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A TEXTBOOK OF PHYSIOLOGY 



is the supplemental air, also 1,500 to 2,000 c.c. in volume. These 

 three together give the vital capacity of an individual. 



Tidal air . . 

 Complemental air 

 Supplemental air 



300-500 c.c. 

 1,500-2,000 c.c. 

 1,500-2,000 c.c. 



Vital capacity . . . . 3,300-4,500 c.c. 



The vital capacity can be measured by expiring to the greatest 

 extent after the fullest possible inspiration into a spirometer (Fig. 149} 

 a form of gasometer. The vital capacity of an individual can be 



FIG. 149. SriKOMETEit. 

 T, Mouth-piece; M, manometer; Cp, counterpoise; K, scale. 



greaty increased by practice. Some athletes (swimmers) have a. 

 capacity of 6 litres. The amount breathed out depends largely upon 

 proper muscular co-ordination. It also varies with posture, being 

 greatest when standing, and least when lying down. 



The amount of air left in the lungs after the greatest expiration 

 possible has been taken is termed the residual air, and measures 

 1,500 to 2,000 c.c. The expired tidal air comes from the trachea, 

 bronchi, and alveoli. It is calculated, of 500 c.c. expired, 140 to 

 160 c.c. comes from the trachea and bronchi, forming the " dead- 



