Nerve-currents run in through sense-organs, and whilst provoking reflex acts in the lower centres, they arouse ideas in the hemispheres, which either permit the reflexes in question, check them, or substitute others for them. All ideas being in the last resort reminiscences, the question to answer is: How can processes become organized in the hemispheres which correspond to reminiscences in the mind?[6]
Nothing is easier than to conceive a possible way in which this might be done, provided four assumptions be granted. These assumptions (which after all are inevitable in any event) are:
1) The same cerebral process which, when aroused from without by a sense-organ, gives the perception of an object, will give an idea of the same object when aroused by other cerebral processes from within.
2) If processes 1, 2, 3, 4 have once been aroused together or in immediate succession, any subsequent arousal of any one of them (whether from without or within) will tend to arouse the others in the original order. [This is the so-called law of association.]
3) Every sensorial excitement propagated to a lower centre tends to spread upwards and arouse an idea.
4) Every idea tends ultimately either to produce a movement or to check one which otherwise would be produced.
Fig. 3. Suppose now (these assumptions being granted) that we have a baby before us who sees a candle-flame for the first time, and, by virtue of a reflex tendency common in babies of a certain age, extends his hand to grasp it, so that his fingers get burned. So far we have two reflex currents in play: first, from the eye to the extension movement, along the line 1—1—1—1 of Fig. 3; and second, from the finger to the movement of drawing back the hand, along the line 2—2—2—2. If this were the baby's whole nervous system, and if the reflexes were once for all organic, we should have no alteration in his behavior, no matter how often the experience recurred. The retinal image of the flame would always make the arm shoot forward, the burning of the finger would always send it back. But we know that 'the burnt child dreads the fire,' and that one experience usually protects the fingers forever. The point is to see how the hemispheres may bring this result to pass.
Fig. 4.—The dotted lines stand for afferent paths, the broken lines for paths between the centres; the entire lines for efferent paths. We must complicate our diagram (see Fig. 4). Let the current 1—1, from the eye, discharge upward as well as downward when it reaches the lower centre for vision, and arouse the perceptional process s1 in the hemispheres; let the feeling of the arm's extension also send up a current which leaves a trace of itself, m1; let the burnt finger leave an analogous trace, s2; and let the movement of retraction leave m2. These four processes will now, by virtue of assumption 2), be associated together by the path s1—m1—s2—m2, running from the first to the last, so that if anything touches off s1, ideas of the extension, of the burnt finger, and of the retraction will pass in rapid succession through the mind. The effect on the child's conduct when the candle-flame is next presented is easy to imagine. Of course the sight of it arouses the grasping reflex; but it arouses simultaneously the idea thereof, together with that of the consequent pain, and of the final retraction of the hand; and if these cerebral processes prevail in strength over the immediate sensation in the centres below, the last idea will be the cue by which the final action is discharged. The grasping will be arrested in mid-career, the hand drawn back, and the child's fingers saved.
In all this we assume that the hemispheres do not natively couple any particular sense-impression with any special motor discharge. They only register, and preserve traces of, such couplings as are already organized in the reflex centres below. But this brings it inevitably about that, when a chain of experiences has been already registered and the first link is impressed once again from without, the last link will often be awakened in idea long before it can exist in fact. And if this last link were previously coupled with a motion, that motion may now come from the mere ideal suggestion without waiting for the actual impression to arise. Thus an animal with hemispheres acts in anticipation of future things; or, to use our previous formula, he acts from considerations of distant good and ill. If we give the name of partners to the original couplings of impressions with motions in a reflex way, then we may say that the function of the hemispheres is simply to bring about exchanges among the partners. Movement mn, which natively is sensation sn's partner, becomes through the hemispheres the partner of sensation s1, s2, or s3. It is like the great commutating switch-board at a central telephone station. No new elementary process is involved; no impression nor any motion peculiar to the hemispheres; but any number of combinations impossible to the lower machinery taken alone, and an endless consequent increase in the possibilities of behavior on the creature's part.
All this, as a mere scheme,[7] is so clear and so concordant with the general look of the facts as almost to impose itself on our belief; but it is anything but clear in detail. The brain-physiology of late years has with great effort sought to work out the paths by which these couplings of sensations with movements take place, both in the hemispheres and in the centres below.
So we must next test our scheme by the facts discovered m this direction. We shall conclude, I think, after taking them all into account, that the scheme probably makes the lower centres too machine-like and the hemispheres not quite machine-like enough, and must consequently be softened down a little. So much I may say in advance. Meanwhile, before plunging into the details which await us, it will somewhat clear our ideas if we contrast the modern way of looking at the matter with the phrenological conception which but lately preceded it.