86 AMPHIBIA. 
each leaf of the gills or branchize. From the branchial 
capillaries, the aérated blood is received by the branchial 
veins, which unite to form an aorta without the interven- 
tion of a second ventricle. Every tyro im 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 branchie, 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 
