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 shallow or slow until carbon dioxid accumulates again.

The Four Kinds of Nerve Action and the Centers that control them.—The cord controls chiefly reflex action; the medulla controls chiefly automatic action; the cerebellum controls chiefly coördinate, or harmonizing, action; the cerebrum controls the purely voluntary acts, for it is the seat of consciousness and thought. The medulla, like the cord, has the gray matter on the inside (Fig. 109). (Jegi.) Structure of the Cerebellum.—The cerebellum, like the cerebrum, has the gray matter or cells on the outside. The gray matter is folded into furrows that are not nearly so winding as the folds in the cerebrum (see Fig. 115). The fibers going to the surface cells have a branched arrangement called the arbor vitæ, or tree of life, which is shown where the cerebellum is cut. The cerebellum, like the cerebrum, is deeply cleft and thus divided into halves, called hemispheres, connected by a band of white matter. Numerals show location of motor centers. (See Fig. 115.) The work of the cerebellum is to aid the cerebrum in controlling the muscles. It coördinates the muscular move-*