IV THE AMOUNT OF AIR RESPIRED 149 



Thus it follows that, after an ordinary inspiration, 

 1,500 + 1,500 + 500 = 3,500 c.c. (100 + 100 + 30 = 230 cubic 

 inches) may be contained in the lungs. By taking the 

 deepest possible inspiration, another 1,500 c.c. (100 cubic 

 inches), called Complemental air, may be added. 



The sum of the supplemental, tidal, and complemental 

 air amounts to about 3,500 to 4,000 c.c. (230 to 250 cubic 

 inches), and is a measure of what is known as the reapiratory 

 or intal capacity. It varies according to a person's height, 

 weight, and age. 



It results from these data that the lungs, after an 

 ordinary inspiration, contain about 3,500 c.c. (230 cubic 

 inches) of air, and that only about one-seventh to one- 

 eighth of this amount is breathed out and taken in again at 

 the next inspiration. Apart from the circumstance, then, 

 that the fresh air inspired has to fill the cavities of the 

 hinder part of the mouth, and the trachea, and the 

 bronchi, if the lungs were mere bags fixed to the end of 

 the bronchi, the inspired air would descend so far only 

 as to occupy that one-fourteenth to one-sixteenth part of 

 each bag which was nearest to the bronchi, whence it 

 would be driven out again at the next exjiiration. But as 

 the bronchi branch out into a prodigious mnnber of 

 bronchial tubes, the inspired air can only peneti'ate for a 

 certain distance along these, and can never reach the 

 air-cells at all. 



Thus the residual and supplemental air taken together 

 are, under ordinary circumstances, stationary — that is to 

 say, the aij comprehended under these names merely 

 shifts its outer limit in the bronchial tubes, as the chest 

 ddates and contracts, without leaving the lungs, and is 

 hence called stationary air ; the tklal air, alone, being 

 that which leaves tlie lungs and is renewed in ordinary 

 respiration. 



It is obvious, therefore, that the business of respiration 

 is essentially transacted by the stationary air, which plays 

 the pai-t of a middleman between the two parties — the 



