THE MOLECULAR THEORY OF EXCITATION 613 



set free a local store of latent energy. The conductor, 

 moreover, is seemingly unlike the conductor in an electrical 

 circuit, where the line wire and return wire must be 

 periodically connected with terminals of an electro-motive 

 source, for any message to be transmitted. In the nerve 

 we have only a single conductor, without a return, an 

 arrangement by which it would appear as difficult to send 

 a message, as it would be to apply the two poles of a 

 battery at the end of a single wire, in the expectation of 

 a signal from the recorder at the far end. Inorganic matter 

 again, is popularly regarded as susceptible only of impulses 

 from the grosser physical forces, while the nerve the 

 vehicle of psychic impulses is conceived of as played 

 upon by forces of a finer order, and as itself modifiable, 

 by subtler influences, notably that of its own previous 

 history, or memory. There are, as we know, some conditions 

 which induce such changes in the nervous channel itself, 

 that messages from outside, previously scarcely perceptible, 

 are accentuated. Under opposite influences, again, the 

 conduction of impulse is interrupted. Similar results a're 

 brought about by certain agents of a polar character, ; like 

 the action of anode and kathode. Under electrotonic action 

 the transmission of impulses through the nerve may be 

 blocked, conduction being renewed as soon as the electric 

 block is removed. Or electrotonic action, again, may be 

 used, for the opposite purpose, of accelerating the trans- 

 mission of impulses. Nothing more convincing than such 

 facts could have been urged in support of the hyper-physical 

 character of the phenomena in question. 



But the experiments which I have described, relating 

 to the conduction of excitatory molecular changes in a 

 piece of iron wire, show that parallel phenomena occur in the 

 physical domain also ; and in order to demonstrate this in 

 a striking manner, I cannot do better than describe an 

 arrangement which I have devised, and which may be 

 taken as an artificial nerve-and-muscle preparation. This 

 consists of a thin iron rod for the transmission of magnetic 



