Whitney^ on tht Circulation in the Tadpole. 3 
■w ith the pure_, aerated blood returned from the luugs^ and 
■\v hicli also flows at the same instant into the ventricle from 
the left auricle. Thus the habitual circulation of this " cold- 
blooded " mixture is the cause of the low tone of vitality that 
distinguishes the perfect reptile. 
We now arrive at the consideration of the tadpole^, who^ 
during the middle and latter stages of its metamorphosis, 
assumes a condition whicli is neither that of fish or reptile,, 
but something which differs considerably from both. 
For the purpose of observation the tadpole must^ of course, 
be selected during the period in which the skin is perfectly 
transparent. The first examinations revealed plainly enough 
the appearances already described of the form and situation 
of the heart, and the three great arterial trunks proceeding 
(right and left) from it. The course and destination of these 
were not so easily arrived at. Many observations were 
required, on tadpoles of various sizes and carefully prepared, 
to arrive at what I believe to be the true anatomical and 
physiological arrangement of these vessels. First, all these 
are closely connected with the corresponding lung. The 
upper one (the cephalic) runs along the upper edge of the 
lung, and gives off", in its course, a branch which ascends to 
the mouth, and is mainly distributed to the thick-fringed lip 
which surrounds the mouth, and is evidently an organ of 
sense, playing the part of judge and sentinel to this important 
entrance. It is profusely supplied with blood by this artery, 
and the blood is returned by a corresponding vein. We may 
call these the labial artery and vein. The cephalic artery 
continues its course around the lung (forming, to all appear- 
ance, the edge of that organ), until it suddenly curves 
upwards and backwards, and reaches the upper surface of 
the head, where it dips between the eye and the brain, 
towards which it is evidently travelling. We may now call 
it the cerebral artery. 
Dr. Carpenter speaks of the second main trunk as con- 
tributing with the third to form the great systemic aorta, 
which descends into the abdomen. The evidence w^e obtained 
appears to be at issue with this statement. To us the micro- 
scope revealed the course and distribution of this artery 
through the lung, freely communicating, by a network of 
vessels, with the upper or cephalic branch, and with the 
lower one, whicli I am next to descrilje. This middle one, 
therefore, would seem to be the true pulmonary artery. The 
third main trunk (which we have been at great pains to 
trace) clearly enters the lung, distributes branches, but con- 
tinues its course as a large trunk, bending downwards to the 
