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IX. Neurology

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IX. Neurology IX. Neurology THE NERVOUS SYSTEM is the most complicated and highly organized of the various systems which make up the human body. It is the 1 mechanism concerned with the correlation and integration of various bodily processes and the reactions and adjustments of the organism to its environment. In addition the cerebral cortex is concerned with conscious life. It may be divided into two parts, central and peripheral. The central nervous system consists of the encephalon or brain, contained within the cranium, and the medulla spinalis or spinal 2 cord, lodged in the vertebral canal; the two portions are continuous with one another at the level of the upper border of the atlas vertebra. The peripheral nervous system consists of a series of nerves by which the central nervous system is connected with the various tissues of the body. For descriptive purposes these nerves may be arranged in two groups, cerebrospinal and sympathetic, the arrangement, however, being an arbitrary one, since the two groups are intimately connected and closely intermingled. Both the cerebrospinal and sympathetic nerves have nuclei of origin (the somatic efferent and sympathetic efferent) as well as nuclei of termination (somatic afferent and sympathetic afferent) in the central nervous system. The cerebrospinal nerves are forty-three in number on either side—twelve cranial, attached to the brain, and thirty-one spinal, to the medulla spinalis. They are associated with the functions of the special and general senses and with the voluntary movements of the body. The sympathetic nerves transmit the impulses which regulate the movements of the viscera, determine the caliber of the bloodvessels, and control the phenomena of secretion. In relation with them are two rows of central ganglia, situated one on either side of the middle line in front of the vertebral column; these ganglia are intimately connected with the medulla spinalis and the spinal nerves, and are also joined to each other by vertical strands of nerve fibers so as to constitute a pair of knotted cords, the sympathetic trunks, which reach from the base of the skull to the coccyx. The sympathetic nerves issuing from the ganglia form three great prevertebral plexuses which supply the thoracic, abdominal, and pelvic viscera; in relation to the walls of these viscera intricate nerve plexuses and numerous peripheral ganglia are found. 2. Development of the Nervous System The entire nervous system is of ectodermal origin, and its first rudiment is seen in the neural groove which extends along the dorsal aspect of the 1 embryo (Fig. 17). By the elevation and ultimate fusion of the neural folds, the groove is converted into the neural tube (Fig. 19).The anterior end of the neural tube becomes expanded to form the three primary brain-vesicles; the cavity of the tube is subsequently modified to form the ventricular cavities of the brain, and the central canal of the medulla spinalis; from the wall the nervous elements and the neuroglia of the brain and medulla spinalis are developed. FIG. 640– Section of medulla spinalis of a four weeks’ embryo. (His.) (See enlarged image) FIG. 641– Transverse section of the medulla spinalis of a human embryo at the beginning of the fourth week. The left edge of the figure corresponds to the lining of the central canal. (His.) (See enlarged image) FIG. 642– aged about four and a half weeks. (See enlarged image) FIG. 643– aged about three months. (See enlarged image) The Medulla Spinalis.—At first the wall of the neural tube is composed of a single layer of columnar ectodermal cells. Soon the side-walls 2 become thickened, while the dorsal and ventral parts remain thin, and are named the roof- and floor-plates (Figs. 640, 642, 643). A transverse section of the tube at this stage presents an oval outline, while its lumen has the appearance of a slit. The cells which constitute the wall of the tube proliferate rapidly, lose their cell-boundaries and form a syncytium. This syncytium consists at first of dense protoplasm with closely packed nuclei, but later it opens out and forms a looser meshwork with the cellular strands arranged in a radiating manner from the central canal. Three layers may now be defined—an internal or ependymal, an intermediate or mantle, and an external or marginal. The ependymal layer is ultimately converted into the ependyma of the central canal; the processes of its cells pass outward toward the periphery of the medulla spinalis. The marginal layer is devoid of nuclei, and later forms the supporting framework for the white funiculi of the medulla spinalis. The mantle layer represents the whole of the future gray columns of the medulla spinalis; in it the cells are differentiated into two sets, viz., (a)spongioblasts