42 THE NERVOUS SYSTEM AND ITS CONSERVATION 



direct effect of external stimuli or by the chemical changes 

 occurring in the perikarya. The impulses that run along 

 the afferent (receptive) channels are originated in a 

 relatively direct manner by external forces. Those that 

 traverse the adju<tors and run out to the effectors may be 

 thought of as the afferent currents which have continued 

 through the synapses of the central axis, hut we shall 

 probably do well to regard them rather a> relays, that is, 

 impulses started anew from perikarya excited to "dis- 

 charge." This view, according to which the energy of the 

 transmission is reinforced by each neuron in the series, 

 is the older doctrine and is still dominant. 



Attention must now be called to the relative ease with 

 which neurons will respond to stimuli applied at different 

 points. It is most necessary for the orderly and purposeful 

 working of the nervous system that nerve-fibers shall not 

 be easily excited save at their specialized beginnings or 

 endings. It is, in fact, difficult to stimulate a nerve al 

 any intermediate point in its course. When we do use 

 sufficiently positive means to make the stimulation 

 effective the result is such as to remind us that it would 

 be undesirable to have it a common incident. A blow 

 on the elbow may be forcible enough to start impulse- 

 in the afferent fibers of the nerve which lies near the 

 surface at that spot, and it instantly seems to the 

 subject that he has sensations pertaining to the wrist and 

 lingers. He has excited to act ivity t he axons which have 

 habitually brought impulses from the hand, and they still 

 awaken the same associations as though they had come 

 over the whole distance. This is a very significant 

 mattei' and it will be wise to place further emphasis 

 upon it. 



When an electric door-bell rings, the householder is 

 usually justified in the assumption that someone is out- 

 side ri Hiring it. Yet it might happen that a slmrt-cireuit 

 at some point along the course of the wire- should cause 

 the signal. The ringing of the bell when occasioned in 

 this abnormal way would have the same qualities to the 



