral hemispheres, responsible for the regulation and coordination of complex voluntary muscular movement and the maintenance of posture and balance.

Primary hyperalgesia: Pain sensitivity that occurs directly in the damaged tissues.

Primary motor area: A group of networked cells in mammalian brains that controls movements of spe­cific body parts associated with cell groups in that area of the brain. The area is closely linked by neural networks to corresponding areas in the primary somatosensory cortex.

Primary visual cortex: The part of the cerebral cortex that is responsible for processing visual stimuli. It is the simplest, earliest cortical visual area. It is highly specialized for processing information about static and moving objects and is excellent in pattern recognition.

Primiparous: A term indicating a mammal that is experiencing its first pregnancy or that is in the period following the birth of its first offspring.

Proerythroblasts: The precursor of the erythroblast, which in turn produces red blood cells or erythrocytes.

Proestrus: The stage of the estrus cycle between Iute- olysis and onset of estrus (heat).

Progenitor cells: A less mature cell in a develop­mental pathway. For example, in the immune system, B lymphocytes are the precursors of plas­ma cells.

Progesterone: A steroid hormone primarily produced by the ovary (corpus luteum) and placenta. It is necessary for maintenance of pregnancy as well as normal mammary development.

Prohormone: A precursor of a hormone, it usually refers to a larger protein structure, which is cleaved to produce the active agent.

Prolactin (Prl): A protein produced in the anterior pituitary gland, it is a critical regulator of mammary gland function and other physiological processes, including regulation of fluid balances and some aspects of behavior.

Pronation: To turn or rotate (the foot) by abduction and eversion so that the inner edge of the sole bears the body's weight.

Proprioceptor: A sensory receptor, commonly found in muscles, tendons, joints, and the inner ear, that detects the motion or position of the body or a limb by responding to stimuli within the organism.

Prosencephalon: The most anterior of the three primary regions of the embryonic brain, from which the telencephalon and diencephalon develop.

Prostacyclin: A prostaglandin produced in the walls of blood vessels that acts as a vasodilator and inhib­its platelet aggregation.

Prostaglandin (PG): A large group of structurally related hormone- or growth factor-like regulators (including PGE, PGF, PGA, and PGB) found in tissues throughout the body. They are derived from arachidonic acid and exhibit multiple actions.

Proteoglycan: An example of a common extracellular matrix molecule, it consists of a glycoaminoglycans (GAG) chains attached to a protein core. These mol­ecules are important in maintenance of intercellular spaces.

Protraction: The extension of teeth or other maxillary or mandibular structures into a position anterior to the normal position.

Proventriculus: A section of the avian digestive tract located before the gizzard.

Pseudo-unipolar neuron: Another term for unipolar neuron.

Pulmonary arteries: Vessels that deliver blood to the lungs to be oxygenated.

Pulmonary circuit: Blood vessels between the pulmo­nary semilunar valve of the right ventricle and the entrance to the left atrium of the heart. It describes the blood flow to and from the lungs.

Pulmonary circulation: The closed loop of blood vessels carrying blood between the heart and lungs.

Pulmonary trunk: The large elastic artery that carries blood from the right ventricle of the heart to the right and left pulmonary arteries.

Pulmonary valve: A one-way valve that permits the flow of blood from the right ventricle into the pul­monary artery during ventricular emptying but pre­vents the backflow of blood from the pulmonary artery into the right ventricle during ventricular relaxation.

Pulmonary veins: Vessels that deliver freshly oxy­genated blood from the respiratory zones of the lungs to the heart.

Pulmonary ventilation: The movement of air into and out of the lungs.

Pulp cavity: The internal chamber in a tooth, contain­ing blood vessels, lymphatic vessels, nerves, and the cells that maintain the dentin.

Pulse pressure: Difference between systolic and dia­stolic pressure.

Pupil: The opening in the middle of the iris. It appears black because most of the light entering is absorbed by the tissues inside the eye.

Purine: One of two categories of nitrogenous ringed compounds found in RNA and DNA (the others are the pyrimidines); examples are adenine and guanine.

Purkinje cells: Any of numerous neurons of the cere­bral cortex having large flask-shaped cell bodies with massive dendrites and one slender axon; also called the Purkinje corpuscles.

Putamen: The outer, larger, and darker gray of the three portions into which the Ientiform nucleus of the brain is divided.

Pyloric region: The region of the stomach that con­nects to the duodenum.

Pyloric sphincter: A sphincter of smooth muscle that regulates the passage of chyme from the stomach to the duodenum.

Pylorus: The gastric region between the body of the stomach and the duodenum; it includes the pyloric sphincter.

Pyramidal system: a massive collection of axons that travel between the cerebral cortex of the brain and the spinal cord.

Pyrimidine: One of two classes of nitrogenous bases found in DNA and RNA examples; a sample is cyto­sine. The other class is purine.

QRS complex: The principal deflection in the elec­trocardiogram, representing ventricular depolarization.

Quiescent period: The time interval of no activity occurring between each pulse during transmission.

Radioimmunoassay (RIA): A sensitive assay method to measure the concentration of hormones and other factors in biological fluids. The technique depends on the ability to produce antibodies against the substance under study and to label it with a radioisotope.

Ramp retina: The human retina has a smooth con­cave surface, but the horse has a ramp retina, which is irregular and inconsistent in its concave appearance.

Ras protein: An example of a large family of GTP- binding proteins that serve to relay signals from cell surface receptors to the cell nucleus. It was named for the ras gene, first identified in viruses that cause sarcoma in rats.

Reabsorption: The net movement of interstitial fluid into the capillary.

Receptive fields: A sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron. Receptive fields have been identified for neurons of the auditory system, the somatosensory system, and the visual system.

Receptor: A specialized cell or group of nerve endings that responds to sensory stimuli.

Receptor potential: The transmembrane potential difference of a sensory cell.

