QUANTITY OF AIR RESPIRED. 167 



shown. After ordinary expiration, such as that which expels 

 the breathing air, a certain quantity of air remains in the lungs, 

 which may be expelled by a forcible and deeper expiration : 

 this he terms reserve air. But, even after the most violent ex- 

 piratory effort, the lungs are not completely emptied ; a certain 

 quantity always remains in them, over which there is no vol- 

 untary control, and which may be called residual air. Its 

 amount depends in great measure on the absolute size of the 

 chest, and has been variously estimated at from forty to two 

 hundred and sixty cubic inches. 



The greatest respiratory capacity of the chest is indicated 

 by the quantity of air which a person can expel from his lungs 

 by a forcible expiration after the deepest inspiration that he 

 can make. Mr. Hutchinson names this the vital capacity : it 

 expresses the power which a person has of breathing in the 

 emergencies of active exercise, violence, and disease; and in 

 healthy men it varies according to stature, weight, and age. 



It is found by Mr. Hutchinson, from whom most of our in- 

 formation on this subject is derived, that at a temperature of 

 60 F., 225 cubic inches is the average vital capacity of a 

 healthy person, five feet seven inches in height. For every 

 inch of height above this standard the capacity is increased, 

 on an average, by eight cubic inches ; and for every inch be- 

 low, it is diminished by the same amount. This relation of 

 capacity to height is quite independent of the absolute capacity 

 of the cavity of the chest; for the cubic contents of the chest 

 do not always, or even generally, increase with the stature of 

 the body ; and a person of small absolute capacity of chest may 

 have a large capacity of respiration, and vice versa. The ca- 

 pacity of respiration is determined only by the mobility of the 

 walls of the chest; but why this mobility should increase in a 

 definite ratio with the height of the body is yet unexplained, 

 and must be difficult of solution, seeing that the height of the 

 body is chiefly determined by that of the legs, and not by the 

 height of the trunk or the depth of the chest. But the vast 

 number of observations made by Mr. Hutchinson seem to leave 

 no doubt of the fact as stated above. 



The influence of weight on the capacity of respiration is less 

 manifest and considerable than that of height: and it is diffi- 

 cult to arrive at any definite conclusions on this point, because 

 the natural average weight of a healthy man in relation to 

 stature has not yet been determined. As a general statement, 

 however, it may be said that the capacity of respiration is not 

 affected by weights under 161 pounds, or 11 1 stones; but that, 

 above this point, it is diminished at the rate of one cubic inch 

 for every additional pound up to 196 pounds, or 14 stones; so 



