No living physiologist is better entitled to speak with authority upon the structure and functions of the brain than Dr. Luys. His studies on the anatomy of the nervous system are acknowledged to be the fullest and most systematic ever undertaken. He begins by treating the soft and delicate material of the brain-tissues with chromic acid, which hardens it so as to fix it sufficiently for the purposes of laboratory work, without altering or distorting its essential constitution. He then cuts off very thin slices of the tissue one after another, and, by employing different chemical reagents for which the various minute elements of the brain have varying susceptibilities, he obtains transparent colored sections of the nervous matter, which throw into strong relief the distinction between cells and fibers, besides exhibiting clearly the nature and direction of their intricate ramifications. In this manner he has systematically made many thousand delicate sections of brains, horizontally, vertically, and laterally, at distances of a millimetre from one another, each of which he photographs, till at last he has succeeded in producing a series of maps of its entire structure which place the relations of its organs in strikingly novel lights. The first division of his present volume is devoted to summing up briefly the main results of these important researches. The late Professor Clifford has already popularized them in part for the English reader; but we believe this is the first time that they have been definitely set forth in any fullness before the general public on this side of the Channel. Confining his attention to the cerebral hemispheres alone, without entering into any particulars as to the cerebellum and other minor appendages, Dr. Luys begins by pointing out the fundamental distinction between the nerve cell or real central organ and the nerve-fiber or connecting thread. The first answers to the telegraph-office, the second to the wire uniting one office with another. The gray matter which forms the outer covering of the convolutions consists of closely packed cells, and is thus really the essential brain; the white matter in the center consists of fibers aggregated into bundles, and is thus really a mass of large nerves. Of the single cells themselves, with their numerous converging fibers, as well as of their arrangement in superimposed layers, Dr. Luys gives very graphic and instructive diagrams. The business of the cells individually and of the gray matter as a whole is to receive sensory messages from the external organs of the senses and to transform or to co-ordinate their impulses into the proper movements—as, for example, when we see a fruit or flower, and stretch out our hands to pick it. The white substance is shown to consist of numerous interlacing fibers, having for their function the conveyance of such information from without inward, or the carrying down of such motor impulses from within outward. Their definite arrangement in regular lines between the two hemispheres, as well as between the surface of the convolutions and the optic thalami and corpus striatum, is admirably shown by diagrammatic figures. This is the most important result of all Dr. Luys's work. He has made it clear that sense-impressions traveling from the eyes, ears, or skin, arrive first at the bodies known as the optic thalami; that they are there re-enforced and worked up, as it were, in special ganglia; and that they are thence reflected to the surface of the hemispheres, where they are finally converted into appropriate movements. He has also fairly settled the fact that certain minor bodies within the optic thalami are closely connected with the main nerves of sight, smell, taste, and hearing respectively, and that they must be considered as subordinate or intermediate centers where the data supplied by those senses are put into shape for consideration on the surface of the brain. The normal course of an excitation in the sense-organs seems to be this: it first proceeds along the fibers to its own subordinate center in the thalami; it then passes up to the corresponding portion of the convolutions; it there for the