Rectum: The inferior 15 cm (6 in) of the digestive tract. Red bone marrow: Bone marrow characterized by meshes of the reticular network that contain the developmental stages of red blood cells, white blood cells, and megakaryocytes.

Red muscle fibers: Those fibers that have a red appearance; they contain high levels of myoglobin and oxygen-storing proteins and tend to have more mitochondria and blood vessels than the white ones.

Red nucleus: A large, well-defined, somewhat elon­gated cell mass of reddish-gray hue that is located in the mesencephalic tegmentum, receives a massive projection from the contralateral half of the cerebellum, receives an additional projection from the ipsilateral motor cortex, and whose effer­ent connections are with the contralateral half of the rhombencephalic reticular formation and spinal cord.

Reduced hemoglobin: Hemoglobin that is not com­bined with O₂.

Referred pain: Pain that is felt in a part of the body at a distance from its area of origin.

Reflex arch: The receptor, sensory neuron, motor neuron, and effector involved in a particular reflex; interneurons may be present, depending on the reflex considered.

Refracted: The ability of the eye to bend light so that an image is focused on the retina.

Relative refractory period: The period that follows the absolute refractory period; the interval during which a threshold stimulus is unable to trigger an action potential unless the stimulus is particularly strong.

Releasing hormones: The general name given a number of small peptides synthesized by hypotha­lamic neurons whose release into the hypothalami- Chypophyseal portal blood system controls secretion of anterior pituitary hormones.

Renin: The enzyme released by cells of the juxtaglo­merular apparatus when renal blood flow declines; it converts angiotensinogen to angiotensin I.

Residual volume: The amount of air remaining in the lungs after maximum forced exhalation.

Resistance: Hindrance of flow of blood or air through a passageway (blood vessel or respiratory airway, respectively).

Resorption: The act or the process of resorbing.

Respiratory bronchiole: The smallest bronchiole (0.5 mm in diameter) that connects the terminal bronchiole to the alveolar duct.

Respiratory burst: The rapid release of reactive oxygen species (superoxide radical and hydrogen peroxide) from different types of cells.

Respiratory capacities: The amount of air that can be forcibly expelled from the lungs following breath­ing in as deeply as possible.

Respiratory membrane: The membrane that consists of the epithelial cells of the alveolus, the endothelial cells of the capillary, and the two fused basement membranes of these layers. Gas exchange occurs across this respiratory membrane.

Respiratory pump: A mechanism by which changes in the intrapleural pressures during the respiratory cycle assist the venous return to the heart; also called the thoracoabdominal pump.

Respiratory rhythmicity center: An area of the brain stem that is involved in the control of respiration.

Resting membrane potential: The voltage that exists across the plasma membrane during the resting state of an excitable cell. Values range from -50 to -200 mv, depending on cell type.

Reticular activating system (RAS): The name given to part of the brain (the reticular formation and its connections) believed to be the center of arousal and motivation in animals. It is situated at the core of the brain stem between the myelen­cephalon (medulla oblongata) and metencephalon (midbrain).

Reticulocyte: An immature erythrocyte.

Reticulorumen: The first chamber in the alimentary canal of ruminant animals, it is composed of the rumen and reticulum.

Retina: A thin layer of neural cells that lines the back of the eyeball of vertebrates and some cephalopods.

Retinal: A carotenoid constituent of visual pigments.

It is the oxidized form of retinol, which functions as the active component of the visual cycle. It is bound to the protein opsin, forming the complex rhodop­sin. When stimulated by visible light, the retinal component of the rhodopsin complex undergoes isomerization at the 11-position of the double bond to the czs-form. This is reversed in "dark" reactions to return to the native trans configuration.

Retinal isomerase: An enzyme that catalyzes the con­version of the trans form of retinaldehyde to ll-czs- retinal, a reaction needed in the regeneration of the visual pigments.

Retraction: The act of pulling apart, usually as part of a surgical procedure.

Retroperitoneal: Located behind or outside the peri­toneal cavity. For example, both male and female re­productive tracts and the kidneys are retroperitoneal.

Rhinencephalon areas: A part of the brain involved with olfaction.

Rhodopsin: Also known as visual purple, it is expressed in vertebrate photoreceptor cells. It is a pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light.

Rhombencephalon: The portion of the embryonic brain from which the metencephalon and myelen­cephalon develop, including the pons, cerebellum, and medulla oblongata; also called the hindbrain.

Rib: One of a series of long, curved bones occurring in pairs and extending from the spine to or toward the sternum.

Right colic (hepatic) flexure: Abend in the colon that is adjacent to the liver, and is therefore also known as the hepatic colic flexure.

Rigor mortis: Muscular stiffening following death; also called postmortem rigidity.

Rods: Photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells.

Root canals: The narrow extension of the pulp cavity that projects into the roots.

Rotation: Regular and uniform variation in a sequence or series, as in the recurrence of symptoms of a disease.

Rough endoplasmic reticulum (RER): The cellular organelle involved in translation of mRNA for syn­thesis of proteins for secretion from cells. It appears in transmission electron micrographs as parallel arrays of intracellular membranes studded with ribosomes.

Round ligament: The Iibromuscular band that is attached to the uterus on either side in front of and below the opening of the uterine tube; it passes through the inguinal canal to the labium majus.

Ruffini ending: One of the four main cutaneous mechanoreceptors. Named after Angelo Ruffini, they are slowly adapting receptors found in the dermis and subcutaneous tissue of the skin. These thin, cigar-shaped encapsulated sensory endings measure pressure when the skin is stretched. Their main function is thermoreception.

Rugae: Mucosal folds in the lining of the empty stomach that disappear as gastric distension occurs.

Rumen: The larger part of the reticulorumen, which is the first chamber in the alimentary canal of rumi­nant animals.

Ruminant: Any hoofed animal that digests its food in two steps, first by eating the raw material and regurgitating a semidigested form known as cud, and then eating the cud, a process called ruminating.

