568 THE SPINAL CORD. 



is constant, being the same as that given forth by a muscle thrown into con- 

 traction by the will. From which we infer, even bearing in mind the dis- 

 cussion in 78 concerning the nature of the muscular sound, that in a re- 

 flex action the afferent impulses do not simply pass through the centre in 

 the same way that they pass along afferent nerves, but are profoundly modi- 

 fied. And in accordance with this we find, as we shall see, that a reflex 

 action takes up an amount of time, the greater part of which is spent in the 

 carrying out of the central changes, and which though variable is always 

 much longer, and may be very much longer, than that taken up by the mere 

 passage of a nervous impulse along a corresponding length of nerve-fibre. 

 The term reflex action is therefore an unsuitable one. The afferent impulse 

 is not simply reflected or turned aside into an efferent channel ; on its arrival 

 at the centre, it starts changes of a different nature from and more complex 

 than its own ; and the issue of efferent impulse is the result of those more 

 complex changes, not the mere continuation of the simpler afferent impulse. 

 In other words, the interval between the advent at the central organ of 

 afferent, and the exit from it of efferent impulses, is a busy time for the ner- 

 vous substance of that organ ; during it many processes, of which we have 

 at present very little exact knowledge, are being carried on. 



499. The character of the movement forming part of a reflex action 

 is also influenced by the intensity of the stimulus. A slight stimulus, such 

 as gentle contact of the skin with some body, will produce one kind of 

 movement ; and a strong stimulus, such as a sharp prick applied to the same 

 spot of skin, will call forth quite a different movement. When a decapi- 

 tated snake or newt is suspended and the skin of the tail slightly touched 

 with the finger, the tail bends toward the finger ; when the skin is pricked 

 or burnt, the tail is turned away from the offending object. And so in 

 many other instances. It must be remembered, of course, that a difference 

 in the intensity of the stimulus entails a difference in the characters of the 

 efferent impulses ; gentle contact gives rise to what we call a sensation of 

 touch, while a sharp prick gives rise to pain, consciousness being differently 

 affected in the two cases because the afferent impulses are different. Hence 

 the instances in question are in reality fuller illustrations of the dependence, 

 to which we called attention above, of the characters of a reflex movement 

 on the characters of the afferent impulses. 



Further, as we have already pointed out ( 97), while the motor impulses 

 started by a weak stimulus applied to an afferent nerve are transmitted along 

 a few, those started by a strong stimulus may spread to many efferent nerves. 

 Granting that any particular afferent nerve is more especially associated with 

 certain efferent nerves than with any others, so that the reflex impulses gen- 

 erated by afferent impulses entering the cord by the former pass with the 

 least resistance down the latter, we must evidently admit further that other 

 efferent nerves are also, though less directly, connected with the same affer- 

 ent nerve, the passage into the second efferent nerve meeting with a greater 

 but not an insuperable resistance. When a frog is poisoned with strychnine, 

 a slight touch on any part of the skin may cause convulsions of the whole 

 body ; that is to say, the afferent impulses passing along any single afferent 

 nerve may give rise to the discharge of efferent impulses along any or all 

 of the efferent nerves. This proves that a physiological, if not an ana- 

 tomical, continuity obtains between all parts of the spinal cord which are 

 concerned in reflex action, that the nervous network intervening between 

 the afferent and efferent fibres forms along the whole length of the cord a 

 functionally continuous field. This continuous network, however, we must 

 suppose to be marked out into tracts presenting greater or less resistance to 

 the progress of the impulses into which afferent impulses, coming along this 



