THE NEURAL BASIS OF ASSOCIATION.--The association of thoughts--ideas, images, memory--or of a situation with its response, rests primarily on a neural basis. Association is the result of habit working in neurone groups. Its fundamental law is stated by James as follows: "When two elementary brain-processes have been active together or in immediate succession, one of them, on recurring, tends to propagate its excitement into the other." This is but a technical statement of the simple fact that nerve currents flow most easily over the neurone connections that they have already used. It is hard to teach an old dog new tricks, because the old tricks employ familiar, much-used neural paths, while new tricks require the connecting up of groups of neurones not in the habit of working together; and the flow of nerve energy is more easily accomplished in the neurones accustomed to working together. One who learns to speak a foreign language late in life never attains the facility and ease that might have been reached at an earlier age. This is because the neural paths for speech are already set for his mother-tongue, and, with the lessened plasticity of age, the new paths are hard to establish. The connections between the various brain areas, or groups of neurones, are, as we have seen in an earlier chapter, accomplished by means of _association fibers_. This function requires millions of neurones, which unite every part of the cortex with every other part, thus making it possible for a neural activity going on in any particular center to extend to any other center whatsoever. In the relatively unripe brain of the child, the association fibers have not yet set up most of their connections. The age at which memory begins is determined chiefly by the development of a sufficient number of association fibers to bring about recall. The more complex reasoning, which requires many different associative connections, is impossible prior to the existence of adequate neural development. It is this fact that makes it futile to attempt to teach young children the more complicated processes of arithmetic, grammar, or other subjects. They are not yet equipped with the requisite brain machinery to grasp the necessary associations. [Illustration: FIG. 18.--Diagrammatic scheme of association, in which V stands for the visual, A for the auditory, G for the gustatory, M for the motor, and T for the thought and feeling centers of the cortex.]