86 AMPHIBIA. 



each leaf of the gills or branchiae. From the branchial 

 capillaries, the aerated blood is received by the branchial 

 veins, which unite to form an aorta without the interven- 

 tion of a second ventricle. Every tyro in comparative 

 anatomy will at once perceive that this is an exact de- 

 scription of the circulation in fishes ; and yet it is taken 

 from that of the Tadpole of the Newt. I shall not enter 

 here into a detail of the changes which take place in the 

 various vessels, by which the branchial vessels become ob- 

 literated or altered in their course, and minute branches 

 are augmented in volume, and enter upon new functions ; 

 such details could only be understood by the experienced 

 anatomist, and would be out of place. It is sufficient to 

 say, that by the dilatation of one vessel a second auricle is 

 produced; that from the last branchial artery a small 

 branch passes to the air-sac or rudimentary lung, which 

 ultimately becomes the pulmonary artery ; and that by 

 other no less astonishing alterations, the transformation of 

 the branchial into the pulmonary circulation is effected, 

 and the heart assumes its new character of a trilocular 

 cavity; possessing, that is to say, two auricles and a 

 single ventricle, by which the blood which is sent to 

 the lungs, and that which is distributed to the system 

 at large, is alike of a mixed character, as in the true 

 Reptilia. 



The respiratory organs are no less surprisingly modified 

 during the progress of the changes just described in those 

 of circulation. The total loss of the branchiae, which are 

 removed by absorption, and the development of the little 

 rudimentary air-sac, so exactly analogous to the air-bag of 

 fishes, into the most beautiful cellular lungs, are changes 

 which, but for the ocular demonstration to which they are 

 yearly submitted, could scarcely be believed. Into the 