Rumination: An eating disorder characterized by having the contents of the stomach drawn back up into the mouth, chewed for a second time, and swal­lowed again. In some animals, known as ruminants, this is a natural and healthy part of digestion and is not considered an eating disorder.

Sacroiliac joint: The joint between the sacrum, at the base of the spine, and the ilium of the pelvis, which are joined by ligaments. Inflammation of this joint is known as sacroiliitis, one cause of disabling low back pain.

Sagittal plane: A sectional plane that divides the body into left and right portions.

Saltatory conduction: Transmission of an action poten­tial along a myelinated fiber in which the nerve impulse appears to leap from node to node.

Sarcolemma: A thin membrane enclosing a striated muscle fiber.

Sarcomere: One of the segments into which a fibril of striated muscle is divided.

Sarcoplasm: The cytoplasm of a striated muscle fiber. Sarcoplasmic reticulum: The endoplasmic reticulum found in striated muscle fibers.

Satellite cells: Any of the cells that encapsulate the bodies of nerve cells in many ganglia.

Scala media: An endolymph-filled cavity inside the cochlea, located between the scala tympani and the scala vestibuli, separated by the basilar membrane and Reissner's membrane (the vestibular mem­brane), respectively.

Scala tympani: The name of one of the perilymph- filled cavities in the cochlear labyrinth. It is sepa­rated from the scala media by the basilar membrane, and it extends from the round window to the heli­cotrema, where it continues as scala vestibuli.

Scala vestibuli: A perilymph-filled cavity inside the cochlea of the inner ear. It is separated from the scala media by Reissner's membrane and extends from the oval window to the helicotrema where it joins the scala tympani.

Scapula: Either of two large, flat, triangular bones forming the back part of the shoulder. More com­monly called the shoulder blade.

Schwann cells: Neuroglia responsible for the neuri­lemma that surrounds axons in the peripheral nervous system.

Sclera: The (usually) white outer coating of the eye made of tough fibrin connective tissue, which gives the eye its shape and helps protect the delicate inner parts.

Scleral venous sinus (canal of Schlemm): A circular channel in the eye that collects aqueous humor from the anterior chamber and delivers it into the bloodstream.

Seasonal anestrus: A period of anestrous produced by exposure of either short (mare) or long (ewe) photoperiods.

Seasonal polyestrus: The term for exhibiting multi­ple estrous cycles during certain times of the year.

Sebaceous ciliary gland: Holocrine glands found in the skin of mammals. They secrete an oily substance called sebum (Latin, meaning "fat" or "tallow") that is made of fat (lipids) and the debris of dead fat­producing cells.

Second messenger: An intracellular molecule whose concentration changes as a consequence of a hormone binding to a cell surface receptor. The released molecule serves to carry the signal of the hormone (first messenger) into the target cell to elicit a reaction. The first second messenger identi­fied in conjunction with the effects of glucagon was cyclic AMP.

Secondary hyperalgesia: Pain sensitivity that occurs in surrounding, undamaged tissues.

Secondary ossification center: About the time of birth, a secondary ossification center appears in each end (epiphysis) of long bones. Periosteal buds carry mesenchyme and blood vessels in and the process is similar to that occurring in a primary ossification center.

Secretin: A hormone secreted by the duodenum that stimulates the production of buffers by the pancreas and inhibits gastric activity.

Secretion: The process of segregating, elaborating, and releasing chemicals from a cell, or a secreted chemical substance or amount of substance.

Segmentation: A morphogenesis process that divides a metazoan body into a series of Semirepetitive segments.

Selectins: A family of transmembrane molecules expressed on the surface of leukocytes and activated endothelial cells.

Self-propagating: Propagating by one's self or by itself. Semicircular canals: A group of three half-circular, interconnected tubes located inside each ear that are the equivalent of three gyroscopes located in three planes perpendicular to one another.

Semicircular ducts: Consists of three oval ducts arranged at right angles to one another; an integral part of the equilibrium mechanism.

Seminiferous tubules: The highly convoluted tubules, located in the testes, that produce spermatozoa.

Sensations: A perception associated with stimulation of a sense organ or with a specific body condition.

Sensory areas: The main cerebral areas that receive sensory information from thalamic nerve projections.

Sensory input: Input that includes somatic sensation and special senses.

Sensory nerve: An afferent nerve conveying impulses that are processed by the central nervous system to become part of the organism's perception of itself and of its environment.

Sensory neurons: Nerve cells within the nervous system responsible for converting external stimuli from the organism's environment into internal elec­trical motor reflex loops and several forms of invol­untary behavior, including pain avoidance.

Septum pellucidum: A thin membrane of nervous tissue that forms the medial wall of the lateral ven­tricles in the brain. It is also called the septum lucidum.

Serosa: See serous membrane.

Serosal fluid: Clear, watery fluid secreted by cells of a serous membrane.

Serotonin: A neurotransmitter in the central nervous system; a compound that enhances inflammation and is released by activated mast cells and basophils.

Serous cell: A cell that produces a serous secretion.

Serous membrane: A squamous epithelium and the underlying loose connective tissue; the lining of the pericardial, pleural, and peritoneal cavities.

Serous pericardium: The lining of the pericardial sac composed of a serous membrane.

Sertoli cells: Cells located in the seminiferous tubules of the testes that are thought to control spermato­genesis. They contain FSH receptors and were named for the Italian reproductive physiologist Enrico Sertoli.

Serum: The ground substance of blood plasma from which clotting agents have been removed.

Sesamoid bone: A bone that forms within a tendon.

Sharpey^,s fibers: A matrix of connective tissue con­sisting of bundles of strong collagenous fibers con­necting periosteum to bone. They are part of the outer fibrous layer of periosteum, entering into the outer circumferential and interstitial lamellae of bone tissue.

Short bone: A bone whose dimensions are approxi­mately equal, consisting of a layer of cortical sub­stance enclosing the spongy substance and marrow.

