148 ELEMENTARY PHYSIOLOGY 



close together so as to force out all, or nearly all, the air 

 wliich they contjiin ; while tlie walls of the chest, when 

 approximated as much as possible, still inclose a very con- 

 siderahle cavity (Fig. 48, B) ; so that, even after the most 

 violent expiratory effort, a very large quantity of air is 

 left in the lungs. 



If an adult man, breathing calmly in the sitting position, 

 be watched, the respiratory act will be observed to be 

 repeated on an average about fifteen to seventeen times 

 every minute ; but the frequency of repetition is very 

 variable. Each act consists of certain components which 

 .succeed one another in a regular rhythmical order. 

 Fir.st, the breath is drawn in or inspired, immediately 

 afterwards it is driven out or expired ; and these suc- 

 cessive acts are followed by a brief pause. Thus, just as 

 in the rhythm of the heart, the auricular systole, the ven- 

 tricular systole and then a pause follow in regular order ; 

 so in the chest, the inspiration, the expiration, and then a 

 pause succeed one another. But in the chest, unlike the 

 case of the heart, the pause is generally very short com- 

 pared with the active movement ; indeed, sometimes it 

 hardly exists at all, a new inspiration following imme- 

 diately on the close of expiration. 



6. The Amount of Air Respired. — At each inspiration 

 of an adult well-grown man about 500 c.c. (20 to 30 cubic 

 inches) of air are inspired ; and at each expiration the 

 same, or a slightly smaller, volume (allowing for the increase 

 of temperature of the air so expired) is given out of the 

 body. To this the name of tidal air has been con- 

 veniently given. 



The amount of air which, as already pointed out, 

 cannot be got rid of by even the most violent expiratory 

 effort and is called Residual air, is, on the average, 

 about 1,500 c.c. (from 75 to 100 cubic inches). 



About as much more in addition to this remains in the 

 chest after an ordinary expiration, ahd is called Supple- 

 mental air. 



