Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/915

Rh SYSTEM.] ANA T M Y 861 cell Si, straight nerve fibre; Sp, spiral nerve fibre ; C, capsule of connective tissue around nerve cell. (After Scale.) of the surface of the body a strong process is given off, which is directly continued into a nerve fibre. The axial cylinder of the fibre is continuous with the cell substance, and Schultze has shown that both exhibit a delicate fibril- lated structure. The medullary sheath and the primitive membrane are also usually continued from the fibre over the nerve cell Hence these bipolar cells seem to be, as Schultze expressed it, nucleated enlargements of the axial cylinder. A remarkable modification of the bipolar nerve cell, carefully studied and described by Lionel Beale, is found in the sympathetic ganglia of the frog. The cells are pear-shaped, and from the narrow end of the pear two nerve fibres arise, one of which, called the straight fibre, forms, as it were, the stalk of the pear; whilst the other, or spiral fibre, winds spirally round the straight fibre, and then passes away from the cell in the opposite direction. Both fibres are nucleated, and at their origin consist, apparently, of axial cylinder substance only; but in their course they may acquire both a medullary sheath and a primitive membrane. The straight fibre passes into the interior of the cell sub stance, and Arnold and Courvoisier be lieve that they have traced it into the nucleus ; but the spiral fibre apparently arises nearer the periphery of the cell. The pyriform cells are invested by a dis tinct capsule of connective tissue. The nerve fibres of these pyriform cells, although they both arise close together from one end of the cell, represent its poles. Should one of the poles, either in this, or in the bipolar form of nerve cell described in the preceding paragraph, be from any cause removed or not developed, then the cell would be unipolar ; and if both poles were absent it would be apolar. In other localities, as in the sympathetic ganglia of man and many other vertebrates, and in the several subdivisions of the cerebro-spinal nervous axis, the nerve cells have more than two poles or processes projecting from them. Cells of this kind are called multipolar, and in many localities they present characteristic forms. In the grey matter of the spinal cord, more especially in its anterior horn, they give rise to numerous processes, and have a stellate or radiate form. In the grey matter on the surface of the convolutions of the cerebrum they are pyramidal in shape. The apex is directed to the surface of the convolution, the base towards the white matter. The processes arise from the base, apex, and sides of the pyramid. In the grey matter on the surface of the cerebellum the body of the cell is almost globular. From that aspect of the cell which is directed towards the white matter FIG. eo. Multipolar ceil a slender central process arises; from the [h c hu ^ a T H S n mi cf opposite or peripheral aspect of the cell capsule of connective two strong, many-branched processes ex- tlssue tend for a considerable distance. In the human sympa thetic ganglia, again, the stellate form of cell prevails, and the existence of a capsule of connective tissue around the individual cells can be recognised. The processes which arise from a multipolar nerve cell, as a rule, divide and subdivide as they pass away from the body of the cell, until at last they give rise to branches of extreme tenuity. These branching processes apparently consist exclusively of cell protoplasm, and have been called protoplasm processes. Gerlach has described the prutoplasm processes of the multi-polar nerve cells of the brain and spinal cord as forming an excessively minute network, from which minute medullatecl nerve fibres arise; and F. Boll conceives that a simi lar arrangement occurs in the cells of the cerebellum. One, at least, of the pro cesses of a multipolar nerve cell does not branch, but becomes directly con tinuous with a nerve fibre, and has been named the axial - cylinder process. This process was first re cognised byDeiters in the cells of the spinal cord ; but Hadlich and Kosch- FIG. 61. Multipolar cell from the grey matter ro of anterior cornu in the spinal cord. AC, ae- non-branchedaxial-cylmderprocessdlrcctly Scribed the Central process continuous with a nerve fibre. of the cells of the cerebellum as continuous with a medullated nerve fibre; and the latter observer has pointed out, that from the base of a pyramidal nerve cell in a cerebral con volution a process may be traced directly into a nerve fibre, Hence it would appear that the multipolar nerve cells may have two modes of union with nerve fibres one directly through the passage of the non-branched axial-cylinder pro cess into a fibre, the other through the origin of fibres from the minute network in which the branched protoplasm processes terminate. The branched processes of adjacent nerve cells may also blend with each other, so as to form an anastomosing cell network, though these anastomoses are, in all probability, not so frequent as was at one time supposed. Schultze has pointed out that not only the protoplasm substance of the body of a multipolar nerve cell, but both the non-branched and branched processes, possess a fibrillated structure similar to that described by him in the axial cylinder of the nerve fibres. Peripheral End-Organs or End Bodies. Nerve fibres at their peripheral extremities terminate in connection with peculiar structures, named end-bodies, terminal bodies, or peri pheral end-organs,v?}iich are situated in the several organs of the body. The motor nerves end in the voluntary and in voluntary muscles; the vaso-motor nerves end in the mus cular coat of the blood-vessels; the sensory nerves end in the skin, mucous membranes, and organs of special sense; and it has been stated that secretory nerves terminate in connection with the ultimate cell elements of the secreting glands. These end-organs possess certain structural pecu liarities, which are by no means uniform in the different parts, so that the end-body connected with the peripheral termination of a nerve is distinctive of the organ in which it is situated. It will be a matter of convenience to defer the consideration of the peripheral end-bodies in the skin, organs of special sense, coats of the blood-vessels, and the several glands, until these parts are described. In this place the mode of termination of the motor nerves in the voluntary and involuntary muscles, of the sensory nerves in the mucous membranes, and of the ending of the nerves in the remarkable bodies named Pacinian corpuscles, will alone be examined. After a nerve has entered a voluntary muscle it ramifies in the connective tissue, which lies between the fasciculi, and at the same time divides and subdivides into smaller branches. These branches interlace with each other and form plexuses, from which slender nervous twigs, often consisting of only a single medullated nerve fibre, pro ceed, which ramify in the connective tissue, separating the individual muscular fibres from, each other. The single