406 TEXT-BOOK OF PHYSIOLOGY 



and the sternum advances, the result of which is an enlargement in the 

 diameters of the thorax. Coincidently with the enlargement of the thorax 

 through muscle activity there goes a corresponding increase in the size and 

 capacity of the lungs, in consequence of the expansion and pressure of the 

 air in the pulmonary alveoli. 



During expiration the diaphragm ascends in consequence of the return 

 of the displaced abdominal viscera, the ribs descend and inwardly rotate 

 and the sternum recedes from the recoil of the elastic tissues, the result of 

 which is a diminution in the diameters of the thorax. Coincidently with the 

 diminution of the thorax there goes a decrease in the size and capacity of the 

 lungs in consequence of the recoil of their elastic tissue whereby the air in 

 the lungs is compressed. 



The intra-pulmonic pressure in consequence of the alternate expansion 

 and compression of the intra-pulmonic air also undergoes a considerable 

 variation. 



During inspiration the intra-pulmonic air expands. With the expansion 

 its pressure falls; but though it is now less than atmospheric pressure it is 



?63 



*- Inspiration* /^^ 



760 m?n. ^ ^ 



< 



~~~7J8 

 A. INTRA-PULMONIC PRESSURES. 



760 mm, j*~ fnspi ration -^-Expiration 



7J1 



B. INTRA-THORACIC PRESSURE. 



Fro. 1 88. REPRESENTING THE CHANGES, i, IN THE INTRA-PULMONIC, AND 2, IN THE INTRA-THO- 

 RACIC PRESSURES DURING INSPIRATION AND EXPIRATION. 



yet much greater than the opposing force of the lung tissue. As a result of 

 the fall of intra-pulmonic pressure, there is a rapid inflow of air which con- 

 tinues until atmospheric pressure is restored; that is, at the end of the 

 inspiration. 



During expiration the intra-pulmonic air becomes compressed. With 

 the compression its pressure rises above that of the atmosphere and in conse- 

 quence there is a rapid outflow of air, which continues until atmospheric 

 pressure is again restored; that is, at the end of the expiration. (Fig. 188, A .) 



The cause for the fall of intra-pulmonic pressure during inspiration and 

 the rise during expiration is to be found in the resistance offered by the air- 

 passages to the movement of the air, throughout their entire extent, and 

 especially at the level of the vocal bands. The greater the resistance, from 

 whatever cause, physiologic or pathologic, the greater the variations of 

 the pressure. If the inspiratory and expiratory movements take place slowly 

 the intra-pulmonic pressure may scarcely vary in either direction. 



In quiet inspiration the fall of pressure, as indicated by a manometer 

 inserted into one nostril, seldom amounts to more than 1.5 mm. of Hg., the 



