Page:Popular Science Monthly Volume 16.djvu/611

Rh could be called) would be shut up to the punctiform instant; he would obey, without noticing, the current which swept him on; drift to his conclusions, but never know why; and act upon the suggestions of experience with a fatality which would be inwardly all the blinder in proportion as it was the more rational to outward semblance. I simply assume for his benefit the possession of a consciousness. I beg that much from the reader’s liberality; and limit my ambition to showing (the consciousness being granted) with what objects it is at any given moment most likely to be filled.

The laws of motor habit in the lower centers of the nervous system are disputed by no one. A series of movements repeated in a certain order tend to unroll themselves with peculiar ease in that order for ever afterward. Number one awakens number two, and that awakens number three, and so on, till the last is produced. A habit of this kind once become inveterate, like the manipulations of certain trades, the balancings of the body in standing or walking, the varying pressure of the legs in response to the swayings of a horse’s gait, may go on automatically while the mind concerns itself with far other affairs. And so it is with thoughts. Not only poems, but the multiplication-table, Greek verbs, and formulas of gibberish like "ana, mana, mona, mike," etc., cohere in the self-same order in which they have once been learned. If we have blundered once in a certain place, we are prone to repeat the mistake again. The higher and the lower nerve-centers, then, are subject to one and the same law; and the reason of the law must be in both cases the same. The fact that there are isolated tracts of conduction in all the centers, and that as we pass from below upward the different centers have in the main different characteristic functions, leads to the notion that each function, ideational or motor, is dependent on a certain tract localized somewhere, which tract when once excited may propagate the excitement to other outlying tracts. The reason for the law of habit would, then, seem to be that the propagation occurs easiest through those tracts of conduction which have been already most in use. Descartes and Locke hit upon this explanation, which modern science has not yet succeeded in improving. "Custom," says Locke, "settles habits of thinking in the understanding as well as of determining in the will, and of motions in the body; all which seem to be but trains of motion in the animal spirits (by this Locke meant identically what we understand by the words neural process), which, once set agoing, continue in the same steps they have been used to, which by often treading are worn into a smooth path, and the motion in it becomes easy and as it were natural."

Let us, then, assume as the basis of all our subsequent reasoning the following law: When two brain tracts or processes have occurred together or in immediate succession, any one of them, on reoccurring, tends to propagate its excitement into the other.

Now, as a matter of fact, things in the brain are much less simple