CHAPTER III THE RULER OF THE BODY
ALL parts of the body must be controlled and made to work together. Over them all a ruler must be set. Not only must the different organs be kept at work, but each must be made to do the proper amount of work, and to do it at the right time. If the digestive organs should begin to work when nothing had been eaten, their work would be useless. If the sweat glands should begin to pour. out sweat on the skin when the body was not hot, their work would be not only useless, but even harmful. If all the muscles should begin to pull and jerk without any order or system (as they do in convulsions), they would succeed only in throwing the body to the ground.
The ruler of the body is the nervous system. When we walk, it is the nervous system that causes the right muscles to move. When we eat, the nervous system sets the diges- tive organs to work. It keeps the heart and lungs going, and governs all the body. The function of the nervous sys- tem is to govern all the organs of the body, and to cause them all to work together for the common good.
The Divisions of the Nervous System. The nervous sys- tem has two great divisions,—the central nervous system and the sympathetic nervous system. The great centers of the central nervous system are the brain and the spinal cord. The central nervous system controls the voluntary
23
Fic. 12. The nervous system, From the brain and spinal cord, nerves run to all parts of the body, 24
THE RULER OF THE BODY 25
muscles (those which we can move when we wish), and its higher centers act as the organ of the mind.
The sympathetic nervous system controls the glands! of the body and the involuntary muscles (page 226),—the muscles which we cannot control by the will, as those of the stomach, intestine, heart, and blood vessels.
The Brain and the Spinal Cord. The brain has three divi- sions,—the cerebrum, the cerebellum, and the medulla oblon- gata, The spinal cord is the soft white substance that you
MEDULLA OBLONGATA
Fi. 13. The brain,
may have seen in the backbone of an animal. In the back- bone of a man it is a little thicker than a lead pencil. At the base of the skull is a great opening, through which the spinal cord enters the cranium (Fig. 14) and joins the brain.
The Membranes of the Brain and Cord. Around the brain and cord are three connective tissue membranes. The outermost membrane, which is thick and tough, is called
1 The salivary glands, glands of the stomach and intestine, the pancreas, liver, kidneys, and sweat glands are some of the glands in the body.
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the dura mater. It lines the entire dorsal cavity of the body, both the cavity of the cranium and the cavity in the spinal column, The innermost membrane, which is thin and delicate, is called the pia mater. It lies close to the surface of the cord and brain and dips down into all the wrinkles and folds in the surface of the brain. In it are the blood vessels that nourish the outer parts of the brain and cord. Between the dura mater and the pia mater is a third membrane, the arachnoid membrane. These membranes? hold the very soft and delicate brain and cord in place and keep them from being shaken about within the dorsal cavity.
The Cerebro-spinal Fluid. The brain and spinal cord are still further protected by a layer of liquid around them called the cere-
Fic.14. The baseoftheskull, bro-spinal fluid. This fluid is on showing thes opening through both sides of the arachnoid mem- which the spinal cord enters the .
‘panic. brane —that is, between the dura
mater and the pia mater — and en- tirely surrounds the cord and brain. It acts as a cushion inside the walls of the dorsal cavity and keeps the brain and cord from striking against the walls of the cavity.
Nerves. From the under surface of the brain and from the spinal cord, shining white nerves pass out to every part of the body. If you should examine one of these nerves under
1 The three membranes taken together are called the meninges of the brain and cord. Cerebro-spinal meningitis isa disease caused by germs growing in these membranes and in the cerebro-spinal fluid,
THE RULER OF THE BODY 27
a microscope, you would find that it is made of many hun- dreds of very fine fibers, bound together by connective tissue. Although these fibers are so slender that they cannot be seen without a microscope, some of them are of great length, the longest reaching from the spinal cord to the hands and feet. In the larger nerve trunks the fibers are bound up in a number of bundles, which are all wrapped together in a com- mon sheath (Fig. 15). The larger nerves divide into smaller
Fic. 15, A is a cross-section of a nerve, showing the bundles of nerve fibers that make up the nerve, wrapped in the connective tissue sheath. 2 is one of the bundles of nerve fibers shown in A, enlarged to show the individual fibers.
branches containing only a few bundles, or sometimes only one bundle. In the finest nerve branches, the bundles of nerve fibers finally break up into the separate fibers, many thousands of which end in the skin and muscles, and in the other organs of the body.
