Modern studies in neurology have contributed much to our knowledge of the function of the nervous system as a whole and of its several parts, and also of the relation of psychical activity to cerebral conditions and processes. The architecture of the neural mechanism delineated by these investigations is not only interesting in itself on account of the marvelous unity of things apparently diverse, but it is at the same time suggestive respecting its office as the physical instrument through which mind must express itself in this world. Psychologists now quite generally conceive of a living being, human or otherwise, as a reacting organism, receiving impressions from its environment and responding to them in some characteristic manner. To be fitted for this office an individual must be provided with appliances alike for the reception of stimulations and for their transformation into incitements to muscular activity. In the human species Nature has ordained that action need not follow immediately and inevitably upon any sense stimulus; fortunately, it may be deferred, so that when it does finally occur it will be the resultant of any given present impression modified by others previously received and treasured up in memory, as we say. To accomplish this really great feat, Nature had to devise an elaborate contrivance, interposed between incoming messages and outgoing impulses, to act as a moderator or transformer of a very extraordinary and intricate character--the central nervous system, comprising the brain and spinal cord. That it may be able to meet the requirements of its office, this system must be equipped with two principal kinds of apparatus--cells, which will serve as storehouses of energy to be employed in keeping the machinery running, and association fibers or pathways, which will put any one cell into communication with others in the cerebral community.