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 (br\ br 2 , br*), which convey the 

 blood to the gills or branchice; and after being aerated bypassing 

 through their filaments, the blood is collected by the bran- 



ab 



a ap av c ab 2 vb 



Fig. 136. BLOOD-VESSELS OF 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. Ey 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, 



