Читать книгу The Body at Work: A Treatise on the Principles of Physiology онлайн

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The most remarkable variations in size are to be found amongst the cells of the nervous system. It may be given as one of the most distinctive characters of nervous tissue that its cells have no fixed or standard dimensions. A nerve-cell enters into connection with other nerve-cells and with muscle-fibres by means of branches, or cell-processes, as they are termed. The cells may be globular, as in the sympathetic system, or star-shaped. Each cell gives off a certain number of processes, which divide like the branches of a tree, and one process which may run for a very long distance without dividing. This latter thread-like process places it in communication either with a distant part of the central nervous system or with the muscle-fibres which it controls. By means of such a thread a cell in the spinal cord may be connected with muscle-fibres of the hand or of the foot. The thread is really a bundle of filaments (neuro-fibrillæ) which separate to supply a number of muscle-fibres. It is, in its whole length, a part of the cell in which it originates. The size of the cell varies as the number of filaments in this bundle (termed the “axon”), and possibly also as their length. Hence it comes about that nerve-cells may be amongst the smallest, or they may be the very largest, in the body. The so-called “granules” of the cortex of the cerebellum and of the cerebrum are almost as small as red blood-corpuscles (Fig. 23). Each of them has five or six minute branched processes and an exceedingly delicate axon. The large cells of the cerebral cortex, which send their axons far down the spinal cord, and the large cells of the spinal cord which supply the muscles of the body, have a diameter ten or twelve times as great as that of a granule. But larger still are the nerve-cells which supply the electric organs of the torpedo and other electric fishes (p. 295); and largest of all are the cells which innervate the curious “fishing-rods” of the strange angler fish (Lophius piscatorius). It is difficult, owing to their irregular shape, to say how large these cells are; but they are visible to the naked eye.