332 THE RESPIRATION 



Stimulation of the endings of the glossopharyngeal nerve inhibits res- 

 piration, which explains the holding of the breath that occurs in swal- 

 lowing. 



The Continuously Acting Afferent Impulses 



The continuously acting afferent impulses are transmitted to the chief 

 respiratory center by the vagi and their branches, the superior laryngeal 

 nerves. If the vagus nerves are cut or their continuity severed by 

 freezing a portion of them, the respiratory movements become markedly 

 slower. Evidently, the vagus nerves in some way hurry up the respira- 

 tory movements. Again, if the central end of either vagus is stimu- 

 lated with the ordinary interrupted f aradic current, a profound effect- 

 on the respiratory movements is usually observed. This effect is how- 

 ever not strictly predictable. Usually there is a quickening of respira- 

 tion, and if the stimulus is a strong one, there may be a standstill of the 

 thorax in the inspiratory position. On the other hand, if the central 

 end of the nerve is stimulated with other types of stimuli, as by slow, 

 weak faradic shocks or by the stimulus produced by the closure of an 

 ascending voltaic current, the effect may be to stimulate expiration 

 rather than inspiration. Such results would seem to indicate that the 

 vagus contains two kinds of afferent fibers to the respiratory center, one 

 kind stimulating inspiration, the other, stimulating expiration. 



Supposing that such fibers exist, the next question is, how do they 

 become stimulated at their terminations -in the lungs? The most nat- 

 ural assumption is that the mechanical distention and collapse of the 

 alveoli which occurs with each respiratory act, serves as the stimulus 

 an hypothesis to which support is offered by the observation that, when 

 air is bloAvn into the lungs so as to distend the alveoli, the animal im- 

 mediately makes a forced expiratory movement, Avhereas when the air 

 is sucked out, the thorax assumes the inspiratory position. 



Of the many methods that have been employed to produce disten- 

 tion of the alveoli, the best is undoubtedly that recently employed by 

 Haldane and Boothby. 13 The person or animal is made to respire through 

 a tube in which is inserted a three-waj^ stopcock, which communicates 

 either with the outside air or with a side-tube leading to a spirometer 

 or bag containing air under slight pressure, so that when the stopcock 

 is turned breathing takes place against a definite positive pressure. 

 Such a method is obviously much more physiologic than one in which 

 the air-tube is suddenly clamped at the end of inspiration and the lungs 

 left in a distended condition. 



The term used to designate the cessation of breathing is called apnea. 

 The extent to which it occurs varies very considerably in different an- 



