RIBS 67 



process which becomes connected with the neural arch and on 

 further development may serve as the chief point of attachment 

 for the rib. The proximal part of the primitive basal stump is 

 correspondingly reduced, and, with rare exceptions, is no longer 

 developed : in its' place is formed a bony bar, arising from the 

 centrum, and generally not preformed in cartilage. 



The ribs of the Urodela and Gymnophiona are bifurcated 

 at their proximal ends, the ventral bar corresponding to the 

 primary rib-rudiment, while the dorsal bar is a secondary 

 structure formed in order to give the rib a firmer connection with 

 the vertebra : in Urodeles it becomes connected with the rib- 

 bearing portion of the vertebra, and in the Gymnophiona with the 

 neural arch itself. 1 



The ribs of Amphibians are never very highly developed : 

 they are only slightly curved and do not encircle the body-cavity 

 to any extent. In Anurans they are not bifurcated and are 

 very short and stump-like (Fig. 45), usually becoming fused 

 with the transverse processes : they have doubtless undergone 

 reduction. 



In many Urodeles the ribs are limited to the trunk, but 

 occasionally one or more pairs occur in the anterior part of the 

 tail, where the basal stumps have already extended ventralwards 

 to form the hasmal arches. 



Finally, reference must be made to the cartilaginous " abdominal 

 ribs " (cf. p. 42) developed in the ventral intermuscular septa in 

 many Amphibians (Necturus, Menopoma, Bombinator). 



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

 comparable to those of the Amphibia, but they grow further 

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

 extent. Ribs may also be present in the tail : in Hatteria, for 

 instance, there are seven or more pairs of caudal ribs. 



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

 segmented from the distal (ventral) portion, 2 and the former is 

 plainly homologous with the Urodele-rib. As a rule a certain 

 number of the ribs unite together ventrally to form a sternum 

 (cf. p. 72 and Fig. 56) : 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 vertebra to the anal region, having 

 a similar form and size throughout. In Lizards, in which a dorsal, 

 unforked. bony portion and a ventral, cartilaginous portion can be 

 distinguished, three or four ribs reach the sternum, and are not 

 always completely segmented off from it. The proximal ends of 



1 According to another view, the bifurcated amphibian rib is originally a 

 double structure, the dorsal bar of the fork originating independently and only 

 uniting secondarily with the ventral bar. 



- An intermediate section also occurs in Crocodiles and man}' Lizards. 



F 2 



