vin CONDUCTIVITY AND EXCITABILITY OF NERVE 73 



the motor nerves from any sensory nerve of the skin, it is obvious 

 that all the prolongations of the same within the reflex centre 

 (spinal cord) must be similarly associated. Clearly the poison 

 cannot alter the structure and direction of the central paths, along 

 which the excitation travels in the spinal medulla. We must 

 rather assume that the new relations into which the central 

 nervous elements have reciprocally entered are due to a chemical 

 alteration of their substance. The action of strychnin proves 

 that the path taken by the excitation in the normal central 

 organ is circumscribed, not by any definite arrangement of fibres, 

 but by the mobility in a definite direction which characterises 

 the mass through which the excitation is transmitted. If we 

 picture the whole of the gray matter as a coherent network of 

 homogeneous, excitable, and conducting plasma, in direct con- 

 nection with the sensory and motor nerve-fibres (a conception 

 that is not indeed borne out by recent histological discoveries), 

 then the only possible explanation of the organised reflex move- 

 ments w r hich regularly follow on a given stimulus must be that 

 there are certain " lines of discharge," along which excitation is 

 normally transmitted, because there is here less resistance the 

 protoplasm is more excitable. The path along which a sensory 

 excitation, calling out a definite reflex movement, travels in the 

 spinal cord, has often been compared to a canalised track, and 

 from a certain point of view the crude illustration suffices. But 

 there is the further question of how this perfectly co-ordinated 

 network of lines of discharge originated, and of whether it is pos- 

 sible during the life of the individual to form new combinations, 

 and new excitatory paths to reflex movements. The question is 

 too wide to be entered upon here ; we can only say that there 

 is some ground for assuming that every impulse traversing the 

 central nervous system, along any path, leaves its traces behind it, 

 inasmuch as it causes certain molecular alterations, which, 

 as they become sharpened by repetition, facilitate the subsequent 

 discharge of action along the same lines (Exner's " Bahnung "). 

 This hypothesis not only accounts for the fact that new reflex 

 combinations of movements may be formed during the life of 

 the individual, but it also gives us the key to an understanding 

 of those co-ordinated reflexes which the individual acquires as an 

 " inheritance " from his ancestors. 



If these facts show that the law of isolated conduction does 