Short-day breeders: Sexually mature females that begin to initiate estrous cycles during periods with reduced photoperiods, that is, short days.

Sigmoid colon: The S-shaped portion of the colon, 18 cm long, between the descending colon and the rectum.

Signal peptide: A small sequence of amino acids in the structure of a newly synthesized protein if the protein will be transported to the Golgi apparatus for packaging and secretion from the cell.

Signal transduction: The relaying of a signal from one form to another. In physiological processes, this is the process whereby extracellular signals are con­verted into intracellular responses.

Signaling transducers and activators of transcription (STATs): Regulators that make a group of transcrip­tion factors (seven are recognized) that are seques­tered in the cytoplasm until activated by the binding of a cytokine or growth factor receptor. Ligand binding causes aggregation of receptor (cytokine) subunits and initiation of a cascade of tyrosine phosphorylation events, during which receptor- linked JAKs become activated to cause phosphory­lation of the receptor. This creates a docking or binding site for a STAT that is, in turn, phosphory­lated by the receptor. Phosphorylated STAT dissoci­ates from the receptor, dimerizes, and translocates to the cell nucleus to interact with the promoters of specific genes. For example, STAT-5 is known to be essential for the hormone prolactin to induce pro­duction of mRNA for milk proteins.

Simple fracture: A bone fracture that causes little or no damage to the surrounding soft tissues; also called a closed fracture.

Single-unit smooth muscle: The most abundant type of smooth muscle. It is made up of muscle fibers that are interconnected by gap junctions so that they become excited and contract as a unit; also known as visceral smooth muscle.

Sinoatrial (SA) node: The natural pacemaker of the heart. It is located in the wall of the right atrium.

Sinusoidal capillary: A capillary with a caliber of 10-20 μm or more; it is lined with a fenestrated type of endothelium.

Sinusoids: An exchange vessel that is similar in general structure to a fenestrated capillary. The two differ in size (sinusoids are larger and more irreg­ular in cross section), continuity (sinusoids have gaps between endothelial cells), and support (sinu­soids have thin basal laminae, if they have them at all).

Skeletal muscle: Muscle composed of cylindrical multi­nucleate cells with obvious striations; the muscle(s) attached to the body's skeleton; voluntary muscle.

Sleep spindles: A burst of brain activity, visible on an EEG, that occurs during stage 2 sleep. It consists of 12-16 Hz waves that occur for 0.5 to 1.5 seconds.

Sliding filament theory: The concept that a sarco­mere shortens as the thick and thin filaments slide past one another in the muscle cell.

Slow oxidative fibers: Fibers that generate energy for ATP resynthesis by means of a long-term system of aerobic energy transfer. They tend to have a low activity level of ATPase, a slower speed of contrac­tion with a less-developed glycolytic capacity. They contain large and numerous mitochondria, and, coupled with the high levels of myoglobin, that gives them a red pigmentation. They have been demonstrated to have a high concentration of mitochondrial enzymes, and thus they are fatigue resistant.

Slow-wave sleep: A term used to describe stages 3 and 4 sleep.

Smell: The ability to perceive odors; also called olfaction.

Smooth muscle: Spindle-shaped cells with one cen­trally located nucleus and no externally visible stria­tions (bands). It is found mainly in the walls of hollow organs.

Soft palate: The fleshy posterior extension of the hard palate, separating the nasopharynx from the oral cavity.

Solitary nucleus: A slender compact bundle of primary sensory fibers that accompany the vagus, glossopharyngeal, and facial nerves and convey information from stretch receptors and chemorecep­tors in the walls of the cardiovascular, respiratory, and intestinal tracts and impulses generated by the receptor cells of the taste buds in the tongue.

Somatic afferent: Fibers that receive information from external sources.

Somatic efferent: Nerve fibers that are responsible for muscle contraction.

Somatic nervous system: That part of the peripheral nervous system associated with the voluntary control of body movements through the action of skeletal muscles, and also the reception of external stimuli.

Somatic reflexes: Reflexes that activate skeletal muscle. Somatomedin hypothesis: An older idea stating that nearly all of the effects attributed to growth hormone were mediated by GH induction of IGF-I in the liver. When first isolated, IGF-I and IGF-II were known as somatomedin A and B, respectively.

Somatostatin: A 14-amino-acid peptide produced in the hypothalamus and other brain areas as well as the pancreas and gut. It is primarily known for its role in inhibiting the secretion of growth hormone, but it is also likely important in GI tract-nervous system interactions.

Somites: A segmental mass of mesoderm in the vertebrate embryo, occurring in pairs along the notochord and developing into muscles and vertebrae.

Spatial summation: A summation of the local poten­tials in which two or more action potentials arrive simultaneously at two or more presynaptic termi­nals that synapse with a single neuron.

Special senses: Any of the five senses related to the organs of sight, hearing, smell, taste, and touch.

Spermatogenesis: The process of creating spermato­zoa. It depends on proliferation, meiosis, and differen­tiation of precursor cells (primary spermatocytes).

Sphenoid bone: A compound bone with winglike processes, situated at the base of the skull.

Sphincter of the hepatopancreatic ampulla (sphinc­ter of Oddi): Controls secretions from the liver, pancreas, and gallbladder into the duodenum of the small intestine.

Spicules: A needlelike structure or part.

Spina bifida: A congenital defect in which the spinal column is imperfectly closed so that part of the meninges or spinal cord may protrude, often result­ing in neurological disorders; also called hydrocele spinalis.

Spinal nerves: Any of 31 pairs of nerves emerging from the spinal cord, each attached to the cord by two roots, anterior or ventral and posterior or dorsal (the latter provided with a spinal ganglion). The two roots unite in the intervertebral foramen but divide again into ventral and dorsal rami, or anterior and posterior primary divisions (the former supplying the foreparts of the body and limbs and the latter the muscles and skin of the back).

Spinal reflex: A reflex arc involving the spinal cord.

