Nerves have two special functions: one to carry impressions made upon the fibres, that end in the different parts and organs of the body, to the brain; another to carry from the nerve cells so-called "nerve impulses," to the different parts and organs of the body. Some nerves have in themselves these two functions, as the nerves that go to the arm or leg; others have but one, as the optic or eye nerve, which can only carry the sensation of sight from the eye to the brain. The nerves that carry sensations to the brain are called _Sensory Nerves_. The nerves that carry motor impulses from the brain are called _Motor Nerves_. There are five special organs of sense, each receiving different impressions, and sending by its sensory nerve or nerves a different character of sensation to the brain, namely: The eye, giving sensations of light and color. The ear, giving sensations of sound. The nose, giving sensations of smell. The mouth, giving sensations of taste. The skin, giving sensations of touch, with ideas of roughness, smoothness, hardness, softness, heat, and cold. There must be, in every case, a direct nerve connection from the organ of special sense to the special group of cells in the brain to which the nerve goes. If the connection is broken at any point, the impression made upon the fibres in the organ of sense cannot reach the brain. Only after the impression reaches the brain and the cells are affected, do we become conscious of a sensation. We then say, as the case may be, I see, or hear, or smell, or taste, or feel something. It thus appears that these organs of sense simply receive the impressions made upon them to transmit to the brain, and it is really the brain that sees, hears, smells, tastes, and feels. By the action of the organs and nerves of special sense we get all our knowledge of the external world, and, probably, if we had no organs of sense, we would have no consciousness of our existence. Pain is due to abnormal action of sensory nerves, caused by disease, injury, or pressure, and the irritation made, being carried to the brain makes us conscious of the peculiar sensation we call pain. So the want of food or water makes an impression upon nerves, which being carried to the brain causes a peculiar sensation, and we say we feel hungry or thirsty.