9. Point out and name two kinds of joints.

10. What are ligaments?

11. Of what is a muscle made?

12. How many muscles in the body?

13. How many tendons can you feel in your wrist?

CHAPTER XIX

THE MUSCLES AND HEALTH

=Making the Muscles Strong.=--No persons use all of the five hundred muscles in the body every day. In slow walking only about twenty muscles are used, while in running more than four times that number are called into action. Muscles which are not used get lazy and weak.

Every time a muscle is made to act the blood vessels enlarge and bring to it more blood to supply food. The more food the muscle has the stronger it grows. The right arm is used more than the left in most persons. This makes it so much stronger that some boys can lift twenty-five pounds more with the right arm than they can with the left.

=Using the Muscles keeps the Body Well.=--All muscles must have more blood when they are used so that the heart is made to beat faster and stronger by exercise. In this way its valves and walls become able to do more work. Such a heart not only does its work better in a well person, but is able to keep pumping when the body is weakened by disease. Many persons die because the heart gets too weak to push the blood through the body.

In all the little s.p.a.ces between the muscles and parts of other organs is some watery part of the blood containing much waste given off from the tissues. Moving the muscles presses on this watery waste in such a way as to move it along into the blood channels. It then can be cast out of the body by the lungs and other organs. One reason why we feel so good after exercise is because the poisonous waste has been taken away.

No one can remain well very long without taking exercise. Children as well as older persons should enjoy one or two hours of outdoor play every day.

[Ill.u.s.tration: FIG. 85.--Various ways of exercising the muscles to keep the body well.]

=How to exercise the Muscles.=--Outdoor games give the best form of exercise. Tennis, baseball, cricket, rowing, and swimming are sports which bring nearly all the muscles into use. Every boy and girl should learn to swim. It is dangerous to go swimming alone or to swim in deep water. Cramp may seize the muscles at any time, so that the limbs cannot be moved. Hundreds of persons are drowned every year by venturing in deep water.

Taking care of the yard and garden and helping with other work about the home is one of the best ways of getting exercise and at the same time doing some good.

=Special Kinds of Exercise.=--A room with ropes, swings, and machines in it for exercise is called a _gymnasium_. Under the direction of a teacher the pupils can get quickly just the right kind of exercise to strengthen the weak parts of the body and keep every organ in health.

The muscles oftenest neglected are those of the chest. Every one should keep his chest full and round by swinging the arms and _practicing deep breathing every day_.

=Danger from too much Exercise.=--Lately it has been learned that very violent exercise for more than a few minutes often injures the heart.

The running of many races until you are all out of breath or much jumping of the rope is likely to strain the heart. It is always harmful to urge the body on until it is completely tired out.

=Alcohol makes the Muscles Weak.=--In the year 1903 two learned men in Switzerland spent much time to determine whether alcohol helped persons do more work. They tried more than two hundred experiments with men to whom they sometimes gave wine and sometimes food, and sometimes both were given together.

The results of these tests showed that when wine was given alone, the man's ability to do work was increased for a short time, but later he could not do so much work as when he had taken no wine. When the man took both food and wine, he could do only about nine tenths as much work as when he took food alone.

The most careful tests by other persons show that whisky will not help a man do more work, lift a heavier weight, or shoot straighter. In fact little or much whisky makes him less able to do any of these things.

=Beer makes the Muscles Lazy.=--Doctor Parkes of Netley secured two gangs of soldiers to do the same kind of work. He allowed the first gang to drink some beer, but the second gang were not allowed to have any. During the first hour the beer gang did the most work, but after that the temperance gang did far more work during the entire day. The next week beer was refused the first gang and given to the second. The beer helped the second gang do more work than the first one for nearly two hours, but after that they did much less than the first gang.

This shows that men who wish to do their best work during the entire day should not use beer.

=Tobacco and the Muscles.=--Many experiments and studies have shown that the body cannot do its best work when even very small amounts of poison are taken day after day. The poison in tobacco is believed to weaken the muscles so much that no man on a football team in any of our large colleges or universities is allowed to smoke or chew during the season. Persons training for any contest where much strength is required do not use tobacco in any form.

=Tobacco prevents Growth of the Muscles.=--The moderate use of tobacco by men has but little effect on the muscles. It may cause them to tire a little more easily when doing very hard work. Tobacco poison does, however, show a marked effect on the muscles of the young.

