THE IvUNGS AND RESPIRATION. 213 



of the amount of air necessary to properly supply the lungs, 

 which includes the rather important subject of ventilation. 



Ventilation. 



As stated before, we take in at each breath about thirty 

 cubic inches of air; that is, about half a liter. As about 

 fifteen breaths are taken on an average per minute, this 

 makes the amount of air taken into the lungs in that time 

 450 cubic inches. But the problem of the amount of air is 

 not so simple as that. If each mouthful of air as it is 

 breathed out could at once be snatched away from the mouth 

 and so enable a fresh mouthful to be- taken in, 450 cubic 

 inches would suffice. By making careful experiments it has 

 been shown that every mouthful of air that we breathe 

 out is mixed with the outside air and vitiates three times as 

 much additional air, to such an extent as to make it no 

 longer fit for respiration. Or, in other words, every mouth- 

 ful we breathe becomes mixed with three additional mouth- 

 fuls outside, in a way to make the four mouthfuls so result- 

 ing perfectly unfit for further respiration. Therefore, the 

 amount of air actually required per minute is not 450 cubic 

 inches, but four times 450 cubic inches, or 1,800 cubic 

 inches per minute. This is just a little over one cubic foot. 

 These figures express rather important results and ought to 

 be kept in mind by persons who have the ventilation of 

 crowded rooms in charge. Fresh air ought to be admitted 

 at the rate of one cubic foot per minute for each person in 

 the house. This does not, of course, mean the production 

 of a draft, it being entirely possible to renew the air at this 

 rate even in a fairly crowded room without subjecting any 

 body in it to exposure to a draft. 



For proper ventilation an amount of space between 

 500 and 1,000 cubic feet for each person ought to be avail- 

 able. In some of the better hospitals where the crowding 

 of patients is not permitted, "and where the subject of proper 

 ventilation is treated as of the primest importance, the 

 amount of space allowed to each patient is often even more 



