Chapter XXXVII: Appendix: 4

A SUMMARY OF ANATOMY

BONES AND JOINTS

[Sidenote: The framework of the body]

The framework of the body is composed of bones. There are 206 bones (not including the teeth) in the body. The bones of the body are divided into four classes--long bones, short bones, flat bones, and irregular bones.

[Sidenote: Construction of bones]

Regardless of their shape, all bones are composed in the same way. Every bone has an outer and an inner portion. The outer portion is a dense layer called _compact bone_. The inner portion is more open, and is much weaker; this is called _cancellated bone_ (from _cancella_, a sponge). In the smaller bones, the marrow, which is quite soft, runs through the spongy bone; in the larger long bones, the marrow is distinct and is enclosed by the spongy bone.

[Sidenote: The periosteum]

Every bone is covered by a thick, tough layer, the _periosteum_, which has three uses. When the bone is injured by disease or accident, the periosteum makes new bone to fill in and repair the break. It also builds new bone on the surface of the old as long as the body is growing. Finally, the periosteum gives strong and firm attachment to the muscles, which send tendons into it.

[Sidenote: Joints]

Every bone in the body (except the hyoid bone, to which the tongue is attached) is joined with some other bone. Most of them join with two or more bones. In most instances the end of a bone which joins with another is rounded off and made very smooth, so that it can slide easily over the other bone. There are three principal kinds of joints in the body. They are called the _ball and socket_ joint, the _hinge_ joint, and the _serrated_ or _saw-tooth_ joint.

[Sidenote: The ball and socket joint]

The ball and socket joint is one that can move freely in all directions. We see it illustrated in the joints of the shoulder and hip. In these joints one of the bones has a deep depression in it, and this depression forms the _socket_. The other bone has a rounded head that fits into the depression. We call this rounded head the _ball_.

[Sidenote: The hinge joint]

The hinge joint is illustrated in the knee and elbow joints. These joints can move backward and forward in one plane like a hinge, but they cannot move in a circle like the ball and socket joints. You cannot swing your forearm about on a pivot at the elbow as you can your whole arm, nor will the knee joint bend in every direction as does the joint at the hip.

[Sidenote: Serrated joints]

Serrated joints do not move. The bones having serrated joints are fitted tightly together so that they form practically one bone. We find this kind of joint illustrated in the way the bones of the skull are put together.

[Sidenote: Ligaments]

The joints of the body are not held together by rivets, pins or bolts as are the joints of a machine, but by bands of very tough tissue placed about a joint in such a way as to allow it to move freely, although the bones are all the time held firmly together. These bands are called _ligaments_. Ligaments are much better than bolts or pegs would be, because they stretch a little, and thus prevent the breaking of the bones when the joint is put under a severe strain.

MUSCLES AND TENDONS

[Sidenote: What muscle is]

The lean meat of any animal is composed entirely of muscle tissue. It is the function of the muscles to move the body.

[Sidenote: How the muscles work]

The muscles are nearly all attached to the bones. They are just long enough to let the joint straighten out when the muscles are at rest, but when the joint bends the muscle contracts. When a muscle contracts it becomes shorter and thicker. Sometimes it becomes very much thicker in one place. Every boy knows how much thicker the arm muscle (_biceps_) becomes when he bends his elbow hard. He calls this "showing his muscle."

[Sidenote: Tendons]

There is not room enough on most of the bones for all the muscles to be attached directly to them. Instead of being thus attached directly to the bone, they end in what we call _tendons_. These tendons are hard and strong, and a very small tendon will lift as much without breaking as quite a large muscle. The muscles are soft and would have to be attached over a large area in order to secure the required strength. The tendons, being so much stronger than the muscles, can be attached to a very small area and yet secure the same amount of power as would result from attaching the muscle itself.

The tendons pass directly into the periosteum, the thick, strong covering of the bones. So strong is this attachment that the bone will often break before the tendon will pull loose.

THE SKIN

[Sidenote: The skin as an armor]

The entire body is covered with skin, which regulates the heat of the body and acts as an armor against blows and cuts which would otherwise injure the delicate nerves and blood vessels beneath. It also serves to some extent to keep out the germs of disease. The skin appears to be smooth, but if you examine it through a strong magnifying glass you will see that it is divided into little areas. The dividing lines do not run straight, however, and the areas are not square like those you find on a checkerboard.

