RESPIRATION 207 



The total quantity of air expired, or, what comes to the 

 same thing, the alteration in the capacity of the chest during 

 expiration, can be measured by means of a spirometer, which 

 consists of an inverted graduated glass bell dipping by its 

 open mouth into water and balanced by weights. The 

 vessel is sunk till it is full of water, the air being allowed 

 to escape by a cock. The expired air is now permitted to 

 enter it through a tube, and displaces some of the water. 

 The spirometer is adjusted so that the level of the water 

 inside and outside is the 

 same, and then the volume 

 of air contained in it is read 

 off. This gives the volume 

 of the expired air at atmo- 

 spheric pressure. Similarly, 

 by breathing air from the 

 spirometer the amount in- 

 spired can be measured. 



From 400 to 500 c.c. of 

 air* are taken in and given 

 out at each respiration in 

 quiet breathing. This is 

 called tidal air. It amounts 



t 35 pounds by weight in 



r , , FIG. 79. DIAGRAM OF SPIROMETER. 



twenty- four hours, or enough A yesse , fi ,, ed wjth wa(er B glass 



tO fill, at atmospheric pres- cylinder with scale C, swung on pulleys 

 . . . . . . . , and counterpoised by weights W. D, tube 



Sure, a Cubical box With a Side f or breathing through. 



of 8 feet. With the deepest 



possible inspiration room can be made for 2,000 c.c. more ; 

 this is called complemental air. By a forced expiration 

 1,500 c.c. can be expelled besides the tidal air ; and to this 

 quantity the name of supplemental or reserve air has been 

 given. After the deepest expiration there always remain 

 about 700 or 800 c.c. of air in the lungs, and this is called 



* The average for 81 healthy students, with an average body- weight of 

 66 kilos, was 460 c.c., or 7 c.c. per kilo. In 4 newborn children the tidal 

 air varied from 20 to 30 c.c., and from 7'6 to 7-3 c.c. per kilo, which is not 

 very different from the amount in the adult. The pulmonary ventilation 

 must therefore be far more rapid in the child, since its respiratory 

 frequency is so much greater. 



