4. The arrangement and grouping of these psychic cells, the number of which runs into millions in the brain of man and the other mammals, is strictly regulated by law, and is distinguished within this highest class of the vertebrates by several characteristics, which can only be explained by the common origin of the mammals from one primitive mammal (or pro-mammal of the Triassic period). 5. Those groups of psychic cells which we must regard as the agents of the higher mental functions have their origin in the fore-brain, the earliest and foremost of the five embryonic brain-vesicles; they are confined to that part of the surface of the fore-brain which anatomists call the cortex, or gray bed, of the brain. 6. Within the cortex we have localized a number of different mental activities, or traced them to certain regions; if the latter are destroyed, their functions are extinguished. 7. These regions are so distributed in the cortex that one part of them is directly connected with the organs of sense, and receives and elaborates the impressions from these: these are the inner sense-centres, or sensoria. 8. Between these central organs of sense lie the intellectual or thought-organs, the instruments of presentation and thought, judgment and consciousness, intellect and reason; they are called the thought-centres, or association-centres, because the various impressions received from the sense-centres are associated, combined, and united in harmonious thought by them.[2] The anatomic distinction between the two regions of the cortex which we oppose to each other as the internal sense-centres and the thought or association-centres seems to me of the highest importance. Certain physiological considerations had for some time suggested this distinction, but the sound anatomic proof of it has only been furnished during the last ten years. In 1894 Flechsig showed that there are four central sense-regions ("internal sense-spheres," or æstheta) in the gray cortex of the brain, and four thought-centres ("association-centres" or phroneta) between these: the most important of the latter, from the psychological point of view, is the "principal brain," or the "great occipito-temporal association-centre." The anatomic determination of the two "psychic regions" which Flechsig first introduced was afterwards modified by himself and substantially altered by others. The distinguished works of Edinger, Weigert, Hitzig, and others, lead to somewhat discrepant conclusions. But for the general conception of psychic action, and especially of the cognitive functions, which interests us at present, it is not necessary to have this delimitation of the regions. The chief point holds, that we can to-day anatomically distinguish between the two most important organs of mental life; that the neurona, which compose both, differ histologically (or in finer structure) and ontogenetically (or in origin); and that even chemical differences (or a different