« PreviousContinue »
The Nervous System coördinates all the various organs and tissues of the body, and brings the individual into conscious relationship with external nature by means of sensation, motion, language, mental and moral manifestations.
The Nervous Tissue may be divided into two systems, the Cerebrospinal and the Sympathetic.
(1) The Cerebro-spinal System occupies the cavities of the cranium and spinal canal, and consists of the brain, the spinal cord, the cranial and spinal nerves. It is the system of animal life, and presides over the functions of sensation, motion, etc.
(2) The Sympathetic System, situated along each side of the spinal column, consists (1) of a double chain of ganglia, united together by nerve cords, and extends from the base of the cranium to the coccyx; (2) of various ganglia, situated in the head and face, thorax, abdomen, pelvis, etc. All the ganglia are united together by numerous communicating fibers, many of which anastomose with the fibers of the cerebro-spinal system. It is the nervous system of organic life, and governs the functions of nutrition, growth, etc.
Nervous Tissue is composed of two kinds of matter, the gray and white, which differ in their color, structure, and physiological endowments; the former consists of vesicles or cells which receive and generate nerve force; the latter consists of fibers which simply conduct it, either from the periphery to the center or the reverse.
Structure of Gray Matter.—The gray matter found on the surface of the brain in the convolutions, in the interior of the spinal cord, and in the various ganglia of the cerebro-spinal and sympathetic nervous systems, consists of a fine connective tissue stroma, the neuroglia, in the meshes of which are embedded the gray cells or vesicles.
The cells are grayish in color, and consist of a delicate investing capsule containing a soft, granular, albuminous matter, a nucleus, and sometimes a nucleolus. Some of the cells are spherical or oval in shape, while others have an interrupted outline, on account of having one, two, or more processes issuing from them, constituting the uni-polar, bi-polar, or multi-polar nerve cells. Cells vary in size ; the smallest being found in the brain, the
largest in the anterior horns of gray matter of the spinal cord. Some of the cell processes become continuous with the fibers of the white matter, while others anastomose with those of adjoining cells and form a plexus.
Structure of the White Matter.—The white matter, found for the most part in the interior of the brain, on the surface of the spinal cord, and in almost all the nerves of the cerebro-spinal and sympathetic systems, consists of minute tubules or fibers, the ultimate nerve filaments, which in the perfectly fresh condition are apparently structureless and homogeneous; but when carefully examined after death are seen to consist of three distinct portions, (1) a tubular membrane; (2) the white substance of Schwann; (3) the axis cylinder.
The tubular membrane, investing the nerve filament, is thin, homogeneous, and lined by large, oval nuclei, and presents, in its course, annular constrictions; it serves to keep the internal parts of the fiber in position, and protects them from injury.
The white substance of Schwann, or the medullary layer, is situated immediately within the tubular membrane, and gives to the nerves their peculiar white and glistening appearance. It is composed of oleaginous matter in a more or less fluid condition; after death it undergoes coagulation, giving to the fiber a knotted or varicose appearance. It serves to insulate the axis cylinder, and prevents the diffusion of the nerve force.
The axis cylinder occupies the center of the medullary substance. In the natural condition it is transparent and invisible, but when treated with proper reagents, it presents itself as a pale, granular, flattened band, albuminous in character, more or less solid, and somewhat elastic. It is composed of a number of minute fibrillæ united together to form a single bundle. (Schultze.)
Nerve fibers in which these three structural elements coexist are known as the medullated nerve fibers. In the sympathetic system, and in the gray substance of the cerebro-spinal system, many nerves are destitute of a me. dullary layer, and are known as the non-medullated nerve fibers.
Gray or Gelatinous nerve fibers, found principally in the sympathetic system, are gray in color, semi-transparent, flattened, with distinct borders, finely granular, and present oval nuclei.
The diameter of the gelatinous fibers is about the sono of an inch; of the medullated fibers from 2500 to 1500 of an inch.
Ganglia are small bodies, varying considerably in size, situated on the posterior roots of spinal nerves, on the sensory cranial nerves, alongside of the vertebral column, forming a connecting chain, and in the different viscera. They consist of a dense, investing, fibrous membrane, containing in its interior gray or vesicular cells, among which are found white and gela. tinous nerve fibers. They may be regarded as independent nerve centers.
Structure of Nerves.--Within the cranial and spinal cavities, the nerve fibers are bound together by connective tissue in the form of continuous bundles. Through the foramina of these cavities the nerve fibers emerge in the form of rounded or flattened cords, which are termed nerves. Each nerve is surrounded by a sheath of connective tissue, the neurilemma, which also forms a stroma in which the blood-vessels ramisy, furnishing nutritive material for the growth and repair of the ultimate nerve fibers.
A nerve consists of a greater or less number of ultimate nerve filaments, separated into bundles by fibrous septa given off from the neurilemma. The nerve filaments pursue an uninterrupted course, from their origin to their termination; branches pass from one nerve trunk into the sheath of another, but there is no anastomosis or coalescence with adjoining nerve fibers. Nerves are channels of connection between the brain and cord, and the muscles, glands, skin, mucous membrane, etc., in which they terminate. Any excitation at either end produces in the nerve an impulse which travels throughout the length of the fiber. If the nerve fibers going to a muscle or gland are stimulated, there is increased muscular movement and increased secretion ; if the nerve fibers distributed to the skin or mucous membranes are stimulated, there is produced in the brain a sensation. This difference in effects produced by irritation has led to a division of fibers into two classes : viz., 1. Efferent or motor. 2. Afferent or sensory. There is no anatomical or chemical difference discoverable between these two classes of fibers.
A Plexus is formed by a number of branches of different nerves interlacing in every direction, in the most intricate manner, but from which fibers are again given off to pursue their independent course, e. g., brachial, cervical, lumbar, sacral, cardiac plexuses, etc.
Nerve Terminations.-(1) Central. Both motor and sensory nerve fibers, as they enter the spinal cord and brain, lose their external investments, and retaining only the axis cylinder, ultimately become connected with the processes of the gray
cells. (2) Peripheral. As the nerves approach the tissues to which they are to be distributed, they inosculate freely, forming a plexus from which the ultimate fibers proceed to individual tissues.
Motor Nerves.-In the voluntary or striped muscles the motor nerves are connected with the contractile substance by means of the “ motorial