Very careful measurements made at one of the large universities showed that the boys who did not smoke grew one tenth more in weight and one fourth more in height than those using tobacco regularly. This slow growth in tobacco users is partly due to the fact that tobacco makes the muscles in the walls of the blood vessels squeeze together so as to shut off some of the blood from the legs, arms, and other parts, so that they get too little food. Tobacco may also cause less food to be digested for the use of the body.

CHAPTER XX

HOW THE BODY IS GOVERNED

=Making the Parts of the Body Work.=--Each of the hundreds of organs in the body has a certain work to do and this must be done at the right time. In order that all may work together and each one do its part when needed, there is a chief manager called the _brain_ and a helping manager named the _spinal cord_. Millions of tiny threads for sending messages connect the two managers with every part of the body.

These threads form the _nerves_.

=The Brain.=--The brain is a soft bunch of matter filling the inside of the skull. The bones of the skull are a quarter of an inch thick and prevent any common knocks from hurting the brain. It is surrounded by three coverings which also help shield it from injury.

The surface of the brain is very uneven. There are a great many folds separated by grooves. Some of these are more than an inch deep.

[Ill.u.s.tration: FIG. 86.--The under side of the brain and the spinal cord with the chief nerves of one side of the body viewed from in front.]

=Parts of the Brain.=--The brain is divided into three chief parts.

The upper and larger part is called the _big brain_ or _cerebrum_.

The lower part behind is the _little brain_ or _cerebellum_. The part under the little brain and round like the thumb is the _stem_ of the brain. It connects the larger parts of the brain with the spinal cord.

The big brain is partly separated into halves by a deep cut called a _fissure_. Each half is a _hemisphere_.

The outer layer of the brain is gray. It is made of millions of tiny lumps of matter which are the bodies of nerve cells. These are connected by threads much finer than hairs with other parts of the brain and spinal cord. Over these threads called _nerve fibers_ one cell can talk to another somewhat as we talk over a telephone wire.

[Ill.u.s.tration: FIG. 87.--Side of the skull cut away to show the brain.

_B_, backbone.]

=The Spinal Cord.=--This is a bundle of nerve matter about as thick as your finger. It extends from the stem of the brain down the ca.n.a.l in the backbone. The outer layer of the spinal cord is white because it is made of the tiny threads, _nerve fibers_. The inner part is made of the bodies of nerve cells and therefore looks gray. The fibers are branching threads from the cells in the cord and brain.

=The Message Carriers or Nerve Fibers.=--In order that the managers may send messages, these fine threads, the nerve fibers, extend from them to all parts of the body. In many places from five to five hundred or more of these fibers are united in one white cord called a _nerve_.

Twelve pairs of nerves are joined to the under side of the brain and thirty-one pairs are connected with the spinal cord (Fig. 86). The nerves of the brain branch to all parts of the head and neck, and one pair goes down to the lungs, heart, and stomach. The nerves connected with the spinal cord branch to every part of the muscles, bones, and skin of the arms, trunk, and legs.

=How the Nerves do their Work.=--On a telephone wire we can send a message in either direction. A message can travel on a nerve in only one direction. For this reason there must be two kinds of nerves. One kind is called _sending nerves_ because the brain and cord send orders over them to make the organs act. The other kind carries messages to the brain from the eyes, ears, skin, or other organs of sense, telling it how they feel. On this account these are named _receiving nerves_.

When we wish to catch a ball, the brain sends an order along the nerve threads down the spinal cord and out through the nerves of the arm to the fingers to get ready to seize a ball. The fingers are spread to grasp the ball, but they do not close until a message goes from the skin of the finger tips to the spinal cord, telling it that the ball is in the hand.

=The Work of the Brain.=--The brain is not only the chief manager of the body, but the home of the mind. The mind acts through the brain.

The mind receives through the brain what the eye sees, the ear hears, the nose smells, and the fingers feel. All this knowledge is stored up in the mind and called _memory_. These facts and others learned later are worked over by the mind. This is called _thought_.

The mind rules and becomes good or bad according to whether it contains good thoughts or bad thoughts. _It is wrong to read books and papers about robberies and murders or to tell or to listen to bad stories_, because in this way evil thoughts get into the mind. The best way of keeping badness out of the mind is to fill it with goodness. It is said that Lincoln was so busy thinking how he could help others that there was no room in his mind for a bad thought.

Doing some kindness every day helps much in the making of a good mind.

=Habit.=--The doing of anything over and over again until the body goes through the same motions without any or very little thought is called _habit_. The brain and nerves are so formed that when they get used to obeying the same order of the mind again and again, they will carry out these orders when the mind no longer gives them. Sometimes they will continue to obey the old orders even when new ones are given.