4 WhitneYj on the Circulatmi in the Tadpole, 
lower surface or floor of the lung^ whence it emerges, curving 
towards the spine, and becomes, as I shall presently show, 
the great abdominal aorta. If these statements are correct, 
they exhibit a different vascular arrangement to that which 
has hitherto been represented, and they have an important 
bearing upon certain physiological points presently to be 
mentioned. 
I will now speak of the means by which we were enabled 
to trace the existence and course of blood-vessels ordinarily 
invisible. It would be a mistake to suppose that you can 
make out distinctly, in the average of tadpoles taken, with- 
out preparation, and placed under the glass, the plan of the 
blood-vessels, even as far as I have yet described them. The 
great obstacle is the large coil of intestines, usually distended 
with dark- coloured food, undergoing digestion and travelling 
along the bowels. This, of course, eflPectually screens every 
thing behind it from the eye of the observer. Moreover, by 
distension the bowels stretch upwards to the chest, reaching 
the apex of the heart, and often concealing a part of the 
lungs. To empty the bowels, therefore, was the great de- 
sideratum. I immersed tadpoles in solutions of many of the 
known aperient drugs — Epsom salts, colocynth, aloes, ela- 
terium, &c., but no purgative effect was produced. But as 
the contents of the bowels are, in the natural state, per- 
petually passing away, I thought that by giving them an 
opportunity of clearing themselves, and at the same time 
preventing an accumulation of fresh matter by putting the 
tadpoles upon low diet, we should extend our field of vision. 
This experiment succeeded. The tadpoles lived on plain 
water for some days ; much of the intestinal contents passed 
away, the canal became comparatively, though not entirely, 
empty, and the bowels, shrinking in proportion, occupied a 
smaller space. 
Figure 1 (PI. I) exhibits what we see ordinarily when a 
tadpole is looked at — heart, the main arterial trunks, and 
the greater part of both lungs. But under the influence 
of low diet we gain a much clearer and more extended view 
of the vascular system. Water diet, of course, impoverishes 
the blood, which is rendered paler, as well as reduced in 
quantity. This effect upon the lungs increases their trans- 
parency, and by diminishing the quantity and redness of the 
blood in the minute vessels, lessens the dazzling, dancing 
movement of red globules, which, most beautiful to look at, 
is yet perplexing to the eye in its endeavour to catch dis- 
tinctly and trace the course of the three large arteries which, 
I have said, traverse these organs. But in proportion as the 
