Physiology and health

Preface

The student in late junior high school or early senior high school is ready for a serious course in the science of physiology. This book, therefore, makes a direct approach to this subject. Provision is made, however, through the list of health habits in the appendix and through the references in the text, to secure continuity with the program of health training which the pupil has followed in the earlier grades.

Like the other books in this series, Physiology and Health has been developed by the classroom method. On the basis of experience thus gained, the discussions of structure have been intentionally limited. The illustrations have been carefully selected to present anatomical facts and should be regarded as study material. Obviously, this course cannot be built upon the foundation of biophysics and biochemistry which underlies courses in college and professional school. It does seek to present in understandable form those fundamental physio¬ logical facts which should be a part of the knowledge of every well-informed individual.

I wish to express particular appreciation to Dr. Percy G. Stiles, Assistant Professor of Physiology, Harvard University, for extensive participation in the preparation of the manuscript. Valuable assistance has also been rendered by Miss Georgie B. Collins and Miss Adelaide Ross, of the Malden School Department, and by Miss Marjory Doherty, Head of the Hygiene Department at the Washington Irving Junior High School, Boston, in testing and developing the material.

IV

PREFACE

Thanks are also due to Superintendent F. G. Marshall and the School Department of the City of Malden for continued cooperation.

C. E. T.

Contents

CHAPTER PAGE

I. The Study of Physiology and Health ... i

II. Living Cells and Tissues .. . io

II What is the unit of structure in the human body?

How is it possible that the parts of the body can differ so much in firmness and strength?

In a building there are relatively few kinds of materials — bricks, wood, iron, glass, for example; is this the case with the body?

How many different kinds of material are in the human body?

If a boy weighs ninety pounds, what part of this weight is water?

In studying any mechanism we first learn the units of structure and how they are put together. The study of biology has shown us that the human body, the lower animals, and the plants are all composed of similar structural units.

Cells. — More than two hundred years ago an Englishman who had a chance to use a better microscope than had previ¬ ously been available made pictures of what he saw through the lenses. One of his drawings was of a thin slice of cork. This familiar material is taken from the bark of the cork tree. Under the microscope it was found to resemble a honeycomb; there were rows and rows of tiny compartments. These have since been called the cells of the cork. The bark was dead and dry when examined; we know now that if the bark had been fresh and living, these compartments would have been full of fluid or at least a very soft gelatinlike substance. This colorless jelly is the living stuff, or protoplasm.

Students who have continued to use the microscope through

IO ii

PLANT CELLS

These are typical plant cells. Note the thick walls and gray pro¬ toplasm and the space filled with clear fluid at the center of some cells.

all these years have found out that leaves and stems of plants show a structure which is much like that of cork. They are made up of units each one of which is walled off from its neighbors, and we still call these units cells. By degrees we have become more interested in the parcels of protoplasm than in the partitions between them. A wise teacher has said

THE AMOEBA (Greatly Magnified)

This is a microscopic free-living single-cell animal which lives in pools and streams.

that it is not the cell but the prisoner in it that we want to learn about.

When the parts of animal bodies came to be studied, it was discovered that they, too, are composed chiefly of units which we call cells, but these are not separated by noticeable walls. The packets of protoplasm are often about the same size as the cells of plants. Generally they are pressed close to¬ gether, and it seems as if they might run into one mass as drops of jelly would be expected to do, yet somehow they re¬ main separate.

Free-living cells. — It is not usually possible for any cell of the human body to be separated from its neighbors and to live long by itself, although by taking the greatest care scien¬ tific workers have kept bits of animal tissue living and even growing in warm and nourishing liquids. There are, how¬ ever, millions of cells which do live apart from others, both in the earth and in the water. Since most of these very simple forms of life are so small as to be invisible to the naked eye, we should know very little about them if it were not for the microscope. Some of these free-living cells are green plants; some are supposed to be colorless plants; some are spoken of as animals. In many cases it is not easy to say which they are. Such cells find food of some sort, if they are fortunate (enough, and make use of it for their growth. Sooner or later each well-grown cell divides to make two. Thus they live on as long as conditions favor. Many of them die as the re¬ sult of accident, but not of old age.

The free-living cells that we talk about most often are the ones called bacteria . These are very tiny cells which swarm in the water and in the surface layers of the soil. The main reason why we hear so much about them is that certain kinds can live and multiply in the bodies of men and animals and cause a number of diseases. Other cells which flourish in enormous numbers are those which we call yeasts. While these are growing and carrying on their life processes in solu¬ tions containing sugar, they produce the changes which we know as fermentation. Alcohol is formed by the action of yeasts on different kinds of sugar.

