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 municate with every other subscriber, would need one thousand wires running into his house; all together, there would have to be several hundred thousand (to be exact, 499,500) wires. With a central office only one thousand are needed. As a telephone system has central offices, so the nervous system has nerve centers. Nerve centers contain nerve cells. Although there are some subordinate nerve centers in the spinal cord, the greatest collection of nerve centers in our bodies is in the skull, and is called the brain. Fishes were the lowest animals studied in animal biology found to possess a true brain.

The nervous system, unlike a telephone system, has other duties besides allowing communication. It enables us to think, and, after reflection, to will and to act by controlling the various organs.

—Showing a , A, or nerve cell with all its parts—dendrites, cell body, and axon; B, a portion of a white fiber highly magnified. (Jegi.)

The Units of which the Nervous System is Constructed.—A nerve cell with all its branches, or fibers, is called a neuron (see Fig. 103); some neuron branches are several feet long. Neurons are the units that compose the nervous system. The living substance in cells is