Reflex Action, Consciousness, and Will.—Usually not all of the force of the impulse is transferred to the motor cell. The sensory cell by means of another of its many branches may transfer part of the impulse to a cell which sends it to the brain. Hence a reflex act is not necessarily an unconscious one. If you unintentionally touch the hand to a hot stove pipe, you may be conscious of the pain and the involuntary jerking away of the hand at the same time.
Reflex Action and the Will.—The will may inhibit, or prevent, an expected reflex act. Yet many reflex acts occur in spite of the effort of the will to prevent them. One cannot always keep from closing the eyes before a threatened blow even if from the other side of a plate glass window, and it is known there is no danger. Sneezing is a reflex act and cannot always be prevented. The forming of saliva and other secretions are reflex acts. Reflex acts are quicker than voluntary acts. An eighth of a second is about the time required for a person to press an electric button after seeing a signal; a reflex act may occur in a shorter time.
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Fig. 109.—Brain and Spinal Cord.
The Brain consists of Three Chief Parts.—(1) There is an enlargement at the top of the spinal cord called the medulla, or the medulla oblongata. It may be regarded as the part of the spinal cord within the skull (see Figs. 109, 110, 114). (2) Above the medulla is the cerebellum, or little brain. (3) The cerebrum, or large brain, fills all the skull except the small part occupied by the medulla and cere-
