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 covering, and are called gray fibers. Both kinds of fibers have connective tissue on the outside to strengthen them. If we let a lead pencil represent a white fiber, the lead corresponds to the axis of protoplasm; the wood corresponds to the white, shiny fat that surrounds it; and the varnish corresponds to connective tissue on the surface of the fiber. A number of white fibers together makes a white mass that is called white matter. The axis of a white fiber, of course, is not white. A mass of cells or of gray fibers is called gray matter. The oxidation of the gray matter, or protoplasm, in neurons gives rise to nerve energy.

Feeling Cells and Working Cells.—Nerve cells are divided into two classes: sensory cells, which feel or receive impressions; and motor cells, which send out impressions to the working organs. Those fibers which carry impressions to the receiving cells are called sensory fibers; those which carry impulses from the cells to the working organs are called motor fibers.

Ganglia and Nerve Centers.—Nerve cells are not scattered uniformly in nervous tissue, but are gathered into groups. A group of nerve cells is called a ganglion (Fig. 106). One or more ganglia having a single function, such as to control the muscles of breathing, form what is called a nerve center. The brain consists of a number of nerve centers with their connecting fibers.

Gross Structure of the Spinal Cord.—The nerve fibers from nearly all over the body lead to cells situated in a large cord in the spinal column called the spinal cord. The spinal cord is separated by a deep fissure almost into halves (Fig. 107). The cells