CAPACITY OF THE LUNGS. 133 



Extreme Breathing Capacity. By the extreme breathing capacity is 

 meant the volume of air which can be expelled from the lungs by the 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 complemental 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 readily be determined 

 by an appropriate apparatus, the spirometer, and comparisons can thus be 

 made between different individuals, both healthy and diseased. The number 

 of observations on this point made by Hutchinson amounts in all to a little 

 less than five thousand. 



The extreme breathing capacity in health is subject to variations which 

 have been shown to bear a very 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 centimetres), it is equal to two hundred and 

 thirty cubic inches (3,768-6 c. c.). 



The most striking result of the experiments of Hutchinson, with regard 

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

 stature, without being affected in a very marked degree 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 upon the length of the body as upon 

 the length of the lower extremities. He ascertained that for every inch 

 (2-5 centimetres) in height, between five and six feet (152-4 and 182-9 centi- 

 metres), the extreme breathing capacity is increased by eight cubic inches 

 (131-1 c. c.). 



Age has an influence, though less marked than stature, upon the extreme 

 breathing capacity. As the result of 4,800 observations on males, it was ascer- 

 tained that the volume increases with age up to the thirtieth year, and pro- 

 gressively decreases, with tolerable regularity, from the thirtieth to the six- 

 tieth year. These figures, though necessarily subject to certain individual 

 variations, may be taken as a basis for examinations of the extreme breath- 

 ing capacity in disease. 



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

 portion of the inspired air is lost in respiration, so that the air expired is 

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

 loss was put by Davy at fa and by Cuvier at -g^ of the volume of air intro- 

 duced. Observations on this point, to be exact, must include a considerable 

 number of respiratory acts ; and from the difficulty of continuing respiration 

 in a perfectly regular and normal manner when the 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 2,990 cubic inches 

 (49,000 c. c.) of air, and ascertained that the volume had diminished by 

 sixty-one cubic inches (1,000 c. c.), or a little more than one-fiftieth. Adopt- 

 ing the approximations of Davy and Cuvier, applied to the human subject, as 