[Sidenote: Sweat glands]

After looking at the skin with a strong magnifying glass you will think that you must have seen all its irregularities, but if you will look at it with a powerful microscope you will find out many other things. In the first place you will see many little openings in the skin. These little openings make the ridges which divide the skin into little areas. Some of the openings are _sweat glands_, and there is always some perspiration coming out of them. When you are very warm you can see, without the aid of the glass, the drops of perspiration as they come out on the surface of the skin. When you are not very warm you cannot see these drops of perspiration, but they are nevertheless coming out all the time. When the perspiration comes so slowly that you cannot see it, it is called _insensible perspiration_.

[Sidenote: Hair]

The whole body is covered with hair. You can see the hair on your head and some of the hair on your arms and the backs of your hands, but most of the hair on the body is so fine that you cannot see it without a microscope. Each of the fine hairs on the body has a root that goes through the skin just as the root from each hair on your head goes through it.

[Sidenote: Sebaceous glands]

Opening into the little pockets in which the hairs stand, are glands that secrete a kind of oily material. They are called _sebaceous glands_ or _follicles_. Sometimes these follicles become stopped up; then the material they secrete becomes thick and cheesy, and the little black points appear on the skin which we call _blackheads_. The white matter which comes out of these blackheads is merely the secretion of the glands from which the water has been absorbed, leaving the solid or cheesy portion.

[Sidenote: Nails]

There is a part of the skin that we do not usually think of as skin. We mean the _nails_. The finger nails and the toe nails do not look like the rest of the skin of the body, but they are made of just the same kind of cells. The cells of the nails are flat, dead, and closely packed together. There are no sweat glands, or sebaceous glands, or any hairs in the nails.

THE DIGESTIVE SYSTEM

[Sidenote: The alimentary canal]

The digestive system is the part of our bodies in which the food we eat is so changed that it can be made use of by the little cells in the body. It is composed of a long canal with many parts and enlargements, each part necessary for a certain required work. This canal as a whole is called the _alimentary canal_.

[Sidenote: Teeth]

The mouth does the first part of the work for the digestive system. Here we find the teeth, which are used for grinding the food. The teeth are composed of three parts, the _head_ (or _crown_), the _neck_ and the _root_. The head, or crown, is very hard. Each tooth is hollow, and in the hollow portion there are nerves and blood vessels.

[Sidenote: The salivary glands]

Opening into the mouth are three pairs of glands known as _salivary glands_. One pair of glands is located just above the angle of the jaw. It is these glands that become swollen when we have mumps. Another pair of glands is placed just inside the jaw bone, near the root of the tongue, and the third pair is located under the tongue. These three pairs of glands secrete the saliva which moistens the food and aids in digesting the starch.

[Sidenote: The esophagus]

The food passes from the mouth; through a passage called the _esophagus_, or gullet, to the _stomach_.

[Sidenote: The stomach]

The stomach is one of the enlarged parts of the alimentary canal. Its walls are quite thick, and in these walls are thousands of little glands. These glands secrete a fluid called _gastric juice_. When the food enters the stomach it is held there for a long time, and the walls of the stomach squeeze upon it so that the food is mixed with the gastric juice until every bit of it that will be of any use to the body has become fluid in character. Not only does the gastric juice make the food liquid, but it acts on it and changes some of it so that it will be suitable for use by the little cells of the body. As fast as the food is made liquid by the juices of the stomach it is allowed to pass into the intestine through an opening called the _pyloric opening_.

[Sidenote: The intestine]

The intestine is a long, narrow, twisting and turning tube that is divided into two principal parts, the large and small intestine. In the walls of the intestine are many little glands that secrete a fluid that helps in digesting the food. Two fluids, one made by the liver, the other by the pancreas, are brought into the intestine by two small tubes, which come together in the wall of the intestine six or seven inches below the pyloric opening. These fluids perform a very important part in the digestion of all the different foodstuffs.

