332 ZOOLOGY. 



communication is established between the arteries which 

 carry the blood to the gills, and those which receive it from 

 those organs, so that the blood is no longer obliged to 

 traverse the gills in order to reach the dorsal artery, and 

 thence to be distributed to the various parts of the body. 

 The artery (a) which springs from the ventricle, and which 

 may at this time be called a branchial artery, becomes then 

 the origin of the dorsal vessel, and constitutes with it a true 



ab 



3 a ap av c ab 2 vb 



Fig. 300. Bloodvessels of the Tadpole of the Frog.* 



aorta, certain branches of which proceeding to the lungs, thus 

 develop and establish a pulmonary circulation. Finally, the 

 branchial vessels are obliterated, and then the circulation takes 

 place nearly as in other reptiles. The venous blood returning 

 from all parts of the body, is poured into the ventricle by one 

 of the auricles, and is therein mingled with the arterial blood 



* a, artery arising from the single ventricle, and dividing into six 

 branches (ab), which go to the three pairs of branchiae, and ramify there 

 they are called branchial arteries ; br, the branchiae, showing the distribu- 

 tion of the branchial arteries upon them, and carrying the blood to them and 

 the branchial veins (vb), bringing back the blood from them after it has been 

 exposed to the oxygen of the atmosphere contained in the water sent across 

 the gills : those or the two last pairs of branchiae unite to form on each side a 

 vessel (c) which, anastomosing in its turn with that of the opposite side, forms 

 the ventral aorta or dorsal artery (ar), which, proceeding backwards, dis- 

 tributes the blood to the greater part of the body. The branchial veins of 

 the first pair of branchiae curve forwards, and carry the blood towards the 

 head (t,t) ; 1, a very fine and anastomotic branch unites the branchial artery 

 and vein together at the base of the first pair of branchiae, and this enlarging 

 afterwards, allows the blood to pass between those two vessels without pass- 

 ing across the gill ; 2, a small anastomotic branch, uniting in the same way 

 the artery and vein of the second pair of branchiae ; 3, a vessel which, by 

 uniting with a small branch situated more inwards, connects the artery and 

 vein of the posterior branchiae; o, orbitar artery ; ap, rudimentary pulmonary 

 arteries. 



