xn MECHANICS OF KESPIKATION 423 



enters and leaves the pulmonary air- passages during a normal 

 inspiration and expiration. It can be measured by a well- 

 calibrated and graduated glass bell, which Hutchinson (1860) 

 termed a spirometer (Fig. 187). A properly constructed gasometer, 

 which offers minimal resistance to the passage of the air, can 

 be substituted (Mosso^. 



The amount of tidal air varies, according to Vierordt, between 

 367 and 699 c.c. in an adult. The average generally taken 

 is 500 c.c. With an average frequency of 16 respirations per 

 minute, the amplitude of pulmonary ventilation (Eosenthal's 

 respiratory capacity) amounts therefore to 8000 c.c. This 

 amount increases in proportion as the respirations are more 

 intense and deeper. Hutchinson gave the name of complementary 

 air to that amount which, after a normal respiration, may still be 

 breathed in by a maximal inspiration : of reserve air to that 

 which may be expelled after a normal expiration by a maximal 

 expiration ; and, lastly, he termed the sum of tidal, complement- 

 ary, and reserve air obtained on following a maximal inspira- 

 tion by a maximal expiration, the vital capacity. The values of 

 the determinations (by means of spirometers) of the volumes of 

 these different measures of air, vary considerably in experiments 

 undertaken on different individuals. According to Haeser's obser- 

 vations, the mean vital capacity of Germans is 3222 c.c. : of English 

 (who are taller on an average;, 3772 c.c. The vital capacity is 

 affected not only by stature, but also by volume of trunk, body, 

 weight, age, sex, profession or trade, condition of digestion or 

 inanition, etc. 



In order to avoid a gross error in spironietry, it is necessary, as v. Hoessliii 

 pointed out, to breathe into a receiver warmed to body temperature (by 

 heating the bottom or walls of the spirometer, or filling it with warm water). 

 Thus with a spirometer warmed to 37 C. the vital capacity amounted to 

 2850c.c.,whilr with the spirometer at 6 C. it was only 2375 c.o., a difference 

 of 16 - 5 per cent. 



The air left in the lungs after a maximal expiration is termed 

 the residual air. It can be determined on the living by the 

 methods of H. Davy and Grehant, which consist, after making a 

 maximal expiration, in breathing for a certain time from a rubber 

 balloon containing a known quantity of hydrogen. When it is 

 supposed that all the residual air has mixed with the hydrogen, 

 the percentage analysis of the air in the balloon is taken, and the 

 value of the residual air is then found by an easy calculation. 

 In different experiments these observers found it to be 1230- 

 1640 c.c. On an average it can be assumed that the residual air is 

 equal to half the vital capacity (Gad). 



It appears from the total of spironietric observations that in 

 each normal respiratory cycle or revolution only a portion of the 



