It is thus evident that research in experimental physiology has had an extent and an application to human psychology which were hardly dreamed of by the original investigators twenty years ago; and it is clear that as, little by little, the facts to which I have just alluded have been established, they have been seen to throw much light upon psychological processes, and to make our knowledge of the mechanism of thinking both wider and more precise.
Secondly: There is another field of investigation from which rich results for mental science have recently been reaped—namely, physiological psychology. The determination of the special function of different parts of the brain, and the fact ascertained by anatomists that each of these parts is related to other parts by means of great bundles of nerve fibers which pass throughout the brain in many directions, joining the different functional areas with one another, have led to the study of the association processes which lie at the basis of most of our thinking.
Mental images never occur singly, but are usually in close relation with other images, the result of simultaneous perceptions. The various qualities of an object perceived by different senses are united in our concept of the object. The beautiful form of the rose, its charming color, its delicate odor, the soft, velvety feel of its petals, and the sharp prick of its thorns all come into my consciousness through various channels, but, being simultaneous in their perception, are all joined with one another in a complex unit—the concept; and when I call to mind a rose, it is not one memory of a single sensation which comes into my consciousness, but it is the associated memory pictures of sight and smell and touch which, by a flash of consciousness, rise together into the mind. And since it is possible to analyze these sensations, it is also possible to trace the association between them. I do not hesitate to call to mind the appearance of the rose, even though I merely perceive its delicate perfume, and there is hardly a flower whose name is not brought up the moment I see or smell it. Yet this process of calling up the image of the flower from its odor, or of calling up its name when I see it, involves a process of transmission of physical impulses from one region of the brain to another—a process of which the physiological psychologist has actually determined the time. We measure associations in hundredths of a second, and with decided accuracy. The mental act of ordinary single association may be said to occupy an eighth of a second The time of the transmission of these impulses varies decidedly at different periods of life, though it requires no delicate apparatus to convince one of the contrast between the quick, acute association of the young man and the slow, uncertain, halting memory of the aged. It has been found by Kroepelin that