Spinothalamic tract: A large ascending bundle of fibers in the ventral half of the lateral funiculus of the spinal cord, arising in the posterior horn at all levels of the cord and continuing into the brain stem. It is composed in the spinal cord of a lateral part that conveys impulses associated with pain and temperature sensation and of an anterior part that is involved in tactile sensation.

Spiral ganglion: Cell bodies of sensory neurons that innervate hair cells of the organ of Corti are located in the spiral ganglion.

Spiral organ or organ of Corti: The organ in the inner ear of mammals that contains auditory sensory cells, or "hair cells."

Splanchnic circulation: The blood vessels serving the digestive system.

Splanchnic nerves: Part of the sympathetic nervous system, which is part of the autonomic nervous system. Most sympathetic preganglionic nerves synapse in the sympathetic trunk lying beside the spinal cord, but splanchnic nerves pass through the trunk, travel near their target organ, and synapse in prevertebral ganglia.

S-T segment: The part of an electrocardiogram imme­diately following the QRS complex and merging into the T wave.

Stapedius muscle: The smallest striated muscle in the body. At just over 1 mm in length, its purpose is to stabilize the smallest bone in the body, the stapes.

Stem cells: Primal undifferentiated cells that retain the ability to produce an identical copy when they divide (self-renew) and differentiate into other cell types.

Stercobilin: A tetrapyrrole chemical compound created by bacterial action on bilirubin and subsequent oxidation.

Stereocilia: Mechanosensing organelles of hair cells, which respond to fluid motion or fluid pressure changes in numerous types of animals for various functions, primarily hearing.

Sternebrae: One of the four segments of the primor­dial sternum of the embryo, the fusion of which forms the body of the adult sternum.

Sternum: A long flat bone, articulating with the car­tilages of the first seven ribs and with the clavicle, forming the middle part of the anterior wall of the thorax, and consisting of the corpus, manubrium, and xiphoid process; also called the breastbone.

Stimulus transduction: The conversion of a stimulus from one form to another.

Streptokinase: An extracellular metallo-enzyme pro­duced by beta-hemolytic streptococcus and used as an effective and cheap clot-dissolving medication— in some cases, of myocardial infarction (heart attack) and pulmonary embolism.

Stretch reflex: See myotatic reflex.

Striated muscles: Muscles that are marked by stripes or bands.

Stroke volume (SV): The amount of blood pumped out of a ventricle during one contraction.

Stroma: The supporting elements in a tissue or organ. For example, in the mammary gland this would be the connective tissue elements that surround and support the ducts and alveoli.

Sty: An inflammation of the sebaceous glands at the base of the eyelashes.

Subarachnoid space: The space between the arach­noid membrane and pia mater that is filled with cerebrospinal fluid and contains the large blood vessels that supply the brain and spinal cord.

Submucosa: The region between the muscularis mucosae and the muscularis externa.

Submucosal glands: Glands that are situated below or underneath (sub) the mucosal tissue.

Submucosal (or Meissner's) plexus: A plexus of unmyelinated nerve fibers, derived chiefly from the superior mesenteric plexus and ramifying in the intestinal submucosa.

Subscapular fossa: A depression in either of two large, flat, triangular bones forming the back part of the shoulder or scapula.

Substance P: A short-chain polypeptide that func­tions as a neurotransmitter, especially in the trans­mission of pain impulses from peripheral receptors to the central nervous system.

Substantia gelatinosa: The apical part of the poste­rior horn of the gray matter of the spinal cord com­posed largely of very small nerve cells and whose gelatinous appearance is due to its very low content of myelinated nerve fibers. It functions in the inte­gration of sensory stimuli that gives rise to the sen­sations of heat and pain; also called Rolando's gelatinous substance.

Substantia nigra: A layer of large pigmented nerve cells in the mesencephalon that produce dopamine and whose destruction is associated with Parkin­son's disease; also called nigra.

Subthalamic nucleus: A Circumscript nucleus that is located in the ventral part of the subthalamus, receives a massive projection from the lateral segment of the globus pallidus, and projects to both pallidal segments and to the mesencephalon.

Sucrase: The enzyme involved in the hydrolysis of sucrose to fructose and glucose. It is secreted by the tips of the villi of the epithelium in the small intestines.

Sulci (singular = sulcus): Depressions or fissures in the surface of an organ, most especially the brain.

Summation: An accumulation of effects, especially those of muscular, sensory, or mental stimuli.

Superior cervical ganglion: The largest of the cervical ganglia, it is placed opposite the second and third cervical vertebrae. It is reddish-gray, usually fusi­form in shape, sometimes broad and flattened, and occasionally constricted at intervals. It is believed to be formed by the coalescence of four ganglia, cor­responding to the upper four cervical nerves.

Superior colliculi: The part of the brain that sits below the thalamus and surrounds the pineal gland in the mesencephalon of vertebrate brains. This structure comprises the rostral aspect of the mid­brain, anterior to the periaqueductal gray and adja­cent to the inferior colliculus. The inferior and superior colliculi are known collectively as the corpora quadrigemina, or four twins.

Superior oblique muscle: A fusiform muscle in the upper, medial side of the orbit whose primary action is intorsion and whose secondary actions are to abduct (laterally rotate) and depress the eyeball (i.e., make the eye move outward and downward).

Superior rectus muscle: A muscle in the orbit. It is one of the extraocular muscles.

Superior sagittal sinus: An unpaired dural sinus in the sagittal groove.

Superior vena cava: The vein that carries blood to the right atrium from parts of the body that are superior to the heart.

Superovulation: The ovulation of an abnormally large number of ova for a given species. This is usually a result of exogenous hormonal treatment. It is often used to produce ova for embryo transfer, cloning, or transgenic manipulation of animals.

Supination: The rotation of the forearm such that the palm faces anteriorly.

Supporting cells: Cells that serve to provide support and protection for and perhaps contribute to the nutrition of principal or other cells of certain organs; such cells are found in the labyrinth of the inner ear, organ of Corti, olfactory epithelium, taste buds, and seminiferous tubules.

