THE KESPIEATOEY SYSTEM. 



In the mammal the breathing organs are lodged in a closed 

 cavity, separated by a muscular partition from that in which 

 the digestive and certain other organs are contained. This 

 thoracic chamber may be said to be reserved for circulatory 

 and respiratory organs which, we again, point out, are so related 

 that they really form parts of one system. 



The mammal's blood requires so much aeration (ventilation) 

 that the lungs are very large and the respiratory system has 

 become greatly specialized. We no longer find the sldn or ali- 

 mentary canal taking any large share in the process ; and the 

 lungs and the mechanisms by which they are made to move the 

 gases with which the blood and tissues are concerned become 

 very complicated. 



Our studies of muscle physiology should have made clear 

 the fact that tissue-life implies the constant consumption of 

 oxygen and discharge of carbonic anhydride, and that the pro- 

 cesses which give rise to this are going on at a rapid rate ; so 

 that the demands of the animal for oxygen constantly may be 

 readily understood if one assumes, what can be shown, though 

 less readily than in the case of muscle, that all the tissues are 

 constantly craving, as it were, for this essential oxygen — well 

 called '■ vital air." 



Respiration may, then, be regarded from a physical and 

 chemical point of view, though in this as in other instances we 

 must be on our guard against regarding physiological processes 

 as ever purely physical or purely chemical. The respiratory 

 process in the mammal, unlike the frog, consists of an active 

 and a (largely) passive phas.e. The air is not pumped into the 

 lungs, but sucked in. So great is the complexity of the lungs 

 in the mammal, that the frog's lung (which may be readily 

 understood by blowing it up by inserting a small pipe in the 

 glottic opening of the animal and then ligaturing the distended 



