iv GENERAL PHYSIOLOGY OF NERVOUS SYSTEM 201 



their internal constitution. That under normal conditions the 

 former conduct centrifugally and the latter centripetally depends 

 not ou any intrinsic difference, but on the specific nature of the 

 organ with which they are related at the centre or the periphery, 

 and to which they transmit the excitation. If experimental efferent 

 excitation of sensory nerves and afferent excitation of motor 

 nerves produces no perceptible motor or sensory effects, there must 

 at the peripheral end of the former and central end of the latter 

 be some apparatus, as to the nature of which we are entirely 

 ignorant, which hinders the excitation from being propagated, as 

 a system of valves determines the direction of flow of a current. 

 There is thus no intrinsic contradiction 

 between the law " of the forward direc- 

 tion of normal excitations " and that " of 

 the double direction of experimental 

 Vexcitation," i.e. such as is artificially 

 produced along the course of the nerve. 

 Intimately connected with this law 

 is the other which Hermann (1879) 

 termed "law of the constant effect of 

 nervous excitation." Whether a nerve 

 be excited at its end or at any point 

 along its course, the effect on the organ 

 of reaction is invariably the same, viz. 



I muscular movement for motor nerves, 



sensation for sensory nerves. The local- F io. 132. -Kuime's experiment 

 isation and character of the muscular 

 movement are determined not by the 

 site of stimulation, but by the number 

 of fibres excited and their peripheral 

 distribution to the muscle. So, too, 

 the location and specific quality of 

 the sensation, e.g. pressure, heat, and 

 pain, which occurs on stimulating a sensory cutaneous nerve 

 at any point, is identical with that produced by the action 

 of natural stimuli upon the end-organ in the skin. The most 

 striking example that can be adduced in proof of this law is that 

 observed when a limb has been amputated. " When the member to 

 which a nerve trunk is distributed," says Johannes Miiller, " is 

 removed by amputation, the stump of the nerve which contains 

 the whole of the shortened nerve-fibres is capable of the same 



\sensations as if the amputated limb were still present. This 

 persists all through life." If the stump becomes inflamed, such 

 persons complain of sharp pains in the entire lost limb. Upon 

 recovery they have the same sensations that normal people feel in 

 a healthy limb, and there is often a persistent sensation of itching, 

 or discomfort, which appears to be localised in the limb that no 



frog's graeilis muscle, halt' of which 

 had been poisoned with curare and 

 then severed, so that only the 

 nerve was left as a connecting 

 bridge. N, nerve that gi\es 

 branches to the poisoned L and 

 non - poisoned K, halves of the 

 muscle ; k, connecting bridge ; 

 Z, nerve -muscle biidgr that is 

 mechanically excited. 



