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day or two after the change. Watch your acts, and see how many of them are really reflex. These reflex acts that you learned to do, are called acquired reflexes. What does this mean? We also call them habits.
402. Can We Make and Break Habits ? — You can see that those things we learn to do so well that they become habits with us, like the reflex actions that are entirely natural, are a great saving of effort, and spare the cerebrum an enormous amount of work. It is thus left free to learn new things. If we " set our mind” to do an act, we can usually master it, until it becomes almost “second nature,” or a habit. So if we have will power, we can choose what our habits shall be. Do you see the danger there is in this freedom to choose? We can make bad habits, too, can we not, and so break up our good ones? Bad habits, like good ones, become easier and easier; if we do not break them, they master us. It is in this
that man can do so much for his success or failure. Having the will always to make a good habit to take the place of a bad one, will help us to forget the bad one, and to be free from it.
403. Why Should We Control Ourselves? - How we admire a person who has perfect control of his body; whose eye is clear, whose arm is steady, whose movement is sure, whether on the athletic field, or in running a machine, in sailing a yacht, or in knowing just what to do in an accident ! Do we not really admire even more a person who has control over his temper, who looks on the bright side of things, who does not go into tantrums, who does not give up a hard task without a struggle, who thinks twice before he speaks, who is considerate of persons older and weaker than himself ?
Do the great athletes have the control over their bodies from their childhood, or must they learn it? You know how hard men work to train themselves to be “ stars
in athletics. Has the person you recognize as a great musician risen to his position without effort ? No, it is
necessary for such a person to work hard to get his wonderful power. So it is with good nerve habits. They are not always easy to learn. Are they not worth the effort ?
404. What are Our Senses ? — How do we know about the world around us? It is through some of our senses, is it not? We have eyes to recognize light and its effect upon objects, ears to be set in vibration by the air movements which make up our sounds, delicate touch cells in the skin, to enable the mind to judge about the
shape, size, temperature, and weight of objects, and along the two channels by which air and food enter the body we have the sentinels of smell and taste, to help us to distinguish the good from the bad, and so to protect the lungs and the digestive tract from harm.
We have already learned about the
wonderful organs of sight (see § 252) and Fig. 284.- A of hearing (see § 266). The sense of touch corpuscle in
touch is in the skin and the tongue. the skin, and the nerve fibers which Small elevations contain touch corpuscles
(Fig. 284), which in their turn contain ends of the nerve fibers. When we touch an object, the change in pressure excites the nerve fiber to carry a mes
sage to the cerebrum. We have to decide by our judgment whether the object is smooth or rough, large or small, hot or cold, hard or soft, according to what the cerebrum learns from the nerve cells. Watch a blind person, and see how carefully he examines a new object. Do our eyes help our touch corpuscles, if we look at the object, as well as feel of it?
Is the sense of touch equally good everywhere ? By no means. Some regions of the body have many more touch corpuscles in a given area than others. One way to test how sensitive a given region is, is to use a pair of dividers, or two pins stuck through a piece of cardboard. The tongue can feel that there are two points, when they are only is of an inch apart; the finger tips, when they are in inch apart. The touch corpuscles of the back, however, report to the brain that there is only one point there, even if the two points are 11 or 2 inches apart.
405. How Do We Smell and Taste ? — In what con
? dition must a substance be, in order that we can smell it? You can realize that it must
Nasal Cavity be a gas, and must pass into the nose along with the air we inhale. We call the cells having the sensitive ends of Nasal Cavity
Olfactory the nerves of smell, the olfac
Fig. 285. The cells of smell, contory (vl-făk'tő-rỉ) cells (Fig. necting the nasal cavity with the
brain. 285). You know how acute the sense of smell of dogs is; in cats this sense is much less developed. Wild animals and primitive men can easily recognize friends or foes at a distance, if the wind blows the odor toward them. Civilized man has largely lost the power to use this sense, except as he smells something very pleasant or very disagreeable.
Taste is the result of messages carried to the cerebrum by nerves in the tongue (Fig. 286) and back of the mouth. In what condition must a substance be, in order that it may have a taste? It must be in a liquid condition. If
it is not liquid when we take it into the mouth, the saliva dissolves it. If the saliva cannot dissolve a substance, it has no taste. What is the taste of sand? Of a silver coin?
The taste cells can recognize sweet and sour tastes, salty and bitter tastes, and combinations of these. If you hold your nose, and then taste of some
strong food, such as cheese or bacon, Fig. 286. The tongue with the struc
the taste will seem unnatural; because tures containing the what we call taste is often a combination ends of the nerves of
Of what other of taste and smell. In fact, some very sense is the tongue an
· strong kinds of cheese are said to be organ?
pleasant to taste, if we do not smell them at the same time. We can develop the sense of taste greatly by practice. Did you know that the quality of a food, coffee, or tea, is often decided by a professional "taster," who has trained his taste cells for the purpose ?
406. Exercises. 1. Why does your mouth “water” when you smell cooking food ?
2. Suppose an American baby were brought up in a family speaking only French, what language would it speak? Why?
3. When an accident occurs, and splinters fly toward the eye, we
find that the eyelids close before the splinter reaches them. Is this voluntary action, or reflex action?
4. When a grown person who spoke incorrect English in his youth becomes excited, he often forgets his correct English, and speaks incorrectly. How do you account for this?
5. Why is it that you can recall a fact you thought you had forgotten?
6. What ways have you found successful in controlling your temper?
7. Why can you learn a lesson by going over it several times ?
8. How is the nervous system like a telephone system? How unlike it?
9. The image formed on the retina is inverted (see § 246); how do we know that the object is right side up?
10. Telephone poles in a row seem to grow shorter the farther they are away; how do we know they are of the same height?
11. Why does a spark on the end of a stick which is being turned rapidly in a circle look like a ring of light?
Summary. — The parts of the body work together because we have a nervous system to organize the body's work.
Nerve cells consist of cell bodies and nerve fibers.
The brain consists of a large number of nerve cells. Its chief parts are called cerebrum, cerebellum, bulb, and
pons. The spinal cord is made up of many nerve cells protected by the backbone.
The sympathetic system assists the central system.
Voluntary acts, which have become involuntary by being performed often, are called habits.
Habits leave the cerebrum free to learn new things. They can be made, or broken, by the will.
Our senses are hearing, seeing, feeling, smelling, tasting.
Our judgment must always decide what the messages from the sense organs mean.