248 
CHANGE OP CIRCULATION IN TADPOLE. 
its fish-like condition. At a is seen the large trunk which 
issues from the ventricle, forming a bulbous enlargement like 
that which is seen in the corresponding part of the Fish. 
From this enlargement proceed three trunks on each side, 
called the branchial arteries (frr 1 , br 2 , hr 3 ), which convey the 
blood to the gills or branchiae; and after being aerated bypassing 
through their filaments, the blood is collected by the bran- 
1 tot o ah 
vb 3 a ap av c ah 2 vb 
Fig. 136. —Blood-vessels op the Tadpole, in first State. 
chial veins (vb, vb). Of these, the first pair transmits its blood 
by the vessels o, o, t, (which are also formed in part by the 
^econd pair) to the head and upper extremities; whilst the 
greater part of the blood of the second pair, with the whole 
of that of the third, is discharged into the trunk c on either 
side. By the union of that vessel with its fellow, the trunk 
a v is formed; which conveys the blood that has been aerated 
in the gills, to the general system, and is thus to be evidently 
regarded as the aorta. But we find here three small vessels 
(1, 2, and 3), which do not exist in the Fish; and which 
establish a communication between the branchial arteries and 
the branchial veins, in such a manner, that the blood may 
pass from the former into the latter, without going through 
the filaments of the gills. These communicating vessels are 
very small in the Tadpole, and scarcely any blood passes 
through them ; but it is chiefly by their enlargement, that the 
course of the blood is subsequently altered. There is also a 
fourth branch, ap, which proceeds to the lungs on either 
side; and as these organs are not yet developed, this pulmo¬ 
nary artery also is at first of very small size. 
288. As the metamorphosis of the other parts proceeds, 
