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• bellum. The cerebrum covers the cerebellum. (Fig.

110.) Is this true of the monkey's brain? (See Fig. 113.)

The work of the medulla is chiefly to control the vital functions (see Figs. 110, 114). Here are located the centers for regulating the breathing, the heart beat, the size of the blood vessels (thus regulating nutrition), and also the less important centers that control swallowing, secretion of saliva, and vomiting. The center for breathing is sometimes called the vital knot, because although the cerebrum and cerebellum may be removed from an animal without causing immediate death, the slightest injury to the vital knot kills the animal at once. In cases of hanging, death is caused by injury to this center.

Fig. 110.—The Brain (cerebrum, cerebellum, medulla).

Fig. 111.—Association Fibers, connecting cells within the cerebrum. (Jegi.)

Automatic Action.—The center called the vital knot is said to regulate the breathing automatically, not reflexly. Reflex acts start in the skin; automatic acts start in the interior of the body. The condition of the blood regulates the breathing automatically during sleep, and partly regulates it during waking. If too much carbon dioxid accumulates in the blood this excites the vital knot, which sends out stronger impulses to the respiratory muscles. Deeper breathing follows, which purifies the blood, and the breathing is then