CHAP, i.] THE SPINAL CORD. 907 



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 decapitated snake or newt 

 is suspended and the skin of the tail lightly touched with the 

 finger, the tail bends towards 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 afferent 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 ( 101) 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 generated 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 afferent nerve, the passage 

 into the second efferent nerve meeting with a greater but not an 

 insuperable resistance. When a frog is poisoned with strychnia, 

 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 im- 

 pulses along any or all of the efferent nerves. This proves that a 

 physiological if not an anatomical continuity obtains between all 

 the 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 or that afferent nerve, 

 are transformed on their advent at the network ; and accordingly 

 the path of any series of impulses in the network will be deter- 

 mined largely by the energy of the afferent impulses. And the 

 action of strychnia may be in part explained by supposing that 

 it reduces and equalises the normal resistance of this network, so 

 that even weak impulses travel over all its tracts with great ease. 



587. Further, the movement, forming part of a reflex 



