[X.] 



THE COMMON FROG. 



139 



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 botalli. 



Fig. 82. —The Circulation of a 'ladpole in its primitive sta^e.'when nearly all the 

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

 and the vessel (or ductus botalli) connecting together the branchial artery and 

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

 br^, br^, br^, the three gills (or branchiae of each side); b7<, 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 {aa), 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 ; pa, pulmonary^ artery : i, 2, 3, anastomosing branches connecting 

 together the ar^jacent branchial arteries and veins. 



? I 



Fig. 84. 



Fig. 83.— The Circulation in a Tadpole at a more advanced stage, when the gills 

 have begun to be absorbed, the pulmonary arteries to increase, as also the 

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

 and branchial veins. 



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

 and the blood passes directly from the heart to the head, the dorsal aorta, 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 



