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for managing the business? But note this fact: while a great many things can be done without the knowledge of the president, a good organization has ways by which an occurrence, even though small, which needs the attention of the president can get to him at once.
Do you realize that some such organization as this exists in the body? At the head of the nerve system is the brain, which is especially the organ of the mind; but the body does so many things that need not go to the mind, that our minds never know of them at all. But if anything unusual happens, or something goes wrong, and the mind needs to know of it, the message can go directly to the highest part of the brain, just as a request made at the door of the bank can, if necessary, go directly to the president himself.
397. What is the Brain Like? We have already learned of the hard case, or skull, which protects the
brain (see $358). If Cerebrum
you could look at the brain inside your skull (Fig. 283), you would see a rounded mass of grayish material having in it a number of folds.
There is a deep dent Spinal
from front to back. This Fig. 283. — Side view of the brain and upper, rounded part of upper end of the spinal cord.
the brain is called the cerebrum (ser'ē-brům). Back of the cerebrum, and below it, at what we call the" base of the brain," is the cere
“ bellum (sěr'ē-běl'ům), or "little brain.” In front of the cerebellum, and below it, like a large knob at the top of
the spinal cord, is the bulb. Another part of the brain, in front of the cerebellum, is called the pons, or“ bridge.” This connects the other three parts of the brain. When a man is full grown his brain weighs from 3 to 4 pounds.
Why is the cerebrum gray? Because of the nerve cells on its surface (see § 395). It has been estimated that there are over 1,000,000,000 of them in an adult person. The inside of the cerebrum is white, because of the nerve fibers it contains (see $ 395). Through the cerebrum we get the sensations of heat, light, sound, touch, taste, and the like, and are able to use our muscles when we wish. Nerves like the nerve of sight (see § 252), and the nerve of hearing (see § 266), go directly from the organ (that is, the eye and ear, respectively) to the cerebrum and back to the organ, without passing through the other parts of the brain or the spinal cord. A healthy cerebrum is necessary for intelligence and judgment. Without it we should be likely to starve, even if food were set before us, for we should not know enough to eat.
The cerebellum has special control of our movements. It makes the muscles contract properly and in pairs, so that we can move in an orderly way.
398. What is the Spinal Cord Like ? - What is the backbone, or spinal column, for (review § 358)? It is for the attachment of the rest of the body's framework, is it not? But it has also the duty of protecting the spinal cord, which is inside it. In an adult the cord passes through the upper 20 of the 24 vertebræ of the spinal column. At its top, the cord is enlarged to form the bulb, which is inside the skull, and is a part of the brain. The lower end of the cord branches out into large nerves which go to the hips and lower limbs.
The spinal cord is white on the outside, from the covering of the nerve fibers, and gray on the inside. Is
this true of the cerebrum ? A cutting across the cord would look like capital H.
399. How are the Internal Organs Controlled ? — How does the stomach get gastric juice when we begin to eat? Does the juice just flow of its own accord, or must it be ordered to flow? We have learned that the body is organized, and that its parts are directed through the nervous system. The glands that secrete gastric juice must get their orders from some nerve center, such as the spinal cord or bulb, before they can go to work. But the mind need not order the flow of gastric juice, for this can be performed by the lower parts of the nerve system just as well. So the message is turned back in the spinal cord or bulb, and the gastric glands get word to secrete the needed juice. But suppose the stomach is injured. Then the messages it sends along the nerves are so unusual that they go right on, past the spinal cord and bulb, to the cerebrum, and we feel pain.
We call the brain and spinal cord, with the great nerve groups directly connected with them, the central nervous system. The brain and cord act like “ Central ” of a telephone system. Whenever one organ wishes another to perform its function, it sends the message to the central system. The proper part of this system sends back the message to the muscles or glands that are to do the thing required.
In addition to the central nervous system, the body has a branch which is called the sympathetic system, and which aids the central system in controlling the important organs of digestion and respiration. It also controls the skin.
400. What is Voluntary Action ? - What does voluntary action mean? It means action that we will to do, does it not? When we see an apple hanging from a tree, and decide that we want it, the cerebrum directs the muscles of the arm to get it. If this is not enough, the cerebrum directs other muscles to act, so that we stand on tiptoe and stretch the body to its greatest height, in our effort to get the apple. The muscles that can act in this way are called voluntary muscles. We cannot stop the muscles in the esophagus from pushing food down into the stomach, nor can we control the muscles of the stomach or the heart by willing to do so; these are involuntary muscles, and the movements they produce are involuntary movements. Involuntary action is also called reflex action.
What causes blushing? Can you control the rushing of blood to your face, so that you will not blush? The skin and the blood vessels that are in it (see $ 390) are partly under control of the sympathetic system (see § 399). Suppose someone pays you an unexpected compliment (or insult). The unusual act so upsets you that your sympathetic system loses control of the blood vessels, and the blood rushes into them, until the skin is red and hot. But in a moment the blood vessels are again under control, and grow smaller, so that only the usual amount of blood can go into the skin.
A good illustration of involuntary, or reflex, action is seen when you unexpectedly bring your finger into a flame, or against a hot, stove. You jerk the finger away before you feel pain. Why is this true? In order that your mind may know that an object is too hot, the message of pain must get to the cerebrum; but the lower part of the nervous system gets word that something is wrong before the brain does, and sends the message to the muscles of your arm to contract. If a fly alights upon a dog that is asleep, does he not try to remove the intruder even in his sleep? Is this voluntary, or reflex, action?
401. Why are So Many Actions Reflex?— Why is it that when we have a cerebrum, through which the mind can work, we do so many things without the order of the brain? All the organs of digestion, respiration, and circulation carry on their usual acts without our knowing it. These are acts inside the body. But there are a great many outside actions that are reflex, too. Do you have to make your eyelids wink? No, this is an act that does not usually need the will. In fact it is hard for you to keep them from winking. The eyes need cleaning, and we might forget to clean them, if the operation were left to our cerebrum to attend to.
You can see why so many acts are reflex. It is so that the brain may be left free from the ordinary duties of the body, and may have time for the unusual work which no other part of the nerve system can do. Learning, thinking, remembering belong to the cerebrum alone.
Are there any acts which once required the full effort of your mind, but which you now do without thinking ? Of course there are. How about walking, skating, writing, which were so hard to learn? Or turning down a certain street to your house? After you have done these a great many times, you do them almost, or quite, without thinking. In fact, when you are changed from one room to another in school, you turn involuntarily,
catch” yourself, into the old room the first