Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/39

Rh vertebrae, each one fastened to its neighbor by connective tissue. The back-bone supports the neck, trunk, and tail, and to it are attached the ribs, pelvic girdle and numerous muscles. It comprises seven cervical, thirteen thoracic, six lumbar, four sacral, and about twenty-eight caudal vertebrae.

A typical vertebra possesses a solid, more or less cylindrical ventral portion, the centrum, or body. The vertebral, or neural, arch is attached to the upper side of the body, the two inclosing the vertebral foramen, through which the spinal cord passes. The roots of the vertebral arches (pedicles) are the two lateral vertical plates of the vertebral arch. The two horizontal plates forming the roof of the arch are the laminae. The neural spine, or spinous process, projects upward from the junction of the laminae. The vertebra bears on each side a transverse process. The spinous and transverse processes serve for the attachment of spinal muscles. Each vertebral arch bears two pairs of articular processes, or zygapophyses. Of these the anterior one, or prezygapophysis, articulates with the preceding vertebra ; the posterior surface, or post-zygapophysis, articulates with the succeeding vertebra.

The essential features of the vertebral column may be brought out best by describing vertebrae from each region of it. The seven cervical vertebrae are quite similar to one another, except the first two, the atlas and epistropheus (axis). The atlas is essentially a ring of bone. It lacks a centrum, this element having been separated from the atlas and attached to the epistropheus as the dens (odontoid process). This process rests in a semi-circular depression, the fovea dentis, on the ventral floor of the atlas. The dorsal surface of the atlas bears a tubercle comparable to the spinous process of an ordinary vertebra. The ventral surface bears a similar tubercle. The atlas articulates