202 RESPIRATION. 



cated by the quantity of air which a person can expel from 

 his lungs by a forcible expiration after the deepest inspi- 

 ration that he can make. Mr. Hutchinson names this the 

 vital capacity : it expresses the power which a person has- 

 ofTreathing in the emergencies of active exercise, violence, 

 and disease; and in healthy men it varies according to- 

 stature, iceight, and age. 



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

 information on this subject is derived, that at a tempera- 

 ' "ture of 60 F., 225 cubic inches is the averate 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 below, 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 gener- 

 ally, 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 capacity of respiration is 

 determined only by the mobility of the walls of the chest j. 

 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 iceiglit on the capacity of respiration is 

 less manifest and considerable than that of height : and it 

 is difficult 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 nj stones; but that, above this point, it 



