HABIT. 107 



struck into new paths ? In other words, can we say just 

 what mechanical facts the expression ' change of habit ' 

 covers when it is applied to a nervous system ? Certainly 

 we cannot in anything like a minute or definite way. But 

 our usual scientific custom of interjDreting hidden molecular 

 events after the analogy of visible massive ones enables us to 

 frame easily an abstract and general scheme of processes 

 which the physical changes in question may be like. And 

 when once the possibility of some kind of mechanical inter- 

 j)retation is established, Mechanical Science, in her present 

 mood, will not hesitate to set her brand of ownership upon 

 the matter, feeling sure that it is only a question of time 

 when the exact mechanical explanation of the case shall be 

 found out. 



If habits are due to the jjlasticity of materials to out- 

 ward agents, we can immediately see to what outward 

 influences, if to any, the brain-matter is plastic. Not to 

 mechanical pressures, not to thermal changes, not to any 

 of the forces to which all the other organs of our bod\- are 

 exposed ; for nature has carefully shut wp our brain and 

 spinal cord in bony boxes, where no influences of this sort 

 can get at them. She has floated them in fluid so that 

 only the severest shocks can give them a concussion, and 

 blanketed and wrapped them about in an altogether excep- 

 tional way. The only impressions that can be made upon 

 them are through the blood, on the one hand, and through 

 the sensory nerve-roots, on the other ; and it is to the infi- 

 nitely attenuated currents that pour in through these latter 

 channels that the hemispherical cortex sIioavs itself to be so 

 peculiarly susceptible. The currents, once in, must find a 

 way out. In getting out they leave their traces in the paths 

 which they take. The only thing they can do, in short, is 

 to deepen old paths or to make new ones ; and the whole 

 plasticity of the brain sums itself up in two words when 

 we call it an organ in which currents pouring in from the 

 sense-organs make with extreme facility paths which do 

 not easily disappear. For, of course, a simple habit, like 

 ever}^ other nervous event — the habit of snuffling, for 

 example, or of putting one's hands into one's pockets, or of 

 biting one's nails — is, mechanically, nothing but a reflex 



