234 



THE ANATOMY OF VEKTEBEATED ANIMALS. 



and the odontoid vertebra are similar to those of the 

 Lizards, and the atlas is the only precaudal vertebra which 

 is devoid of ribs. The centra have nearly hemispherical 

 articular surfaces, and thus differ from those of ordinary 

 Lacertilia, while the superadded articular processes found 

 only in certain Lizards attain a great development in the 

 Snakes. The zygapophyses are broad and flattened, and the 

 outer sui'faces of the anterior pair are commonly prolonged 

 into a process. The anterior surface of the arch above the 



Fifr. 71. 



Fig. 71.— Auterior and posterior views of the dorsal vertebra of a 

 Pj'thon : — z. s., zygosphene; z. a., zygantrum ; p. z., prezygapo- 

 physes ; pt. z., postzygapophyses ; t.p., transverse processes. 



neural canal is produced into a strong wedge-shaped zygo- 

 sphene, which fits into a con-esponding zygantrum of the 

 next preceding vertebra ; and, on the posterior surface of 

 the arch, there is a zygantrum for the zygosphene of the 

 next preceding vertebra. (Fig. 71.) 



^ The transverse processes are short and tubercle-like, and 

 the heads of the ribs which articulate with them are simple. 

 Each rib usually gives off a short upward process at a little 

 distance from its head; it is curved, usually hollow, and 

 terminates, inf eriorly, in a cartilage which is always free, no 

 trace of a sternum existing. Strong descending processes 

 are given off from the undersides of many of the presacral 



