276 



CHORDATE ANATOMY 



consequently the aortic arches are reduced. In fishes the persistent aortic 

 arches are broken up into a capillary net-work in the gills. In urodeles 

 the last four pairs of aortic arches persist, the blood from the ventral aorta 

 largely short-circuiting the gills. In Anura gills are lost and blood passes 

 directly through the aortic arches to the dorsal aorta. In the Anura the 

 fifth pair of aortic arches degenerate. That portion, the ductus arteriosus, 



Fig. 250. — Modifications of the aortic arches in difTerent vertebrates. A, primi- 

 tive scheme; B, dipnoan; C, urodele; D, frog; £, snake; F, lizard; G, bird; H, mam- 

 mal, c, celiac artery; da, dorsal aorta; dh. ductus Botalli; ec, ic, external and internal 

 carotids; p, pulmonary artery; s, subclavian; va, ventral aorta. Vessels carrying venous 

 blood, black; those with mixed blood, shaded; those which disappear, dotted outlines. 

 (From Kingsley's "Comparative Anatomy of Vertebrates," after Boas.) 



which connects the pulmonary branches of the sixth aortic arch with the 

 dorsal aorta also atrophies. The third pair of arches persist as the roots 

 of the carotid arteries. In reptiles as in Anura three pairs of aortic arches 

 persist, the 3rd, 4th, and 6th in part as pulmonary. In reptiles the com- 

 mon carotid arteries connect with the right and not with the left systemic 

 or fourth aortic arch. In mammals portions of three aortic arches persist 

 in the adult, the 3rd, 4th, and 6th as in reptiles. The systemic arch of the 



