Page:Popular Science Monthly Volume 35.djvu/765

 Plato carries out the same figure to explain different degrees of memory. When the wax is deep, abundant, smooth, and of the right quality, the impressions are lasting. Such minds learn easily, retain easily, and are not liable to confusion. But, on the other hand, when the wax is very soft, one learns easily but forgets as easily; if the wax is hard, one learns with difficulty, but what is learned is retained.

In some way or other, we do not know exactly how, the sensations leave behind them impressions or memory pictures. And these separate memory pictures are associated together, as they have all come from the same object; so that, the association being once made, any one will bring to mind the others, and hence if you perceive the fragrance you remember the appearance of the flower from which it comes—its color or its feel. This association of separate memory-pictures is secured by means of fine nerve-threads, which pass between the various areas of the brain and join the parts of the mental image with each other. This may be represented in the diagram (Fig. 5) by placing a circle for each memory-picture in its appropriate place and joining the circles by lines. The circles represent those little round masses of brain substance called nerve-cells, and the lines the association nerve-fibers uniting the cells (Fig. 6). The diagram shows the physical basis of the mental image of a rose—what has been called by Romanes a "recept," since its elements have been received by the senses. What is true of the rose is true of every other object which we have ever learned to know, for of every object we have a recept, or a series of mental images in the brain.