Health, public and personal

Preface

This book aims to promote good health in individuals and in communities. To keep ourselves healthy in all situations of our complicated society to-day requires a clear understand¬ ing of the normal activities of the human body. For this reason the book starts with a brief explanation of the work¬ ing of the vital machine.

The functions of the body are then taken up in greater detail and are related to the life of the community. Rules of conduct for healthful living are developed and applied, with special emphasis on the reasons for these rules. Ancient traditions of doubtful value are discarded and all precepts are modern and scientific.

The topics are made as practical and interesting as possible, not only by simple, common-sense treatment, but by copious illustrations and searching questionnaires. These latter are put, not at the ends of chapters, but after each topic, so as to assure a mastery of the subject in hand, before the pupil proceeds.

Every effort has been made to furnish material which will enable the pupil to safeguard his own and the community’s health; yet the importance of the physician is recognized and consultation with him is urged in all but the very sim¬ plest cases of illness.

The development of civic spirit is one of the definite aims of this book. The dependence of community health upon personal health is constantly emphasized and the importance of quarantine and of precautions against contagion is con¬ stantly brought out.

iii

IV

PREFACE

The author is indebted to his sister, Miss Selma Anderson, for aid in planning illustrations, to Miss Mary Dixon, whose skillful pen and brush prepared the drawings, and to many friends who supplied photographs.

Thanksgiving, 1922.

R. E. B.

Chapter II. The Body at Work

1. Movements. 2. Food and digestion. 3. Oxygen, breath¬ ing. 4. Transportation, blood and lymph. 5. Elimination of wastes. 6. Regulation of temperature. 7. Control of the body. 8. Sensation. 9. Protection.

Chapter III. The Treatment of Disease .... 1. From superstition to science. Charms, mysteries, tradi¬ tion, pseudo-science, true science — its devoted workers and their wonderful accomplishments. 2. Health and sickness. Sickness and remedies, bacteria, the body’s defense, curing and avoiding disease.

Chapter XVIII. The Government and Health

Water supply, sewage, food inspection, building regulation, hospitals and sanatoria, health centers, health in school, quaran¬ tine, animal pests, nuisances, protection of workers, recreations, practitioners of healing, “patent medicines.” An ideal for the race.

PAGE

LIST OF ILLUSTRATIONS

FIGURE

The Sower.

•

Frontispiece

PAGE

Thatcher’s Glen ....

A Girl Athlete ....

McKinley Park, Sacramento, California

Examples of Cells

Cell Division ....

A Municipal Playground

The Framework of Man

Arm Muscles . .

Tendon Function

The Wrist.

Muscle Cells ....

Diagram of the Digestive Tract .

Gastric Glands ....

The Salivary Glands .

X-ray of the Chest

Respiratory Organs

Air Sacs of the Lungs .

Blood in a Capillary .

Phagocytosis ....

Iliya Metchnikoff

Diagram of Capillaries

Diagram of the Blood Circulation. Cobred

Kidneys .

Cross Section of a Nerve

Spinal Cord.

The Epidermis ....

Louis Pasteur ....

Sir Joseph Lister ....

Mold, Yeast, and Bacteria .

Effect of Soil Bacteria .

