The Skeleton 53 



processes that bear the laterally and ventrally 

 facing postzygapophyses (Fig. 17, 6). 



The atlas, as in other vertebrates, is highly 

 specialized. It consists (Fig. 1 7) , even in the adult 

 animal, six feet or more in length, of four distinct 

 portions, a ventral (4), a dorsal (i), and two lateral 

 (2) parts. The ventral portion is relatively more 

 massive than in most animals ; its anterior surface 

 is concave and forms the main part of the articular 

 surface for the occipital condyle of the skull. Its 

 postero-dorsal surface articulates with the odontoid 

 process of the axis. On its postero-lateral surfaces 

 are the facets for articulation with the first ribs, 

 which, unlike the other cervical ribs, have but one 

 articular surface. Articulating dorsally with this 

 ventral element of the atlas are the two rather 

 heavy lateral elements which form the neural 

 arch. Anteriorly they form the lateral parts of the 

 articular surface for the condyle and dorsally they 

 unite for a short distance with each other. Pro- 

 jecting ventrally from the posterior part of their 

 dorsal portion are the small postzygapophyses. 

 Ventrally and laterally they articulate with the 

 odontoid process (Fig. 17, 3). Projecting dorsad and 

 cephalad from the dorsal surface of these lateral 

 elements is the dorsal element of the atlas (Fig. 

 17, i), the pro-atlas, which may not be properly 

 a part of the vertebral column at all, since it is said 

 to be merely a membrane bone. Gadow says it 

 is the detached neural spine of the atlas. It is 