[Sidenote: Villi and lymphatics]

Besides the little glands in the walls of the intestine there are many thousands of little finger-like projections standing up from the walls. These are called _villi_. Each villus has in it very small vessels, into which the food passes after it has been digested. These vessels are of two sorts: blood vessels, which take up the digested starch and proteid foods, and another sort known as _lymphatics_, which take up the fats. All the lymphatics combine into a single vessel which empties into the great vein at the base of the neck. Thus the fatty foods pass into the blood and are mingled with the food materials taken up directly by the blood vessels.

CIRCULATION OF THE BLOOD

[Sidenote: Two kinds of blood vessels]

There are two kinds of blood vessels in the body. We call them _arteries_ and _veins_. The arteries serve to carry the blood from the heart to all parts of the body, and the veins serve to carry the blood back to the heart. The heart is really a part of the blood vessels, half of each side being like the veins and half like the arteries.

[Sidenote: Arteries and veins]

The walls of the arteries are thicker than those of the veins. Two sets of arteries leave the heart, one from each side. The artery that starts from the right side of the heart goes to the lungs and carries _venous blood_, which has a very poor supply of oxygen and is full of impurities. We call this the _pulmonary artery_. The artery that leaves the left side of the heart goes to all parts of the body but the lungs and carries _arterial blood_, which has much more oxygen and is much more free from impurities.

[Sidenote: Capillaries]

If we follow the blood as it circulates we will see how it reaches all parts of the body. The big artery that leaves the left side of the heart divides into smaller and smaller arteries until there are branches going to every part of the body. These branches keep dividing until they are so small that we call them _arterioles_, and these little arterioles again divide and become so small that we call them _capillaries_.

[Sidenote: Changes in the blood]

While the blood is passing through the arterioles and the capillaries, something is happening to it. The little cells with which these small vessels come in contact have been taking the oxygen and the nourishing material out of the blood. At the same time they have been putting something into the blood. In place of the oxygen they have been putting in carbon dioxid and in place of the nourishing material they have been putting in the worn-out materials from the cells. As the blood passes through certain parts of the body, such as the kidneys, the worn-out materials from the cells are taken out of the blood and sent out of the body. The carbon dioxid is left in the blood until it goes to the lungs.

[Sidenote: Venous blood]

The blood has now been followed to the capillaries, where oxygen and nourishing material have been taken out of it, and where something else has been added to it. As we follow a little capillary, we find that instead of dividing again, it joins others and gradually grows larger. As these blood vessels grow larger the walls do not become so thick as the walls of the arteries of the same size. These larger vessels formed by the union of capillaries are called _veins_. In other words, the veins are simply continuations of the arteries that have divided into extremely small branches and have now come together again. The blood which has been changed is now called _venous blood_. It has much less oxygen in it, but has received a great deal of carbon dioxid. The veins continue to come together, until finally they form a single large vein which empties into the upper half of the right side of the heart. From there the blood is driven into the lower half of the right side of the heart and thence to the pulmonary artery, which goes to the lungs. The blood is not changed in the heart, so what goes into the pulmonary artery is still venous blood.

[Sidenote: Arterial blood]

The blood goes straight from the heart to the lungs and there it is changed into arterial blood. The change consists in taking oxygen from the air and giving off carbon dioxid to the air. From the lungs the blood goes through the capillaries again into the veins, the small capillaries in the lungs uniting to form the pulmonary veins. These veins finally unite into one vein that empties into the upper half of the left side of the heart, and from there the blood goes to the lower half of the same side. The blood has now reached its starting point and is ready to begin its journey again. The journey of the blood is as follows:

[Sidenote: Entire circulation]

From the right side of the heart to the arteries; from arteries into arterioles; from arterioles to capillaries; from capillaries into veins; from veins into the heart; from the heart to the lungs; and from the lungs back to the heart again.

THE RESPIRATORY SYSTEM

[Sidenote: Use of the respiratory system]

That portion of the body by which we breathe is called the respiratory system. This system is composed of the _nasal passage_, the _pharynx_, the _larynx_, the _trachea_, the _bronchi_, and the _lungs_. The mouth is not a part of the respiratory system; we should never breathe through our mouths.

