tJ4 THE ANATOMY OF VERTEBRATED ANIMALS. 



vieeeral cleft is left unnumbered, and one must be added to tne nnmner of each bran* 

 chiai cleft to give its number in tlie series of visceral clefts. 



B. Hypothetical diagram of the aortic arches In the Shark Ilepianchun, which has seven 



branchial clefts. Sp. The remains of the fh-st visceral cleft as the spiracle. Branchia? 

 are developed on all the arches. 



C. LepidoslrtTi. — The first arch has disappeared as such, and the first ^^sceral cleft is ob- 



literated. Internal branchiae are developed in connection with the second, fifth, sixth, 

 and seventh aortic arches; external branehise in connection with the fourth, fifth, and 

 sixth. PA. — The pulmonaiy artery. The posterior two visceral clefts are obliterated. 



r A Teleostean Fish. — The first aortic arch and first visceral cleft are obhterated as before. 

 The second aortic arcb bears the psendo-branchia {Ps.B.\ whence issues the ophthalmic 

 artery, to terminate in the choroid gland {Oh.). The next four arches bear gills. Tho 

 .seventh and eighth arches have been observed in the embryo, but not the nintli, and 

 the included clefts are absent in the adult. 



E. The Axolotl {Siredon)^ a, perennibranchiate amphibian. The third, fourth, fifth, and 

 sixth aortic arches, and the anterior four branchial clefts, persist. The first visceral 

 cleft 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 mirabile (carotid glands Ca. G.) which terminates 

 it. The fourth pair of aortic arches persist. The fifth and sixth pair lose their connec- 

 tions with the subvertebral aortic trunk, and become the roots of the cutaneous and 

 pulmonary arteries. The first visceral cleft becomes the tympanum, but ail the others 

 are obhterated in the adult. 



The embryonic aorta gives off omphalomeseraic branches 

 (Fig. 26, o) to the umbilical vesicle ; and ends, at first, in the 

 hyjyogastric arteries (which are distributed to the allantois in 

 the abranchiate Vertebrata), and a median caudal continuation. 

 The blood from the umbilical vesicle is brought back, as before 

 mentioned, by the omphalomeseraic veins (Fig. 26, o'), which 

 unite in a dilatation close to the head ; the dilatation (sinus 

 venosus) receives, on each side, a short transverse venous 

 trunk, the ductus Cuvieri (Fig. 26, JDG), which is itself 

 formed, upon each side, by the junction of the anterior and 

 posterior cardinal veins, which run backward and forward, 

 parallel with the spine, and bring back the blood of the head 

 and of the trunk. 



The blood of the allantois is returned by the umbilical 

 vein, or veins (Fig. 26, u), which are formed in the anterior 

 wall of the abdomen, and open into the venous sinus before 

 mentioned. The blood of the posterior extremities and kid- 

 neys is, after a while, brought to the same point by a special 

 median vein, the vena cava inferior (Fig. 26, cv). 



The development of the liver effects the first great change 

 in the arrangements now described. It, as it were, interrupts 

 the course of the omphalomeseraic vein, which is not only the 

 vein of the umbilical sac but also that of the intestine, and 

 converts it into a meshwork of canals, which communicate, on 

 one side, with the cardiac part of the vein, and, on the other 

 Bide, with its intestinal part. The latter is thus converted into 

 the vena portce (Fig. 26, vp), distributing the blood of the 

 Btomaoh and intestines to the liver ; while the former becomes 



