CH. XXVI.] 



NERVOUS FACTOR IN RESPIRATION 



379 



inspiratory and expiratory acts. It was sought and found in the 

 alternate distension and contraction of the air-vesicles of the lungs 

 where the vagus terminations are situated. 



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. 301, 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. 301, B). 



Head regards ordinary respiration as 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 re- 

 spiration with a stimulus that 

 leads to expiration. Expiration 

 is a negative ventilation, and 

 so provides the stimulus that 

 leads to inspiration. 



We must naturally be on 

 our guard against regarding the 

 forcible inflations and deflations 

 produced by a pump as com- 

 pletely analogous to the changes 

 produced in the lungs by or- 

 dinary breathing ; nevertheless, 

 the two sets of impulses are 

 undoubtedly called into action 

 if the respiratory processes are 

 sufficiently energetic, and 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 expira- 

 tory movement, so much so that in unlaboured breathing they alone 

 need be considered. 



Apncea. If positive and negative ventilation are used together 

 rapidly and alternately at a rate quicker than the respiratory rhythm, 

 both inspiratory and expiratory processes are inhibited, and the respira- 

 tion ceases for a short time. This follows naturally from the experi- 

 ments previously described. This can be done on an animal with a 

 pair of bellows fixed to a tube in the trachea ; or voluntarily by one- 



. 301. 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.) 



