50 COMPARATIVE ANATOMY. 



or four ribs reach to the sternum, 1 and are not always completely 

 segmented off from it. 



The ribs of Birds exhibit a much more marked segmentation 

 into vertebral and sternal portions, and this evidently stands in 

 relation to respiration; they moreover develop so-called unci- 

 nate processes (Fig. 36, Uri). In this latter, as in many other 

 points, they show a relation to certain Reptiles (viz. Hatteria and 

 Crocodiles). 



The ribs of Archseopteryx are of special interest, as they are more simi- 

 lar to those of Reptiles than to those of Birds, though they do not closely 

 resemble the former. Their structure is delicate, their ends are pointed, 

 and no uncinate processes have been observed : in transverse section they are 

 ellipsoidal, and not flattened like those of Birds. Whether a connection with 

 a breast-bone existed is not certainly proved, as nothing is known of a sternum 

 or of sternal ribs. The breast-bone must at any rate have been very small, as 

 the " abdominal ribs " extend far forwards ; it was probably provided with a 

 keel, for the quills of the wing are well developed. 



FIG. 37. COSTAL ARCH OF MAN. 



WK, centrum of vertebra ; Pt, transverse process ; Ps, neural spine ; Cp, body of 

 rib ; Ca, capitulum ; Co, neck ; Tb, tuberculum ; Kn, cartilaginous (sternal) 

 rib ; St, sternum. 



It has already been mentioned that the cervical ribs and transverse 

 processes may become united together in representatives of all the Amniota, 

 and the fusion between the ribs and dermal plates in Chelonians may be 

 here noted. 



In the true ribs of Mammals, and especially in those of Man, 

 a capitulum, a neck, a tuberculum, and a body may be distinguished 

 (Fig. 37). The capitulum articulates with the centrum, and the 

 tuberculum with the transverse process of the vertebra. v The 

 number of ribs which reach the sternum varies considerably. 



1 In Crocodiles, eight to nine ribs reach the breast-bone ; in Birds, three to 

 eight. An ossification of the inscriptiones tendineae of the rectus abdominis takes 

 place in Crocodiles and Hatteria, and similar structures (so-called "abdominal 

 ribs") occur in numerous fossil Reptiles (Nothosaurus of the Trias, Enaliosaurus of 

 the Jurassic, Pterodactylus, &c.). Archseopteryx also possessed twelve to thirteen 

 veil-developed "abdominal ribs." These must not be confounded with the remains 

 of true abdominal ribs, which persist without the corresponding vertebral portion 

 in the Chameleon and certain Birds (W. K. Parker). 



