ORGANS OF CIRCULATION. 



I8 7 



portion of the arch and the pulmonary artery increase in size, 

 while that portion of the arch between the pulmonary artery 

 and the radix remains undeveloped (ductus Botalii) or becomes 

 entirely aborted. In the amphibia also the radix disappears be- 

 tween the first and third arches, so that blood forced through 

 the anterior arches can only go to the head through the carotids. 



MAMS. 



FIG. 200. Diagrams of the aortic arches in different groups of vertebrates. 

 A, fishes; B, amphibia; C, reptiles; D, birds; E, mammals; a, ventral aorta; 

 c, internal carotids ; e, external carotids ; /, pulmonary arteries ; s, subclavian 

 arteries. After Lambert. 



In the urodeles two arches on either side (three and four) con- 

 nect with the dorsal aorta by means of the radices ; but in the 

 anura, on the assumption of the adult condition, the fourth arch 

 degenerates. 



In the reptiles the conditions are somewhat complex. In 

 certain lizards the third arch as well as the fourth may be con- 



