358 NERVOUS MECHANISM. [BOOK n. 



main trunk of the vagus also causes a slowing or even standstill of 

 the respiration, as for instance in deep chloral narcosis or when 

 the nerve has become exhausted by previous stimulation. Stimu- 

 lation of the superior laryngeal frequently produces not only a 

 complete cessation of all inspiratory movements, as indicated by 

 the perfectly lax diaphragm, but also contractions of the abdominal 

 muscles indicating an expiratory effort; and it is obvious that the 

 commencement of an expiration must be preceded by a cessation of 

 inspiratory effects, just as similarly inspiration must be preceded 

 by the cessation of expiration. Hence the influences which inhibit 

 inspiration are often spoken of as expiratory though they may not 

 go so far as to produce an actual expiration. 



Corresponding to these antagonistic influences we may suppose 

 the existence of separate fibres, augmentative or inspiratory fibres, 

 the stimulation of which leads to inspiratory movements, and 

 inhibitory or expiratory fibres the stimulation of which checks 

 inspiration and subsequently gives rise to expiration. But it 

 must be remembered that the existence of these fibres is hypo- 

 thetical, and that some other explanation may eventually be given 

 of the facts which we have just described. Indeed we are not able 

 at present to give a wholly consistent and satisfactory explanation of 

 the nature and working of the respiratory centre. Apparently we 

 must conceive of its consisting of two parts, an inspiratory and 

 an expiratory: and direct stimulation of the medulla produces 

 sometimes inspiration, sometimes expiration; but the two parts 

 must be considered as co-ordinated in such a way as to act 

 alternately. Of the two the inspiratory centre is in ordinary life 

 the more important, the more sensitive and the more active, since 

 in normal breathing active expiratory effects are scanty, and the 

 emptying of the chest is chiefly the result of the cessation of 

 inspiration. Under conditions, however, which we shall speak of 

 presently under the name of dyspnoea, the expiratory centre 

 comes distinctly into play, since actual expiratory efforts come to 

 the front and, as we shall see, the greater the difficulty of breathing 

 the more and more prominent they become. We may picture to 

 ourselves, as Eosenthal has done, that the inspiratory centre is the 

 seat of two conflicting processes, one tending to the discharge of 

 inspiratory impulses and the other offering resistance to that 

 discharge, the former gathering head during a period of rest and so 

 at last overcoming the latter, and effecting an actual discharge. 

 After this the accumulation of inspiratory processes once more 

 begins, and once more terminates in a discharge, thus leading to 

 the rhythm of respiration. We may further suppose that the aug- 

 mentative impulses ascending the vagi, produce their effect by 

 diminishing the processes of resistance, and thus bring about move- 

 ments which are at once quicker and less ample. But we have to add 

 to this conception some view as to the relation of the expiratory to 

 the inspiratory centre in order to explain why the impulses inhibitory 



