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.
[Illustration: 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.
[Illustration: 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 canal 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.
Many persons would like to break off the habit of drinking beer or
whisky, of chewing tobacco, and using bad language, but they find it
very hard to make the mind rule the body because they have let the
nerves have their own way so long.
Speaking cheerfully to those we meet, giving a kind word to our
friends, and looking pleasant are good habits which every one ought to
form in youth. They not only make the mind better, but they help the
body to keep well and will prepare the way for success in life later.
Nobody wants a grumbling clerk or a sour-faced housekeeper.
[Illustration: FIG. 88.--The difference in appearance between a
pouting and a pleasant expression.]
=Parts of the Body work without Orders from the Brain.=--A snake with
its brain crushed will still squirm and a chicken with its head cut
off jumps about. These movements are caused by orders sent from the
spinal cord. When the hand or foot is being hurt, the spinal cord
orders the muscles to draw the limb away even before we feel the pain
in the brain. Many of the movements of the body which are often
repeated may be directed by the spinal cord, while the brain is left
free to do other work. This is why the spinal cord is called the
helping manager.
The action of the muscles in the walls of the blood vessels, the
working of the stomach, the liver, pancreas, and other glands are not
directed by the brain, but by the _sympathetic nerves_. These extend
from a little cord on either side of the backbone to all parts of the
body and make the organs, such as the heart and sweat glands, which we
cannot make obey our will, do their work.
=Injury to the Nerves.=--The nerves are so important for the welfare
of the body that all the chief ones are placed deep in the flesh,
where they are not likely to be hurt. If the nerves leading to the arm
were cut, it could not be moved, and we should have no feeling in it.
The hurting of a part of the brain, the spinal cord, or the nerves may
cause loss of feeling or motion in the leg, arm, or other part of the
body. Such a part then seems asleep or dead and is said to have
_paralysis_.
Pressing on a nerve prevents it from acting. Sitting so as to press on
the nerve of the leg often makes the foot go to sleep. The bursting of
a blood vessel in the brain may let a blood clot form and press on the
nerves which govern the arm or the leg. This pressure may cause
paralysis.
=Resting the Brain.=--When there is no food in the stomach, it has
time to rest. When we sit down or lie down, the muscles get rest. The
brain is always busy except when we are asleep. No one can live even a
week without sleep. If a dog is kept awake five days, it will die.
[Illustration: FIG. 89.--Sleeping in the position shown in the lower
figure prevents free breathing and tends to cause round shoulders. The
upper figure shows correct position.]
Children need much more sleep than older persons. Men and women who
work should have about eight hours of sleep daily to remain in good
health. Children of twelve years should sleep nine hours each day;
those of ten years, ten hours; those of seven years, eleven hours; and
those of four years, twelve hours.
=Getting the most out of Sleep.=--You should go to bed every night at
about the same hour. This will help you to fall asleep as soon as you
are in bed. Do not sleep in the clothes which you have worn during the
day, but hang them up to air, and put on a night robe.
Children should use a very low pillow, so that the body can lie
straight in the bed. This gives the lungs and heart freedom to act. Do
not lie on the back as this causes some of the organs to press on
certain nerves and makes you dream. The windows should be opened wide
because fresh air is the best aid to rest and health and keeps away
tuberculosis.
PRACTICAL QUESTIONS
1. What makes the parts of the body work together?
2. Describe the surface of the brain.
3. Name the three parts of the brain.
4. Of what is the outer layer of the brain made?
5. Where is the spinal cord?
6. What are nerve fibers?
7. What work does the brain do?
8. What makes the mind good or bad?
9. What is habit?
10. How long should children sleep?
11. How can you get the most good out of sleep?