The Function of Nerves. Zhe function of the nerves is to carry messages between the different parts of the body and the brain and spinal cord.
Some nerves carry messages fo the brain and cord. By these messages we learn when anything touches the body,
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when the body is hungry or thirsty, or hot or cold, when it is in pain, and about the things that we see, hear, taste, and smell. Other nerves carry messages outward from the brain and cord, causing the muscles to move, and making all the parts of the body to work together in harmony. The power by which we feel, think, and will lies in the brain. The commands that we send out, causing voluntary movements of the muscles, start from the brain. The nerves are useful only to carry to the cord and the brain messages which tell us about the body and the outside world, and to carry com- mands outward from the brain and cord to the muscles and to the other organs of the body.
The Telegrapher and the Telegraph Wires. The brain and spinal cord are often compared to a telegrapher in an office, and the nerves in the body to telegraph wires that run out in all directions from the office. Over the wires the tele- grapher receives messages that tell him what is going on about him, and he sends out messages commanding that cer- tain things be done. So through some of the nerves the brain and cord receive messages which tell them about the body and the things going on around it; and over other nerves they send messages out commanding the muscles to move the body, and the different organs to do their work according to the body’s needs.
Necessity for a Nervous System. From this you will under- stand that the nervous system connects all parts of the body and causes all the organs to work together for the good of the body as a whole. Without a nervous system, the human body would not be one body at all, but a mass of flesh and blood and bones, the different organs working not at all, or working without plan or system, and the whole body certain todie ina few minutes. With a nervous system it is the living,
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moving, speaking human body, the most wonderful thing in all the world.
Summary. The nervous system controls all parts of the body and causes them to work together. The great divisions of the nervous system are the central nervous system and the sympathetic nervous system. The central system controls the voluntary muscles; its chief center (the brain) is the organ of the mind. The sympathetic system controls the involuntary muscles and the glands of the body.
The brain has three divisions —the cerebrum, the cere- bellum, and the medulla oblongata. Around the brain and cord, and protecting them, are three membranes — the dura mater, pia mater, and arachnoid membrane —and a layer of cerebro-spinal fluid.
Nerves are composed of nerve fibers. Nerve fibers carry messages between the different parts of the body and the brain and spinal cord. Some fibers take messages to the brain and cord, and some fibers carry messages away from the brain and cord. The nerve centers (brain and cord) may be compared to a telegrapher, and the nerve fibers to telegraph wires. The incoming messages bring information about the body and the world around the body, and the outgoing messages are commands from the nerve centers to the muscles, glands, and other organs.
Without a nervous system the different organs of the human body would work without system, or would not work at all, and the body would die. By the nervous system, all the body parts are made to work harmoniously together, — all are united into one wonderful body.
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QUESTIONS
What is the function of the nervous system? Name the two divisions of the nervous system. What are the chief centers of the central nervous system? What is one of the great functions of this system? With what part of the nervous system is the mind associ- ated? What is the function of the sympathetic nervous system?
Name the three divisions of the brain, Name the three mem- branes that are around the spinal cord and brain. What is the function of these membranes? What is between the arachnoid mem- brane and the pia mater? What is its function?
Describe the structure of a nerve. How long are the longest nerve fibers in the body? What is the function of a nerve?
In comparing the nervous system to a telegrapher and a tele- graph system, what part of the nervous system corresponds to the telegrapher? What is the function of this part of the nervous system? What part of the nervous system corresponds to the tele- graph wires? What is the function of this part of the nervous system?
To what part of the body are the incoming messages carried? What is the effect of these messages? To what organs are the out- going messages taken? What is the effect of these messages?
Why is a nervous system necessary in the body?
Why is it possible for a tree to live without a nervous system, when a man cannot do so? If the cells in the human brain should become cold, would they die? Do the cells in the feet or ears die if they become cold? If a fish should be frozen in the ice, would the cells of its brain be killed? Are little one-celled animals killed by cold water? Ask a physician which cells in the body are most affected by such poisons as strychnin, opium, nicotine (the poison in tobacco), and alcohol.