Moreover, most of the acts we perform during a day's work are results of the automatic working of this bodily machine. The heart pumps; the blood circulates its load of food, oxygen, and wastes; the movements of breathing are performed; the thousand and one complicated acts that go on every day within the body are seemingly undirected.
The central nervous system.
Automatic Activity.—In addition to this, numbers of other of our daily acts are not thought about. If we are well-regulated body machines, we get up in the morning, automatically wash, clean our teeth, dress, go to the toilet, get our breakfast, walk to school, even perform such complicated processes as that of writing, without thinking about or directing the machine. In these respects we have become creatures of habit. Certain acts which once we might have learned consciously, have become automatic.
But once at school, if we are really making good in our work in the classroom, we begin a higher control of our bodily functions. Automatic control acts no longer, and sensation is not the only guide—for we now begin to make conscious choice; we weigh this matter against another,—in short, we think.
Parts of the Nervous System.—This wonderful self-directive apparatus placed within us, which is in part under control of our will, is known as the nervous system. In the vertebrate animals, including man, it consists of two divisions. One includes the brain, spinal cord, the cranial and spinal nerves, which together make up the cerebro-spinal nervous system. The other division is called the sympathetic nervous system and has to do with those bodily functions which are beyond our control. Every group of cells in the body that has work to do (excepting the floating cells of the blood) is directly influenced by these nerves. Our bodily comfort is dependent upon their directive work. The organs which put us in touch with our surroundings are naturally at the surface of the body. Small collections of nerve cells, called ganglia, are found in all parts of the body. These nerve centers are connected, to a greater or less degree, with the surface of the body by the nerves, which serve as pathways between the end organs of touch, sight, taste, etc., and the centers in the brain or spinal cord. Thus sensation is obtained.
Sensations and Reactions.—We have already seen that simpler forms of life perform certain acts because certain outside forces acting upon them cause them to react to the stimulus from without. The one-celled animal responds to the presence of food, to heat, to oxygen, to other conditions in its surroundings. An earthworm is repelled by light, is attracted by food. All animals, including man, are put in touch with their surroundings by what we call the organs of sensation. The senses of man, besides those we commonly know as those of sight, hearing, taste, smell, and touch, are those of temperature, pressure, and pain. It is obvious that such organs, if they are to be of use to an animal, must be at the outside of the body. Thus we find eyes and ears in the head, and taste cells in the mouth, while other cells in the nose perceive odors, and still others in the skin are sensitive to heat or cold, pressure or pain.
But this is not all. Strangely enough, we do not see with our eyes or taste with our taste cells. These organs receive the sensations, and by means of a complicated system of greatly elongated cell structures, the message is sent inward, relayed by other elongated cells until the sensory message reaches an inner station, in the central nervous system. We see and hear and smell in our brain. Let us next examine the structure of the nerve cells or neurons part of which serve as pathways for these messages.
Diagram of a neuron or nerve unit.