Whitney^ on the Circnkttion in the Tadpole. 5 
minute brandies and their contents are less conspicuous, the 
large trunks become traceable ; and we have repeatedly been 
able to follow their course as they have lain naked and un- 
encumbered beneath the eye (see figures) . The bearing of 
this in a physiological point of view I shall reserve till we 
liave completed the circle of the vascular system. 
Figure 2 exhibits the more extended view of the 
vessels obtained under the influence of low diet. Here are 
vessels revealed which, till now, were invisible. But we will 
continue our course along the arteries. The third trunk, 
traversing the lung, is now seen to emerge from its lower 
edge or floor, and descends into the abdomen. There could 
be no doubt that this contributed to form the great ab- 
dominal aorta, conveying the blood which supplies the bowels 
and all the abdominal organs. Still it was partially hidden. 
We had not yet seen the vessel on each side descend and 
unite with its fellow to form the abdominal trunk. But this 
desideratum was at last vouchsafed to us in a most unex- 
pected manner. It happened that on one occasion a small, 
black-looking little fellow was put under the microscope. 
He was taken from a jar of fresh tadpoles who had 
been regaling upon green weeds. To all appearance 
opaque, he proved, to our astonishment, transparent from 
head to tail ! He was an animated case, and nothing 
more. His heart was beating and his blood circulating, 
but the latter was quite colourless, not a single red 
globule was visible anywhere. The creature was a mass of 
outlines merely. The globules chased one another like 
globules of water, the heart was a colourless globe, the lungs 
were two transparent ovals, and the bow^els a colourless, 
empty, transparent coil. Our animated case, however, proved 
to be " a case in point.^^ That which we had long been 
searching for was now before us. Through the empty coil 
we saw the artery on each side descending from the lungs, 
converging to the spine, meeting its fellow, and with it 
uniting to form the abdominal aorta (fig. 3). After the 
aorta has supplied the abdominal viscera, we find a prolonga- 
tion, or caudal artery, descending to the tail, the all-impor- 
tant organ of locomotion in the tadpole. This artery, 
entering the root of the tail, is imbedded deeply in the 
flesh, whence it emerges, and then continues its course, 
closely accompanied by the vein, to within a short distance 
of the tail's extremity, where, being reduced to a state of 
extreme fineness, it terminates in a capillary loop, which is 
composed of the end of the artery and the beginning of the 
vein. The artery, in its course, gives off branches con- 
tinually to supply the neighbouring tissue. You may often 
