360 RESPIRATION. [CH. xxiv. 



In one series of experiments positive ventilation was performed ; 

 that is, air was pumped repeatedly into the lungs, and so 

 increased their normal distension; this was found to decrease 

 the inspiratory contractions of the diaphragm, until at last they 

 ceased altogether, and the diaphragm stood still in the expiratory 

 position (fig. 323, A). 



In a second series of experiments, negative ventilation was per- 

 formed ; that is, the air was pumped repeatedly out of the lungs, 

 and a condition of collapse of the air-vesicles produced. This was 

 found to increase the inspiratory contractions of the diaphragm, 

 expiration became less and less, and at last the diaphragm 

 assumed the position of inspiratory standstill (fig. 323, B). 



Fig. 323. Tracings of diaphragm. The upward movements of the tracings represent 

 inspiration ; the downward movements, expiration. A, result of positive, B, of 

 negative ventilation. (After Head.) 



Distension of the air-vesicles therefore stimulates the fibres of 

 the vagus which excite the expiratory phase of respiration ; 

 collapse stimulates those which excite the inspiratory phase. 



Ordinary respiration is an alternate positive and negative 

 ventilation, though not so excessive as in the experiments just 

 described. Inspiration is positive ventilation, and so provides the 

 nervous mechanism of respiration with a stimulus that leads to 

 expiration. Expiration is a negative ventilation, and so provides 

 the stimulus that leads to inspiration. 



It is probable that of the two sets of impulses, those which 

 are started by the inspiratory movement play a more active 

 part in the regulation of respiration than those started by the 

 expiratory movement. Gad explains the latter by supposing they 

 are simply due to a cessation of the former, or, in other words, 