Chaulmoogra Tree, Leaves and Fruit

vii viii LIST OF ILLUSTRATIONS

FIGURE PAGE

34. A Sanitary Fountain.56

35. A Chaulmoogra Forest in Burma.57

36. Cost of Growth-food . ....... 62

37. Interior of Milk Station.67

38. A Food Preserver.72

39. Packing House Workers.73

40. Milk Receiving Station at Palatine, Ill.74

41. A Sanitary Counter.75

42. Home Pasteurizing ........ 77

43. Breakfast Bacon.78

44. Pork and Beans..80

45. Packing Houses.82

46. Skill and Endurance Impaired by Drink .... 87

47. Alcohol Impairs Scholarship.89

48. Alcohol and Pneumonia ....... 90

49. Smoking Handicaps Students.91

50. School Lunch. 94

51. Sections of Teeth ........ 98

52. Professor Theobald Smith.100

54. Diagram of a Villus.108

55. Junction of Small and Large Intestines . . . .110

56. Diagram of the Heart. Colored, .113

57. Valves in a Vein.115

58. Arteries of the Arm and Hand.115

59. The Malaria Mosquito.119

60. Major-General William C. Gorgas.121

61. A Section of the Head and Throat.125

63. Air Cavities of the Head.127

64. Before and After Removal of Adenoids .... 128

66. Air Good to Breathe.133

67. Conserving the Workman’s Health.135

68. Pollen Grains.13(j

69. The Smoke Nuisance.137

70. Smoke-begrimed.138

71. Window Open.140

72. A Humidifier. 142

73. Fresh-Air School.. . . .143

74. Breathing Movements ........ 144

LIST OF ILLUSTRATIONS ix

FIGURE PAGE

75. Diagram of Kidney Tubules.147

76. A Kidney Cut in Two.148

77. Diagrammatic Cross Section of the Skin . . • . 153

78. Parasites Which Infest the Human Skin .... 156

79. Paul Ehrlich.161

80. A Contrast.163

81. X-ray of a Foot in a Foolish Shoe.164

82. X-ray of a Foot in a Sensible Shoe.165

• 85. X-rays of Arches . '.167

86. Barefoot Tracks.168

87. Bobby Anderson.171

88. Hercules and Jason.172

89. A Young Champion.174

90. Home Exercises.179

91. Stand Straight.180

92. Sit Straight.181

93. Hygienic Desks.182

94. A Good Bearing.183 -

95. How to Bend Forward.183

96. Section through the Knee Joint.185

97. Both Legs Broken.186

98. Functional Areas of the Cerebrum.189

99. Nerve Currents.196

100. A Bird Haunt.200

101. Diagram of a Camera.202

102. The Eye in Its Socket. . . . . . . . 203

103. Changing the Focus.204

104. Near-sighted Vision.207

105. Far-sighted Vision.208

106. A Section through the Ear.212

107. Vaccination Protects.217

108. Swat the Fly.220

109. Emil Adolph Behring.221

110. Antitoxin Production.222

111. Antitoxin.223

114. Stop and Look!.239

115. Dangerous Play.241

X

LIST OF ILLUSTRATIONS

FIGURE

116. Machine Guard .

PAGE

. 244

Artificial Respiration .

. 255

A Poor Place to Live .

. 263

A Good Place to Live .

. 265

Zone the City!

. 266

Air and Sunshine.

. 267

A Hot Air Furnace

. 268

Hot Water Heat .

. 269

O-Cedar Oil Mop

. 270

A Sanitary Well .

. 271

Her Back Yard .

. 273

A Jacketed Stove in a One-Room !

School

. .277

Training for Health

. 278

Community Service

. 280

Garbage Collectors

. 281

United States Inspection

. 282

The Stamp of Approval

. 283

Sore Throat.

. 284

Crowded

. 286

A Healthful Home

. 287

A Fresh Air Hospital .

. 288

Homes for Health

. 289

Baby Day .

. 290

Medical Inspection

. 291

Measurements

. 292

Guarding the Threshold

. 294

The Paper Burner

. 296

Clean the Streets!

. 298

Drudgery That Destroys

. 299

Child Labor

. 300

Filling In

. 301

A Public Playground .

. 302

A Lake in Deer Park, Cook County, Chicago

. 303

A School Playground .

. 305

Rudolph Virchow

. 306

Camp Fire Girls in a Publicly Owned “Forest Preserve”

. 307

HEALTH

PUBLIC AND PERSONAL

Chapter I Life

Reflect that life, like every other blessing,

Derives its value from its use alone.

_ —Samuel Johnson.

The Boon of Health. — We want to work — to earn our living; we want to play — to enjoy our leisure; we want

Figure 2.— Thatcher’s Glen.

Here, under giant elms and cottonwoods, flanked by hawthorns and with the river in the rear, Chicago parties make holiday, enjoying their leisure.

LIFE peace and content in our resting hours. These we can have only as we have health. The material luxuries that money can buy are insignificant as com¬ pared with the boon of health.

