CIRCULATORY ORGANS. 



carrying blood from the ventral to the dorsal aortae, while the fifth, re- 

 duced in size, perform a similar function in a few urodeles (fig. 280, C), 

 but elsewhere they entirely disappear. The fourth arches show a dif- 

 ferentiation between the two sides in many reptiles. That on the left 

 side becomes separated from the rest of the ventral aorta (fig. 280, E, F) 

 and has its own trunk connecting with the right side of the partially 

 divided ventricle, and, as will be understood from the relations of the 

 heart (p. 281), it may carry a mixture of arterial and venous blood. 

 From the dorsal side, this blood of the left fourth arch is largely dis- 

 tributed to the digestive tract, the cceliac axis arising from its radix, 

 while the part connecting it with the dorsal aorta is reduced in size. 

 The right arch and the carotids are connected with the left side of the 



FIG. 288. Aortic arches of amniotes, after Hochstetter. .4, Varanus; B, snake; C, 

 alligator; D, bird; E, mammal, b, basilar artery; cc, common carotid; ci, ce, internal and 

 external carotids; rfa, dorsal aorta; p, pulmonary; s, subclavian. 



heart and hence are purely arterial, the arch forming the main trunk 

 connecting the heart w r ith the dorsal aorta. In the birds (fig. 280, G) 

 the radix of the left side of the adult disappears distal to the origin of 

 the subclavian artery, so that this arch supplies only the fore limb of 

 that side, while the right arch is purely aortic in character. In the 

 mammals (fig. 280, H) these relations are exactly reversed, the right 

 arch being subclavian, the left supplying the dorsal aorta and the 

 subclavian of that side. 



With the development of lungs (dipnoi, tetrapoda) a pair of pul- 

 monary arteries are developed from the sixth pair of arches on the 

 ventral side of the pharynx. These grow back into the lungs, while the 

 rest of the arch, dorsal to their origin, becomes reduced to a small vessel 

 the ductus arteriosus (d. Botallii) in some urodeles, and persists 

 occasionally vestigially in higher vertebrates. Elsewhere it entirely dis- 

 appears. In the dipnoi and amphibia, where the ventricle remains 



