CENTRAL NERVOUS SYSTEM. 667 



can thus be accomplished in a few seconds by cutting off the afferent impulses 

 from the skin may of course follow any slow diminution in these impulses, 

 although all such slow changes are much more likely to be accompanied by 

 some sort of compensation whereby other afferent impulses in a measure take 

 the place of those which have been suppressed. The loss of these impulses 

 which rouse the cells to activity is usually a more important condition than 

 direct nutritive change, and must for this reason always be kept in view. 



Inhibition. On the other hand, let one leg of a reflex frog be stimulated 

 in the usual manner by pinching or by acid, and then the experiment repeated, 

 while the other leg is lightly pinched at the same time, and it will be found 

 that either the latent period preceding the response is increased or, with the 

 strength of stimulus employed, the reaction does not occur. This is an ex- 

 ample of inhibition which can be caused by the simultaneous excitement of a 

 nerve-cell in several ways. 



To obtain inhibition there must be at least two pathways by which impulses 

 reach a given cell, and the two stimuli must tend to excite different reactions. 

 When they tend to excite the same reaction a reinforcement follows. The inhi- 

 bition, therefore, is connected with the effect of these two sets of impulses upon 

 the responding cell, and that is always associated with the fact that as the two 

 paths end in different relations to the cell, the impulses must enter it at differ- 

 ent points, and hence in the first instance tend to act on different portions of 

 the cell-contents. 



Though at the present time it is not possible to give a theory of inhibition 

 that will be general and satisfactory, there is enough known to indicate that 

 this effect, when developed in the central nervous system, is not produced by 

 a special set of nerve-fibres, but is the result of the action of several incom- 

 ing impulses, arriving by different paths, on the responsiveness of a given 

 cell. 



E. VOLUNTARY ACTIONS. 



On attempting to distinguish between a voluntary and reflex act from the 

 physiological standpoint, we find the chief difference to be that the voluntary 

 act is not predictable, because, according to the capabilities of the animal, it 

 may be more variable in form than is the reflex response, and also because, 

 instead of occurring within a short interval after the stimulus, as does the 

 reflex, the voluntary response may be delayed even for years. For example, 

 we read in a book some statement that makes us desire to question the author. 

 The question is a response to the stimulus given by the printed page, and it 

 may be carried out by writing a letter within a few hours, or delayed until a 

 meeting with the author years hence. During this interval, and in the absence 

 of the author, the reaction which will take the form of a question remains 

 incomplete, while his presence is sufficient to set in motion the train of stimuli 

 which shall cause it. Moreover, consciousness enters as an element into such 

 reactions, and there is present a mental image of the act to be accomplished, 

 together with some remembrance of its execution. 



