CAPACITY OF THE LUNGS. 131 



quantity of air in the lungs which can be expelled by a forced expiration. In 

 ordinary respiration a comparatively small volume of air is introduced with 

 inspiration, and a nearly equal quantity is expelled by the succeeding expira- 

 tion. By the extreme action of all the inspiratory muscles in a forced inspi- 

 ration, a supplemental quantity of air may be introduced into the lungs, which 

 then contain much more than they ever do in ordinary respiration. For 

 convenience of description, physiologists have adopted the following names, 

 which are applied to these various volumes of air : 



1. Residual Air ; that which is not and can not be expelled by a forced 

 expiration. 



2. Reserve Air ; that which remains after an ordinary expiration, deduct- 

 ing the residual air. 



3. Tidal, or Ordinary Breathing Air ; that which is changed by the 

 ordinary acts of inspiration and expiration. 



4. Complemented Air ; the excess over the ordinary breathing air, which 

 may be introduced by a forcible inspiration. 



In measuring the air changed in ordinary breathing, it has been found 

 that the acts of respiration are so easily influenced and it is so difficult to 

 experiment on any , individual without his knowledge, that the results of 

 many good observers are not to be relied upon. This is one of the most 

 important of the questions under consideration. The difficulties in the way 

 of estimating with accuracy the residual, reserve or complemental volumes, 

 will readily suggest themselves. The observations on these points which 

 may be taken as the most definite and exact are those of Herbst and of 

 Hutchinson. Those of the last-named observer are very elaborate and 

 were made on a large number of subjects of both sexes and of all ages and 

 occupations. They are generally accepted by physiologists, as the most ex- 

 tended and accurate. 



Residual Air. Perhaps there is not one of the questions under consider- 

 ation more difficult to answer definitely than that of the quantity of air 

 which remains in the lungs after a forced expiration ; but it fortunately is 

 not one of any great practical importance. The residual air remains in the 

 lungs as a physical necessity. The lungs in health are always in contact 

 with the walls of the thorax ; and when this cavity is reduced to its smallest 

 dimensions, it is impossible that any more air should be expelled. The vol- 

 ume which thus remains has been variously estimated. The residual volume 

 has been estimated at about one hundred cubic inches (1,639 c.c.), but the 

 quantity varies very considerably in different individuals (Hutchinson). 

 Taking everything into consideration, it may be assumed that this estimate 

 is as nearly correct as any. 



Reserve Air. This name is given to the volume of air which may be ex- 

 pelled and changed by a voluntary effort, but which remains in the lungs, 

 added to the residual air, after an ordinary act of expiration. It may be 

 estimated, without any reference to the residual air, by forcibly expelling air 

 from the lungs, after an ordinary expiration. The average volume, accord- 

 ing to Hutchinson, is one hundred cubic inches (1,639 c.c.). 



