The Vertebral Column. 121 



vertebrae. In each vertebra the transverse process is perforated 

 by a costo-transverse foramen (foiamen transversarium) , the 

 latter serving for the passage of the vertebral artery forward to the 

 head. Through the presence of this aperture, the base of the 

 transverse process is divided into two parts, namely, a dorsal, or 

 posterior root (radix posterior), and a ventral, or anterior root 

 (radix anterior). The anterior root is a coalesced rib, and is com- 

 parable in its general relations to the normal ribs of the thoracic 

 vertebrae. 



The first vertebra is the atlas (Fig. 56, A). It is peculiar in 

 lacking the vertebral body, the latter being represented by the 

 odontoid process of the epistropheus (cf . Plate II) ; also in possessing 

 special articular surfaces, and in having its transverse process 

 greatly flattened in the dorsoventral direction. It consists of a 

 ventral half-ring, the anterior arch (arcus anterior), a dorsal 

 half-ring, the posterior arch' (arcus posterior) , with paired lateral 

 masses (massae laterales) uniting them. The lateral masses also 

 form the bases of the transverse processes. The anterior arch 

 bears on its ventral side a small backwardly-directed process, the 

 anterior tubercle (tuberculum anterius). A similar posterior 

 tubercle (tuberculum posterius) on the dorsal surface of the 

 posterior arch is comparable to the spinous process of an ordinary 

 vertebra. The anterior surface of the atlas bears on either side an 

 extensive concave smooth surface, the superior articular pit (fovea 

 articularis superior), for articulation with the convex occipital 

 condyles of the skull. Its posterior surface bears on either side a 

 somewhat triangular inferior articular facet (fades articularis 

 inferior) for articulation with the epistropheus. These surfaces 

 take the place of the arch articulations of ordinary vertebrae. 

 Through the compression of the transverse process, the costo- 

 transverse foramen is converted into a canal. The anterior aper- 

 ture of this leads by a shallow groove,- the sulcus arteriae 

 vertebralis, into a foramen perforating the posterior arch. 

 This aperture, represented in some mammals by separate alar 

 and intervertebral foramina serves to transmit the vertebral artery 

 and the first cervical nerve. 



The space enclosed by the atlas is divided into a dorsal portion, 

 corresponding to the vertebral foramen of other vertebrae, and a 



