RESPIRATION OF REPTILES. 113 



eating together, and occupying considerable space on 

 the sides of the body. The air is swallowed through 

 the nostrils by a peculiar working of the jaws, and is ex- 

 pelled by the action of the abdominal muscles. This 

 sort of lungs is capable ol acting on but a comparatively 

 small portion of blood, and now if we look at the circu- 

 lation of these creatures, we shall see how admirably 

 these two functions, respiration and circulation, corres- 

 pond to each other. 



The circulation of Reptiles has this peculiarity which 

 distinguishes it from that of all other classes the blood 

 which is brought from both the body and the lungs, is 

 poured into a single reservoir, or into two or three, 

 which finally communicate, where it is mingled together, 

 and thence sent back again to the lungs and body. 



Emily. Then some portion of the blood sent away 

 to nourish the body, is venous, and some of that sent to 

 the lungs to be renovated by their action, is arterial, and 

 of course does not need their action. Why, what can be 

 the reason of this ? I should think they would suffer the 

 fate of the blue-boys, whom you spoke of yesterday. 



Dr. B. The nature of their lungs is such, that they 

 are unfitted for acting on blood which is wholly venous ; 

 it would require a greater amount of action than they 

 are capable of. So too, if the arteries carried only ar- 

 terial blood to the body, it would be renovated and ex- 

 cited beyond its proper measure. 



Emily. Still, I do not understand clearly, why this 

 inactivity of respiration and circulation, should be so 

 particularly necessary to Reptiles, or, to use the lan- 

 guage of writers on natural theology, I do not perceive 

 the final cause. 



Dr. B. This is not very obvious, though generally 

 it does appear very clearly to depend on their habits of 

 life. Most of them live in the water, and are obliged to 

 remain under the water for very long periods before com- 

 ing up to breathe ; an active respiration, like man's or the 

 birds', would obviously be incompatible with such a mode 

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