THE RESPIRATORY CAPACITY 



261 



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

 measured by a modified gasometer or spirometer, 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 con- 

 nection 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 diminished by the same amount. 



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

 ably less than that of height. It is difficult to arrive at any definite conclusions on this 



FIG. 235. Diagram of Hutchinson's Spirometer. (Landois.) A , Graduated cylinder serving 

 as a receiver for the breath; it is supplied with a stopcock at the top for the ready expulsion of air, 

 and is balanced by weights passing over pulleys. B, Mouthpiece with tube reaching nearly to the 

 top of the graduated receiver (A) when the latter is sunk in the reservoir ready for an experiment; 

 there is a stopcock in this tube near the first angle, to prevent regurgitation of air. C, Reservoir 

 for the graduated receiver. In using the spirometer the reservoir and graduated receiver are filled 

 with water, or, to prevent the absorption of carbon dioxide, with a saturated aqueous solution of 

 common salt (NaCl). When ready for an experiment, the stopcock at the top of the receiver 

 is closed and that in the tube of the mouthpiece opened, and the breath forced into the receiver. 

 The receiver rises as fast as the breath displaces the water. After the breath is forced into the re- 

 ceiver the stopcock in the tube of the mouthpiece is closed, and the water outside and inside the 

 receiver brought to the same level, so that the air within the receiver shall be at the atmospheric 

 pressure. The amount of breath within the receiver is then read directly from the scale attached 

 to the receiver. For accurate measurement the breath should stand a few minutes to acquire the 

 temperature of the liquid over which it is collected ; then the various corrections for aqueous vapor 

 tension, and the variations from the standard temperature and pressure, should be made. 



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 di- 

 minishes 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.) 



