IX.] 



THE COMMON FROG. 



»39 



fishes. Minute vessels, however, directly connect 

 together, at the root .of each gill, the branchial artery 

 and branchial vein of each gill. Such a connecting 

 vessel is termed a ductus boialli. 



I\/ A 

 pa. dcL cuL 



br3 



Fic;. 82 — The Circulation of a Tadpole in its pr mitive stage, when nearly all the 

 blood is distributed to the gills ; the pulmonary arteries being quite rudimentary, 

 and the vessel (or ductus bota'li) connecting together the branchial artery and 

 vein at the root of each gill being minute, a, bulbus aortse ; b, branchial arteries ; 

 br^. br^, br^, the three gills (or branchiae of each side); bii, the branchial veins 

 which bring back the blood from the gills — the hindermost pair of branchial veins 

 on each side unite to form an aortic arch {na), which again unites with its fellow 

 of the opposite side to form da, the descending (or dorsal) aorta. The branchial 

 veins of the foremost gills give rise to the carotid arteries cc ; o, artery going 

 to the orbit ; /rt, pulmonary artery: 1, 2, 3, anastomosing branches connecting 

 logether the adjacent branchial' arteries and veii.s. 



Fig. 84. 



Fig. 83. — The Circulation in a Tadpole at a more advanced stage, ^«'he^ the gills 

 have begun to be albsorbed, the pvilmonary "Arteries to increase, us also the 

 connecting branches (at the root of the gills) between the branchial arteries 

 a"d branchial veins. 



Fig. 84. — The Circulation in a young Frog. Here the gills have been absorbed, 

 pnd the blood passes directly from the heart to the head, the dorsal aoria, the 

 lungs, and the skin. 



A minute vessel given off from the third branchial 

 artery, is the incipient pulmonary artery. 



As development proceeds, as the gills diminish by 



