portion, the postero-internal column, or the column of Goll, bordering the posterior median fissure, and (b) an external portion, the postero-external column, the column of Burdach, lying just behind the posterior roots. They are composed of long and short commissural fibers which connect together different segments of the spinal cord. Structure of the Gray Matter.-The gray matter, arranged in the form of two crescents, presents an anterior and posterior horn. It is made up of a delicate network of fine nerve fibers (axis cylinders), supported by a connective tissue framework of nucleated nerve cells, which in the anterior horns are large and multipolar, and connected with the anterior roots of spinal nerves; in the posterior horns the nerve cells are smaller, and situated along the inner margin, and in the caput cornu. Small cells are also found in the posterior vesicular columns, and in the intermediary lateral tract. SPINAL NERVES. Origin. The spinal nerves are thirty-one in number on each side of the spinal cord, and arise by two roots, an anterior and posterior, from the anterior and posterior aspects of the cord respectively: the posterior roots present near their emergence from the cord a small ganglionic enlargement; outside of the spinal canal the two roots unite to form a main trunk, which is ultimately distributed to the skin, muscles, and viscera. The Function of the Anterior Roots is to transmit motor impulses from the centers outward to the periphery. Irritation of these roots, from whatever cause, excites convulsive movements in the muscles to which they are distributed; disease or division of these roots induces a condition of paresis or paralysis. The Function of the Posterior Roots is to transmit the impressions made upon the periphery to the centers in the spinal cord, where they excite motor impulses; or to the brain, in which they are translated into conscious sensations. Irritation of these roots gives rise to painful sensations; division of the roots abolishes all sensation in the parts to which they are distributed. The ganglion on the posterior root influences the nutrition of the sensory nerve; for if the nerve be separated from the ganglion, it undergoes degeneration in the course of a few days, in the direction in which it carries impressions, i. e., from the periphery to the centers; if the nerve be divided between the ganglion and the cord, the central end only undergoes degeneration. The nutrition of the anterior root is governed by nerve cells in the gray matter of the cord; for if these cells undergo atrophy, or if the nerve be divided, it undergoes degeneration outward. COURSE OF THE ANTERIOR AND POSTERIOR ROOTS. The Anterior Roots pass through the anterior columns, horizontally, in straight and distinct bundles, and enter the anterior cornuæ, where they diverge in four directions. (1) Many become connected with the prolongations of the multipolar nerve cells. (2) Others leave the gray matter, pass through the anterior white commissure, and enter the anterior columns of the opposite side. (3) A considerable number enter the lateral columns of the same side, through which they pass to the medulla oblongata, where they decussate and finally terminate in the corpus striatum of the opposite side. (4) Others traverse the gray matter horizontally, and come into relation with the posterior roots. The Posterior Roots enter the posterior horns of the gray matter (1) through the substantia gelatinosa, (2) through the posterior columns; of the former, some bend upward and downward, and become connected with the anterior cornua; others pass through the posterior commissure to the opposite side; of the latter, fibers pass into the gray matter to the posterior vesicular columns, passing obliquely through the posterior white columns upward and downward for some distance, and enter the gray matter at different heights. Decussation of Motor and Sensory Fibers.-The Motor fibers, which conduct volitional impulses from the brain outward to the anterior cornuæ, arise in the motor centers of the cerebrum; they then pass downward through the corona radiata, the internal capsule, the inferior portions of the crura cerebri, the pons Varolii, to the medulla oblongata, where the motor tract of each side divides into two portions, viz.: J. The larger, containing 91 to 97 per cent. of the fibers, which decussates at the lower border of the medulla and passes down in the lateral column of the opposite side, and constitutes the crossed pyramidal tract. 2. The smaller, containing 3 to 9 per cent. of the fibers, does not decussate, but passes down the anterior column of the same side, and constitutes the direct pyramidal tract, or the column of Türck. Some of the motor fibers of these two tracts, after entering the anterior cornuæ of the gray matter, become connected with the large multipolar nerve cells, while others pass directly into the anterior roots. Through this decussation each half of the brain governs the muscular movements of the opposite side of the body. |