334 THE RESPIRATION 



excitability, and being active to a greater or less extent all the time; 

 while the other acts only occasionally on the "tuned up" center. There 

 is, of course, no doubt that it is through the nerves that the occasional 

 alterations of respiration occur. They appear also to have a certain 

 influence on the rhythm, for Stewart, Pike and Guthrie 17 observed that, 

 after resuscitation from acute brain anemia, the respirations when they 

 returned were of the same rhythm as that of the artificial respirations 

 employed during the resuscitation. 



The usually accepted hypothesis as to the mechanism by which the 

 nerve impulses hasten the respiratory movements is that an afferent 

 impulse is transmitted to the respiratory center towards the end of each 

 inspiration, which has the effect of inhibiting the inspiratory discharge 

 from the center and thus cutting short the act of inspiration so that ex- 

 piration automatically supervenes. This explanation is in agreement 

 with the fact that quiet inspiration involves activity on the part of the 

 respiratory muscles, whereas expiration is usually almost entirely pas- 

 sive, being due to the return to a resting position of the stretched and 

 displaced structures. On the other hand, in forced respiration and in 

 certain animals under normal conditions, expiration becomes active, in 

 which event a current of action becomes evident in the vagus nerve dur- 

 ing the expiratory phase. 



The superior laryngeal branch of the vagus should really be classified 

 as one of those nerves that have an occasional influence on the respiratory 

 center, its particular function being in connection with the act of cough- 

 ing. When a foreign body irritates the mucous membrane of the larynx, 

 the nerve fibers transmit impulses to the respiratory center which ex- 

 cite a violent expiration and at the same time cause the glottis to close. 

 The closure of the glottis lasts, however, only during the first part of 

 the expiration; it then opens, with the result that the sudden release of 

 intrapulmonic pressure causes the expulsion of the foreign substance 

 in the air passages. 



