THE ELEMENTS OF NERVE PHYSIOLOGY 43 



ear that had previously become familiar. So it is with our 

 sensations; we rely on the regularity of their external 

 causation, but we are always liable to be misled by an 

 experience like that of the tingle that follows the rap on 

 the elbow. Impulses started anywhere between the 

 periphery and the central stations have the same conse- 

 quences for us as though they had been initiated at the 

 remotest terminal structures. The more one reflects on 

 this fact, the more one marvels at the freedom from con- 

 fusing hallucinations which .we usually enjoy. 



Are nerve-impulses all alike? This is a most interesting 

 question. The first disposition will be for one to say that 

 they must vary widely in nature to produce such dis- 

 similar effects. But most of our physiologic progress has 

 been found consistent with the idea that they vary chiefly 

 in intensity and frequency and not in the more subtle 

 ways which might be conceived. The reader must be 

 reminded that two electric currents precisely alike in 

 themselves may cause utterly unlike phenomena if they 

 are led through different fixtures. The closing of one 

 circuit, for example, may ring a bell; the closing of another 

 may light a gas-jet. It is, for the most part, quite as 

 unnecessary to suppose that different nerves carry im- 

 pulses of contrasted character as to assume that the 

 electric currents are mysteriously differentiated in the 

 case cited. The clarifying view that they are essentially 

 similar throughout the whole system is one which we owe 

 primarily to the great physiologist, Johannes Miiller 

 (1801-1858). The theory encounters many difficulties, 

 but is not to be prematurely abandoned. 



Teaching experience shows that students find it ex- 

 ceedingly hard to believe that the impulses passing along 

 various nerve-trunks are of one unvarying character. The 

 instructor is at great pains to dissuade them from speaking 

 of "messages" or "sensations" in this connection. Mes- 

 sages and sensations are evolved from nerve-impulses 

 only by a process of translation; these highly colored 

 words cannot properly be applied to the energy in transit. 



