RESPIRA TION. 461 



thus establish an exciting electric current which affects expiratory fibres. If 

 the strength of the current be increased, these effects give place to those of an 

 opposite character, the respirations becoming more frequent and the inspi- 

 rations more marked in depth and force, the explanation of this difference 

 being that the stronger current has also excited inspiratory fibres, so that now 

 both expiratory and inspiratory impulses are generated, but the latter, being 

 more potent in their influences, cause acceleration of the rate and accentuated 

 inspirations. The effects following stimulation of the central end of the cut 

 vagus by a current of moderate strength are best observed after both nerves have 

 been divided and when there exist slow, deep, powerful respirations. Under 

 such circumstances stimulation of the central end of one of the vagi is followed 

 at once by an increase in the respiratory rate and a return of the general char- 

 acters of the inspiratory and expiratory phases toward the normal ; and if the 

 degree of excitation be properly adjusted, the normal rate and normal charac- 

 ter of breathing may be restored. Still stronger excitation further accelerates 

 the rate, causing the respiratory acts to follow each other with such frequency 

 that inspiration begins before the expiratory act (relaxation of the inspiratory 

 muscles) has been completed. The inspiratory muscles are therefore never 

 completely relaxed. With a further increase of stimulus the expiratory 

 relaxation becomes less and less, until finally the respirations are brought to a 

 standstill in the inspiratory phase, the inspiratory muscles being in tetanus. 



If the nerves be fatigued from over-excitation or if the animal be 

 thoroughly chloralized, stimulation of the central end of the cut nerve by a 

 strong current is no longer followed by inspiratory stimulation, but is followed 

 by expiratory stimulation (the inspirations being shortened and weakened, the 

 expirations prolonged and spasmodic) and by long pauses between expiration 

 and inspiration. If the excitation be sufficiently strong, arrest of respiration 

 occurs in the expiratory phase. 



It will be observed from the above results that electrical irritation of the 

 central end of the cut pneumogastric may be followed by effects of an oppo- 

 site character, extremely weak irritation causing expiratory stimulation (weaker 

 and shorter inspirations, prolonged and active expirations, expiratory pan--. 

 and diminished respiratory rate) ; whereas moderate irritation causes inspiratory 

 stimulation (stronger and deeper inspirations and increased respiratory rate). 

 These diverse results are explained by the fact that these nerves contain two 

 kinds of fibres having opposite functions: fibres of one kind convey impulses 

 which affect the expiratory centre ; those of the other kind convey impulses 

 which affect the inspiratory centre. The former are more susceptible to weak 

 electrical stimulation, and thus their presence may be elicited by the weakest 

 stimulus capable of causing any response. At the same time they arc less 

 readily exhausted, so that if the vagi be subjected to prolonged stimulation 

 by a strong current, the inspiratory fibres are exhausted before the expiratory 

 fibres. For moderate and strong currents the inspiratory fibres are affected 

 to a greater degree than the expiratory fibres, therefore inspiratory stimula- 

 tion predominates. 



