2 Whitney, on the Circulation in the Tadpole. 
most delicate, transparent tissues, tbrough wliicli are seen 
the globules of the blood, perpetually, but alternately, enter- 
ing by one orifice and leaving it by another. The heart 
appears to be slung, as it were, between two arms or branches, 
extending right and left. From these trunks the main 
arteries arise. The heart is enclosed within an envelope or 
pericardium. This pericardium is, perhaps, the most deli- 
cate and is, certainly, the most elegant beauty in the 
creature's organism. Its extreme fineness makes it often 
elude the eye under the single microscope, but under the 
binocular its form is distinctly revealed. Then it is seen as 
a canopy or tent, enclosing the heart, but of such extreme 
tenuity that its folds are really the means by which its exist- 
ence is recognised. 
Passing along the course of the great blood-vessels to the 
right and left of the heart, the eye is arrested by a large, oval 
body, of a more complicated structure and dazzling appear- 
ance. This is the lung, which, in the tadpole, is a cavity 
formed of most delicate, transparent tissue, traversed by 
certain arteries, and lined by a crimson network of blood- 
vessels, the interlacing of which, with their rapid currents 
and dancing globules, form one of the most beautiful and 
dazzling exhibitions of vitality. 
The tadpole is hatched with respiratory and circulating 
organs that resemble those of the fish. It lives in the water, 
breathes oxygen from the air contained in the water, and 
during the early period of its existence respires exclusively by 
gills. Our inquiries do not apply to this the earliest stage 
of tadpole life, but to the middle and later periods, the stage 
of transition between the fish and the reptile, when, as Dr. 
Carpenter expresses it, ^' there is a kind of balancing between 
the organs which are disappearing and those which are being 
evolved.^' 
It will be remembered that in the fish the heart has two 
cavities, an auricle and ventricle; that the blood, returned 
by the veins of the body to the auricle, passes into the 
ventricle, and is then transmitted to the gills, where, being 
exposed to the air contained in the water, it becomes deprived 
of carbonic acid, aerated, and rendered fit for re-circulation 
through the system. In the reptile we find a modification 
of plan coincident with the lower tone of vitality which 
distinguishes these cold-blooded creatures. Their heart has 
three cavities, two auricles and one ventricle ; and by this 
contrivance there is a perpetual mixture in the heart of the 
impure carbonized blood which has already circulated through 
the body, and flows into the ventricle from the right auricle, 
