OH. XXIV.] VITAL CAPACITY. 355 



is about 225 to 250 cubic inches, or 3,500 to 4,000 ccm. It is 

 the sum of the complements,!, tidal, and supplemental air. 



The respiratory capacity, or as John Hutchinson called it, vital capacity, 

 is usually measured by a modified gasometer or tpirometer, into which the 

 experimenter breathes, making the most prolonged expiration possible 

 after the deepest possible inspiration. The quantity of air which is thus 

 expelled from the lungs is indicated by the height to which the air-chamber 

 of the spirometer rises : and by means of a scale placed in connection with 

 this, the number of cubic inches is read off. 



In healthy men, the respiratory capacity varies chiefly with 

 the stature, weight, and age. 



It was found by Hutchinson, from whom most of our informa- 

 tion on this subject is derived, that at a temperature of 15 '4 C. 

 (60 F.), 225 cubic inches is the average vital or respiratory 

 capacity of a healthy person, five feet seven inches in height. 



Circunixtanccx a(Tccting the amount of respirator y 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 

 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 state- 

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

 by weights under 161 pounds, or ii 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. 



Ser. 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 circum- 

 stances, such as exercise or rest, health or disease, fec. Varia- 

 tions in the number of respirations correspond ordinarily with 

 similar variations in the pulsations of the heart. In health the 

 proportion is about i to 4, or i 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 propor- 

 tion. It happens occasionally in disease, especially of the lungs 

 or air-passages, that the number of reajnratory acts increases in 

 quicker proportion than the beats of the pulse ; and, in other 



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