270 HANDBOOK OF PHYSIOLOGY. 



mercury. The force manifested in the strongest expiratory acts is, on 

 the average, one-third greater than that exercised in inspiration. But 

 this difference is in great measure due to the power exerted by the 

 elastic reaction of the walls of the chest; and it is also much influenced 

 by the disproportionate strength which the expiratory muscles attain, 

 from their being called into use for other purposes than that of simple 

 expiration. The force of the inspiratory act is, therefore, better adapted 

 than that of the expiratory for testing the muscular strength of the 

 body. (John Hutchinson.) 



The instrument used by Hutchinson to gauge the inspiratory and expiratory 

 power was a mercurial manometer, to which was attached a tube fitting the 

 nostrils, and through which the inspiratory or expiratory effort was made. 

 The following table represents the results of numerous experiments : 



Power of . Power of 



Inspiratory Muscles. Expiratory Muscles. 



1.5 in. . . Weak . . . . 2.0 in. 



2.0 

 2.5 

 3.5 

 4.5 

 5.5 

 6.0 

 7.0 



Ordinary . . . 2.5 



Strong . . . .3.5 



Very strong . . . 4.5 



Remarkable . . .5.8 



Very remarkable . . 7.0 



Extraordinary . . .8.5 



Very extraordinary . 10.0 



The greater part of the force exerted in deep inspiration is employed 

 in overcoming the resistance offered by the elasticity of the lungs. 



The amount of this elastic resistance was estimated by observing the elec- 

 tion of a column of mercury raised by the return of air forced, after deaih, 

 into the lungs, in quantity equal to the known capacity of respiration during 

 life ; and Hutchinson calculated, according to the well-known hydrostatic law 

 of equality of pressures (as shown in the Bramah press), that the total force to 

 be overcome by the muscles in the act of inspiring 200 cubic inches of air is 

 more than 450 Ibs. 



The elastic force overcome in ordinary inspiration is, according to the same 

 authority, equal to about 170 Ibs. 



Douglas Powell has shown that within the limits of ordinary tran- 

 quil respiration the elastic resilience of the walls of the chest favors in- 

 spiration ; and that it is only in deep inspiration that the ribs and rib- 

 cartilages offer an opposing force to their dilatation. In other words, 

 the elastic resilience of the lungs, at the end of an act of ordinary 

 breathing, has drawn the chest-walls within the limits of their normal 

 degree of expansion. Under all circumstances, of course, the elastic 

 tissue of the lungs opposes inspiration and favors expiration. 



It is possible that the contractile power which the bronchial tubes 

 and air- vesicles possess, by means of their muscular fibres may (1) assist 

 in expiration; but it is more likely that its chief purpose is (2) to regu- 

 late and adapt, in some measure, the quantity of air admitted to the 