or young neuroglia cells, and (b) germinal cells, which are the parents of the neuroblasts or young nerve cells (Fig. 641). The spongioblasts are at first connected to one another by filaments of the syncytium; in these, fibrils are developed, so that as the neuroglial cells become defined they exhibit their characteristic mature appearance with multiple processes proceeding from each cell. The germinal cells are large, round or oval, and first make their appearance between the ependymal cells on the sides of the central canal. They increase rapidly in number, so that by the fourth week they form an almost continuous layer on each side of the tube. No germinal cells are found in the roof- or floor-plates; the roof-plate retains, in certain regions of the brain, its epithelial character; elsewhere, its cells become spongioblasts. By subdivision the germinal cells give rise to the neuroblasts or young nerve cells, which migrate outward from the sides of the central canal into the mantle layer and neural crest, and at the same time become pear-shaped; thetapering part of the cell undergoes still further elongation, and forms the axiscylinder of the cell. The lateral walls of the medulla spinalis continue to increase in thickness, and the canal widens out near its dorsal extremity, and assumes a 3 somewhat lozengeshaped appearance. The widest part of the canal serves to subdivide the lateral wall of the neural tube into adorsal or alar, and a ventral or basal lamina (Figs. 642, 643), a subdivision which extends forward into the brain. At a later stage the ventral part of the canal widens out, while the dorsal part is first reduced to a mere slit and then becomes obliterated by the approximation and fusion of its walls; the ventral part of the canal persists and forms the central canal of the adult medulla spinalis. The caudal end of the canal exhibits a conical expansion which is known as the terminal ventricle. The ventral part of the mantle layer becomes thickened, and on cross-section appears as a triangular patch between the marginal and ependymal 4 layers. This thickening is the rudiment of the anterior column of gray substance, and contains many neuroblasts, the axis-cylinders of which pass out through the marginal layer and form the anterior roots of the spinal nerves (Figs. 640, 642, 643). The thickening of the mantle layer gradually extends in a dorsal direction, and forms the posterior column of gray substance. The axons of many of the neuroblasts in the alar lamina run forward, and cross in the floor-plate to the opposite side of the medulla spinalis; these form the rudiment of the anterior white commissure. About the end of the fourth week nerve fibers begin to appear in the marginal layer. The first to develop are the short intersegmental fibers from 5 the neuroblasts in the mantle zone, and the fibers of the dorsal nerve roots which grow into the medulla spinalis from the cells of the spinal ganglia. By the sixth week these dorsal root fibers form a well-defined oval bundle in the peripheral part of the alar lamina; this \??\ gradually increases in size, and spreading toward the middle line forms the rudiment of the posterior funiculus. The long intersegmental fibers begin to appear about the third month and the cerebrospinal fibers about the fifth month. All nerve fibers are at first destitute of medullary sheaths. Different groups of fibers receive their sheaths at different times—the dorsal and ventral nerve roots about the fifth month, the cerebrospinal fibers after the ninth month. By the growth of the anterior columns of gray substance, and by the increase in size of the anterior funiculi, a furrow is formed between the 6 lateral halves of the cord anteriorly; this gradually deepens to form the anterior median fissure. The mode of formation of the posterior septum is somewhat uncertain. Many believe that it is produced by the growing together of the walls of the posterior part of the central canal and by the development from its ependymal cells of a septum of fibrillated tissue which separates the future funiculi graciles. Up to the third month of fetal life the medulla spinalis occupies the entire length of the vertebral canal, and the spinal nerves pass outward at 7 right angles to the medulla spinalis. From this time onward, the vertebral column grows more rapidly than the medulla spinalis, and the latter, being fixed above through its continuity with the brain, gradually assumes a higher position within the canal. By the sixth month its lower end reaches only as far as the upper end of the sacrum; at birth it is on a level with the third lumbar vertebra, and in the adult with the lower border of the first or upper border of the second lumbar vertebra. A delicate filament, the filum terminale, extends from its lower end as far as the coccyx.
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