92 THE ANATOMY Or VERTEBRATED ANIMALS. 



or subvertebral aorta, i.— ix. the aortic arches, corresponding with 

 Ifft., the mandibular; Hy.^ the hyoidean, and Br.\-~Br.-, the seven 

 branchial visceral arches, i. ii. in. iv. v. vi. vii., the seven 6ranc/i(a/ 

 clefts. The first visceral cleft is left unnumbered, and one must be 

 added to the number of each branchial cleft to give its number in 

 the series of visceral clefts. 



B. Hypothetical diagram of the aortic arches in the Shark Hepianclms, 

 which has seven branchial clefts. Sp. The remains of the first 

 visceral cleft as the spiracle. Branchiae are developed on all the 

 arches. 



C. Lepidosiren. — The first arch has disappeared as such, and the first 

 visceral cleft is obliterated. Internal branchiae are developed in 

 connection with the second, fifth, si.xth, and seventh aortic arches ; 

 external branchiae in connection with the fourth, fifth, and sixth. 

 PA. the pulmonary artery. The posterior two visceral clefts are 

 obliterated. 



D. A Teleostean Fish.— The first aortic arch and first visceral cleft 

 are obliterated, as before. The second aortic arch bears the pseudo- 

 branchia {Fs.B.), whence issues the ophthalmic artery, to terminate 

 in the choroid gland (^Ch.) The next four arches bear gills. The 

 seventh and eighth arches have been observed in the embryo, but 

 not the ninth, and the included clefts are absent in the adult. 



E. The Axolotl (S/rerfora), a perennibranchiate amphibian. The third, 

 fourth, fifth, and sixth aortic arches, and the anterior four branchial 

 clefts, persist. The first visceral clefc is obliterated. 



F. The Frog. — The three anterior aortic arches are obliterated in the 

 adult. The place of the third, which is connected with the anterior 

 external gill in the Tadpole, is occupied by the common carotid and 

 the rete mirahile (carotid gland, Ca. G.) which terminates it. The 

 fourth pair of aortic arches persist. The fifth and sixrh pair lose 

 their connections with the subvertebral aortic trunk, and become 

 the roots of the cutaneous and pulmonary arteries. The first 

 visceral cleft becomes the tympanum, but all the others are oblite- 

 rated in the adult. 



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 Meptilia ; 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 backwards ; 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 f oui-th pair. The fifth jDair of arches give off the 

 pulmonary arteries, the so-called " ductus arteriosus" repre- 



