346 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



anterior, and their union in many cases with the 

 sternum in the ventral median line. A definite 

 thorax may henceforward be recognised, and the 

 appearance of this cage associated with the complete 

 absence of branchial respiration. In front of the 

 thorax more or less well developed ribs may be 

 developed, but these never become connected with the 

 sternum ; in the Chelonia there are no cervical ribs. 

 The sternal ribs are jointed, and in Reptiles the 

 sternal portion remains cartilaginous ; where the 

 sternum is short many of the ribs are connected with 

 the cartilaginous rod that lies behind it. In Birds 

 the connection between the ribs and the sternum is 

 more complete, and the whole apparatus is firmer and 

 stronger (see Fig. 135), while in them, as in crocodiles, 

 the side walls of the thorax are strengthened by the 

 development of backwardly directed processes from 

 the ribs (uncinate processes) (up). Cervical ribs 

 are very rare among Mammals, though occasionally 

 found, as in Bradypus. The thoracic ribs are so 

 articulated as to be capable of a considerable amount 

 of movement, thanks to which the thoracic cavity 

 may be increased or diminished in size ; they are 

 either attached directly to the sternum, or connected 

 with it by ligament, or are quite free at their distal 

 end. In the Cetacea some of the hinder ribs lose 

 even their attachment to the vertebrae. 



In the Ophidia, where the primitive condition of a 

 large number of similar ribs is retained, there is a 

 single articular head for connection with the vertebral 

 column ; this is of a kind to allow of considerable 

 freedom of movement, and the snake uses its successive 

 pairs of ribs as stilts by which it may move along the 

 ground. 



In the Amniota the ventral ends of the ribs 

 become constricted off, and fuse with one another in 

 the median line ; the cartilaginous plate thus formed 



