284 RESPIRATION 



capacity of the chest is indicated by the quantity of air which a person can 

 expel from his lungs by a forcible expiration after the deepest possible in- 

 spiration. The vital capacity is the sum of the reserve, tidal, and comple- 

 mental airs. It expresses the power which a person has of breathing in the 

 emergencies of active exercise, violence, and disease. The average capacity 

 of an adult, at 15.4 C. (60 F.), is about 225 to 250 cubic inches, or 3,500 

 to 4,000 c.c. 



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

 usually measured by a modified gasometer or spirometer, into which the experi- 

 menter 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 or centimeters is read off. 



In healthy men, the respiratory capacity varies chiefly with the stature, 

 weight, and age. 



Circumstances Affecting the Amount of Respiratory Capacity. For every 

 inch of height above the standard the respiratory capacity is increased, on an 

 average, by eight inches ; and for every inch below it is diminshed by the same 

 amount. 



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

 considerably less than that of height. 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. 



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 inches per 

 year; so that the capacity of respiration of a man sixty years old would be 

 about thirty cubic inches less than that of a man forty years old, of the same 

 height and weight (John Hutchinson). 



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 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 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 

 which the inspiratory muscles are capable of exerting on the chest is greatest 



