56 COMPARATIVE ANATOMY 



In the Urodele Necturus four cartilaginous "abdominal ribs" (see 

 below) may be present in the septa between the ventral parts of the 

 myotomes on the level of the shoulder-girdle. Bony abdominal ribs also 

 occur in certain Stegocephala. 



Reptiles. As already mentioned, the ribs of the Aniniota are 

 comparable to those of the Amphibia, but they grow further 

 ventral wards and so encircle the body-cavity to a greater or less 

 extent. The dorsal (proximal) section of the rib may also become 

 segmented from the distal (ventral) portion ; and as a rule a certain 

 number of the ribs unite together ventrally to form a sternum 

 (Fig. 44) : these are usually distinguished as " true " ribs from the 

 others, or " false " ribs. 



The ribs of Snakes show the least amount of differentiation ; for, 

 without giving rise to a sternum, they extend along the whole 

 trunk from the third cervical vertebra to the anus, and retain 

 throughout a similar form and size. In Lizards, where a dorsal, 

 unforked, bony and a ventral cartilaginous portion can be distin- 

 guished, three or four ribs reach the sternum, and are not always 

 completely segmented off from it. 



In Chelonians the cervical ribs unite with the, vertebras more or 

 less completely, and in the region of the trunk the ribs become fused 

 with the costal plates of the carapace (p. 33). Their proximal 

 unbifurcated ends are attached between the centra, at the junction 

 of centrum and arch. There is no sternum. 



The proximal ends of the cervical ribs in the Crocodilia are 

 bifurcated, in correspondence with the double transverse processes 

 in this region, and thus a vertebrarterial canal is formed. Further 

 back, the ribs increase in length, and become segmented into two 

 or three articulated portions. In passing from before backwards, 

 their point of origin becomes gradually shifted, so that while the 

 anterior thoracic ribs are attached to the centra, the posterior ones 

 arise entirely from the transverse processes, which increase in size 

 correspondingly. Eight or nine ribs reach the sternum, and from 

 the eighteenth vertebra backwards the transverse processes no 

 longer bear ribs, but only short cartilaginous apophyses. 



Uncinate processes (see below) are present in connection with 

 the ribs in the Crocodilia as well as in Hatteria. 



"Abdominal ribs," arising as ossifications of the inscriptiones tendinea$ 

 of the ventral muscles, occur in Crocodiles and in Hatteria, as well as in 

 numerous fossil Reptiles. 



Birds. The ribs of Birds exhibit a much more marked 

 segmentation into vertebral and sternal portions, both of which 

 become ossified, and this evidently stands in relation to their 

 more active respiration. Uncinate processes, moreover, arise from 

 the vertebral portions in nearly all Birds, and overlap the 

 ribs next behind them (Fig. 41). The whole costal apparatus 



