THE SKELETAL SYSTEM 25 



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 cylin- 

 drical ventral portion, the centrum, or body. The verte- 

 bral, 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 zyg apophyses. 

 Of these the anterior one, or prezyg apophysis, 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 (odon- 

 toid process). This process rests in a semi-circular depres- 

 sion, 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 ven- 

 tral surface bears a similar tubercle. The atlas articulates 



