316 



RHYTHM OF RESPIRATION. 



[BOOK ii. 



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. The quantity left in the lungs after the deepest 

 expiration amounts to about 1400 2000 c.c. 



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. Thus various authors have given figures varying from 53 c.c. 

 to 792 c.c. The statement made above is that given by Vierordt as the 

 mean of observations varying from 177 to 699 c.c. 



The Rhythm of Respiration. If the movements of the column 

 of tidal air, or the movements of expansion and contraction, or the 



FIG. 55. TRACING OF THORACIC EESPIRATORY MOVEMENTS OBTAINED BY MEANS OF 

 MAREY'S PNEUMATOGRAPH. (To be read from left to right.) 



A whole respiratory phase is comprised between a and a ; inspiration, during which 

 the lever descends, extending from a to 6, and expiration from 6 to a. The 

 undulations at c are caused by the heart's beat. 



