ADVANCED EXPERIMENTAL PHYSIOLOGY 



133 



low resistance are more convenient than the special instrument 

 known as the spirometer (Fig. 130), although the latter is very 

 useful for some experiments. 



The subject of the experiment should breathe through the mask 

 and meter for a minute or two before the record is taken, in order 

 that he may become accustomed to the novel conditions. Then 

 the volume of each breath and the number in periods of consecutive 

 minutes should be determined. A table should be made to show 

 the results obtained with each member of the class, for the differ- 

 ences in the rate and depth of 

 breathing in healthy men are 

 considerable ; some men breathe 

 slowly 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 remem- 

 ber as a general rule that what 

 is lost in frequency is compen- 

 sated in depth, so that the 

 volume breathed per minute by 

 a man with a frequency of respi- 

 ration 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 different individuals. After 

 breathing out the tidal air the 

 subject should expire as deeply 

 as possible ; an additional 1,500 

 to 2,000 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 1,500 to 2,000 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 50Q c.c. -f- complemental air 1,500 c.c. -f- supplemental 

 air 1,500 c.c. It is about 3,500 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. 



FIG. 130. Spirometer. 



T, mouthpiece ; M, manometer ; 

 Cp, counterpoise ; R, scale. 



