called protoplasm. The protoplasm in nerve cells possesses the most marvelous and varied powers of any known substance, for the nerve cells are the seat of the mind.
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Fig. 104.—Large Nerve Trunk, such as supplies the muscles. Cross-section (magnified 6 diameters), showing bundles of nerve fibers. (Peabody.)
Nerve Cells and Fibers.—The many branches of nerve cells make them the most remarkable of all cells for irregularity in shape. Since the protoplasm of the cell continues into the fibers, it is plainly wrong to consider the nerve cell as something apart from its fibers. It is not a complete cell without them. A cell usually has many short branches called dendrons or dendrites (see Fig. 103) for communicating with near-by cells, and one long branch called an axōn (Fig. 103) for communicating with distant parts. The axons form the fibers that go to the skin, muscles, and other organs.
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Fig. 105.—c, a white fiber with its fatty sheath (dark); d, two gray fibers (without sheath).
A Nerve.—These long branches, or axons, of nerve cells go all over the body and are often bound together into visible cords called nerves, or nerve trunks (Fig. 104).
White and Gray Fibers (Fig. 105).—Some fibers have a fatty covering surrounding the thread of protoplasm; they are white and glistening, and are called white fibers. Others are without this fatty
