RESPIRATION. 97 



Capacity of the Lungs. Have the class stand, and each pupil raise 

 his right hand. 



1 . Tidal Air. Let all breathe together, at the ordinary rate and 

 depth, and let the hand rise about three inches during inspiration, and 

 fall again during expiration. The amount of air taken in at an ordinary 

 breath is from 20 to 30 cubic inches, or about a pint. This is called 

 tidal air. 



2. Complemental Air. As before, let the hand go up and down 

 with the breathing, but at the end of the third inspiration, instead of 

 stopping with the usual amount, keep on breathing in as much as pos- 

 sible, letting the hand rise accordingly. This air that can be taken in 

 above the ordinary breath is called the complemental air, and it is 

 estimated to be, on the average, about 120 cubic inches. 



3. Reserve Air. Begin as before, and at what would be the end 

 of the third expiration continue to drive out as much air as possible, 

 indicating the degree by correspondingly lowering the hand. This air 

 that can be breathed out beyond the ordinary expiration is called the 

 reserve air, and is reckoned at about 100 cubic inches. 



4. Residual Air. The air cannot all be breathed out. The re- 

 mainder is called the residual air, and is computed to be about 100 

 cubic inches. 



The Vital Capacity. All the air that can be breathed out after a 

 full inspiration, i.e. the sum of the complemental, tidal, and reserve 

 air, would be about 240 to 250 cubic inches, and is called the vital 

 capacity. Of course these figures represent only the average of cer- 

 tain experiments and observations. By practice any one can con- 

 siderably increase his vital capacity. 



A Test of the Capacity of the Lungs. A simple method of 

 measuring these stages of respiration is to take a gallon bottle and 

 first carefully graduate it to pints by pouring in water and marking on 

 the outside with a file. Then invert the bottle in a trough of water, 

 and inhale from it by means of a rubber tube. Or fill the bottle, in- 

 vert in water, and exhale into it. 



Hygiene of Breathing. Those persons who take con- 

 stant exercise in the open air are likely not to suffer much 

 from deficient respiration. But persons following seden- 



