NERVOUS SYSTEM (ante). The nerve fibers have a great diversity of size; some of them found between the muscles are as much of an inch in diameter, having a tubular membrane. medullary contents, and an axial band. Other nerve fibers having the same structure are not more than of an inch in diameter. Other fibers again are composed of an axial band and a medullary covering only; and still others have no other element than the axial band. The important element, and the one necessary to the conducting function, is the axis cylinder. In regard to the gelatinous nerve fibers, or the fibers of Remak, which are found, as stated in the preceding article, chiefly in the sym pathetic system, there is a difference eif opinion. By some physiologists they are believed to be connective tissue elements, not possessing the peculiar properties of nerves; but the predominant view is that they have peculiar nerve properties. These gelatinous fibers are particularly found in that portion of the sympathetic system connected with involun tary contraction of non-striated muscles, viz., in the gray filaments sent to these muscles, and not in the white filaments of the sympathetic, which are regarded as incapable of exciting motion. Microscopic investigation has shown that, in the development of the embryo, the first formed nerve fibers are identical with the fibers of Remak, and that the new nerve elements which are formed in the reparation of injuries are also of this char acter. The gelatinous nerve fibers of the sympathetic system are of a flattened form, with smooth, sharp borders, having a grayish pale color, and containing, within a sheath, munerons fine granulations, and, at intervals, oval cells or nuclei of the same diameter as the caliber of the fibers, and composed of fine granular matter, without nucleoli. The dimensions of these "nuclei" or cells are about of an inch in their short, and of as inch in their long diameter. The mcdullated nerve fibers, in passing into certain organs, such as the paeinian corpuscles (q.v.), the tactile corpuscles (see Toccu), and s inc glandular structures, lose the tubular sheath and medullary investment of the axis cylinder, while nothing but the latter element composes the termination. It was formerly thought that the tubular elements also entered the pacinian corpuscle, but recent observa tions have led to other conclusions.
Nene cellar and their relations.—The cellular elements of the nervous system, which are regarded as the "generating elements" of nervous force, are composed of four kinds of cells—apolar, unipolar, bipolar, and multipolar; and these are of various sizes and diverse appearances. The apolar cells are rounded bodies composed principally of gran ular matter, and having nuclei and nucleoli. They are always present in the ganglia of
the sympathetic nerve, and have been found in the cerebro-spinal centers. It is not certain that unipolar cells exist iu man, but they are not uncommon in the lower animals. Bipolar cells are found in the ganglia of the posterior roots of the spinal nerves, and also in ganglia of the sympathetic. Small cells, with three and sometimes four poles or con nections, are found in the posterior horn of the gray matter of the spinal cord (q v.). while large, irregularly shaped multipolar cells with numerous prolongations are found in the anterior horns of gray matter. Some of the cells have ten or twelve poles, and have received the name of motor cells. The general anatomical elements am, however, in all these forms of cells essentially alike. Except the apolar and unipolar cells, they are of irregular shape; and have highly refracting granular contents. In adult animals there is usually but one nucleus, but the nerve cells of young animals often have multiple nuclei. The nucleoli arc generally single, but sometimes they are also multiple. The size of nerve cells varies, ranging from less than -/-46- to more than of an inch in diameter. The nuclei are not so variable in size, ranging from to T of an inch. In examining these delicate organisms with the microscope it is usual to harden the tissues with chemical substances, such as chromic acid, and make very thin sections with a sharp knife; but, notwithstanding all the care that may be taken, the precise rela tions ifetween the nerve cells and nerve fibers have not been satisfactorily determined. Some of the German physiologists, among them Max Schultze, have become satisfied that multipolar cells have one prolongation unlike the rest, which is continuous with the axis cylinder of the nerve fiber, the other prolongations being of a different character, and, although branching out to is considerable extent, not connected with the nerve fibers. In tracing a nerve fiber towards its origin in the white portion of a nerve center, it is observed to lose its sheath, and consist only of an axis cylinder and investing medullary substance; the white portion of the center being composed of these simply medullated, non-sheathed fibers. When a fiber passes from these white portions into the gray substance it loses its medullary investment, and passes among the cells simply in the form of an axis cylinder; and although it is certain that many fibers are prolongations of the poles of the cells, it is not clearly made out that this is always the case.