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360. What is the Use of Tendons ? — Clench your right fist, and with your left hand feel the drawn-up muscles of your right forearm. Also feel the tough cords on the inside of your wrist. Are these muscles? Then feel the cords on the inside of your elbow joint. Are these muscles, too?
Suppose that all the muscles which move the hand (there are about 30) were in the hand itself. What a clumsy affair the hand would be, and how unable to do fine, delicate work! But nature has put into the forearm the strong muscles that move the hand; in the forearm their thickness does not matter. What passes into the hand is not these large muscles, but long, slender, very strong extensions of them; thus the hand may remain small and skillful. The extensions of the muscles are called tendons. They are the extensions of the connective tissue (see $$ 195 and 359) of the muscles, and attach a muscle to the bone it is to move, even when the bone is some distance away. There are also many strong tendons passing from the lower limbs through the ankles; these tendons move the bones of the feet.
How can tendons be kept in place at such important joints as those of the wrist and ankle? Encircle your right wrist with the thumb and forefinger of your left hand, as if your left hand were a bracelet. There are “bracelets,” or bands, of tough, strong tissue under the skin. These are called ligaments. They keep the tendons at the wrist and the ankle from slipping out of place. Usually they do their work properly, but sometimes they are strained or torn.
361. Is Care of the Body Worth While ? - Does it matter what position you take when you walk, stand, or
sit? Why do some people
gracefully, while others are so awkward in their movements ? Part of the difference, at least, comes from the way in which people learn to use their bones and muscles. A correct carriage of the body is a matter of health as well as of looks. If the body is not carried erect, round shoulders are the result, and the chest is likely to be too small for the lungs and heart inside it. Slumping down into a chair throws the organs of the abdomen out of place. Our bodies are beautifully built machines, but and they are delicate, grow easily in any position in which we hold them. We should help them to become erect and graceful by carrying ourselves correctly (see § 262).
Have we any duty to our muscles? Two things are absolutely necessary for a healthful body; they are rest and exercise in the proper proportions. If the body is not allowed to rest long enough, and if the hours of sleep are too short, the whole body becomes worn out. The muscles become sore, and one feels as if he were not strong enough to walk briskly, or even to hold his body erect when he is sitting in a chair. Just as important as rest, however, is exercise. Exercise is best when it is carried on out of doors. No amount of indoor work can take its place. Why? The clothing should always be sufficient for protection in stormy weather. Brisk exercise is needed every day, and the person who neglects it will suffer by having weak muscles, a poor digestion, or some other form of bodily weakness or discomfort.
362. Exercises. 1. Between the vertebræ (věr'tē-brē), or separate bones of the backbone, there are pads of cartilage. How will they affect the jarring of the body in walking? The ability of the back to bend?
2. Do you think you are taller at night, or in the morning? Why?
3. From the way in which you can move your fingers and thumb, what kinds of joints do you think they have?
4. How has the shape and structure of man's hands made his advancement possible ?
5. What is the effect of high heels upon the way in which the wearer walks?
6. What is meant by the division of labor? How is the body like a village of people?
7. Name some of the kinds of cells in the body, and the part they take in keeping the body alive.
8. Of what advantage is it to man that he can walk upright?
Summary. Vertebrates are animals with an interior bony framework for the support of the body.
The cell is the unit of living matter. It consists of protoplasm.
When the cells of a many-celled animal divide, they remain attached, and the animal grows in size.
A group of cells of the same kind, and doing one kind of work, is called a tissue.
Bones consist mainly of connective tissue and of mineral matter.
A child's bones become stiff because the bone cells secrete mineral matter from its food.
The joints of the body are flexible and inflexible joints.
The three kinds of flexible joints are hinge, ball-and-socket, and gliding joints.
The bones of the head and trunk are chiefly for protection; those of the limbs for movement.
Muscles are organs that have the duty of producing movement in the parts of the body. They do their work because their cells have the power to contract and then to relax.
Tendons are extensions of the connective tissue of muscles.
Ligaments are bands and cords that hold tendons and bones in place.
The body needs proper amounts of exercise and rest.
HOW FOOD IS DIGESTED
363. What Organs Digest Our Food ? - If someone
were to ask you what organs digest food, would you answer : “ The stomach”? Do you realize that the organs of digestion form a long tube about 30 feet long, and that the stomach does only a part of the work? This long tube is called the digestive tract. When food is taken into the mouth, it is not in a condition to be used by the body. The digestive tract consists of organs that act upon the foods and change them, so that they can be used. As the food is mostly insoluble, it has to be put into such form that it can dissolve. The digestive tract is lined throughout with
269. — A a continuation of the skin which covers simple gland is made
by fold in the the outside of the body. This interior .
membrane. skin is called mucous (mūk'ŭs) mem
In a complex gland
the many folds pour brane. The mucous membrane begins their secretion out with the lips, where it shows a thin pink through one tube, or covering. The network of blood vessels under it causes the color. The surface of the mucous membrane is made up of flat cells. In many places these cells are folded in; thus glands are formed (Fig. 269).
The glands take certain liquids from the blood, and store them in pockets from which they are poured out upon the foods that pass by. We say that the glands secrete these liquids. The liquid secreted is called a secretion (sē-krē'shŭn).
364. How Does the Mouth Help Digestion ? — The mouth is the first part of the digestive tract. The nerves of taste are located on the tongue, and the nerves of smell are just above the mouth, in the nose; so that the sentinels of taste and smell can pass upon the food, and tell us whether we ought to eat it or not. Many people gauge the amount of food to be eaten by its attractiveness. Such habits often result in overeating. Overeating gives the digestive tract too much to do, and puts it out of order. The tongue, besides containing the nerves of taste, is of such a muscular structure that it is very skillful in pushing the food about in the mouth. After the teeth have ground the food thoroughly, the tongue pushes it back into the mouth, so that it may be swallowed.
When you are hungry and smell cooking food, your mouth “waters." The water in the mouth is called saliva (să-li'vă). Three pairs of glands in the lining of the mouth secrete this liquid. Besides softening and moistening the food, the saliva acts upon it to change its nature. Saliva is like a base (see § 177), and has an alkaline reaction. What does this mean? It also contains a substance called ptyalin (ty'ă-lin), which is a ferment and simply by its presence brings about a partial change of the starch into sugar. Hence a cracker becomes sweet when chewed. Try it.
365. What is the work of the Teeth? --- How many teeth have you? When you are a grown-up," or an