Body cells. — The human body is entirely made up of cells or cell products. The skin will serve to illustrate cell structure. The cells of the skin are in many layers. Those near the surface are dead and dry. They form scales which i 4 PHYSIOLOGY AND HEALTH

are constantly falling off or being washed away. Those which lie deeper are alive, and are oval rather than flat in outline. You know that if you scrape the skin a little, but not enough to make it bleed, you will find moisture. You will also feel pain because you are injuring the fine beginnings of sensory nerves which are among these deeper cell layers.

SECTION THROUGH THE CELLS OF THE OUTER SKIN (Greatly Magnified)

Note the thin dead cells at the outside and the thin cell walls.

The living cells in the skin take in food from the neighbor¬ ing fluid, and this enables them to grow, much as the whole animal does. But they do not keep growing to an unusual size, for now and then each one divides into halves. The new cells which are formed in this way are at first smaller than the others, but they soon grow to average size. The forming of fresh cells makes up for the loss of those which have been pushed to the surface and detached.

We ought not to think of the living cells as being hard bodies. They are in fact quite yielding. If we could en¬ large an average cell a thousand times in each dimension, it would probably resemble a raw oyster as much as anything. It would be very juicy and easily pressed out of shape. Three-quarters of such a cell is water. We shall find that

DIAGRAM OF TYPICAL CELL (After Wilson)

1. Main body. 2. Nucleus. 3. Attraction sphere. 4. Food par¬ ticles and waste. 5. Cell wall. 6. Masses of active material found in certain cells, called plastids.

what gives strength and fixed form to the human body is not its cells, but other materials which they deposit about them. In spite of the presence of these harder substances — of which the mineral part of bone is an example — about twothirds of the weight of the body is due to water.

When cells are viewed through the microscope, they seem at first to have no definite structure. We have said before that they appear like tiny drops of jellylike material. But i6 when these same cells have been carefully prepared for study by coloring or staining them with suitable dyes, we find that their substance is not the same in every part. Each cell has certain parts which can best be understood by referring to the picture. The surface is probably different from the interior: we say that the cell has a membrane, though we do not mean to suggest anything more substantial than a most delicate film. Almost always the stain shows a small mass inside the

STEPS IN CELL DIVISION (After Wilson)

Note that the material of the nucleus divides before the cell body.

cell which looks as if it were denser than the rest. This is called the nucleus.

When a cell is about to break in two, the nucleus generally divides first. At this time it looks as if it contained a snarled or knotted thread. The twisted thread breaks up into short pieces and these seem to be exactly shared between the two new nuclei which are made from the old one. For a little while the cell may exist with two nuclei, but soon a crease, or groove, runs around it, and this is the beginning of a separa¬ tion into two complete cells.

Muscle fibers are really cells.

These cells are of unusual length, but they are the units, the parcels of protoplasm, which make up the living part of the muscle. Unlike the cells of the skin, muscle fibers do not divide so as to increase in number. When a muscle becomes larger as a result of exercise, it is supposed that the number of the fibers remains just the same as be¬ fore, but that the average fiber be¬ comes thicker. In the nervous sys¬ tem there are peculiar cells of a branching form; these do not di¬ vide once the brain is fully formed.

A baby a year old has as many cells in his brain as he will ever have; his education is carried out by adjusting these cells to act together, not by adding new ones.

Tissues. — The various kinds of material which we find in the body are called tissues. Each is com¬ posed of groups of similar cells.

There are four principal sorts, two of which, muscular and ner¬ vous, have been mentioned. The others are the connective and the epithelial .

(Courtesy of D. Appleton & Co.)

MUSCLE FIBERS (Greatly Magnified)

These cells have been torn to show the covering sheath.

Note : A, The broken end of the muscle fibers; B, A bundle of fibrils at the torn end; N, A nucleus; S, The sheath. (From Jordan’s A Textbook of Histology.)

i8

Connective tissues, as the name suggests, hold the parts of the body together and in shape. Bone is an example. The general form of the body depends, of course, on the skeleton. Just as the body as a whole has a skeleton, so each organ has a framework, though of a more flexible kind. Think, for in¬ stance, of a muscle. It could not accomplish anything by its

A NERVE CELL (Greatly Magnified)

Note the finely branching structure of the cell.

shortening if its fibers were not bound together and given something to pull upon. Tendons unite muscles with bones and thus give the muscle fibers a chance to pull upon the bones. These white cords, the tendons, are composed of a very tough variety of connective tissue. The same tissue in the form of thin layers covers the whole muscle on the out¬ side and divides it internally into bundles of fibers. You have seen these thin white partitions in meat. What we call a muscle is really a combination of two kinds of tissue: the true muscular part which does the work and the connective tissue which makes the work effective.

