96 



and the circulation of their blood is therefore 

 uninterrupted ; but reptiles were destined to live 

 at one time in the air, and at another in the 

 water, and it became necessary, therefore, to give 

 them a circulating apparatus, which could per- 

 form its office equally well, whether the animal 

 was respiring, as it does while in the air, or had 

 for a time ceased to respire, as is the case when 

 it is beneath the water. Some of the higher or- 

 ders of reptiles, as the tortoise and the alligator, 

 make an approach to the double heart of birds 

 and quadrupeds, that is to say, while it is in 

 structure still more or less a single heart, it is in 

 function at least so long as the respiratory or- 

 gans are in action almost a double one ; the 

 right side of it receiving principally the impure 

 blood from the large veins of the body, and send- 

 ing it through the lungs, while the left side re- 

 ceives chiefly the purified blood from the lungs, 

 and transmits it by the main artery to the rest of 

 the system. This, however, is the case only 

 when respiration continues unobstructed ; since, 

 upon the animal's descending below the water, 

 both sides of the heart co-o^brate more or less in 

 receiving the blood from the body in general, and 

 in transmitting it again to the body, the right 

 side, which, under other circumstances, trans- 

 mits it principally to the lungs, now sending it, in 

 common with the left, to the body in general ; and 

 the left side, which, under other circumstances, 

 receives it chiefly from the lungs, now receiving 

 it, in common with the right, from the large veins 



