Illustration: Fig. 113.—Non-Medullated Fibers. Two nerve fibers, showing the nodes or constrictions of Ranvier and the axis cylinder. The medullary sheath has been dissolved away. The deeply stained oblong nuclei indicate the nerve corpuscles within the neurilemma. 264. The Nervous System Compared to a Telegraphic System. In men and other highly organized animals, nerves are found in nearly every tissue and organ of the body. They penetrate the most minute muscular fibers; they are closely connected with the cells of the glands, and are found in the coats of even the smallest blood-vessels. They are among the chief factors of the structure of the sense organs, and ramify through the skin. Thus the nervous system is the system of organs through the functions of which we are brought into relation with the world around us. When we hear, our ears are bringing us into relation with the outer world. So sight opens up to us another gateway of knowledge. It will help us the better to understand the complicated functions of the nervous system, if we compare it to a telegraph line. The brain is the main office, and the multitudes of nerve fibers branching off to all parts of the body are the wires. By means of these, nerve messages are constantly being sent to the brain to inform it of what is going on in various parts of the body, and asking what is to be done in each case. The brain, on receiving the intelligence, at once sends back the required instructions. Countless messages are sent to and fro with unerring accuracy and marvelous rapidity. Thus, when we accidentally pick up something hot, it is instantly dropped. A nerve impulse passes from the nerves of touch in the fingers to the brain, which at once hurries off its order along another set of nerves for the hand to drop the burning object. These examples, so common in daily life, may be multiplied to any extent. Almost every voluntary act we perform is executed under the direction of the nervous system, although the time occupied is so small that it is beyond our power to estimate it. The very frequency with which the nerves act tends to make us forget their beneficent work. 265. Divisions of the Nervous System. This system in man consists of two great divisions. The first is the great nerve center of the body, the cerebro-spinal system, which rules the organs of animal life. This includes the brain, the spinal cord, and the cerebro-spinal nerves. Nerves are given off from the brain and the cord, and form the mediums of communication between the external parts of the body, the muscles or the sense organs, and the brain.