Other examples of connective tissue are found in the liga¬ ments which unite one bone to another at the joints. What we call fat is a form of connective tissue in which the cells are swollen with oil drops.

Connective tissues differ from muscular and nervous tis¬ sues in that most of the substance of connective tissue is apparently quite lifeless material. It is not composed of cells which are taking food and growing, but rather of deposits, or cell products. Living cells have made these deposits by their own activity, but when the tissue has been fully de¬ veloped, the cells no longer seem im¬ portant. Under the microscope the dense, lifeless matter is found to oc¬ cupy most of the space. Suppose that we are examining a specimen of ten¬ don. We find that it is mainly com¬ posed of slightly wavy white fibers closely pressed together to form bundles. Here and there among these fibers may be seen rows or chains of cells. The most remarkable property of the tendon is its great strength, and this is not due at all to the cells, but to the tough white fibers. Still, it is true that the cells made the fibers in the first place, and that if the tendon is to be repaired after an injury, these cells have to be depended on to provide the new material.

The structure of bone as revealed by the microscope is very interesting. Bone is so hard as to seem almost like a mineral

CARTILAGE (Greatly Magnified)

Cartilage is a tissue in which the cells have been separated by the large amount of lifeless ma¬ terial which they have deposited.

substance. In fact, a large part of it is a form of lime, some¬ what like the material in limestone. This mineral matter re¬ mains in its place in a bone which has been in a hot fire, but a burned bone is very light and por¬ ous. The loss of weight shows that originally there must have been other substances there. In dried bone there are microscopic cavities in the hard mass. In life each of these cavities is occupied by a cell. These cells developed the bone by depositing lime salts.

They continue to live their shel¬ tered lives within the bone. Sim¬ ilar cells repair injured bone by producing a new deposit to cement the broken ends together.

Epithelial tissues serve as cov¬ erings, or linings. The outer skin is a familiar example. But there are other kinds of epithelial tissues very different from this. In many cases they are composed of cells in a single layer, as, for instance, the Still more delicate—.suggesting films of bubbles — are the partitions between the air and the blood in the lungs.

It might seem that epithelial tissues, like connective tissues, have little or no action of their own. A mere covering or lin¬ ing for a part of the body need not necessarily be alive. Yet most of the epithelial cells are probably living. This makes

CROSS SECTION OF BONE SHOWING MINUTE STRUCTURE (Greatly Magnified)

i. Surface layer of bone. 2. Deeper portion. 3. Canals. 4. Location of cell.

lining of the intestine.

it possible not only for new ones to take the place of old, but also for other activities to go on. Epithelial cells have at least two forms of activity which we must not neglect to mention: they have to do with absorption and with secretion.

These two processes seem to be opposites, yet in some re¬ spects they are very much alike. Absorption is the taking in, through layers of cells, of substances dissolved in fluids; secretion is the passing out, through layers of cells, of other dissolved materials. The food we have eaten and digested has to be absorbed through the epithelium of the intestine.

Secretion is illustrated by the working of the concealed organs (glands) such as those which produce the tears and the epithelial cells saliva. To absorb or to secrete, the cells or THE CHEEK

must be truly alive. _ (Greatly Magnified)

Perhaps we should explain what is meant by a gland. You have seen drops of perspiration ris¬ ing from the pores of your skin. Each of these pores is the opening of a tiny tube made of epithelial cells. The tube is much coiled and closed at the deep end. It is wrapped around with a network of fine blood vessels. The coiled tube is a sweat gland; its cells receive water and some dissolved ma¬ terial from the blood, and they discharge this fluid through the slender outlet which leads up to the pore. In the lining of the stomach there are glands opening to the surface in much the same way. They secrete the gastric juice.

The glands we have just mentioned are very small. The body also contains large glands, like the liver. This is a mass of seemingly solid tissue weighing as much as three pounds. When thin sections of it are examined with the microscope,

Nucleus the whole substance is seen to be made up of secreting tubes of epithelium, closely packed together. In this case the neighboring tubes unite to form larger ones, and these join in turn until finally the secretion, or bile, is all conducted out of the liver by a single passage. The outlet of such a compound gland is called a duct. The main duct with its branches ex-

(Greatly Magnified)

The ducts through which the secretion escapes are marked with “+”.

tends to all parts of the organ as the roots of a plant might reach to all parts of a pot filled with earth. Ducts from the salivary glands send saliva into the mouth through small openings in the cheeks and under the tongue.

