82 THE ANATOMY OF VERTEBRATED ANIMALS. 



passes into a cardiac aortic trunk, wliile the posterior end i3 

 continuous with the great veins which bring back blood from 

 the umbilical vesicle — the omplialomeseraic veins. 



The cardiac aorta immediately divides into two branches, 

 each of which ascends, in the first visceral arch, in the form 

 of a forwardly convex aortic arch^ to the under side of the 

 rudimentary spinal column, and then runs, parallel with it a 

 fello'W, to the hinder part of the body, as a prbniti-ce subverted 

 bral aorta. The two primitive aortas very soon coalesce 

 throughout the greater part of their length into one trunk, 

 the defiiiitive subvertebral aorta ; but the aortic arches, sepa- 

 rated by the alimentary tract, remain distinct. Additional 

 arterial trunks, to the number of four in the higher Verte- 

 hrata^ and more in the lower, are successively developed, 

 behind the first, in the other visceral arches, and further con- 

 nect the cardiac and subvertebral aortse. 



In the permanently branchiate Vertebrata^ the majority of 

 these aortic arches persist, giving off vessels to the branchial 

 tufts, and becoming converted into afferent and efferent 

 trunks, which carry the blood to and take it from these tufts. 

 (Fig. 25, A, B, C, D, E.)^ 



In the higher Jimphibia, which, though branchiate in the 

 young state, become entirely air-breathers in the adult con- 

 dition, such as the JBatrachia (Fig. 25, F) and Ccecilia, the 

 permeable aortic arches are reduced to two (the middle pair 

 of the three which supply the external gills, and the fourth 

 pair of embryonic aortic arches) by the obliteration of the 

 cavities of the dorsal ends of the others. Of the posterior 

 arches, the remains of the fifth and sixth become the trunks 

 which give off the pulmonary arteries, and, in the JBatrachia, 

 cutaneous branches. The anterior, or third, primitive aortic 

 arch becomes the common carotid trunk, and ends in the 

 carotid gland^ whence the internal and external carotids 

 arise. In those Vertebrata which never possess gills, the 

 arches become reduced either to two pair, as in some Lacer- 

 tilia / or to one pair, as in other Jiej^tilia / or to a single 

 arch, as in A.ves and 3IaimnaUa. The aortic arches tlms 

 retained are, in the Lizards in question, the third and the 

 fourth pairs iu order from before backward ; but the fourth 

 pair only, in other Reptiles ; in Birds, the right arch only of 

 the fourth pair ; and in Mammals, the left arch only of the 

 fourth pair. The fifth pair of arches giv^e off the pulmonary 

 arteries, the so-called " ductus arteriosus " representing the 

 remains of the primitive connection of these arches with the 



