Chapter summary

Classification of bones

1. Osteology is the study of bones.

2. Bones are classified as long, short, flat, or irregu­lar or sesamoid on the basis of their shape and their proportion of compact or spongy bone.

Bone structure

Gross anatomy

1. Bones consist of compact (dense) and cancellous (spongy) bone.

2. Spongy bone consists of a network of bone called trabeculae or spicules interspersed with spaces filled with red or yellow marrow. Compact bone is found on the surface while spongy bone is found on the interior.

3. A long bone consists of a diaphysis (shaft) and epiphyses (ends), with a metaphysis joining the two. The medullary cavity of the diaphysis contains yellow marrow; the epiphyses contain spongy bone. The bone elongates in the region of the epiphyseal plate, which, after growth ceases, is replaced by the epiphyseal line.

4. The fibrous covering surrounding the bone is the periosteum while the internal cavity is lined with the endosteum. Hyaline cartilage covers joint surfaces.

5. Flat bones consist of two thin plates of com­pact bone enclosing a diploe, or spongy layer. Short and irregular bones resemble flat bones structurally.

6. In adults, hematopoietic tissue is found within the diploe of flat bones and occasionally within the epiphyses of long bones. In young animals, red marrow is also found in the medullary cavity.

Microscopic anatomy of bone

1. The structural unit of compact bone is the osteon, or Haversian system. Each osteon consists of a central canal running parallel to the long axis, which is surrounded by concentric lamellae of bone matrix. Osteocytes, embedded in lacunae, are connected to each other by sending processes that project into canaliculi, which are narrow canals connecting the lacunae.

Chemical composition of bone

Bone is composed of organic and inorganic compo­nents.

Bone development

1. Osteogenesis, or bone formation, occurs in two manners. The flat bones of the skull, face, man­dible, and clavicle form through intramembra- nous ossification in which the bones develop from a fibrous membrane. The cells of the con­nective tissue differentiate into osteoblasts.

2. Other bones develop via endochondral ossifica­tion in which cells first become chondroblasts and make a hyaline cartilage matrix. Eventually, near the center of the bone, a primary ossification center forms in which cells from the periosteum migrate inward and become osteoprogenitor cells, which then become osteoblasts. Secondary ossification centers develop in each epiphysis.

3. During endochondral ossification, the cartilage at each end of the diaphysis forms five regions: (a) zone of reserve cartilage (most distal from the diaphysis), (b) zone of proliferation, (c) zone of hypertrophy, (d) zone of calcified cartilage, and (e) zone of resorption.

4. Cartilages grow from within (interstitial growth) and by addition of new cartilage tissue at the periphery (appositional growth).

Bone growth, remodeling, and repair

1. Long bones increase in length by interstitial growth of the epiphyseal plate cartilage and its

replacement by bone. Eventually, the epiphyseal cartilage is replaced by bone and epiphyseal closure occurs, which marks the end of long bone lengthening.

2. Appositional growth increases bone diameter/ thickness.

3. New bone is continually deposited and resorbed in response to hormonal and mechanical stimuli, a process called bone remodeling.

4. Osteoclasts release lysosomal enzymes and acids on bone surfaces that digest and resorb bone. The dissolved products are transcytosed to the opposite side of the osteoclast and released into the interstitial space.

5. Eventually, osteoclasts are replaced by osteo­blasts, which rebuild the bone.

6. When blood calcium levels decline, PTH is released from the parathyroid glands and stimu­lates osteoclasts to digest bone matrix, releasing ionic calcium. When blood calcium levels rise, calcitonin is released from the thyroid glands, stimulating deposition of calcium into the bone.

Bone repair

1. Fractures can be classified as a displaced or non­displaced fracture, complete or incomplete frac­ture, linear or transverse fracture, or an open or closed fracture.

2. The bone repair process involves four steps: (a) hematoma formation, (b) fibrocartilaginous callus formation, (c) bony callus formation, and (d) bone remodeling.

Homeostatic imbalances of bone

1. Osteomalacia is a condition in which the bones are insufficiently mineralized. Rickets is a name for the same condition when it is present in pre­pubertal animals.

2. Parturient paresis (milk fever) occurs at the beginning of lactation when the cow cannot mobilize enough calcium to meet the needs for milk production. Symptoms include loss of appetite, followed by muscle weakness, decreased body temperature, labored breathing, and paralysis of hind legs.

3. Egg laying fatigue in birds is similar to milk fever in cows. High-producing egg-laying hens can develop weak and brittle bones. A hen must deposit as much as 8-10% of her total calcium into the eggshell each day.

4. Osteoporosis is any condition in which bone breakdown outpaces bone formation, causing bones to become weak and porous. Postmeno­pausal women are particularly susceptible.

