CIRCULATORY SYSTEM 



787 



into two portions. An afferent branchial artery which carries the blood 

 from the ventral aorta to the gills and an efferent branchial artery which 

 carries it from the gills to the radix aortae. Both afferent and efferent 

 vessels run parallel to each other for a part of their course, and are con- 

 nected with each other by numerous capillary loops running through 

 the gill filaments. As the blood passes through the gills it loses its 

 carbon dioxide and takes up oxygen so that it is changed from venous 

 to arterial blood. In all animals that develop an amnion one cannot 

 distinguish between afferent and efferent branchial arteries, the aortic 

 arches running directly from the ventral aorta to the dorsal longitudinal 

 radices aortae. 



With the possible exception of cyclostomes, no gills are ever de- 

 veloped in the region of the first arch, so no afferent and efferent vessels 



ir. 



Fig. 448. 



I, Aortic arches of amniotes. Compare with Figure 309. A, African Lizard 

 (Varanus) ; B, Snake; C, Alligator; D, Bird; E, Mammal. 6, basilar artery; ec, 

 common carotid ; ei, ce, internal and external carotids ; da, dorsal aorta ; p, pul- 

 monary, , subclavian. (From Kingsley after Hochstetter. ) 



II, Comparison of Heart and aortic arch of crocodile and bird, a, right auricle ; 

 a', left auricle; ao, descending aorta; c, small connecting vessel; m, intestinal 

 branches. (c and m, before they separate, form the left aortic arch) ; v, right 

 ventricle: v' , left ventricle; 1 and 1', carotid arteries; 2 and 2', right and left 

 aortic ardh ; 4 and 4', pulmonary arteries. The right aortic arch, together with the 

 tiny branch, c. form the descending aorta. (After Boas.) 



