538 BREATHING CAPACITY. [BOOK n. 



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 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 or 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 the mean of observations 

 varying from 177 to 699 c.c. 



328. 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 tambour or 

 with a receiver which in turn is connected with a tambour, see Fig. 

 37, and Fig. 71. The movements of the column of air in the 

 trachea are transmitted to the tambour, the consequent expansions 

 and contractions of which are transmitted to the recording drum by 

 means of a lever resting on it. 



