THE VERTEBRAL COLUMN. 121 
vertebrae. In each vertebra the transverse process is perforated 
by a costo-transverse foramen (foramen 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. lis posterior surface bears on either side a 
somewhat triangular inferior articular facet (facies 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 