[Sidenote: The use of the nose in breathing]

As the air passes through the nasal passage it is warmed and moistened, and a great deal of dust and dirt is taken out of it. Thus the nasal passage serves to warm, to moisten, and to clean the air we breathe, and is a very important part of the respiratory system, since either cold or dry air is very irritating to the lungs.

[Sidenote: Pharynx, larynx, and trachea]

There is only one tube leading from the back of the nasal passage to the lungs. Different parts of this tube are given different names. The pharynx is that part that extends from the back of the nose to the vocal chords. The larynx is the part of the throat where the vocal chords are located. We sometimes call it the "Adam's apple." It is very prominent in some men, but seldom noticeable in women. The trachea is the part of the tube leading down from the larynx. At the lower end of the trachea the tube divides into two parts that we call the bronchi, one leading to each lung.

[Sidenote: The bronchi]

The bronchi carry the air from the trachea to the lungs. They divide again and again until they become so small that there is a branch for each little air cell in the lungs.

[Sidenote: The lungs]

The lungs are the most important part of the respiratory system. They are made up of lobes. There are two lobes in the left lung and three in the right. Each lobe is divided into _lobules_, which means small lobes. Each lobule is divided into air spaces. In these air spaces, or cells, the work of the lungs is performed.

[Sidenote: Air cells]

The capillaries run in the thin walls of the air spaces. The walls of these fine blood vessels are so very thin that the air in the air cells comes in almost direct contact with the blood in the vessels. While the blood is passing through the vessels in the walls of the air spaces, something happens to both the blood and the air. The air we take into our lungs contains a great deal of oxygen and very little carbon dioxid. The air that comes out of our lungs contains a great deal of carbon dioxid and much less oxygen. In other words, the oxygen from the air goes into the blood, and the carbon dioxid from the blood goes out into the air.

[Sidenote: Necessity of pure air]

If the air we breathe is not pure and does not contain enough oxygen, the blood cannot get all the oxygen we need, and the cells of the body become sick and die. If, when we breathe, we do not fill each little air cell in our lungs with air, a great deal of the blood sent to the lungs for oxygen cannot get it. If we wear very tight clothing we cannot take a full breath and hence cannot fill all the air spaces with air. If the air spaces in the lungs are not filled, the blood does not get the oxygen it needs, as there is no other place in the body to get it.

THE EYE AND THE EAR

[Sidenote: The eye compared to a camera]

The eye is one of the most important organs in the body and also one of the most delicate. It is very much like a camera.

[Sidenote: The cornea and the sclera]

When you look at an eye you are likely to think that the front of it is blue or brown. The colored part is not the front of the eye. If you look at the eye from the side you will see that there is a curved part in front of the colored part and that the curved part is perfectly clear. This curved clear part of the eye we call the _cornea_. The cornea connects with the white part of the eye, and this white part extends all around the rest of the eye, except at a small point in the back where the _optic nerve_ comes through. This white part we call the _sclera_.

[Sidenote: Aqueous humor]

The space between the cornea and the colored part of the eye is filled with a clear fluid that is called the _aqueous humor_, which means watery fluid. The space occupied by this fluid is called the _anterior chamber_ of the eye.

[Sidenote: The iris]

The iris, which is the colored portion of the eye, is a curtain that is hung between the anterior and posterior chambers of the eye. It prevents any light getting into the posterior chamber except that which passes through a round hole in the iris called the _pupil_. The pupil grows larger or smaller according to the amount of light needed by the eye. If you look away from a bright light at something in the dark, the pupil grows larger; if you look back at the light, it grows smaller. You can see this in a hand mirror.

[Sidenote: Lens]

Behind the pupil is a clear mass shaped like a very strong magnifying glass. This is the _lens_. The lens causes to be formed on the back of the eye a clear picture of whatever you are looking at. When you focus a camera, you move the back towards or away from the lens. When you focus your eye you cannot move the back of the eye, but you can make the lens more or less convex as may be needed to make a clear picture.

[Sidenote: Vitreous humor]

Behind the iris and the lens, we find the posterior chamber of the eye. This occupies by far the greater portion of the ball and corresponds to the dark chamber of a camera. This chamber is filled with a clear fluid called the _vitreous humor_, which means jelly-like fluid. It is a clear, gelatinous substance.