It is told that John D. Rocke¬ feller once said he would give a million dollars if he could eat cheese.

The activities of a healthy body in work and in play are a source of satisfaction and joy, depend¬ ent on no one's gift. Elemental satisfaction lies not in what we get but in what we do. We can experience this satisfaction only by keeping ourselves able to do, by living in perfect health.

Health means a condition in which every part of the body is acting normally, doing its work perfectly. The working of the body, activity, is life. Therefore our first step in the study of keeping in health is to learn how life manifests itself in the body.

Keystone View Co. of N. Y.

Figure 3. — A Girl Athlete.

This high school girl of Panama is an ideal of physical excellence which any girl might well emulate.

How may the most elemental sat¬ isfaction in life be secured? What is health ? What is the first step in our study of maintaining health?

The Living Substance.— There

is a substance called protoplasm which exists throughout the bodies of plants and animals, and which is the only living material we know. In com¬ mon plants some of the protoplasm has a coloring sub-

ELEMENTS AND COMPOUNDS 3

stance in it which makes the leaves look green. In the blood most of the protoplasm contains a coloring substance which makes that fluid look red. Otherwise most protoplasm is nearly colorless; it has the appearance of raw white of egg.

What is protoplasm? How may its appearance vary in living things ?

Molecules and Atoms. — In order to understand the make-up of protoplasm we must know something of what chemists have learned about matter. Imagine a drop of water divided into particles as small as dust, then each of these particles again divided into many smaller particles, and these smaller particles divided again and again. This imaginary process would in time bring us to the smallest possible particle of water. This is called a molecule of water. The smallest particle of salt is a molecule of salt. The smallest particle of sugar is a molecule of sugar.

Every substance is composed of molecules, but they are too small to be seen by the most powerful microscope. How¬ ever, by means of an electric current we can divide the molecule of water into still smaller particles called atoms. But when we divide the molecule of water into atoms, wfe no longer have water. We have two gases, hydrogen and oxygen. Every molecule of water is made up of two atoms of hydrogen and one atom of oxygen. In chemistry H stands for hydrogen and O for oxygen, so chemists indicate this make-up of the water molecule by the formula H 2 0.

Explain what a molecule is. Explain what an atom is.

Elements and Compounds. — Since water is composed of two substances we call it a compound. But no one has ever been able to separate hydrogen and oxygen into any other substances, so we call them simple substances or elements. There are only about ninety elements known. All other substances in the world are compounds or mixtures of com¬ pounds. In nature compounds are constantly being broken down into their elements, and the elements reunited into

LIFE other compounds. This is called chemical change. It accompanies every activity of life.

Why is water called a compound? Why is oxygen called an element? About how many elements are known to chemists? Why is bread a mixture rather than a compound? What have compounds to do with life?

Composition of Protoplasm. — Since the activities of life consist (from the chemises standpoint) in building up compounds and breaking them down, and since these changes take place within the molecule of protoplasm, we must know something of the composition of that molecule if we are to understand the activities of the body. The molecule of protoplasm is one of the largest and most complex molecules known. Though it contains many hundred atoms it is chiefly composed of four elements, — carbon, hydrogen, oxygen, and nitrogen. These elements you have perhaps already learned, — carbon as charcoal, pencil leads, or di¬ amond, the other three as gases, hydrogen the lightest of all substances, not occurring free in nature, oxygen forming one fifth and nitrogen four fifths of the atmosphere.

These four elements are all contained in the food we eat. In the process of building up the protoplasm molecule, atoms of each element in the required number are taken from the food and joined together to form the living com¬ pound. In the breaking down process these atoms are released from their union in the large protoplasm molecule and make new combinations in smaller molecules. This is explained more fully on pages 10, 12, and 20.

Name the four chief elements of protoplasm. In what forms have you seen carbon? What sort of substance is hydrogen? What is oxygen and where does it exist free? Nitrogen?

Cells. — The protoplasm of a plant or animal is found in very small particles called cells. Each cell has a very thin covering or cell wall. Commonly some of the protoplasm which composes the cell is denser than the remainder and

CELLS is gathered together into a little body called the nu'cle-us. The nucleus can move about within the cell; it changes its form very much with some of the cell’s activities. These cells together with the fluids, fibers, and minerals which lie between them make up the body. They have been likened to the bricks which compose a wall. The comparison is

Figure 4. — McKinley Park, Sacramento, California.

