The bones are grouped into the axial skeleton (skull, vertebrae, scapulae, sternum, ribs, and pelvis) and the appendicular skeleton (limbs). Bones can also be classified as long bones, short bones, flat bones, and irregular bones. This classification is important in understanding development of the skeletal system (flat bones undergo intramembranous ossification and the other types undergo endochondral ossification) and because susceptibility to some disease processes varies among the bone types.

The skull (Figure 5.1) consists of 14 paired and 7 unpaired bones, with more than 40 foramina that allow for the passage of vessels and nerves. These bones can be divided into the calvaria or skull cap (dorsal parts of the frontal, occipital, and parietal bones), the base of the skull (ventral parts of the frontal, parietal, and occipital; sphenoid, temporal, and ethmoid), and facial bones (maxilla, mandible, nasal, lacrimal, palatine, incisive, pterygoid, vomer, zygomatic, and nasal conchae). The only synovial joints in the head are the temporomandibular joints and the atlanto-occipital joint. Other joints are synarthroses or sutures, which may be open at birth, but typically close by 6 weeks of age or, less commonly, by 6 months of age. Initially, these joints are fused by fibrous connective tissue, but they may become ossified in older animals.

The vertebral column consists of cervical, thoracic, lumbar, fused sacral, and coccygeal or caudal vertebrae. Species variations are highlighted in Table 5.1. Vertebrae are classified as irregular bones and are formed through endochondral ossification. The parts of a typical vertebra are labeled in Figure 5.2. The first cervical vertebra, or the atlas, articulates with the occipital condyles of the skull (Figure 5.3). Other vertebrae articulate with their cranial and caudal neighbors via facets. The transverse processes of the thoracic vertebrae articulate with the heads of the ribs. Ribs are flat bones, composed of a dorsal ossified part and a ventral cartilaginous part. The ribs also articulate with the bones of the sternum (sternebrae), which are connected by intersternebral synchondroses. The manubrium is the first sternebra, and the xiphoid process is the last sternebra.

The hyoid apparatus (stylohyoid, epihyoid, ceratohyoid, basihyoid, and thyrohyoid; see Figure 7.3) and the ossicles of the inner ear (incus, malleus, and stapes) are also part of the axial skeleton. The hyoid apparatus supports the tongue, pharynx, and larynx, and the ossicles transmit and amplify sound waves from the tympanic membrane, through the middle ear, to the interface with the fluid-filled inner ear.

The bones of the forelimb include the scapula, humerus, radius, ulna, carpal bones (radial, ulnar, accessory, first, second, third, and fourth), metacarpal bones, and phalanges (proximal, middle, and distal). The clavicle or collarbone is only well developed in animals that use the forelimb for climbing or grasping, such as primates, bats, and some rodents. Cats and dogs have a rudimentary clavicle, while the bone is absent in ruminants, pigs, camelids, and equids. The hindlimb bones are the pelvic bones (ischium, ilium, and pubis), femur, patella, tibia, fibula, tarsal bones (talus, calcaneus, central, first, second, third, and fourth), metatarsal bones, and phalanges (proximal, middle, and distal). Sesamoid bones are embedded in tendons of the forelimb and hindlimb to improve leverage and alleviate stress. The number of digits varies between species, as illustrated in Table 5.1 and Figure 5.4.