3T0 COMPARATIVE PHYSIOLOGY. 



a 



E ^ « \ . 



Vr - * 



Fig. 292.— Section of the parenchyma of the hnman lung, injected throngh the pul- 

 monary artery (Schulze). a, a, a, c, c, c, walla of the air-cells; 6, small arterial 

 branch. 



THE ENTRANCE AND EXIT OF AIR. 



Since the lungs fill up so completely the thoracic cavity, 

 manifestly any change, in the size of the latter must lead to 

 an increase or diminution in the quantity of air they contain. 

 Since the air within the respiratory organs is heing constantly 

 rohhed of its oxygen, .and rendered impure by the addition of 

 carbonic dioxide, the former must be renewed and the latter 

 expelled ; and, as mere diffusion takes place too slowly to ac- 

 complish this in the mammal, this process is assisted by the 

 nervous system setting certain muscles at work to alter the size 

 of the chest cavity. Because of the ribs being placed obliquely, 

 it follows that their elevation will result in the enlargement of 

 the thoracic cavity in the antero-posterior diameter ; and, as the 

 chest, in conseqitence, gets wider from above downward, also in 

 the transverse diameter; which is moreover assisted by the ever- 

 sion of the lower borders of the ribs ; and, if the convexity of the 

 diaphragm were diminished by its contraction and consequent 

 descent, it would follow that the chest would be increased in 



