LUNG CAPACITY. 119 



4. The air cannot all be breathed out. The remainder is 

 called the Residual Air, and is computed to be about 100 cubic 

 inches. 



All the air that can be breathed out after a full inspira- 

 tion, 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 

 considerably increase his vital capacity. 



A simple method of measuring these stages of respiration 

 is to graduate a gallon bottle carefully 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, invert in water, and 

 exhale into it. 



It will be observed, since the same tube that takes in the 

 fresh air also sends out the bad air, that when the current is 

 reversed the first air to be sent out is the last air that came 

 in, which is, therefore, the best air in the lungs. But the 

 amount taken at each inspiration is three or four times as 

 great as the capacity of the trachea and bronchi, so that at 

 each breath two-thirds or more of the fresh air is taken 

 directly into the air vesicles. In addition to this the better 

 air in the trachea is mixing with the poorer air in the air 

 vesicles by diffusion ; the oxygen being more abundant in the 

 trachea, passes down into the deeper parts of the lung, while 

 the carbon dioxid, so abundant in the vesicles, passes up into 

 the trachea. 



At each inspiration the air in the lungs receives about one- 

 eighth of its volume of fresh air, and at each expiration about 

 one-ninth of the air is sent out. But it must be remembered 

 that this one-ninth sent out is not the worst, but the very best, 



