SKELETON AND TEETH 71 



accomplished by the partial rotation of skull and atlas to- 

 gether upon the second vertebra in a manner presently to be 

 explained. On the hinder aspect are two articular surfaces 

 {posterior cotyles) in shape like the anterior pair, but very much 

 less concave, which are in contact with corresponding surfaces 

 on the second vertebra. The neural arch of the atlas is broad 

 and low and the neural canal is apparently much too large 

 for the spinal cord, but, in fact, only a part of the circular 

 opening belongs to the neural canal. In life, the opening is 

 divided by a transverse ligament into an upper portion, the 

 true neural canal, and a lower portion, which lodges a pro- 

 jection from the second vertebra. The atlas usually has no 

 neural spine and never a prominent one ; the transverse 

 processes are broad, wing-like plates and each is perforated by 

 a small canal, which transmits the vertebral artery. 



The second vertebra, or axis, is a little more like the ordi- 

 nary vertebra, having a definite and usually elongate centrum, 

 on the anterior end of which are the two ar- 

 ticular surfaces for the atlas. Between these 

 is a prominent projection, the odontoid pro- 

 cess, which fits into the ring of the atlas and 

 has a special articulation with the lower bar r;/",~'^T°^ ^j*^'^' 



^ left side. od.p. odon- 



of that ring. In most mammals the odon- toid process, cot., an- 



. . 1 • 1 1 j_i • 1 • terior cotyles. n.a., 



toid process is a bluntly conical peg, varying ^^^^^.^j ^^^j^ ,^ ^.^^ 

 merely in length and thickness, but in many neural spine, pt.z., 



, IIP i 1 • J. 1 • - posterior zygapophy- 



long-necked torms the peg is converted into a ses. <r., transver.se pro- 

 semicylindrical spout, convex on the lower ^®^^- «'fl'-. anterior 



1 • /> opening of canal for 



Side and concave above. The neural spine of the vertebral artery. 

 the axis is almost always a relatively large, !" "', posterior open- 



\ _ '' ° ' ing of the same. 



hatchet-shaped plate, which is most developed 

 in the carnivorous forms, and the transverse processes are com- 

 monly slender rods. 



The five succeeding cervical vertebrae are much alike, though 

 each one has a certain individuality, by which its place in the 

 series may readily be determined. The centrum has a convex 



