208 A MANUAL OF PHYSIOLOGY 



the residual air. After a normal expiration following a 

 normal inspiration the lungs still contain stationary air to 

 the amount of about 2,500 c.c. 



The residual air may be measured by causing a person, starting 

 immediately after the deepest possible expiration, to breathe out and 

 in several times into a vessel (a spirometer) filled with hydrogen, till 

 it can be assumed that the hydrogen and the residue of air in the 

 lungs have been completely mixed. Knowing the quantity of 

 hydrogen originally contained in the vessel, we can calculate from 

 the percentage at the end of the experiment the quantity of air with 

 which it has been mixed that is, the residual air (Davy). 



Let V be the quantity of hydrogen in the spirometer at first, and 

 p the percentage amount in it at the end of the experiment. Let x 

 be the volume of residual air in the lungs at the beginning. 



Then, since the quantity of hydrogen remains unchanged after the 



P 



mixture, - (x + V) = V, 



100 



Suppose V = 4,000 c.c., 

 and ^ = 85 per cent., 



we get x= I2>000 = about 705 c.c. 



FIG. 80. DIAGRAM TO ILLUSTRATE THE RELATIVE AMOUNT OF COMPLE- 

 MENTAL, TIDAL, SUPPLEMENTAL, AND RESIDUAL AIR. 



But some carbon dioxide would be given off by the lungs, and some 

 oxygen, and perhaps hydrogen, absorbed, during the experiment, and 

 therefore slight corrections might have to be made. Sir Humphry 

 Davy actually calculated the residual air in his own lungs, as deter- 

 mined by this method, at 672 c.c. 



The coefficient of ventilation, that is, the ratio of the quantity of air 

 taken in at each inspiration to the quantity already in the lungs, has 

 oeen estimated at about \ or %. 



The term vital or respiratory capacity is applied to the 

 quantity of air which can be expelled by the deepest expira- 

 tion following the deepest inspiration, and amounts in an 

 adult of average height to 3,500 or 4,000 c.c. The maximum 



