VITAL CAPACITY OF THE CHEST. 821 



way. Hence, in the use of this apparatus, allowance must be made 

 for all the above-mentioned disturbing influences ; and, it is certain, 

 that the so-called vital capacity is not strictly related to the muscular 

 power of the individual. Nevertheless, it is a valuable addition to our 

 means of diagnosis, as to the condition of the lungs ; the obstructed 

 state of those organs in the earlier stages of phthisis, and the deficient 

 respiratory movement dependent upon this condition, cause a serious 

 diminution in the vital capacity, as compared with the normal standard 

 in persons of the same sex, age, and stature. A diminution of 16 per 

 cent, in the normal capacity, is said to indicate a diseased condition 

 (Hutchinson); but care must be taken to allow for the effects of con- 

 gestion, and of abdominal disease. 



If in a person 5 feet 6J inches in height, the breathing air averages, 

 during the twenty-four hours of work, rest, and sleep, 20 cubic inches 

 at each inspiration, and the number of respirations per minute for the 

 day and night be taken at 18, the quantity of air inspired and expired 

 by an adult man of mean stature, in one minute, would be 360 cubic 

 inches. This would give 518,400 cubic inches or 300 cubic feet, 

 for the twenty-four hours. This amount is less than the total daily 

 quantity as estimated by Valentin, which is 688,348 cubic inches; less 

 also than the quantity, viz., 686,000 cubic inches, found by Edward 

 Smith to be the average in four adults, of a mean height of 5 feet 10J 

 inches, during a state of rest. During ordinary exercise, the estimates 

 of the last-named observer, give 804,780 cubic inches ; in the case of 

 the actively employed laborer, 1,568,390 ; and during a day's work, 

 including twelve hours of Alpine exercise, 1,764,000 cubic inches. 

 Vierordt's estimate for the twenty-four hours, is 732,000 cubic inches ; 

 but that author assumes the quantity of air inspired per minute to be 

 450 cubic inches. Edward Smith found it to average 500 cubic inches 

 in the day, and 400 during the night, in four persons of the mean 

 stature of 5 feet 10J inches. 



It has been estimated that, with a vital capacity of 200 cubic inches, 

 the force employed in making the necessary full inspiration is equal 

 to the raising of 300 Ibs. weight upon the surface of the chest ; but in 

 forcible expiration, the power exerted is much greater. In ordinary 

 breathing, supposing the quantity of air inspired to be 20 cubic inches, 

 the resistance overcome by the inspiratory muscles is equal to a weight 

 of 200 Ibs. 



Changes in the Air from Respiration. 



The air expired from the chest differs in three respects from that 

 which is inspired. It is increased in temperature, except of course 

 when the inspired air is already hotter than the body itself. It con- 

 tains, as a rule, more moisture, unless when it is previously, and ex- 

 ceptionally, saturated with watery vapor. Lastly, it undergoes im- 

 portant changes of composition, the chief of which consist in a loss of 

 oxygen and an addition of carbonic acid. 



The increase of temperature in the expired air is regulated by the 

 temperature of the air taken into the lungs. When the surrounding 

 air is cold, the increase is not quite so great as when its temperature 



