THE VERTEBRAL COLUMN 159 



region and the first two cervical vertebrae (the atlas and the 

 epistropheus) are specially modified to provide for movements of 

 the skull. The posterior vertebrae (Fig. 82, C) are dorsoventrally 

 compressed, with low arches and short spinous processes. In the 

 seventh vertebra, however, the spinous process begins to be elon- 

 gated as in the succeeding thoracic vertebrae. In each vertebra 

 the transverse process is perforated by a costo-transverse foramen 

 (foramen transversarium), which serves for the passage of the 

 vertebral artery forward to the head. This aperture divides the 

 base of the transverse process into a dorsal, or posterior root (radix 

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

 development of these parts shows that the anterior root is really 

 a reduced rib which has become fused to the body and to the 

 transverse process and is comparable in its general relations to the 

 normal ribs of the thoracic vertebrae. 



Atlas 



The first vertebra is the atlas (Fig. 82, 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 possess- 

 ing special articular surfaces, and in having its transverse process 

 greatly flattened dorsoventrally. It consists of a ventral half-ring, 

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

 arch (arcus posterior), and paired lateral masses (massaelaterales), 

 the last being thickened regions of the bone uniting the arches at 

 each side and forming the bases of the • transverse processes. 

 The anterior arch bears on its ventral side a small backwardly- 

 directed process, the anterior tubercle (tuberculum anterius), 

 named from its position in the human body, where the ventral 

 surface is anterior. A similar posterior tubercle (tuberculum 

 posterius) projects forward on the dorsal surface of the posterior 

 arch and 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 one of the 

 convex occipital condyles of the skull. Its posterior surface 

 bears en either side a smaller, somewhat triangular, inferior 

 articular facet (facies articularis inferior) for articulation with 

 the epistropheus. These surfaces take the place of the arch 



