82 THE ANATOMY OF VERTEBRATED ANIMAL9. 



passes into a cardiac aortic trunk, while the posterior end ia 

 continuous with the great veins which bring back blood from 

 the umbilical vesicle — the omphalomeseraic 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 its 

 fello-w, to the hinder part of the body, as a. primitive subverle- 

 hral aorta. The two primitive aortse very soon coalesce 

 throughout the greater part of their length into one trunk, 

 the definitive mbvertehral 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- 

 brata, and more in the lower, are successively developed, 

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

 nect the cardiac and subvertebral aortae. 



In the permanently branchiate Vertebrata, the majority of 

 these aortic arches persist, giving oif 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 Amphibia, which, though branchiate in the 

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

 dition, such as the Satrachia (Fig. 35, F) and Gmcilia, 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 Satrachia, 

 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 Reptilia ; or to a single 

 arch, as in Aves and Mammalia. The aortic arches thus 

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

 fourth pairs in order from before backward ; but the fourth 

 pair onl}', 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 give off the pulmonary 

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

 remains of the primitive connection of these arches with tlie 



