RESPIRATION 403 



quence there is a rapid outflow of air, whiclTcontinues until atmospheric 

 pressure is again restored; that is, at the end of the expiration. (Fig. 192 ?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 

 rise in expiration, 2.5 to 3 mm. of Hg. In forcible inspiratory and expiratory 

 efforts these limits may be largely exceeded. Thus it was found by Bonders 

 that with one nostril closed and a mercurial manometer inserted into the 

 other the pressure by voluntary efforts could be made to fall 57 mm. during 

 inspiration and to rise 87 mm. during expiration. The changes in intra- 

 pulmonic pressure are graphically represented in the upper half of Fig. 191. 



763 



Inspiration -* /^ 



760 m?n. 



Expiration, 



7S8 



A. INTRA-PULMONIC PRESSURES. 



760mm, 



Expiration 



B. INTRA-THORACIC PRESSURE. 



FIG. loi. REPRESENTING THE CHANGES, i, IN THE INTRA-PULMONIC, AND 2, IN THE INTRA-THO- 

 RACIC PRESSURES DURING INSPIRATION AND EXPIRATION. 



The intra-thoracic pressure also varies during both inspiration and expira- 

 tion. As the thorax enlarges and the intra-pulmonic pressure falls, the 

 recoil of the elastic tissue increases, with the result of still further dimin- 

 ishing the intra-thoracic pressure, until its maximum is reached near 

 the end of the inspiration. The fall of intra-thoracic pressure at 

 the end of a quiet inspiration reaches to about 9 mm. Hg. 

 forcible inspiratory efforts this fall in intra-thoracic pressure may 

 amount to 30 or 40 mm. of Hg. As the thorax again diminishes and 

 intra-pulmonic pressure rises above the atmospheric pressure, t 

 of the elastic tissue is again opposed, with the result of mcrea *mg tb 

 intra-thoracic pressure, until the former condition of pressure 

 been regained at the end of the expiration. Neither the fall nor 

 the subsequent rise of the intra-thoracic pressure takes place , howei 

 in a steadily progressive manner for the following reasons . a tracm; 

 were made of the variations in the circumference of the thorai 

 respiratory movement it would resemble in its mam features 

 Fig 191, and variations in any linear dimension of the lung wo 

 course in the same proportion. This amount of elongation of elast 