5. Paget's disease is characterized by excessive and abnormal bone remodeling.

Bones and skeleton

Markings on bones

Bones contain various depressions, bumps, and ridges that serve as sites where muscles and tendons originate or attach and blood vessels and nerves travel.

Skeleton

1. The skeleton includes all of the bones of the body.

2. Functions of the skeleton include (1) support, (2) storage of minerals and lipids, (3) protection, (4) blood cell production, and (5) leverage.

3. There are three types of cartilage found in the skeleton. Hyaline cartilage is the most abundant, and provides support and flexibility for the skel­eton. Elastic cartilage contains more elastic fibers than hyaline cartilage. It is, therefore, better able to withstand bending. It is found in only two places: the external ear and the epiglottis. Fibro- Cartilage is highly compressible, possessing great tensile strength. It is found in the menisci within the knee and intervertebral discs.

4. Cartilage can continue to grow by two processes. Appositional growth occurs when new cartilage forms on the surface of preexisting cartilage. Interstitial growth occurs from inside of the car­tilage mass.

Skeleton classification

The skeleton is divided into the appendicular skel­eton, which includes the bones of the limbs and limb girdles, and the axial skeleton, which includes the skull, hyoid apparatus, vertebral column, ribs, and sternum.

Axial skeleton

1. The skull is a very complex structure made mostly of flat bones. Except for the mandible that is attached via a moveable joint, the bones of the skull are connected by interlocking joints called sutures.

2. The cranium includes those bones that surround the brain.

3. The facial bones include those bones enclosing the nasal and oral cavities. These bones form the structure of the face.

The vertebral column

1. The vertebral column, also called the spine, pro­tects the spinal cord, supports the head, and serves as an attachment site for muscles affecting body movements.

2. The vertebrae are the irregularly shaped bones making up the spinal column. They are grouped into the cervical (neck), thoracic (back), lumbar (loin), sacral, and caudal (tail) vertebrae. The number of different groups of vertebrae differs by species.

3. The common features of a vertebra include the body, vertebral arch, vertebral foramen, and processes. The processes include a midsaggital dorsal projection called the spinous process, two lateral extensions called the transverse processes, and four articular processes.

4. The first and second cervical vertebrae are called the atlas and axis, respectively.

5. The vertebral foramen of each vertebra connects to form the vertebral canal.

Thoracic

1. The thorax is the bony cavity formed by the sternum, ribs, costal cartilages, and bodies of the thoracic vertebrae. The sternum, or breastbone, are the unpaired bones (sternebrae) forming the floor of the thorax. The thoracic cage protects the organs of the thoracic cavity.

2. The manubrium is the enlarged first Sternebra while the xiphoid process is the last Sternebra capped by the xiphoid cartilage.

3. The ribs consist of long, curved bones that form the lateral wall of the thorax. The ribs can be grouped as follows:

a. True ribs. These articulate directly to the sternum via their costal cartilage.

b. False ribs. These include all ribs that are not true ribs. Their costal cartilages merge to form the costal arch, which indirectly joins them to the sternum in all domestic species except the dog.

c. Floating ribs. These include the last false ribs in the dog and man. There is one pair in dogs and two pairs in man.

Appendicular skeleton

Thoracic limb

1. While humans have the clavicle to keep the shoulder in a lateral position, domestic animals lack this bone since their thoracic limb is main­tained under their body. The top of the thoracic limb begins at the scapula.

2. The humerus, sometimes called the brachial bone, is the largest bone in the thoracic limb. It articulates proximally with the scapula in the glenoid cavity forming the shoulder joint, and distally with the radius and ulna forming the elbow joint.

3. The radius and ulna fuse in the horse and ruminants.

4. The distal portion of the thoracic limb is techni­cally the manus, commonly called the forepaw in carnivores. It consists of the carpus, meta­carpus, and digits, the latter with their individ­ual phalanges and their associated sesamoid bones.

The pelvic (hip) girdle

1. The pelvic girdle, or bony pelvis, consists of the two hip bones (ossa coxarum), sacrum, and the first few caudal vertebrae. It encases the pelvic cavity.

2. The hip bone (os coxae) consists of the fused ilium, ischium, pubic, and acetabular bones. The two hip bones are fused at the pelvic sym­physis. The femur, or thigh bone, articulates proximately with the hipbone, forming the hip joint, and distally with the tibia, forming the stifle joint.

3. The tibia and fibula are located between the femur and metatarsal bones.

4. The tarsus, or "hock," consists of the three rows of bones between the tibia/fibula and metatarsal region.

5. Metatarsal bones and digits are located distal of the tarsus.

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