The work of cells. — When a single cell has to get along by itself, it must do many things; it takes in food, throws off waste, breathes, and reproduces itself by the process of divi¬ sion. Many of these single cells can move about, and they

2 3 all respond in some way when things happen to them. When many cells are in a close partnership, as is the case in our own bodies, matters are somewhat different. Here we have what is called ‘a division of labor.’ Certain cells do particular things most of the time and rely on their neighbors for help to make this possible. For instance, the cells of muscles have the power to contract, but they depend on the coopera¬ tion of other tissues to get the food they need, and to attend to the supplying of their oxygen and the removal of their wastes. The cells of the nervous system have a wonderful service to perform, but they have to be fed and sheltered and transported from place to place in the world by cells located in other tissues.

You will see that the body is like a civilized community, or society, of which the cells are members. When it is found that a man has a very remarkable talent — perhaps as a musician, a ball player, or physician — people usually co¬ operate to help him to do his best in his own line. They re¬ lieve him of all other work. They feel fully repaid for all that they do for him because of the pleasure or service he gives them in return. The tissues of the body behave in much the same way. We do not mean that the individual cells feel responsibility or gratitude, but they act much like the citizens in the cases we mentioned. Vast numbers of muscle and gland cells seem to be at the service of the nervous system, but it is not a one-sided relation, for their own wel¬ fare depends on the performances of the brain.

A man may be cast away, like Robinson Crusoe, yet man¬ age to keep himself alive. He can find food and cook it; can shelter and clothe himself. But all of these needs would be better attended to if he could have the cooperation of other men and women with special training. You can see that highly civilized people are safer and happier when they are living in large groups. Savages, on the other hand, may be better off when they are scattered, because they are all after the same things and are not of much help to one another. That is the way with single-cell animals. In the bodies of the higher animals there is much variety in the types of cells and in their work. So, too, there is increased dependence of each part upon all the rest.

Substances in the body. — We have described the living structures of the body without attempting to tell of what ma¬ terials they are made. The protoplasm of living cells is com¬ plex, and a wide knowledge of chemistry would be necessary to begin to understand its exact nature. You are already familiar with some of the substances of the body, such as fat, iron, sugar, lime, sulphur, and water. Your later study of science will give you a knowledge of many other interesting substances which are to be found in the human body.

1. What is the usual meaning of the word ‘cell’? How has

the meaning changed since the word was first used?

2. What do you understand by protoplasm?

3. What do cells need?

4. What are tissues and what are the chief kinds?

5. What is meant by the division of labor among cells? Illus¬

trate.

6. Discuss the nature and the importance of the substances produced by cells in the connective tissues.

Contrast the life processes of free-living cells with those of cells in the human body.

8. What have you learned in this chapter which would suggest the importance of drinking enough water?

1. Burn a piece of bone where there is a good draft. Bring it

to class. Note its lightness and the small cavities all through it. Explain this condition.

2. Let some one go to the butcher shop or market and get a piece

of tendon to bring to the class. Does it stretch? Can you pull it in two? Does it cut easily?

3. If you have a microscope at school, perhaps your teacher

will help you to see some living cells through it. Yeast cells may be seen by soaking a yeast cake in water and putting a drop of the water on a glass slide for study under the microscope. Green plant cells may be secured by scraping off the green growth sometimes seen on the bark of trees and putting these single-cell green plants ( Pleurococcus) under the microscope on a glass slide.

Ill

Can you tell just what happens when you lift your hand?

What is the source of power to move the human body?

Have you ever seen muscle tissue? Where?

Do muscles produce every movement of the body?

What substances are used by muscles?

Does muscle produce waste products?

Every boy knows that his muscles are the motors, or en¬ gines, which set the parts of his body in motion. He feels them tighten and grow hard under the skin and understands that somehow certain muscles move his arms and legs. Other muscles change the expression of his face; still others keep his chest rising and falling as he breathes.

