358 RESPIRATION [CH. XXIV. 



ventilation is secured, but obviously the composition of the expired 

 air is not the same as that of alveolar air, for the latter, though it is 

 ultimately breathed out, is diluted on its upward journey by mixture 

 with the bronchial air, and that in its turn with the air of the upper 

 air chambers ; in other words, the expired air is alveolar air (rich in 

 carbon dioxide) diluted with bronchial air (richer in oxygen) and 

 with atmospheric air (still richer in oxygen). No doubt diffusion of 

 gases occurs as well, oxygen diffusing inwards and carbon dioxide 

 outwards, but this molecular movement is too slow to be of any real 

 use in aerating the blood, for almost immediately the respiratory 

 movements cease, death occurs. 



b. Complemental air is the quantity over and above this which 

 can be drawn into the lungs in the deepest inspiration ; its 

 amount varies, but it may be reckoned as 100 cubic inches, or about 

 1600 c.c. 



c. Reserve or supplemental air. After ordinary expiration, such 

 as that which expels the breathing or tidal air, a certain quantity of 

 air, about 100 cubic inches (1600 c.c.) remains in the lungs, which 

 may be expelled by a forcible and deeper expiration. This is termed 

 reserve or supplemental air. 



d. Residual air is the quantity which still remains in the lungs 

 after the most violent expiratory effort. Its amount depends in great 

 measure on the absolute size of the chest, but may be estimated at 

 about 100 cubic inches, or about 1600 c.c. 



The total quantity of air which passes into and out of the lungs 

 of an adult, at rest, in 24 hours, varies from 400,000 (Marcet) to 

 680,000 (Hutchinson) cubic inches. This quantity, however, is 

 increased, and may be more than doubled by exertion. 



e. Respiratory or Vital Capacity. The vital 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 inspiration possible. 

 The average capacity of an adult, at 154 C. (60 F.), is about 225 to 

 250 cubic inches, or 3500 to 4000 c.c. It is the sum of the com- 

 plemental, tidal, and supplemental air. 



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 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.), 



