81 



same time its venal characters, and becomes arteriali- 

 zed. Since also, it has been shewn (63.), that fishes 

 do not live in water which contains only nitrogen gas, 

 it is reasonable to infer, that, like water-snails and 

 muscles, they produce no change upon it. 



66. The amphibious class of animals, which live 

 partly in air and partly in water, exhibits great va- 

 riety in the structure of their circulating and respi- 

 ratory organs. In the frog and toad, the heart con- 

 sists of one auricle and one ventricle, as in fishes. 

 The auricle receives the venal blood from the body, 

 which passes into the ventricle, and from thence in- 

 to the aorta ; but this aorta soon divides into two 

 branches, one for the body and one for the lungs ; 

 and hence but half of their blood is, in each circula- 

 tion, exposed to the action of the air received into 

 the lungs. In the turtle, the same intention is ef- 

 fected by a different mechanism ; for, though the 

 heart of that animal consists of four cavities, yet the 

 ventricles freely communicate, and therefore the pul- 

 monary artery and aorta arise in fact from one cavi- 

 ty ; so that only half of the blood, thrown out at each 

 contraction of the ventricles, will pass through the 

 lungs, provided the areas of the two arteries corres- 

 pond. As a part only of the blood is thus sent 

 through the lungs in each circulation, it is plain that 

 a cessation of the respiratory function does not neces- 

 sarily put a stop to the circulation of the blood in these 

 animals, as it does in fishes, in the mammalia and in 

 birds ; and such animals are said to possess a jiulmo 

 arbitrarlus^ or are able to live either in water or in 

 air. The length of time which they can live without 

 respiration, has been supposed to depend on the 



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