1 68 RESPIRATION 



trigeminal and cutaneous nerves. Of these the pneumogastric 

 is by far the most important. 



Section of a single pneumogastric is followed by variable res- 

 piratory disturbances which usually disappear in less than an 

 hour. Section of both nerves is followed, after a short interval 

 of increased respiratory activity, by slow and powerful inspira- 

 tions, by forced expiration and an appreciable interval before 

 the next inspiration. Irritation of the central end of the cut 

 nerve by a very weak current seems to stimulate the inhibitory 

 part of the center, for the rate is slowed, the expirations are 

 strenuous and the inspirations weak. When the current is 

 increased to a moderate strength opposite results are obtained, 

 the accelerator portion of the center being stimulated. These 

 facts show that the pneumogastrics possess both inspiratory and 

 expiratory fibers, and that the former are stimulated more by a 

 moderate current and the latter more by a very weak one. 

 The rhythm of respiration, therefore, includes the regular 

 sequence of inspiratory and expiratory movements upon each 

 other. 



Now what is it that, under normal conditions, irritates the 

 terminals of the pneumogastrics and causes them to convey 

 inspiratory and expiratory impressions? It has been held that 

 a change in the composition of the alveolar air an accumulation 

 of carbon dioxide irritates the nerve terminals and explains 

 the conveyance of the inspiratory impressions, while the stretch- 

 ing of the lung tissue originates the expiratory impressions. 

 Others ascribe both inspiratory and expiratory impressions to 

 lung movements movements of inspiration exciting expiratory, 

 fibers, and movements of expiration exciting inspiratory fibers. 

 These observers cite the fact that artificial inflation and aspira- 

 tion excite expiration and inspiration respectively. 



Stimulation of the superior laryngeal, as when foreign bodies 

 accidentally enter the larynx, excites violent expiration. 



The glosso-pharyngeal contains afferent fibers especially impor- 



