Page:Principles of Psychology (1890) v1.djvu/45

time, and, by virtue of a reflex tendency common in babies of a certain age,
Fig. 3. 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.

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;
Fig. 4.—The dotted lines stand for afferent paths, the broken lines for paths between the centres; the entire lines for efferent paths. let the feeling of the arm's extension also send up a current which leaves a trace of itself, m¹; let tha 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