392 THE HUMAN BODY. 



it were broken. The capacity of the chest, and therefore of 

 the lungs, varies much in different individuals, but in a man 

 of medium height there remain in the lungs after the most 

 violent possible expiration, about 1640 cub. cent. (100 cub. 

 inches) of air, called the residual air. After an ordinary 

 expiration there will be in addition to this about as much 

 more supplemental air ; the residual and supplemental to- 

 gether forming the stationary air, which remains in the 

 chest during quiet breathing. In an ordinary inspiration 500 

 cub. cent. (30 cub. inches) of tidal air are taken in, and 

 about the same amount is expelled in natural expiration. 

 By a forced inspiration about 1600 cub. cent. (98 cub. inches) 

 of complemental air can be added to the tidal air. After a 

 forced, inspiration therefore the chest will contain 1640 -f- 

 1640 -f- 500 -f 1600 = 5380 cubic centimeters (328 cubic 

 inches) of air. The amount which can be taken in by the 

 most violent possible inspiration after the strongest possible 

 expiration, that is, the supplemental, tidal, and complemental 

 air together, is known as the vital capacity. For a healthy 

 man 1.7 meters (5 feet 8 inches) high it is about 3700 cub. 

 cent. (225 cub. inches) and increases 60 cub. cent, for each 

 additional centimeter of stature; or about 9 cubic inches for 

 each inch of height. 



The Quantity of Air Breathed Daily. Knowing the 

 quantity of air taken in at each breath and expelled again 

 (after more or less thorough admixture with the stationary air) 

 we have only to know, in addition, the rate at which the 

 breathing movements occur, to be able to calculate how 

 much air passes through the lungs in twenty-four hours. 

 The average number of respirations in a minute is found by 

 counting on persons sitting quietly, and not knowing that 

 their breathing rate is under observation, to be fifteen in a 

 minute. In each respiration half a liter (30 cubic inches) of 

 air is concerned; therefore 0.5 X 15 X 60 X 24 = 10,800 

 liters (375 cubic feet) is the quantity of air breathed under 

 ordinary circumstances by each person in a day. 



Hygienic Remarks. Since the diaphragm when it con- 

 tracts pushes down the abdominal viscera beneath it, these 

 have to make room for themselves by pushing out the soft 

 front of the abdomen which, accordingly, protrudes when the 

 diaphragm descends. Hence breathing by the diaphragm, 

 being indicated on the exter.ior by movements of the abdo- 



