302 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



tract, the coeliac axis arising from its radix (fig. 319, E, F) 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 

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

 connecting the heart with the dorsal aorta. In the birds (fig. 

 319, 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 charac- 

 ter. In the mammals (fig. 319, H) these relations are exactly re- 

 versed, 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, fig. 319, C) in some urodeles, 

 and persists occasionally vestigially in higher vertebrates. Else- 

 where it entirely disappears. 



The ductus Botalli is important in the embryonic circulation of araniotes, 

 as the larger part of the blood goes through it to reach the dorsal aorta, as long 

 as the allantois is the organ of respiration, while only enough blood goes through 

 the pulmonary artery to nourish the lung. With the first inspiration of air, 

 the duct closes and all blood passing into the last arch goes to the lung. 



In the dipnoi and amphibia, where the ventricle remains un- 

 divided, the pulmonary arteries are connected with the same trunk 

 (ventral aorta) as are the other aortic arches (fig. 319, C, D). In the 

 amniotes (£, F, G, H) with partial or complete division of the ven- 

 tricle (fig. 319, Z) to F), the truncus and the ventral aorta are divided 

 in such a manner that derivatives of the sixth arch are connected 

 with the right side of the heart, while the rest of the ventral aorta, 

 save for the exception noted in the reptiles above, receives its blood 

 from the left side of the heart. 



In connexion with tjie almost complete obliteration of the fifth arch, and, in 

 most pulmonate vertebrates, the separation of the sixth from the rest, it is 

 interesting to note that in the lower vertebrates (elasmobranchs) there is already 

 a differentiation of these two arches from the others of the series (fig. 320). 



The dorsal aorta arises by the fusion of two primitive trunks which run pos- 

 teriorly, just dorsal to the mesentery and approximately parallel to the noto- 

 chord, to the end of the body. This fusion may extend only as far forward as 

 the last aortic arch, the parts of the trunks in f^ont of this point being the 



