398 THE HUMAN BODY 



together 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 inspira- 

 tion about 1,600 cub. cent. (98 cub. inches) of complemental air 

 can be added to the tidal air. After a forced inspiration, therefore, 

 the chest will contain 1,000+1,600+500+1,600=4,700 cub. cent. 

 (300 cub. 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 3,700 cub. cent. (225 

 cub. inches) and increases 60 cub. cent, for each additional centi- 

 meter of stature ; or about 9 cub. inches for each inch of height. 

 These figures are, of course, average figures. Individual variations 

 from them are numerous. 



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 respira- 

 tions 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 cub. 

 inches) of air is concerned; therefore 0.5X15X60X24=10,800 

 liters (375 cub. feet) is the quantity of air breathed under ordi- 

 nary circumstances by each person in a day. 



Hygienic Remarks. Since the diaphragm when it contracts 

 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, protudes when the diaphragm descends. 

 Hence breathing by the diaphragm, being indicated on the exte- 

 rior by movements of the abdomen, is often called "abdominal 

 respiration," as distinguished from breathing by the ribs, called 

 " costal " or " chest breathing." In both sexes the diaphragmatic 

 breathing is the most important, but, as a rule, men and children 

 use the ribs less than adult women. Since both abdomen and 

 chest alternately expand and contract in healthy breathing, any- 

 thing which impedes their free movement is to be avoided; and 

 the tight lacing which used to be thought elegant a few years 