[Sidenote: The retina and the optic nerve]

The optic nerve enters the eyeball from the back and spreads its fibers out in a thin membrane called the _retina_, which corresponds to the sensitive plate in a camera. The lens focuses on the retina the image of any object you look at. The impression made on the minute nerve endings in the retina is carried by the optic nerve fibers to the brain. When this impression reaches the brain we see the object.

[Sidenote: Movements of the eye]

Each eye has six muscles that turn it in whatever direction you want to look. These muscles are very delicate, and for true sight they must be exactly adjusted. If the muscles on one side of the eye are stronger than those on the other side, you will be cross-eyed or wall-eyed. If one of the muscles in one eye is stronger than the corresponding muscle in the other eye, it pulls the eyeball out of place and you "see double."

[Sidenote: The use of the outer ear]

The portion of the ear that you see on the side of your head has as much to do with hearing as the outer rim of the horn into which you speak has to do with making a record for the phonograph. You know that the record is really made at the little end of the horn, while the big end simply collects the sound. The outside portion of the ear simply collects sounds, and the real hearing is done with the portion of the ear that is not seen.

The outer ear connects with the short tube that leads to the _drum_, which is a thin membrane that separates the middle ear from the outer ear. This drum does not have so much to do with hearing as is supposed. To have a hole in the drum does not mean that you cannot hear.

[Sidenote: The middle ear]

The middle ear is that portion which is just inside the drum. In this we find a chain of little bones. The one attached to the drum is shaped much like a hammer, and is called the _malleus_, which means mallet or hammer. The next one is shaped like an anvil and is called _incus_, which means anvil, and the third is called _stapes_, which means stirrup. The flat part of the stapes fits into a small opening that leads to the internal ear.

[Sidenote: The inner ear]

The internal ear is shaped like a snail shell. It makes several turns, which are hollow like the ends of a conch shell. In these channels the nerve that receives the impressions made by the sounds is distributed. This nerve carries the impressions to the brain.

THE NERVOUS SYSTEM

[Sidenote: Importance of the nervous system]

This system is so important in its use and so difficult to understand in its details, that a description of it should have more space than can be given in this summary. Can you imagine having no feeling and being unable to move? This would be your condition if you had no nerves.

[Sidenote: Nerves compared to telegraph system]

Briefly, the nerves might be called the telegraph system of the body. There is a great central station called the brain where messages are received and sent out; there are many sub-stations that make up the spinal cord. Twelve great nerves pass directly from the brain to the body; all others pass from the spinal cord.

[Sidenote: Extent of nervous system]

Every nerve leaving these centres divides and sub-divides into little threads as the arteries divide and sub-divide into arterioles and capillaries, until every part of the body--every muscle and part of the skin--has its nerve.

[Sidenote: Voluntary and reflex action]

Every time you _choose_ to move your hand, your brain sends to the necessary muscles an instantaneous order to act. This is called _voluntary action_. If you put your finger against a hot stove you jerk it away before you could have time to _choose_ to do it. This happens as an order from a sub-station and is called _reflex action_.

Messages travel both ways and it is necessary that the nervous connection with every part of the body remain unbroken, and important that the nervous condition be kept healthy.

NOTES TO THE TEACHER

A false delicacy has often prevented the teaching of vital lessons to growing children. The day is how at hand when foolish sentiment must no longer prevent the spread of any knowledge which is necessary to exterminate the plagues that have afflicted the race.

In chapters 17, 18, 19 and 32, unpleasant facts are given in plain language. They are facts that parents do not teach their children and that most teachers will not frankly treat with classes in a school. Though they are disagreeable to discuss, they are essential for children of school age to know. Now that the dependence of public health upon personal hygiene is recognized, personal habits and sanitary conditions are more frankly dealt with than formerly.

It is an excellent thing occasionally, to have one or two points of an assigned lesson answered in writing. Any topic that the teacher may think advisable to treat in this manner may be discussed on paper at the beginning of the recitation, five or six minutes being allowed for that purpose. Even a whole chapter may be assigned to be studied with a view of answering in writing the questions at the end of the chapter. The written test can then take the place of the usual oral recitation. This method is suggested to the teacher who hesitates to discuss orally certain plain but essential facts.