CH. XXVI.] TIDAL AIR 359 



the air which leaves the lungs will come in part from the alveoli, but 

 the effect of the stream of outgoing air is mainly as before, to effect a 

 thorough admixture of the air in the intermediate air-passages ; thus 

 the alveolar air will become mixed with that in the bronchial tubes, 

 and that in turn will be mixed with that in the upper air-chambers. 

 In a succession of alternate ordinary inspirations and expirations 

 adequate 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). 



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

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

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



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

 as that which expels the tidal air, a further quantity of air, about 100 

 cubic inches (1600 c.c.) can be expelled by a forcible deep expiration. 

 This is termed reserve or supplemental air. The last portion of the 

 air thus expelled will consist of air from the alveoli. 



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 15'4 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. Haldane measures the volume of air expired by the reduction which takes 

 place in the volume of the body when placed within a plethysmograph large 

 enough to take a man, with the exception of his head. 



The number of respirations in a healthy adult person usually ranges 

 from 14 to 18 per minute. It is greater in infancy and childhood. 



