180 PRINCIPLES OF PHYSIOLOGY 



is a series of muscular movements by which 

 the chest expands, as in inspiration, and 

 the air rushes into the upper air passages 

 to mix with the air already there : this 

 is followed in ordinary expiration by an 

 elastic recoil of the chest wall by which the 

 air is expelled from the upper air passages. 

 The air in those passages mixes with the air 

 in the ultimate air cells by a physical process 

 of diffusion of gases. The whole of this process, 

 the distension and recoil of both chest wall 

 and lungs, constitutes a pulmonary ventilation. 

 The essential phenomena of respiration are, 

 however, in the air cells and in the tissues. 

 The lungs may eliminate a small amount of 

 water by evaporation from the respiratory 

 gases, and, occasionally, other matters 

 may pass off which taint the breath. The 

 mechanism of external breathing is carried 

 on by a complex system of muscles and by a 

 special innervation. 



68. Kidneys. Excretion is also carried on 

 by the kidneys. These organs, which may be 

 regarded as highly modified tubular glands, 

 separate from the blood, water ; various 