Suppressors of cytokine signaling (SOCS): Intracel­lular proteins that regulate the activity of signaling pathways, especially STAT-associated pathways.

Suprachiasmatic nucleus: A nucleus in the hypothal­amus, so named because it resides immediately above the optic chiasm (OX). It consists of two nuclei, each of which lies on either side of the hemi­sphere separated by the third ventricle (3V). Its principal function is to create the circadian rhythm, which regulates the body functions over the 24-hour period.

Supraglenoid tubercle: A projection of bone located superior to the glenoid cavity. It is the attachment site for the tendon of the long head of the biceps brachiim.

Supraspinous fossa: Smaller than the infraspinatus fossa, it is concave, smooth, and broader at its ver­tebral than at its humeral end; its medial two-thirds gives origin to the supraspinatus.

Surface tension: The force at the liquid surface of an air-water interface resulting from the greater attrac­tion of water molecules to the surrounding water molecules than to the air above the surface; it is a force that tends to decrease the area of a liquid surface and resists stretching of the surface.

Sympathetic chains: Chains that extend from the base of the skull to the coccyx.

Sympathetic division: The division of the autonomic nervous system that is responsible for "fight or flight" reactions; it is primarily concerned with the elevation of metabolic rate and increased alertness.

Sympathetic subdivision: The subdivision that acti­vates what is often termed the "fight or flight" re­sponse. This response is also known as Sympathetico- adrenal response of the body; the preganglionic sympathetic fibers that end in the adrenal medulla (but also all other sympathetic fibers) secrete acetyl­choline, which activates the secretion of adrenaline (epinephrine) and to a lesser extent noradrenaline (norepinephrine) from it.

Symporter: Also known as a cotransporter, it is an integral membrane protein that is involved in sec­ondary active transport. It works by binding to two molecules at a time and using the gradient of one solute's concentration to force the other molecule against its gradient.

Synapse: The junction across which a nerve impulse passes from an axon terminal to a neuron, a muscle cell, or a gland cell.

Synaptic bouton: Axonal terminals; the bulbous distal endings of the terminal branches of an axon; also called synaptic knob.

Synaptic cleft: The fluid-filled space at a synapse.

Synaptic delay: The period between the arrival of an impulse at the presynaptic membrane and the ini­tiation of an action potential in the postsynaptic membrane.

Synaptic plasticity: The ability of the connection, or synapse, between two neurons to change in strength.

Synaptic vesicles: Small membranous sacs contain­ing a neurotransmitter.

Syndesmochorial placenta: A type of epitheliocho­rial placenta in which endothelial epithelium becomes eroded so that maternal capillaries are exposed to the chorionic epithelial tissue.

Synergist: Muscle that aids the action of a prime mover by effecting the same movement or by stabi­lizing joints across which the prime mover acts to prevent undesirable movements.

Syrinx: The name for the vocal organ of birds. Located at the base of a bird's trachea, it produces sounds without the vocal cords that mammals have.

Systemic circuit: The vessels between the aortic valve and the entrance to the right atrium; the system other than the vessels of the pulmonary circuit.

Systemic circulation: The closed loop of blood vessels carrying blood between the heart and body systems.

Systems physiology: The study of the function of specific systems such as the cardiovascular, respira­tory, or reproductive systems.

Systole: A period of contraction in a chamber of the heart, as part of the cardiac cycle.

Systolic blood pressure: The peak arterial pressure measured during ventricular systole.

T tubule (transverse tubule): The extension of the muscle cell plasma membrane (sarcolemma) that protrudes deeply into the muscle cell.

T wave: A deflection of the ECG corresponding to ventricular depolarization.

Tapetum lucidum: A reflecting layer immediately behind, and sometimes within, the retina of the eye of many vertebrates (though not humans); it serves to reflect light back to the retina, increasing the quantity of light caught by the retina.

Tarsal plates: Two thin, elongated plates of dense connective tissue, about 2.5 cm in length; one is placed in each eyelid, and contributes to its form and support.

Taste: One of the most common and fundamental of the senses of animals. It is the direct detection of chemical composition, usually through contact with chemoreceptor cells.

Tectorial membrane: A gelatinous membrane, attached to the bony spiral lamina, which overlies the hair cells within the cochlea of the inner ear.

Tectum: A rooflike structure of the body, especially the dorsal part of the mesencephalon.

Tegmentum: A part of the midbrain consisting of white fibers running lengthwise through gray matter.

Telodendria: The terminal axonal branches that end in synaptic knobs.

Temporal lobe: The lowest of the major subdivisions of the cortical mantle of the brain, containing the sensory center for hearing and forming the rear two-thirds of the ventral surface of the cerebral hemisphere. It is separated from the frontal and parietal lobes above it by the fissure of Sylvius.

Temporal summation: The summation of the local potential that results when two or more action potentials arrive at a single synapse in rapid succession.

Tendon: A band of tough, inelastic fibrous tissue that connects a muscle with its bony attachment and consists of rows of elongated cells; minimal ground substance; and densely arranged, almost parallel, bundles of Collageneous fibers.

Tendon reflex: A myotatic or deep reflex in which the muscle stretch receptors are stimulated by per­cussing the tendon of a muscle.

Tensor tympani muscle: A muscle that originates from the Cartilagenous wall of the eustachian tube (also called the auditory tube) and the bony wall surrounding the tube.

Terminal bouton: An enlarged axon terminal or pre­synaptic terminal.

Testosterone: The major male sex steroid, it is pro­duced in the interstitial tissue of the testes by Leydig cells.

Tetrodotoxin: A potent neurotoxin, found in many puffer fish and certain newts.

Thermoreceptors: A sensory receptor that responds to heat and cold.

Thick filament: A Cytoskeletal filament in a skeletal or cardiac muscle cell, it is composed of myosin, with a core of titin.

