228 



SPIROMETRY AND VITAL CAPACITY. 



(5.) Vital Capacity is the term applied to the volume of air which 

 can be forcibly expelled from the chest after the deepest possible 

 inspiration. It is equal to 3,772 c.c. (or 230 cubic inches) for an 

 Englishman (Hutchinson), and 3,222 for a German (Haeser). 



Hence, after every quiet inspiration, both lungs contain (1 + 2 + 3) 

 = 3,000-3,900 c.ctmr. [220 cubic inches] ; after a quiet expiration 

 (1 + 2) = 2,500-3,400 c.ctmr. [200 cubic inches.] So that about J- 

 to ^ of the air in the lungs is subject to renewal at each respiration. 



Estimation of Vital Capacity. The estimation of the vital capacity 

 was formerly thought to be of great consequence, but at the present time 

 not much importance is attached to it, nor is it frequently measured in 

 cases of disease. It is estimated by means of the SPIROMETER of 



i Hutchinson. This instrument (Fig. 



101), consists of a graduated 

 cylinder filled with water and in- 

 verted like a gasometer over water, 

 6$ and balanced by means of a counter- 

 LJ poise. Into this cylinder a tube 

 projects, and this tube is connected 

 with a mouth-piece. The person 

 to be experimented upon takes the 

 deepest possible inspiration, closes 

 his nostrils, and breathes forcibly 

 into the mouth-piece of the tube. 

 After doing so the tube is closed. 

 The cylinder is raised by the air 

 forced into it, and after the water 

 inside and outside the cylinder is 

 equalised, the height to which the 

 cylinder is raised indicates the 

 amount of air expired, or the vital 

 or respiratory capacity. In a man of average height, 5 feet 8 inches, 

 it is equal to 230 cubic inches. 



The following circumstances affect the vital capacity : 



(1.) The height. Every inch added to the height of persons between 5 and 6 

 feet, gives an increase of the vital capacity =130 c.c. [8 cubic inches.] 



(2.) The body- weight. When the body- weight exceeds the normal by 7 per 

 cent., there is a diminution of 37 c.c. of the vital capacity for every kilo, of increase. 



(3.) Age. The vital capacity is at its maximum at 35 ; there is an annual 

 decrease of 23 "4 c.c. from this age onwards to 65, and backwards to 15 years of age. 



(4.) Sex. It is less in women than men, and even where there is the same cir- 

 cumference of chest, and the same height in a man and a woman, the ratio is 10 : 7. 



(5.) Position- More air is respired in the erect than in the recumbent position. 



(6.) Disease. Abdominal and thoracic diseases diminish it. 



Fig. 101. 

 Scheme of Hutchinson's Spirometer. 



