Experiments and clinical observations lead to the conclusion that the sympathetic system supplies motor power to many of the internal viscera, especially the heart and the intestinal canal; that it also contains sensitive fibers, as is shown by the sufferings of patients during the passage of a gall stone or a renal calculus through a duct, whose sole nervous energy is derived from tide system; that it presides over the process of secretion in the most important glands; and that it operates on the blood-vessels in causing them to contract, while the eerebt.o-spinal nerves produce the opposite effect.
On examining different parts of the nervous system under the microscope, we find that the nervous matter is distributed in two forms, the vesicular and the fibrous. The vesicular matter is gray in color, and granulur in texture, contains nucleated nerve cells, and is largely supplied with blood; it is immediately associated with mental actions, and is the seat in which the force manifested in nervous action originates. The fibrous matter is, in most parts, white and composed 91 tubular fibers, though in some parts it is gray and 'consists of solid fibers; it is less vascular than the former, and is simply the conductor of impressions made upon it. When these two kinds of matter are united together into a mass they form a nervous center, such as the brain or spinal cord, while the nerves passing to and from them arc composed of threads of fibrous matter. The nervous matter of both kinds is a soft, unctuous substance, with very slight tenacity; the softness being in a great measure clue to the large quantity of water which it contains.
The fibrous form is the most extensively diffused thrnighout the body. It forms a large portion of the nervous centers, and is the main constituent of all the nerves. It occurs in two varieties, viz., as the tubular fiber, or the nerve tube, and the gelatinous,fiber, the latter being of comparatively rare occurrence, and being found chiefly in the sympa thetic system.
When a tubular fiber is viewed by reflected light, it pressmts a beautiful pearly luster, and appears to be homogeneous. But if viewed by transmitted light, with a sufficient magnifying power, indications of structure become visible. Externally, there is the tubular membrane, a homogeneous and probably very delicate elastic tissue, according to Todd. Within the edge of the tubular membrane, on either side are seen two thicker and darker lines, which appear to mark the outer and inner 14,149 of the structure known as the white substance of Schwann, which forms a tube within the tubular membrane; and within the white substance of Schwann is a transparent mates al occupying the axis of the nerve tube, and commonly known as the axis cylinder. By the application of
reagents, it is seen that the chemical composition of the white substance is different from that of the axis cylinder, and hence the functions of these two parts are doubtless different; the latter is in general soft and pulpy. The nerve-tub's are cylindrical in form, and lie parallel to one another, without any inosculation, tve except their frequent terminations in loops. Their average diameter is about „WI, of -ku inch.
The gelatinous fibers are flattened, soft, and homogeneous in appearaik-4. and contain numerous round or oval nuclei. Their diameter is about of au inch. in appear ance they much resemble the fibers of unstriped muscle.
The vesicular form of nervous matter is of a dark reddish-gray color, is fonnt: in the nervous centers, is always well supplied with capillaries, and consists esseptiOly of nucleated cells or vesicles, which are most commonly globular or ovoidal, bud often present one or more tail-like processes, when they are termed caudate. These caudate vesicles present great difference in shape and size. The processes are very delicate, and readily break off close to the vesicle. They probably either serve to connect distant vesicles or else become continuous with the axis cylinders of the tubular fibers.
We may now consider the way in which the nerves and nervous centers are made up of these anatomical elements.
A nerve is composed of a bundle of tubular fibers surrounded and connected by areolar tissue, which forms a sheath known as the neurolemma, whose office is to protect. the delicate tubes, and to support the capillaries from which they derive their ment.
The nervous centers exhibit a union of the vesicular and fibrous textures, which may be variously arranged. In the Brain (q. v.) the Yesicultir matter lies 'externally, forming the gray or cineritious substance; in the spinal cord, on the other hand, the vesicular or gray matter lies in the central portion, and the fibrous or white matter is external to it; while in the ganglia the two structures are more or less uniformly associated.