CH. XXIV.] VITAL CAPACITY 359 



225 cubic inches is the average vital or respiratory capacity of a 

 healthy person, five feet seven inches in height. 



Circumstances affecting the amount of respiratory capacity. 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. 



The influence of weight on the capacity of respiration is less manifest and con- 

 siderable 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 

 llij 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. 



By age, the capacity is 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. 



Sex. The vital capacity of an adult man to that of a woman of the same height 

 is 10 to 7. 



The number of respirations in a healthy adult person usually ranges 

 from 14 to 18 per minute. It is greater in infancy and childhood. 

 It varies also much according to different circumstances, such as 

 exercise or rest, health or disease, etc. Variations in the number of 

 respirations correspond ordinarily with similar variations in the 

 pulsations of the heart. In health the proportion is about 1 to 4, 

 or 1 to 5, and when the rapidity of the heart's action is increased, 

 that of the chest movement is commonly increased also ; but not in 

 every case in equal proportion. It happens occasionally in disease, 

 especially of the lungs or air-passages, that the number of respiratory 

 acts increases in quicker proportion than the beats of the pulse ; and, 

 in other affections, much more commonly, that the number of the 

 pulse-beats is greater in proportion than that of the respirations. 



The Force of Inspiratory and Expiratory Muscles. The force with 

 which the inspiratory muscles are capable of acting is greatest in 

 individuals of the height of from five feet seven inches to five feet 

 eight inches, and will elevate a column of nearly three inches (about 

 60 mm.) of mercury. Above this height the force decreases as the 

 stature increases ; so that the average of men of six feet can elevate 

 only about two and a half inches of 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.) 



