178 



PRACTICAL PHYSIOLOGY 



and deeply, others take rapid and shallow breaths. The volume of air 

 breathed per minute varies from 9 to 5 litres, the number of breaths 

 from 23 to 10, and the averages for the volume of each breath from 

 900 to 250 c.c. It is important to remember as a general rule that 

 what is lost in frequency is compensated in depth, so that the volume 



breathed per minute by a man with 

 a frequency of respiration of 10 

 may be the same as that of a man 

 whose ordinary rate of breathing is 

 22 per minute. 



The tidal air is the volume of 

 air breathed at each respiration, and 

 it varies from 900 to 250 c.c. in dif- 

 ferent individuals. After breathing 

 out the tidal air the subject should 

 expire as deeply as possible ; an 

 additional 1500 to 2000 c.c. will 

 be recorded. This is called the 

 supplemental air. Now let the 

 subject take as deep an inspiration 

 as possible; it will be about 1500 

 to 2000 c.c. above the tidal air. 

 This quantity is known as the 

 complemental air. 



The so-called vital capacity is the 

 greatest volume of air that can be 

 expired after the deepest possible 

 inspiration ; it is composed of tidal 

 air 500 c.c. + complemental air 1 500 

 c.c. + supplemental air 1500 c.c. It 

 is about 3500 c.c., but too much 

 importance should not be attached to it, for it depends largely upon 

 practice and control of the inspiratory and expiratory muscles. A 

 broken-winded bandsman, who is accustomed to control the blast of air 

 which he delivers to his instrument, may have a so called vital capacity 

 greater than that of an athlete. 



The Effect of Muscular Exercise upon the Respiration is very great ; 

 within a few minutes, varying according to the severity of the work 

 and the condition of the subject, the volume of air breathed may be 

 doubled, the number of breaths showing a smaller increase. The 

 breathing is deeper, and the mouth is opened to diminish the resist- 

 ance to the passage of the air in and out of the chest. Discomfort 



FIG. 176. Spirometer. T, mouthpiece ; M 

 manometer ; Cp, counterpoise ; R, scale. 



