Sauropsida: Class Reptilia 



171 



In contrast to amphibians, reptiles have well-developed ribs. They 

 may be present on at least some of the vertebrae of each of the four 

 regions of the column (Fig. 363). Cervical ribs are usually short and 

 may be fused to the vertebrae. They may be absent from the first two 

 or three vertebrae, but in crocodilians even the atlas and axis carry 

 ribs. It is in the trunk region, and especially the more anterior part of 

 it, that the ribs are best developed. Here they are so long as to nearly 

 encircle the body. Each rib may consist of two or three segments, the 

 ventral one being cartilaginous. Posteriorly, they diminish in length 



Fig. 365. Sternum, 

 etc., of Iguana luber- 

 culata. (c) coracoid : 

 (cl) clavicle; (e) epi- 

 sternum; (ec) epicora- 

 coid; (//.) humerus; 

 {pc) precoracoid; {s) 

 sternum; (x) xiphi- 

 sternum. (After Blan- 

 chard. Courtesy, 

 Kingsley: '"Compara- 

 tive Anatomy of Ver- 

 tebrates," Philadel- 

 phia, The Blakiston 

 Company.) 



and several of the most posterior trunk vertebrae may lack ribs. Each 

 of the two sacral vertebrae carries a pair of short, stout ribs. The two 

 ribs on each side are joined to the upper end of the dorsal bone (ilium) 

 of that side of the pelvic girdle. The more anterior postsacral vertebrae 

 may carry short ribs. In snakes, usually all vertebrae behind the axis 

 and as far back as the cloacal region carry ribs which nearly encircle 

 the body and are of nearly equal length throughout the entire distance 

 — which is considerable in a snake. 



In reptiles except snakes, some of the "legless lizards," and turtles, 

 there is a median ventral skeletal plate, usually entirely cartilaginous, 

 with which the ventral ends of several of the more anterior pairs of 

 trunk ribs are joined. This is the sternum, corresponding to the 

 "breastbone" of man (Fig. 365). The shorter, more posterior ribs 

 do not meet the sternum. Attached to the anterior end of the sternum 



