A reflex action, in its simplest conception, is a movement (muscular, vascular, or psycho-motor) or a secretion which is the result of the transformation of a centripetal impression into a centrifugal impulse. The apparatus required for the accomplishment of the reflex action consists essentially of a sensitive surface of skin or mucous membrane, of tendinous or other deep structure, to receive the impression; a sensory afferent nerve to convey it; a nervous centre of the simplest structure (one or two ganglion cells) to receive and transform it; a motor or efferent nerve to transmit the resulting impulse to a common muscle, to the vascular wall of a vessel or viscus, or to a gland. These various terminal organs execute their normal functions in response to the centrifugal impulse, and thus give external or tangible evidence that the reflex action or reflex is completed. This mechanism, the reflex arc, is illustrated by the diagram (Fig. 1), which will also serve for the explanation of many of the morbid reflex states. [Illustration: FIG. 1. Diagram illustrating the Arc for Reflex Action. The centripetal and centrifugal paths, the receptive and terminal organs of the arc, are shown connected with the spinal centre, which is a portion of central gray matter containing ganglion cells. The inhibitory cerebral influence is constantly exerted in health to moderate reflexes.] In the healthy animal body many important functions are performed under the law of reflex action; for example, many organic or secretory functions, the movements of hollow viscera, vascular movements, muscular tonus, defensive and expressive muscular movements. In the process of education many acts which at first must be learned by oft-repeated efforts, such as walking, playing upon musical instruments, etc., later become habitual and automatic, and are then largely executed by reflex {51} action unconsciously, co-ordinated by the spinal (?) apparatus. Much of our intellectual activity is governed by the same law operating between the psycho-sensory and the psycho-motor apparatus. Reflex actions are only to a moderate extent under the control of volition—indeed, most of them are subconscious—but a strong restraining action is unconsciously exerted by the cerebrum (vide Fig. 1) upon the spinal reflex centres. This is the so-called inhibitory cerebral influence. The existence of such an influence is shown by experiments upon animals, and by human cases in which, the cerebral activity being reduced or abolished, we observe a great increase in the frequency and extent of reflex movements, particularly in the common muscular apparatus. Clinically, we distinguish cutaneous or superficial, tendinous or deep reflexes, according to the seat of the original excitation or testing-point. Cerebral or psychic reflexes are, however, also to be borne in mind. These reflexes may be abolished or increased.