Thin filament: A Cytoskeletal filament in a skeletal or cardiac muscle cell, consists of actin, troponin, and tropomyosin.

Third ventricle: A narrow, vertically oriented cavity in the midplane below the corpus callosum that communicates with each of the lateral ventricles through the interventricular foramen.

Thoracic cavity: A cavity that is surrounded by the ribs and muscles of the chest.

Thoracic inlet: The superior thoracic aperture refers to the superior opening of the thoracic cavity. It is referred to anatomically as the thoracic inlet and clinically as the thoracic outlet.

Thorax: The chest.

Threshold: The transmembrane potential at which an action potential begins.

Thromboplastin: A protease that converts prothrom­bin to thrombin in the early stages of blood clotting; also called thrombokinase.

Thrombopoietin (TPO): A glycoprotein hormone, produced mainly by the liver and the kidney, that regulates the production of platelets by the bone marrow. It stimulates the production and differen­tiation of megakaryocytes, the bone marrow cells that fragment into large numbers of platelets.

Thyroid: An endocrine gland whose lobes are lateral to the thyroid cartilage of the larynx.

Thyroxin: Also called T4, it is a hormone produced by the thyroid gland; it is less potent than T3 triio­dothyronine, which is also produced in lesser amounts by the thyroid gland.

Tidal volume (TV): The volume of air moved into and out of the lungs during a normal, quiet respira­tory cycle.

Tissue factor (TF): A protein present in subendothe­lial tissue, platelets, and leukocytes necessary for the initiation of thrombin formation from the zymogen prothrombin. Thrombin formation ulti­mately leads to the coagulation of blood.

Tissue plasminogen activator: A thrombolytic agent (clot-busting drug). It is approved for use in certain patients having a heart attack or stroke. This drug can dissolve blood clots, which cause most heart attacks and strokes.

Titin: A giant 3 MDa muscle protein and a major con­stituent of the sarcomere in vertebrate striated muscle. It is a multidomain protein that forms fila­ments approximately 1 μm in length spanning half a sarcomere. Titin has a number of functions, includ­ing the control of assembly of muscle thick fila­ments, a role in muscle elasticity, and the generation of passive tension.

Trabecula: A connective tissue partition that subdi­vides an organ.

Trachea: The windpipe, an airway extending from the larynx to the primary bronchi.

Transducin: The name given to the heterotrimeric G protein that is naturally expressed in vertebrate retina rods and cones (a different transducin gene is expressed in each cell type).

Transferrin: A plasma protein that transports iron through the blood to the liver, spleen, and bone marrow.

Transforming growth factor β (TGF-β): One of a number of structurally related growth factors involved in the regulation of growth, differentia­tion, and development of the many organ systems.

Transverse: Lying across the long axis of the body or of a part.

Transverse colon: The part of the colon from the hepatic flexure (the turn of the colon by the liver) to the splenic flexure (the turn of the colon by the spleen). The transverse colon hangs off the stomach, attached to it by a wide band of tissue called the mesocolon. The transverse colon is mobile (unlike the parts of the colon immediately before and after it), and it is very mobile in the abdomen of some individuals.

Transverse fissure: A short but deep fissure, about 5 cm long, extending transversely across the under­surface of the left portion of the right lobe, nearer its posterior surface than its anterior border.

Transverse fracture: A fracture in which the line of break forms a right angle with the axis of the bone.

Triad: The transverse tubule, and the terminal cister­nae on each side of it, in a skeletal muscle fiber.

Tricuspid valve: The right atrioventricular valve, which prevents the backflow of blood into the right atrium during ventricular systole.

Trigger zone: A specific area that, when stimulated by touch, pain, or pressure, excites an attack of neuro­logic pain.

Triglyceride: A lipid that is composed of a molecule of glycerol attached to three fatty acids.

Trypsin inhibitor: Chemicals that reduce the bio­availability of trypsin, an enzyme essential to nutri­tion of many animals.

Tuber cinereum: A prominence of the base of the hypothalamus, extending ventrally into the infun­dibulum and pituitary stalk.

Tunic: One of the enveloping layers of a part; one of the coats of a blood vessel; one of the coats of the eye; one of the coats of the digestive tract.

Tunica externa: The outermost layer of connective tissue fibers that stabilizes the position of a blood vessel.

Tunica interna: The innermost layer of connective tissue fibers in a blood vessel; it consists of the endo­thelium plus an underlying elastic membrana.

Tunica media: The middle layer of connective tissue fibers in a blood vessel; it contains collagen, elastin, and smooth muscle fibers in varying proportions.

Tympanic membrane: Colloquially known as "the eardrum," it is a thin membrane that separates the external ear from the middle ear. Its function is to transmit sound from the air to the ossicles inside the middle ear.

Type I alveolar cells: The single layer of flattened epithelial cells that forms the wall of the alveoli within the lungs.

Type II alveolar (or septal, cells): The cells within the alveolar walls that secrete pulmonary surfactant.

Ultradian rhythms: The regular recurrence in cycles of less than 24 hours from one stated point to another, as certain biologic activities which occur at such intervals, regardless of conditions of illumination.

Undershoot: A temporary decrease below the final steady-state value that may occur immediately fol­lowing the removal of an influence that had been raising that value.

Unipennate: Of or being a muscle whose fibers are attached obliquely to one side of a lateral tendon.

Unipolar neuron: A neuron in which embryological fusion of the two processes leaves only one process extending from the cell body.

Unmyelinated axon: An axon whose neurilemma does not contain myelin and across which continu­ous propagation occurs.

Upper respiratory system: The nasal cavity, pharynx, and associated structures.

Upstroke: The rising or depolarization phase of the action potential.

Urobilin: A compound derived from urobilinogen and ultimately from the bilirubin excreted in bile.

Urobilinogen: A colorless product of bilirubin reduc­tion. It is formed in the intestines by bacterial action. Part of it is resorbed and returned to the liver, while the rest is excreted in feces. Trace amounts can be detected in urine.

