428 BREATHING CAPACITY. [Book ii. 



case of a distension corresponding to a very forcible inspira- 

 tion, to 30 mm. In the living body this mechanical elastic force 

 of the lungs may be assisted by the contraction of the plain 

 muscular fibres of the bronchi ; the pressure, however, which 

 can be exerted by these probably does not exceed 1 or 2 mm. 



When a manometer is introduced into a lateral opening of 

 the windpipe of an animal, the mercury will fall, indicating a 

 negative 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 manom- 

 eter 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 un- 

 dergoes 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 k the vital capacity ; ' k extreme differential 

 capacity ' is a better phrase. It may be measured by a modifi- 

 cation of a gas-meter called a Spirometer ; and though it varies 

 largely, the average may be put down at 3 — 4000 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 comple- 

 mental and reserve airs. The quantity left in the lungs after 

 the deepest expiration amounts to about 1400 or 2000 c.c. 



Since the respiratory movements are so easily affected by various 

 circumstances, the simple fact of attention being directed to the 

 breathing 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 the 

 mean of observations varying from 177 to 698 c.c. 



§ 259. Graphic Records of Respiratory Movements. These 

 may be obtained in many various ways. 



The simplest, readiest and perhaps the most generally useful 

 method is that of recording the movements of the column of air. 

 This may be effected by introducing a T piece into the trachea, one 

 cross piece being left open, and the other connected with a Marey's 



