relation to certain coloring matters) may be perceived. We may conclude from this that the neurona or psychic cells which compose both organs also differ in their finer structure; there is probably a difference in the complicated fibrils which extend in the cytoplasm of both organs, although our coarse means of investigation have not yet succeeded in detecting this difference. In order to distinguish properly between the two sets of neurona, I propose to call the sensory-cells or sense-centres _æsthetal cells_, and the thought-cells or thought-centres _phronetal cells_. The former are, anatomically and physiologically, the intermediaries between the external sense-organs and the internal thought-organs. To this anatomic delimitation of the internal sense-centres and thought-organs in the cortex corresponds their physiological differentiation. The sensorium, or sense-centre, works up the external sense-impressions that are conveyed by the peripheral sense-organs and the specific energy of their sensory nerves; the _æstheta_, or the central sense-instruments that make up the sensorium, and their organic units, the _æsthetal cells_, prepare the sense-impressions for thought and judgment in the proper sense. This work of "pure reason" is accomplished by the _phronema_ of the thought-centres, the _phroneta_ (or the various thought-organs that compose it) and their histological elements, the phronetal cells, bringing about an association or combination of the prepared impressions. By this important distinction we avoid the error of the older sensualism (of Hume, Condillac, etc.)--namely, that all knowledge depends on sense-action alone. It is true that the senses are the original source of all knowledge; but, in order to have real knowledge and thought, the specific task of reason, the impressions received from the external world by the sense-organs, and their nerves and centres, must be combined in the association-centres and elaborated in the conscious thought-centres. Then there is the important, but frequently overlooked, circumstance that there is in advance in the phronetal cells of the civilized man a valuable quality in the shape of inherited potential nerve-energy, which was originally engendered by the actual sense-action of the æsthetal cells in the course of many generations.