108 PSTCUOLOOY. 



diseliarge ; and its anatomical substratum must be a path 

 in the system. The most complex habits, as we shall 

 presently see more fully, are, from the same point of view, 

 nothing but concatenated discharges in the nerve-centres, 

 due to the presence there of systems of reflex paths, so 

 organized as to wake each other up successiyely — the im- 

 pression produced by one muscular contraction serving as 

 a stimulus to provoke the next, until a final impression 

 inhibits the process and closes the chain. The only diffi- 

 cult mechanical problem is to explain the formation de novo 

 of a simple reflex or path in a pre-existing nervous system. 

 Here, as in so many other cases, it is only the premier pas 

 qui coiite. For the entire nervous system is nothing but a 

 system of paths between a sensory terminus a quo and a mus- 

 cular, glandular, or other terminus ad quern. A path once 

 traversed by a nerve-current might be expected to follow 

 the law of most of the paths we know, and to be scooj)ed 

 out and made more permeable than before ; * and this ought 

 to be repeated with each new jjassage of the current. 

 Whatever obstructions may have kept it at first from being 

 a path should then, little by little, and more and more, be 

 swept out of the way, until at last it might become a natural 

 drainage-channel. This is what happens where either 

 solids or liquids pass over a path ; there seems no reason 

 why it should not happen where the thing that passes is a 

 mere wave of rearrangement in matter that does not dis- 

 place itself, but merel}" changes chemically or turns itself 

 round in place, or vibrates across the line. The most 

 j^lausible views of the nerve-current make it out to be the 

 passage of some such wave of rearrangement as this. If 

 only a part of r the matter of the path were to 'rearrange' 

 itself, the neighboring parts remaining inert, it is easy to 

 see how their inertness might oppose a friction which it 

 would take many waves of rearrangement to break down 

 and overcome. If we call the path itself the ' organ,' and 

 the wave of rearrangement the ' function,' then it is obvi- 



* Some paths, to be sure, are banked up by bodies moving through 

 them under too great pressure, and made impervious. These special cases 

 we disregard. 



