SKELETON. 



145 



In their relationships to the muscles they resemble the ribs 

 of the amphibia, and are in no way differentiations of haemal 

 arches. 



From the amphibia upwards the ribs are typically articulated 

 with the vertebrae by two heads, a dorsal or tubercular head 

 articulating with the diapophysis, a ventral or capitular head 

 resting upon the parapophysis. There is thus formed a skeletal 

 arch (vertebrarterial canal) between rib and vertebra, through 

 which passes a vertebral artery (Fig. 149 VC). In the am- 



FIG. 153. Anterior end of the vertebral column of Polypterus, showing both 

 kinds of ribs from below, from Wiedersheim. Ps, parasphenoid ; R, true ribs 

 (1-V); WK, vertebral centra; + fish ribs. 



phibia the two heads are said to arise separately and to unite 

 later. From these typical conditions various modifications may 

 occur. Thus either head may disappear, while the parapophysis 

 (as in many mammals) may be reduced to an articular surface. 

 Again, as in the anura, the ribs may fuse to the diapophysis, or, 

 as in the neck of mammals, to both di- and parapophysis. In 

 crocodiles both tubercular and capitular heads articulate with 

 the transverse process in most of the thoracic ribs. 



In the amphibia the ribs are usually short, and are confined 



