BLOOD system. 



395 



that on the right (left of the figure) becomes the aorta, 

 and that on the left the subclavian on that side. The 

 subclavian on the other side is a branch of the aorta. 

 The next pair became 

 the carotids. They are 

 already so in the lizard 

 and in the frog (see Fig. 

 270). 



In the mammals the 

 modification is a little 

 different. It is seen 

 (Fig. 273) that of the 

 first pair of arches, that 

 on the left is the pul- 

 monary artery, which in 

 this case supplies both 

 lungs, while that on the 

 right is aborted. Of the 

 second pair, that on the 

 left (right of the figure) 

 becomes the aorta, while 

 that on the right the 

 subclavian on that side. The third pair, as in birds, and 

 as already in reptiles and amphibians, becomes the carot- 

 ids. In Fig. 274 we give a figure of the heart, with its 

 outgoing vessels a little modified, to suggest their ho- 

 mology, and numbered so as to show the corresponding 

 parts. 



We see now that the only difference between bird 

 and mammal in the aorta and other outgoing vessels of 

 the heart is that among the various arches different ones 

 have been selected to be retained in the arch form as 

 aorta and for pulmonary artery and for subclavian. 

 In both cases — i. e., in both birds and mammals — the 

 steps of the change may be traced in the embryo. 



Fig. 273. — Modified for mammal. 



