RESPIRATION. 269 



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 information 

 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. 



Circumstances affecting the amount of respiratory capacity. For every inch 

 of height above this standard the capacity is increased, on an average, by eight 

 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 11^ 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 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 of forty years old, of the same height 

 and weight. (John Hutphinson.) 



The number of respirations in a healthy adult person usually ranges 

 from 14 to 1-8 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 pulses 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 indi- 

 viduals of the height of from five feet seven inches to five feet eight 

 inches, and will elevate a column of three inches of mercury. Above 

 this height the force decreases as the stature increases; so that the aver- 

 age of men of six feet can elevate only about two and a half inches of 



