CHAP, in.] PROPERTIES OF NERVOUS TISSUES. 113 



Inhibition. In speaking of reflex action, we took it for granted 

 that the spinal cord was, at the moment of the arrival of the 

 afferent impulses at the central nerve-cells, in a quiescent state ; 

 that the nerve-cells themselves were not engaged in any auto- 

 matic action. We were justified in doing so, because as far as 

 the muscles generally of the body are concerned, the spinal cord 

 is in a brainless frog perfectly quiescent; an afferent impulse 

 reaching an ordinary nerve-cell of the spinal cord does not find 

 it preoccupied in discharging efferent impulses to the muscles with 

 which by means of nerve-fibres it is connected. But what happens 

 when afferent impulses reach a nerve-cell or a group of nerve-cells 

 already engaged in automatic action ? 



We have already referred to an automatic respiratory centre 

 in the medulla oblongata. We may here premise, what we shall 

 shew more in detail hereafter, that the pneumogastric nerve is 

 peculiarly associated as an afferent nerve with this respiratory 

 centre. Now if the central end of the divided pneumogastric be 

 stimulated at the time when the respiratory centre is engaged 

 in its accustomed rhythmic action, sending out complex co- 

 ordinated impulses of inspiration (and of expiration) at regular 

 intervals, one of two things may happen, the choice of events 

 being determined by circumstances which need not be considered 

 here. 



The most striking event, and the one which interests us now, 

 is that the respiratory rhythm is slowed or stopped altogether. 

 That is to say, afferent impulses which, under ordinary con- 

 ditions, would, on reaching a quiescent nervous centre, give rise 

 to movement, may, under certain conditions, when brought to 

 bear on an already active automatic nervous centre, check or stop 

 movement by interfering with the production of efferent impulses 

 in that centre. This stopping or checking an already present 

 action is spoken of as an ' inhibition ; ' and the effect of the 

 pneumogastric in this way on the respiratory centre is spoken of 

 as ' the inhibitory action of the pneumogastric on the respiratory 

 centre.' 



The other event is that the respiratory rhythm is accelerated. 

 We shall hereafter discuss the explanation of the two events. We 

 may however state that according to one view the pneumo- 

 gastric contains among its afferent fibres two sets, which are either 

 of a different nature from each other, or are so differently connected 

 with the respiratory centre, that impulses arriving along one stop, 

 while those arriving along the other quicken, the action of that 

 centre. Hence, the one set are called ' inhibitory,' the other ' ac- 

 celerating' or 'augmenting' fibres. But we are concerned at present 

 only with the fact that the stimulation of a nerve may produce 

 either inhibitory or augmentative effects. 



Similarly the vaso-motor centre in the medulla may, by im- 

 pulses arriving along various afferent tracts, be inhibited, during 



jr. 8 



