SKELETON. In most animals and plants, the shape could not be maintained without a thickening and hardening of certain parts to form a support for the whole. These hardened parts are the skeleton; they dry and remain after the rest of the body has disappeared. In higher animals the skeleton is always rendered more rigid and permanent by the deposit in it of lime salts, thus leading to the formation of bone. In most of the lower or inverte brate animals, the skeleton is on the surface and acts as a pro tection as well as a framework. This is an exoskeleton. In the higher or vertebrate animals there is an internal or endoskeleton and the exoskeleton is either greatly modified or disappears.
The following account is divided into (I) axial, or skeleton of the trunk, (2) appendicular or skeleton of the limbs, (3) visceral skeleton, or those parts which originally form the gill supports of water breathing vertebrates. The skull and exoskeleton are con sidered separately (see SKULL; SKIN AND EXOSKELETON).
It lies in the middle of the back of the neck and trunk; has the cranium at its summit ; the ribs at its sides, which in their turn support the upper limbs ; whilst the pelvis, with the lower limbs, is jointed to its lower end. The spine consists in an adult of twenty-six bones, in a young child of thirty-three, certain of the bones in the spine of the child becoming ankylosed later. The bones of the spine are arranged in groups, named from their position, cervical, thoracic (form erly called dorsal), lumbar, sacral, and coccygeal or caudal; and the number of vertebrae in each group may be expressed in a formula. In man the formula is as follows = 33 bones, as seen in the child; but the five sacral vertebrae fuse together into a single bone— the sacrum—and the four coccy geal into the single coccyx.
The vertebrae are irregularly-shaped bones, but have certain characters in common. Each possesses a body and an arch, which enclose a ring, with certain processes and notches. The body, or
centrum, is a short cylinder, which by its upper and lower surfaces is connected by fibrocartilage with the bodies of the vertebrae immediately above and below. The arch encloses the spinal mar row or nervous axis, springs from the back of the centrum, and consists of two symmetrical halves united behind in the middle line. Each half has an anterior part or pedicle, and a posterior part or lamina. The processes usually spring from the arch. The spinous process projects backward from the junction of the two laminae, and the collective series of these processes gives to the entire column the character from which has arisen the term "spine." The transverse processes project outward, one from each side of the arch. The articular processes project, two upward and two downward, and are for connecting adjacent vertebrae. The notches, situated on the upper and lower borders of the pedicles, form in the articulated spine the intervertebral foramina through which the nerves pass out of the spinal canal. The vertebrae in each group have special characters.
Cervical Vertebrae.—In man and all mammals, with few excep tions, whatever be the length of the neck, the cervical vertebrae are seven in number. The first, or atlas, has no body or spine: its ring is very large, and on each side of the ring is a thick mass of bone by which it articulates with the occipital bone above and the second vertebra below. The second vertebra, axis, has its body surmounted by a thick, tooth-like odontoid process, which is re garded as the body of the atlas displaced from its proper vertebra and fused with the axis. This process forms a pivot round which the atlas and head move in turning the head from one side to the other; the spine is large, thick and deeply bifid. The seventh is distinguished by its long prominent spine, which is not bifid, and by the small size of the foramen at the root of the transverse process. In the human spine the distinguishing character of all the cervical vertebrae is the foramen at the root of the transverse process.