CHAP. IL] RESPIRATION. 567 



pressure as it is called, during inspiration, and rise, indicating a 

 positive pressure, during expiration, both fall and rise being slight 

 and varying according to the freedom with which the air passes in 

 and out of the chest. When a manometer is fitted with air-tight 

 closure into the mouth, or better, in order to avoid the suction- 

 action of the mouth, into one nostril, the other nostril and the 

 mouth being closed, and efforts of inspiration and expiration are 

 made, the mercury falls or undergoes negative pressure with 

 inspiration, and rises, or undergoes positive pressure during 

 expiration. It has been found in this way that the negative 

 pressure of a strong inspiratory effort may vary from 30 to 74 mm., 

 and the positive pressure of a strong expiration from 62 to 

 100 mm. 



The total amount of air which can be given out by the most 

 forcible expiration following upon a most forcible inspiration, that 

 is, the sum of the complemental, tidal and reserve airs, has been 

 called 'the vital capacity;' 'extreme differential capacity' is a better 

 phrase. It may be measured by a modification of a gas-meter called 

 a spirometer ; and though it varies largely, the average may be put 

 down at 34000 c.c. (200 to 250 cubic inches). 



Of the whole measure of vital capacity, about 500 c.c. (30 c. 

 inch) may be put down as the average amount of tidal air, the 

 remainder being nearly equally divided between the complemental 

 and reserve airs. 



Since the respiratory movements are so easily affected by various 

 circumstances, the simple fact of attention being directed to the breath- 

 ing being sufficient to cause modifications both of the rate and depth of 

 the respiration, it becomes very difficult to fix the volume of an average 

 breath (tidal air). Thus various authors have given figures varying 

 from 53 c.c. to 792 c.c. The statement made above is the mean of 

 observations varying from 177 to 699 c.c. 



The estimates of the volume of residual air made according to 

 different methods by different observers differ extremely ; accord- 

 ing to the method referred to below the volume varies from 500 c.c. 

 to 1000 c.c. or more. 



The volume of residual air may be indirectly determined as follows. 

 A person breathes into a spirometer filled with a known volume of 

 hydrogen several times until the air in the lungs is well mixed with 

 the gas in the spirometer; he finishes up with a powerful expiration. 

 The residual air now contains a certain percentage of hydrogen, the 

 same as that of the gas now contained by the spirometer. The person 

 then instantly shifts his breathing into a second spirometer containing 

 a known volume of pure air, and continues breathing into it until 

 the air in the spirometer is thoroughly mixed with the air in the lungs. 

 Since on starting the second breathing the only air in the lungs was 

 residual air (containing a certain percentage of hydrogen), the per- 

 centage of hydrogen in the spirometer at the close of the experiment, 

 that is the extent to which the known volume of pure air in the 



