52i VETERINARY PHYSIOLOGY 



these vesicles, that, if this air were uniformly distributed, it would 

 add only about -fth to the volume of each. The exchange of 

 gases depends, in fact, largely upon the process of diffusion 

 (fig. 212). 



Oxygen is constantly being removed by the blood from the 

 air in the air vesicles, and carbon dioxide is constantly being 

 added to it. Hence, the pressure of oxygen is lower and the 

 pressure of carbon dioxide higher than in the air breathed, and 

 hence, a diffusion of oxygen to the air in the vesicles and a 

 diffusion of carbon dioxide from it are constantly going on. In 

 the less expansile parts of the lung the exchange of gases will 



Fig. 212. — To show the exchanges of gases by diffusion between 

 the tidal air and the reserve and residual air. 



depend more largely upon diffusion than in the more expansile 

 parts. 



IV. Breath Sounds- 



The air, as it passes into and out of the lungs, produces 

 sounds that may be heard on listening over the thorax. The 

 character of the breath sounds is of the utmost importance in 

 the diaofnosis of diseases of the lungs, and must be studied 

 practically (Practical Physiology). 



On listening over the trachea or over the bifurcation of the 

 bronchi behind (between the 4th and 5th dorsal vertebrae), a 

 harsh sound, something like the guttural ch (German ich), may 

 be heard with inspiration and expiration. This is called the 

 bronchial sound. 



If the ear be applied over a spot under which a mass of air 

 vesicles lies, a soft sound, somewhat resembling the sound of 



