BACKBONED ANIMALS WHICH BREATHE IN AIR 



429 



As in so many other cases, material already present has been 

 modified so as to serve a fresh purpose. It is worth while to note 

 here that the lungs of Mammals, like those of Amphibians and 

 Reptiles, are very elastic, and are kept continually on the stretch. 

 Their elasticity greatly aids in the expulsion of impure air from the 

 breathing passages. 



It would be a mistake to suppose that the breathing move- 

 ments which the walls of the chest execute cause air to rush 

 into the bronchial tubes and air-cells. It is only the air in the 

 larger air-passages which is directly renewed in this way, while 

 purification of the air in the smaller passages and their endings 

 is brought about by gaseous diffusion. 



In Amphibians, Reptiles, and Birds the breathing and feeding 

 tracts are not very completely separated (except in the case of 

 Crocodiles), but in Mammals they are 

 much more perfectly marked off from 

 one another. The internal nostrils do 

 not open on the roof of the mouth, but 

 into the pharynx (fig. 554). It may, in 

 fact, be said that the two tracts cross 

 one another in this part of the digestive 

 tube, on the floor of which is the open- 

 ing {glottis) which leads into the wind- 

 pipe. Mammals also present a very 

 characteristic arrangement by means 

 of which food is prevented from get- 

 ting into the breathing organs. For 

 the front of the glottis is guarded by 



1 . n -i //// 1*1 Fig- 554- Mouth, Nose, &c., of Man, 



an elastic nap, the epiglottis, which in vertical section 



durinO" Swallowing is folded back OVer * Cavity of nose opening into pharynx 



& . CM), and separated from mouth-cavity by 



the breathing opening, and constitutes, hard pa iat e () and soft paiate w, the latter 



,. , . , ending in a rounded projection (the uvula, ), 



aS it Were, a SOrt OI bridge OVer below which is the opening between mouth- 



which the food passes back into the JS^^^SK^S^sTI^ 

 gullet. Human beings sometimes at- 

 tempt to speak when this transit of food is in progress, with the 

 result that the epiglottis springs up and particles make their way 

 into the windpipe. This is what is popularly called " swallowing 

 the wrong way ". 



It would scarcely repay us to consider in detail the various 

 modifications which the breathing organs present in the various 



