CH. XXIV.] 



POSITIVE AND NEGATIVE VENTILATION 



363 



the position of expiration. This result always follows stimulation of 

 the superior laryngeal nerve. 



Most of these facts were known previously, but the interpretation 

 of them, in the light of further experiments immediately to be 

 described, is the following : 



That there are in the vagus two sets of fibres, one of which pro- 

 duces an increased activity of the inspiratory part of the respiratory 

 centre, and the other an increased activity of the expiratory part of 

 that centre. Stimulation of the first stops expiration and produces 

 inspiration ; stimulation of the second does the reverse. 



The question now is, What is it that normally produces this 

 alternate stimulation of the two sets of fibres ? If we discover this 

 we shall discover the prime moving cause in the alternation of the 

 inspiratory and expiratory acts. 

 It was sought and found in the 

 alternate distension and con- 

 traction of the air-vesicles of 

 the lungs where the vagus 

 terminations are situated. 



In one series of experiments 

 positive ventilation was per- 

 formed ; that is, air was pumped 

 repeatedly into the lungs, and so 

 increased their normal disten- 

 sion ; 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. 331, A). 



In a second series of ex- 

 periments, negative ventilation was performed ; 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 con- 

 tractions of the diaphragm, expiration became less and less, and at last 

 the diaphragm assumed the position of inspiratory standstill (fig. 331,B). 



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. 



FIG. 331. Tracings of diaphragm. The upward move- 

 ments of the tracings represent inspiration ; the 

 downward movements, expiration. A, result of 

 positive, B, of negative ventilation. (After Head.) 



