VITAL CAPACITY. 215 



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 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 iij 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 that, for example, 

 while a man of five feet six inches, and weighing less than 

 nj stones, should be able to expire 217 cubic inches, one 

 of the same height, weighing 12^ stones, might expire 

 only 203 cubic inches. 



By age, the capacity appears to be increased from about 

 the fifteenth to the thirty-fifth year, at the rate of five cubic 

 inches per year, from thirty-five to sixty-five it diminishes 

 at the rate of about one and a-half cubic inch per year ; so 

 that the capacity of respiration of a man of sixty years 

 old would be about 30 cubic inches less than that of a man 

 forty years old, of the same height and weight. 



Mr. Hutchinson's observations were made almost exclu- 

 sively on men ; and his conclusions are, perhaps, true of 

 them alone ; for women, according to Bourgery, have only 

 half the capacity of breathing that men of the same age 

 have. 



The number of respirations in a healthy adult person 

 usually ranges from fourteen to eighteen per minute. 



It is greater in infancy and childhood; and of course 



