The law of combination, also, is readily translated into [{416}] neural terms. The "pre-existing loose linkages" which it assumed to exist undoubtedly do exist in the form of "association fibers" extending in vast numbers from any one part of the cortex to many other parts. These fibers are provided by native constitution, but probably terminate rather loosely in the cortex until exercise has developed them. They may be compared to telephone wires laid down in the cables through the streets and extending into the houses, but still requiring a little fine work to attach them properly to the telephone instruments.
Fig. 64.--Diagram for the learning of the name of an object, transformed into a neural diagram. The vocal movement of saying the name is made in response to the auditory stimulus of hearing the name, but when the neurone in the "speech center" is thus made active, it takes up current also from the axon that reaches it from the visual center, even though the synapse between this axon and the speech neurone is far from close. This particular synapse between the visual and the speech centers, being thus exercised, is left in an improved condition. Each neurone in the diagram represents hundreds in the brain, for brain activities are carried on by companies and regiments of neurones. (Figure text: object seen, visual center name heard, auditory center, speech center, name spoken)
The diagrams illustrating different cases under the law of combination can easily be perfected into neural diagrams, though, to be sure, any diagram is ultra-simple as compared with the great number of neurones that take part in even a simple reaction.
The reader will be curious to know now much of this neural interpretation of our psychological laws is observed fact, and how much speculation. Well, we cannot as yet [{417}] observe the brain mechanism in actual operation--not in any detail. We have good evidence, as already outlined, for growth of the neurones and their branches through exercise.
Fig. 65.--Control, in multiplying. The visual stimulus of two numbers in a little column, has preformed linkages both with the adding response and with that of multiplying. But the mental set for adding being inactive at the moment, and that for multiplying active (because the subject means to multiply), the multiplying response is facilitated.
We have perfectly good evidence of the law of "unitary response to multiple stimuli" from the physiological study of reflex action; and we have perfectly good anatomical evidence of the convergence and divergence of neural paths of connection, as required by the law of combination. The association fibers extending from one part to another of the cortex are an anatomical fact. [Footnote: [See p. 56.]] Facilitation is a fact, and that means that a stimulus which could not of itself arouse a response can coöperate with another stimulus that has a direct connection with that response, and reinforce its effect. In short, all the elements required for a neural law of combination are known facts, and the only matter of doubt is whether we have built these elements together aright in our interpretation. It is not pure speculation, by any means.