SPINAL CORD on MARROW, THE STRUCTURE AND FUNCTION'S OF, The spinal cord is that elongated part of the cerebrospinal axis (see SYsTzm) which is con tained in the spinal canal from the foramen magnum, at the base of the skull, superiorly, to the first or second lumbar vertebra inferiorly, where it merges into the fermi nale, which extends to the lower end of the sacral canal, and in no way (liars structu rally from the proper spinal cord, except that no nerve-roots are connected with it. The membranes by which it is protected from danger, and kept in its proper position, are described in the article NERVOUS SYSTEM. Its length varies from 15 to 18 in., and it presents a difference in its diameter in different parts, there being an upper or cervical, and a lower or lumbar enlargement. In form it is a flattered cylinder. It is almost completely divided, along the median plane by an anterior and posterior fissure, into two equal and symmetrical parts. The anterior fissure is more distinct and wider at the sur face than the posterior fissure, but it only penetrates to about one-third of the thickness if the cord, while the posterior fissure extends to about half the thickness of the cord. fhe two halves are hence only united near the center by a commissural band, which is traversed by the " spinal Canal " extending downward from the fourth ventricle-(sec BRAIN), and about one-hundredth of an inch in diameter. A posterior and an interior lateral furrow (two shallow depressions, the latter being scarcely perceptible) further divide each half of the cord into a posterior, a lateral, and an anterior column; these two furrows corresponding with the lines of attachment of the posterior and ante rior nerve-roots. The separation of the antero-lateral columns into the " anterior" and the lateral columns is made more obvious internally by the mode in which the gray or vesicular nervous matter (described in the article NERVOUS SYSTEM) is arranged in rela tion to the white or fibrous matter. Although the distribution of the gray matter differs considerably in different parts of the cord, it usually presents in a transverse section the form of two somewhat crescent-shaped masses, whose convexities are turned toward each other, and are connected by the gray commissure, while their cornua are directed toward the surface of the cord; the posterior peak on each side nearly reaches the posterior lateral furrow, while the anterior, though the larger cornu, does not approach quite so near the surface at the assumed anterior furrow. The enlargement of the cord in the
cervical and lumbar region, where the great nervous plexuses are given off, is chiefly due to the increase, at those points, of gray matter, which is comparatively deficient in the interval between them. The white substance seems to increase regularly from the lower to the upper part of the cord; and this fact, as Dr. Carpenter remarks, seems to indicate the probability that the longitudinal columns serve (as formerly supposed) to establish a direct connection between the encephalic centers and the roots of the spinal nerves. Careful microscopic investigation has revealed the fact that the root-fibers of the spinal nerves run two very distinct courses in the substance of the cord; the first transverse, and the second longitudinal. The transverse fibers traverse the cord horizontally or obliquely, and appear to pass out in the other set of roots connected with the same segment, either en its own or on the opposite side of the median fissure; while the longitudinal fibers in part connect the posterior roots directly with the posterior column without passing into the vesicular matter, but for the most part enter the gray matter, and emerge from it into the posterior column, or into the posterior part of the lateral column of the same or the opposite side. How far these longitudinal fibers run up or down the cord, is undecided. It is probable that some of them are longitudinal commissures, serving to connect the nerve-roots of one segment of the cord with the vesicular matter of another above or below it, and it is possible that all are of this character, in which case the spinal cord will be the real center of all the nerve-fibers conneeted with it.