146 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



to the region near the backbone. 1 In some forms {Megaloba- 

 trachns and some stegocephalans) the ribs are not confined to 

 the trunk region, but from three to eight pairs may occur in 

 the tail. It is to be noted that the pelvis does not articulate 

 directly with the transverse process of the sacral vertebra, but 

 that connection is effected by the intervention of a sacral rib, 

 distinct in many forms. In the caecilians ribs occur on every 

 vertebra except the first and last. 



In the amniotes the ribs in the trunk region acquire a much 

 greater development, and, like the hoops of a barrel, extend 



FIG. 154. Pelvis and sacrum of alligator. /, ilium; R, sacral ribs ; 

 S', S", sacral vertebrae. 



around the body cavity. They may be ossified throughout their 

 extent, in which case each rib is usually divided into several 

 segments (crocodile ), 2 but usually a considerable portion re- 

 mains cartilaginous. Ventrally they may terminate freely, or 

 they may connect with a sternum to be described later. In the 

 great majority of the birds, as well as in some reptiles (croco- 

 dilia, rhynchocephalia), each rib bears a backwardly directed 

 uncinate process, which overlaps the rib behind, thus giving 

 additional strength to the thoracic framework. 



1 Ribs occur in the ventral region of some stegocephali (see p. 147), and cartilaginous 

 ventral ribs have been described in Necturus and Menopoma (urodeles). 



2 The median segment in crocodiles is not truly ossified, but is cartilage partially 

 calcified. 



