According as muscular movements are, or are not, under the influence of the will, they are distinguished as _voluntary_ and _involuntary_. To the latter kind belong the movement of the heart, and the movements of the muscles in the wall of the gut by which the food is made to progress. [Illustration: FIG. 4.—Bending of the Arm by Contraction of the Biceps Muscle. _a_, humerus; _b_, ulna; _c_, elbow-joint; _d_, biceps muscle; _e_, origin; _f_, insertion of the same. In the right-hand figure of the muscle _d_ is contracted; in the left-hand figure it is slackened. ] To destroy the contractile power of a muscle it is not necessary to injure the muscle itself. Every muscle is related to a nerve, which sends its fine branches to the fibres making up the muscle. If we cut the nerve, the corresponding muscle loses its power of contraction. But the nerve arises from the _central nervous system_, which in vertebrates principally consists of the brain and spinal cord. The muscle will therefore lose its contractile power if the connection with these central parts is broken. The true cause of movement resides in these parts. A sort of change, the essential nature of which is unknown to us, takes place in them, and is propagated along the nerve to the muscle, causing it to contract. The central nervous system is, therefore, the origin, the centre from which the order to contract proceeds; hence its name. The nerves which run from these central parts to the muscles are known as the _nerves of movement_ (motor nerves). [Illustration: FIG. 5.—Diagram to explain the Action of the Motor and Sensory Nerves. ] There is still, however, a second group of nerves, the _nerves of sensation_ (sensory nerves), which arise in the sense-organs (skin, mucous membrane of tongue, nose, ear, eye), and convey to the central nervous system the impressions they receive from the outer world by the aid of these sense-organs. In the appended diagram (Fig. 5), _C_ represents the central nervous system; _B.N._, a motor nerve, branching in the muscle _M_; _G.N._, a sensory nerve, which runs from the blood-bathed inner skin or dermis (_L.h._), underlying the outer skin or epidermis (_O.h._), to the central system. (The arrows indicate the direction in which impulses are conveyed along the corresponding nerve.)