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TEXT-BOOK OF PHYSIOLOGY. 



of the thorax through muscle activity, there goes a corresponding in- 

 crease in the size and capacity of the lungs in consequence of the 

 expansion and pressure of the air in the pulmonary alveoli. With 

 the expansion of the air its pressure falls; but though it is now less 

 than atmospheric, it is yet much greater than the opposing force 

 of the lung tissue. As a result of the fall of intra-pulmonary pres- 

 sure there is a rapid inflow of air, which continues until atmos- 

 pheric pressure is restored; that is, at the end of the inspiration. 

 With the diminution of the thorax, through the recoil of the elastic 

 tissue of the thoracic and abdominal walls, there goes a correspond- 

 ing decrease of lung capacity, in consequence of the recoil of 

 the elastic tissue of the lungs. As a result, the air in the lungs 

 becomes compressed, its pressure rises above that of the atmosphere, 

 and a rapid outflow of air takes place, which continues until atmos- 

 pheric pressure is 



Insp. 



Insp. 



Exp. 



Intra-pulmonary pressure. 



Exp. 



760 mm 



C 760 mm 



Intra-thoracic pressure. 



FIG. 167. REPRESENTING THE CHANGES, i, IN THE 

 INTRA-PULMONARY, AND, 2, IN THE INTRA-THO- 

 RACIC PRESSURES DURING INSPIRATION AND EX- 

 PIRATION 



again restored; that 

 is, at the end of the 

 expiration. 



The cause for the 

 fall of intra-pulmon- 

 ary 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 es- 

 pecially at the level of 



the vocal bands. The greater the resistance, from whatever cause, 

 physiologic or pathologic, the greater the variations of the pressure. 



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

 ometer inserted into one nostril, seldom amounts to more than 1.5 mm. ; 

 the rise in expiration, 2.5 to 3 mm. In forcible inspiratory and ex- 

 piratory efforts these limits may be largely exceeded. Thus it was 

 found by Bonders that with one nostril closed and a mercurial man- 

 ometer 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-pulmonary pressure are graphically 

 represented in the upper half of Fig. 167. 



The intra-thoracic pressure also varies during both inspiration 

 and expiration. As the intra-pulmonary pressure falls, the recoil of 

 the elastic tissue increases, with the result of diminishing the intra- 

 thoracic pressure, though not in a steadily progressive manner. The 



