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mouth being thus rarified, a fresh supply enters 

 by the nostrils, which are furnished with a valve, 

 by which its subsequent escape is prevented. 

 The lingual bone is then forcibly raised by the 

 large pendulous muscles of the jaws ; and the 

 air, having no other passage, is necessarily forced 

 into the lungs. Hence reptiles, unlike the high- 

 er classes of animals, can still continue to breathe 

 if their bodies are cut open, because they do not 

 require a vacuum round their lungs, but not if 

 their jaws are held apart, since the air, which 

 should have entered the lungs, is in this case 

 forced out by the mouth. The air, thus received, 

 is subservient to the purification of the blood in 

 the usual manner j but it is not so immediately 

 vitiated as air received into fleshy lungs, owing 

 to the larger size of the cells, which do not im- 

 mediately allow the whole of it to come into con- 

 tact with their sides. This is one reason why 

 reptiles can sustain an impediment to their respi- 

 ration for a much longer time than birds and 

 mammiferous animals ; but another, and a much 

 better reason is to be found in the distribution of 

 their blood-vessels, those going to the lungs, as I 

 have before explained, not forming a necessary 

 part of the general circulating system, but con- 

 stituting, as it were, only an appendage to it, 

 which may for a time cease to transmit blood 

 without inconvenience. In both these respects, 

 however, we cannot but discern the hand of a 

 ruling Providence, adapting the structure of ani- 

 mals to the habits which are to characterize 



