APPARATUS OF RESPIRATION. 



77 



artery are distributed, by means of which, now become capil- 

 laries, the venous blood is exposed to the action of the air. 



The smaller the cells, the greater will be the extent of 

 the membrane, and the more extended the surface upon which 

 the blood is exposed to the action of the air. The smallness 

 of the cells and the activity of respiration are thus in a direct 

 ratio to each other ; and this is proved by contrasting the 



Fig. 59. Thorax of Man.* 



large pulmonary cells in the lungs of the frog with the 

 microscopic cells which we find in the lungs of birds and 

 mammals. 



137. But air, as air, never penetrates beyond the little 



* On the left side the muscles have been removed. The arch forming the 

 diaphragm towards the interior of the chest is seen on the left side, g, and on 

 the right side a dotted line marks the extent of the ascent of the same muscle 

 on the right side ; h, pillars of the diaphragm attached to the lumbar ver- 

 tebrae; i, elevator muscles of the ribs ; d, collar-bone. 