The muscles make up about half the weight of the body. In the newborn baby they probably do not make up so large a part; as a child grows from year to year the gain in muscle goes on faster than the gain in some other kinds of material. The muscles are the only parts of the body which have power to move; the other parts must be carried or moved by them. The muscles of certain animals are familiar to us as meat. To be sure, some portions of the body which are not muscle are eaten as meat — the liver, for instance, or tripe, which is the lining of the stomach — but steaks, chops, and roasts are cut from the large muscles which were used to move the legs or ribs of the steer or the sheep when the animal was alive.

Phalanges

THE HUMAN SKELETON

Serratus

Rectus

Femoris

Tibialis

Anticus of the Toes

Sartorius

Vastus Externus tensors of Fingers Flexors of Fingers

Triceps

Biceps

FRONT VIEW OF IMPORTANT MUSCLES

Latissimus Dorsi

Gluteous Maximus

Vastus Externus

Flexors of Toes

Biceps Cruris

G astrocnemius

Tendon Achilles

Triceps -

Flexors of the Hand

Extensors of the Hand

Deltoid

2 9

BACK VIEW OF IMPORTANT MUSCLES

Nature of muscle. — A muscle may be of almost any shape or size. Suppose we think of the one on the front of the arm between the shoulder and the elbow, which a boy is often proud to show to his friends. This is called the biceps. It is thickest in the middle and narrows toward the ends. If we could see it, we should find its thick middle part reddish in color. Near the ends it becomes slender and very firm and

ACTION OF OPPOSING MUSCLES

This diagram illustrates the opposed relation of the biceps and triceps muscles.

tough. These slender white ends are the tendons of the muscle, and they are attached to the bones. The thick red part of the biceps contracts and makes the movements.

The tendons apply the force to the bones. They are like the traces which make it possible for a horse to pull a wagon. We must remember that muscles do all their work by pulling; they never push. When the biceps shortens and thickens, it raises the forearm and hand as the elbow is bent. The biceps

THE PRINCIPLE OE OPPOSING MUSCLES

The cords which turn a rudder illustrate the principle of opposing muscles.

muscle swings the forearm up in much the same way that a boy raises a fish pole. A study of the diagram will show the similarity of the movements. We can make such movements slowly or quickly, with little force or much, as we choose.

The different muscles of the body are usually so arranged that if one of them can move a part in a certain direction, there will be another of them fitted to make the opposite movement. Think again of the biceps raising the forearm or the fish pole. If the muscle should stop pulling and grow limp and soft, the arm would drop back. The weight would make this happen; but there is a muscle on the back of the upper arm which can give extra force to the downward move¬ ment. This muscle is called the triceps. It is used in straightening the arms, as in striking a blow wfith a hammer. You can see that when the biceps shortens, the triceps must be stretched, while the triceps in turn can pull against the biceps and stretch it. These two muscles act, one against the other, in the same way as do the two lines which turn the rudder of a boat. One line swings the rudder to the one side, the other to the opposite side.

All over the body the muscles are arranged in this way, one balancing an¬ other. There are muscles which pull down the lower jaw and open the mouth, while very much stronger muscles raise the jaw in biting. These latter muscles are so strong that the pressure between the back teeth when one is biting hard may be as much as two or three hundred pounds. There are muscles which bend the body forward and others which straighten it; these two sets work by turns when a man is rowing.

Muscle fibers. —Every muscle is made up of an immense number of tiny parts which we call fibers. These are about as large as hairs, and they are packed in bundles side by side. Each fiber is really a little muscle which can shorten and pull

VOLUNTARY OR STRIPED MUSCLE

Notice the relation between the fine blood vessels and the muscle fibers. (See also the il¬ lustration on page 33.)

MUSCLES .

to do its share in the work of the whole. Muscle fibers are often about an inch in length. Remember that lean meat is muscle. When we look at well-cooked pot roast or corned beef, we see how it falls apart in little strips. We say that it has a ‘grain.’ The fibers of the muscle run lengthwise of the grain, and when living muscle shortens, it does so in this direction.

What makes muscles contract. — The muscles that we have been talking about are often called voluntary. This is to express the idea that they usually do what we wil 1 them to do. The brain is the organ which we use when we think, and there are connections be¬ tween the brain and the muscles.

These connections are made by means of fine threads which we call nerve fibers. A muscle like the biceps has a great many of these fibers leading into it. They come to it from a great bundle of such fibers called the spinal cord, which extends down the back from the brain itself.

Voluntary muscles never work of themselves; they are al¬ ways called into action through their nerve fibers. If the fibers which go into a certain muscle have been accidentally cut or destroyed by disease, we say that the muscle is para¬ lyzed. This expression means that the owner cannot make it work at will. It remains idle and after a time wastes away.