482 SANGUIFEROUS SYSTEM. 



The bulbus arteriosus is considerable in the adult proteus, 

 and gives off, not only the three branchial arteries on each 

 side to the external gills, but also an aortic arch on each 

 side which unite together to form the trunk of the aorta, 

 as shown by Rusconi thus combining in the same animal, 

 the vascular conditions of the tadpole and the adult of 

 the caducibranchiate amphibia, or those of the fishes and 

 the true reptilia; the same plan of structure appears to 

 exist in the other perennibranchiate forms of this class. 

 Even in the adult triton (138. b. b. b.) three aortic trunks 

 continue to originate from each side of the aorta, the third 

 pair (138. i. i.) being chiefly occupied in forming the two 

 pulmonary arteries (138. k. 7c.) which communicate by small 

 ductus arteriorsi (138. h. h.) with the arches of the abdomi- 

 nal aorta (138. d. d.) In the anourous amphibia, as the 

 frogs and toads, the aortic arches still further coalesce by 

 the metamorphosis, and externally appear to constitute, in 

 the adult state, only a single great trunk on each side, 

 divided internally by septa throughout its course, which 

 gives off the usual cephalic, aortic, and pulmonic arteries; 

 and while the external branchiae in the tadpoles of these 

 species are withdrawn, and are becoming absorbed, the 

 branchial apertures, first on the one side of the neck and 

 then on the other, become closed up by a thin fold of the 

 skin which has been mistaken by some for the analogue of 

 the opercular bones which support this part in fishes. Traces 

 of the early branchial capillaries appear to be preserved on 

 the carotid arteries through life, and the posterior or pul- 

 monic arch of the aorta gives off an artery on each side to 

 the back part of the head, as well as the great pulmonary 

 arteries to the lungs. The branchial arteries in the young 

 state of the coecilice are observed to ramify on internal gills, 

 which open on each side of the neck by cutaneous apertures, 

 as in the larvae of other amphibia. The bulbus arteriosus 

 in the menopoma (119. A.), is long, cylindrical, muscular 

 and provided with numerous internal valves, as in the pla- 

 giostome fishes ; these valves are disposed in two transverse 

 rows, with four in each row, and the artery is dilated at the 

 giving off of the aortic arches, which are four on each 

 side. The small posterior or proximal aortic branches, on 

 each side, are distributed chiefly on the simple pulmonary 



