114 RESPIRATION 



most forcible expiration after the most profound inspiration. This has 

 been called by Hutchinson, the vital capacity, as signifying " the volume 

 of air which can be displaced by living movements." Its volume is 

 equal to the sum of the reserve air, the breathing air and the comple- 

 mental air, and it represents the extreme capacity of the chest, less the 

 residual air. Its physiological importance is due to the fact that it can 

 be determined by an appropriate apparatus, the spirometer, and compari- 

 sons may thus be made between different individuals, both healthy and 

 diseased. The number of observations on this point made by Hutchin- 

 son amounts in all to a little less than five thousand. 



The extreme breathing capacity in health is subject to variations 

 that bear a close relation to the stature of the individual. Hutchinson 

 begins with the proposition that in a man of medium height (five feet 

 eight inches, or 170.2 centimeters), it is equal to two hundred and thirty 

 cubic inches (3768.6 cubic centimeters). 



The most striking result of the experiments of Hutchinson, in re- 

 gard to the modifications of the vital capacity, is that it bears a definite 

 relation to stature, without being much affected by weight or by the 

 circumference of the chest. This is especially remarkable, as it is well 

 known that height does not depend so much on the length of the body 

 as on the length of the lower extremities. He ascertained that for 

 every inch (2.5 centimeters) in height, between five and six feet (152.4 

 and 182.9 centimeters), the extreme breathing capacity is increased by 

 eight cubic inches (131.1 cubic centimeters). 



Age has an influence, though less marked than stature, on the 

 extreme breathing capacity. As the result of 4800 observations on 

 males, it was shown that the volume increased with age up to the 

 thirtieth year, and progressively decreased, with tolerable regularity, 

 from the thirtieth to the sixtieth year. The figures given above, al- 

 though subject to certain individual variations, may be taken as a basis 

 for examinations of the extreme breathing capacity in disease. 



Relations in Volume of the Expired to the Inspired Air. A certain 

 proportion of the inspired air is lost in respiration, so that the air ex- 

 pired is always a little less in volume than that taken into the lungs. 

 The loss was put by Davy at one-seventieth, and by Cuvier at one-fiftieth 

 of the volume of air introduced. Observations on this point, to be 

 exact, must include a considerable number of respiratory acts ; and 

 from the difficulty in continuing respiration in a regular and normal 

 manner when attention is directed to the respiratory movements, the 

 most accurate results may probably be obtained from experiments on 

 the lower animals. Despretz caused six young rabbits to respire for 

 two hours in a confined space containing 2990 cubic inches (49,000 



