This requires a general knowledge of anatomy, physiology, and pathology which we shall presuppose the reader to possesses so that we may present only facts to which his attention should be particularly called. Let us begin with the relation of nerve tissue to other tissues where this relation can be most clearly comprehended, namely, with the development of the human embryo. Segmentation The complete human organism represents the snarled fusion of a series of similar, yet specialized, somatic segments, each presenting most of the attributes of a simple animal, though the association and co-ordination of all are required for the production of higher animal phenomena. The embryo is composed of such segments placed with their centers in the same axial line. Each segment contains in association which is morphologic, physiologic, and anatomical, a segment of nerve matter and a somatic (body) segment. The neural segments are arranged end to end so as to form the rudimentary beginning of the complete central nerve axis of the adult human body; the somatic segments blend together with somewhat indefinite lines of cleavage which are to become much more indefinite and obscure by changes in relative form due to differences in the growth rate of different parts or to involuntionary changes following functional inutility at various periods. Gray says, “The intrinsically segmental nature of the spinal cord is expressed by the association of each definite segment with the somatic segment supplied by its nerve.” Within each segment there may be observed at an early period cell migrations from the walls of the primitive neural tube and amoeboid projection of axonic and dendritic processes from these cells, which serve to bring the other tissues of the segment under the control of the nerve elements; there is an assumption of command, as it were, by the nervous system, so that the epithelial, connective, and muscular tissues of each segment are linked in sensomotor and vegetative co-ordination by the contact association of the nerves which ramify them--sensomotor because the nerves are presently to carry the only force capable of inciting activity of any kind in other tissues, vegetative because the functions of growth, nutrition, and repair, in each somatic cell, depend upon the continuity of communication between it and the lowest nerve cell in the nerve pathway which connects it with the higher motor and sensor centers. Development of the Nerve System Already may be noted a hint and a prophecy of that future segmental organization by which it becomes possible for some spinal vertebra to become displaced and thus begin a morbid process which may diffuse itself throughout an entire body segment, involving neural and somatic elements together. Already the simple organization begins to become rapidly complex and difficult to trace.