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ingly commonly said to be a pulmonary heart, in 

 contradistinction to that of reptiles, which is 

 called a systematic one ; and it is evident that 

 the circulation of the blood in these animals 

 must be at once stopped by preventing the action 

 of their gills, since it is through these organs 

 alone that the blood transmitted by the heart can 

 reach the several parts of the body. 



Among reptiles, the frog has its heart situated 

 immediately below its breast-bone ; and it re- 

 ceives its blood from the large veins of the body, 

 in the same manner as that of fishes. It does 

 not, however, like that of fishes, transmit this 

 blood directly through the respiratory organs, 

 and thence indirectly to the rest of the system, 

 but at once, by the large arteries leading to the 

 body in general, of which the arteries leading to 

 the lungs are only branches ; while correspond- 

 ing veins, returning from the lungs, terminate in 

 the large veins of the body in general, already in 

 their course to the heart. It hence follows, that 

 the course of the blood through the respiratory 

 organs is not, in these animals, as it is in fishes, 

 essential to its course through the rest of the 

 system ; since, while in the latter, the vessels of 

 the gills constitute one portion of the great circle 

 in which the blood moves, in the former, those 

 of the lungs constitute only segments of smaller 

 circles, attached to this great one, but not form- 

 ing any part of it. It was the intention of nature 

 that fishes should constantly inhabit the waters, 

 in which their gills play freely and constantly, 



