394 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



the neck, and gill arches through which the blood 

 passes, forming, in fact, several aortic arches on each 



side. But afterward 

 these are modified into 

 the outgoing vessels 

 of the heart, and their 

 original arch form is 

 retained only by one. 

 The manner in which 

 the change takes place 

 is shown in Figs. 271, 

 272, and 273. Fig. 271 

 shows in a schematic 

 way the primitive aor- 

 tic or gill arches (al- 

 most exactly repre- 

 sented now by sharks), 

 and Figs. 272 and 273 

 the same as it is modi- 

 fied for a bird and 

 mammal respectively. 

 Finally, Fig. 274 is the 

 mammalian heart slightly modified to suggest the homol- 

 ogy of the several parts. 



To explain: The gill arches are five in number on 

 each side, as in a shark (Fig. 271). The two upper pairs 

 are soon aborted, even before leaving the class of fishes, 

 and only three on each side remain to be accounted for. 

 These are the three found in the lizard, and inherited 

 with modifications in birds and mammals. These are 

 therefore the only ones we have to deal with. Fig. 272 

 shows the modification of this formula in birds. It is 

 seen that the first arch on each side becomes the two 

 pulmonic arteries, one going to each lung, precisely as in 

 the lizard and in the frog. Of the next pair of arches, 



Fig. 272. — Modified for bird. 