Uterus: A hollow, tubular organ with layers of smooth muscle and internal epithelial lining. It connects the cervix and the oviducts. It is responsible for sperm transport to the site of fertilization in the oviducts, accepts the early embryo, and houses the fetus during gestation.

Uvea: The middle of the three concentric layers that make up an eye.

Vagus nerve: Also called pneumogastric nerve or cranial nerve X, it is the tenth of twelve paired cranial nerves, and is the only nerve that starts in the brain stem (within the medulla oblongata) and extends, through the jugular foramen, down below the head, to the abdomen. The vagus nerve is arguably the single most important nerve in the body.

Variable: A quantity that is capable of assuming any of a set of values.

Varicosities: A varicose enlargement or swelling.

Vascular endothelial growth factor: A substance made by cells that stimulates new blood vessel for­mation, a mitogen for vascular endothelial (vessel lining) cells.

Vascular tunic: The middle of the three concentric layers that make up an eye; also called the uvea.

Vasoconstriction: A reduction in the diameter of arte­rioles due to the contraction of smooth muscles in the tunica media, it elevates peripheral resistance and may occur in response to local factors through the action of hormones, or from stimulation of the vasomotor center.

Vasodilation: An increase in the diameter of arteri­oles due to the relaxation of smooth muscles in the tunica media, it reduces peripheral resistance and may occur in response to local factors through the action of hormones, or after decreased stimulation of the vasomotor center.

Vasomotor center: The chief dominating or general center, which supplies all the unstriped muscles of the arterial system with motor nerves, situated in a part of the medulla oblongata; it is a center of reflex action, by the working of which afferent impulses are changed into efferent, vasomotor impulses leading either to dilation or constriction of the blood vessels.

Vasomotor nerve fibers: Sympathetic nerve fibers that cause the contraction of smooth muscle in the walls of blood vessels, thereby regulating blood vessel diameter.

Vasomotor tone: A relatively constant frequency of sympathetic impulses that keep blood vessels par­tially constricted in the periphery.

Venous return: The volume of blood returned to each atrium per minute from the veins.

Venous valves: A multicuspid structure located inside a vein. The valve cusps are attached to the vein wall and grow in a conjoined state.

Ventral: Pertaining to the anterior surface.

Ventral respiratory group: The area that controls expi­ration and acts to increase the force of inspiration.

Ventral root: The efferent motor root of a spinal nerve. At its distal end, the ventral root joins with the dorsal root to form a mixed spinal nerve.

Ventricles: A fluid-filled chamber; in the heart, one of the large chambers discharging blood into the pul­monary or systemic circuits; in the brain, one of four fluid-filled interior chambers.

Vermis: The narrow middle zone occurring between the two hemispheres of the cerebellum.

Vestibular apparatus: The sensory system that pro­vides the dominant input about one's movement and orientation in space.

Vestibular membrane: A membrane inside the cochlea of the inner ear. It separates scala media from scala vestibuli.

Vestibular nystagmus: Rapid involuntary rhythmic eye movement, with the eyes moving quickly in one direction (quick phase), and then slowly in the other (slow phase).

Vestibule: An enlarged area at the beginning of a canal, that is, inner ear, nose, larynx.

Visceral afferent: A pathway coming into the central nervous system that carries subconscious informa­tion derived from the internal viscera.

Visceral pleura: Serous membrane investing the lungs and dipping into the fissures between the several lobes.

Visceral serosa: The part of the double-layered mem­brane that lines the outer surfaces of organs within the ventral body cavity.

Vision: Visual perception via the visual system; one of the senses.

Visual pigments: Chemicals functioning in the visual cycle in retinal rod cells. Through excitation by visible light, a series of complex molecular changes occur that serve to trigger, in the optical nerve endings, an impulse transmitted to the brain, result­ing in the perception of vision.

Vocal ligament: The band that extends on either side from the thyroid cartilage to the vocal process of the arytenoid cartilage.

Voltage-gated channels: Channels in the plasma membrane that open or close in response to changes in membrane potential.

Voluntary striated: Also called skeletal muscle.

Wandering macrophages: The macrophages that migrate to a wound to effect cellular defenses against invading pathogens.

Wave summation: An increase in the frequency with which a muscle is stimulated, which increases the strength of contraction.

White blood cells (WBCs): Leukocytes; the granulo­cytes and agranulocytes of blood.

White matter: Whitish nerve tissue, especially of the brain and spinal cord, chiefly composed of myelin­ated nerve fibers and containing few or no neuronal cell bodies or dendrites; also called alba, substantia alba, or white substance.

White muscle fibers: Those fibers that contain low levels of myoglobin and oxygen-storing proteins and have a white appearance.

White rami: A bundle of nerve fibers that connect a sympathetic ganglion with a spinal nerve and are divided into two kinds, one consisting of myelin­ated preganglionic fibers.

Withdrawal reflex: A spinal reflex intended to protect the body from damaging stimuli. The classic example is when you touch something hot and withdraw your body part from the hot object.

Woven bone: Bony tissue characteristic of the embry­onic skeleton in which the collagen fibers of the matrix are arranged irregularly in the form of inter­lacing networks; also called nonlamellar bone or reticulated bone.

Xiphoid cartilage: The posterior and smallest of the three divisions of the sternum, below the gladiolus and the manubrium; also called xiphoid or xiphoid process.

Xiphoid process: The cartilage at the lower end of the sternum; also called ensiform cartilage, ensiform process, Xiphisternum, or xiphoid cartilage.

Yellow bone marrow: Bone marrow in which the meshes of the reticular network are filled with fat.

Z discs (or Z lines): Delicate membranelike structures found at either end of a sarcomere to which the actin myofilaments attach.

Zonular fibers: A ring of fibrous strands connecting the ciliary body with the crystalline lens of the eye.

Zygomatic arch: The arch formed by the temporal process of the zygomatic bone and the zygomatic process of the temporal bone; also called zygoma.