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Nerve Cells and Fibres

cell, substance, processes, white, nerve-fibres and rod

NERVE CELLS AND FIBRES.

Nerve-cells have a general resemblance under the microscope to other cells. They vary in size from ,h, to of an inch. They con sist of masses of granular-looking material protoplasm—and contain a nucleus and nucleo lus (see CELLS, p. 53), but have no cell wall. Processes or poles pass from the corpuscle or cell, branching outwards.

There are various kinds of nerve corpuscles, the main differences being in the shape of the cells, and in the number of processes given off from them. Some cells give off two, others many processes. One kind of cell is charac teristic of one part of the nervous system. Thus, in Fig. 80, a and e show the appearances of cells found in the spinal marrow. They are line irregular masses of protoplasm, and give sometimes tripolar, that is, with three poles, though, as we see, they may have four poles. Cells from the lesser brain (cerebellum) are shown at c and g. They are rather oval in shape, and one or two branching processes come off from the small end of the oval. Those with one process are called unipolar, those with two dipolar. So characteristic are these forms of cells of special parts of the nervous system that under the microscope a small piece of spinal marrow could be identified simply by the pres ence in it of a multipolar cell, and a email piece of large brain simply by the presence of a pyramidal cell, and so on.

The processes of cells become important parts of nerve-fibres, as will be seen in considering nerve-fibres. Nerve-cells grow, manifest ac tivity, and decay as do other cells (p. 54).

present the appearances shown in Fig. 81, c and d. They are of glassy trans parency, and have a double outline as repre sented in the figure. They thus resemble tubes. The contents of the tube are of a clear jelly-like character. When stained by colouring agents

nerve-fibres are found to consist of a rod pass ing down the centre, called the axis-cylin der, which is sur rounded on all sides by a white substance, the white substance of Schwann, the whole being inclosed in a delicate sheath (neurilemma). At in tervals gaps occur in the white substance, but the central core is continuous throughout the whole length of the nerve. In a and b, Fig. 81, the gaps in the white substance, which is as if stained black, are shown, and the axis-cylinder is seen crossing the gap. In some fibres (e of Fig. 81) no white substance exists, but only the central rod surrounded by the delicate sheath. A nerve-fibre resembles a wire prepared for conducting electricity, with its central rod of copper and its outer layer of gutta-percha, silk, or cotton, to coat the copper and protect it from contact with other con ductors. The copper rod is the important part of an electrical conductor. Similarly the axis cylinder is the important part of a nerve-fibre, and it is found to be the continuation of a process of a nerve-cell. Thus nerve-cells and nerve-fibres are related in that the process of one is the axis-cylinder and essential part of the other.

A is a group, or several groups, of cells connected together by nerve fibres, and associated together for a common function or purpose.

A Nerve is a cord formed of a bundle, or several btindles, of nerve-fibres, supplied with blood-vessels, and supported and surrounded by connective tissue.

Ganglia is the term applied to distinct and separate little masses of groups of associated nerve-cells with the fibres related to them.