This park, with its ball ground, tennis courts, boats, play apparatus, and club house, well exemplifies the modern thought that wholesome recreation is a necessity in healthful living.

good, though the cells do not have such a regular arrange¬ ment, and the “ mortar ” between them is sometimes more than the “ brick.”

On the other hand, the differences between the cells of the body and the bricks of a wall are very great. Since protoplasm has the power of movement, the cell is an active bit of matter. Muscle cells, for example, have the pecul¬ iar power of contracting to produce movements of the body. The cells of the sweat glands produce perspiration. In

LIFE short, the cells are more than building material; they do. all the work of the body.

Cells have the power of communicating with neighbor cells. They also respond to outside influences. Changes of light and of heat, an electric current, a touch, produce' responses in the protoplasm of cells.

Of what is a cell composed? What is the nucleus of a cell? In what respects are the cells of the body and the bricks of a wall similar, and in what important respects do they differ?

Sizes of Cells. — In size cells differ greatly. A large cell is one fiftieth or one hundredth of an inch in diameter. A common sized human cell is one two-thousandth of an inch across. Some small cells have a diameter of one ten-thou¬ sandth of an inch. The body contains cells of many different sizes. Those of adults are no larger than those of infants, but there are more of them.

How many large cells side by side would extend an inch? How many medium sized cells would there be to an inch? How many small ceffs? Do you grow simply by increase in the size of cells?

Form and Arrangement of Cells. — In form and arrange¬ ment cells are adapted to the work they have to do. If a cell’s work is to exert a pull by shortening itself (muscle) it will be long and narrow. If a cell is to receive and send out communications (nerve cell) it will have branches for many contact points. If a group of cells is to act as a pave¬ ment on an exposed surface, as the skin and the lining of the mouth, each cell will be thin and flat and will overlap its fellows, like shingles. If cells are to secrete a watery fluid they will be plump and arranged in a single layer around the tube which carries off the fluid. If a cell is to be used for storage it will be about as nearly spherical as it can be with the other cells pressing against it.

Why have cells various shapes? Give the reason for the shape of each cell in the accompanying figure.

FORM AND ARRANGEMENT OF CELLS

Figure 5. — Examples of Cells.

A. Three muscle cells, each with a nucleus, magnified 300 times. B. A train cell, magnified 400 times. C . A cross section of a stomach gland magnified 200 times, and a lengthwise section of the same gland. D. Fat cells, each containing a droplet of oil, magnified 400 times. E. A bone cell, magnified 1000 times.

1. About what portion of an inch in diameter is the drawing of a fat cell?

2. Since the drawing is 400 times the size of the real cell how large would the cell be ?

3. Compute the size of each kind of cell here shown.

4. How are the cells of the stomach gland arranged with reference to the tube which carries away the fluid they secrete ?

5. Does the form of a brain cell adapt it to receive nerve currents from one source, or from many sources ?

6. Which kind of cell is best adapted to contracting (shortening) ?

LIFE

Tissue. — Cells in some cases fit tight against one another, and in other cases are separated by a substance lying be¬ tween them. Bones, for example, have a stony matter in the considerable spaces between the cells, and tendons, the straps that fasten muscles to bones, are largely composed of stout threads which lie between the cells. Substances that lie between the cells are called intercellular material. A group of cells of any kind, together with their intercellu¬ lar material, is called tissue. We speak of brain tissue, liver tissue, kidney tissue, bone tissue, cartilage tissue, fat tissue, etc.

Give a definition of tissue. Name examples of tissue. What is the meaning of the prefix inter in the word intercellular %

Connective Tissue. — In bones, in tendons, and in the tough tissue that holds the bones together, the quantity of intercellular matter is large. This makes the tissue very strong. Such tissue is called connective tissue. It supports and binds together parts of the body. The strong inter¬ cellular material is the chief part of this tissue, and its cells serve to produce and repair it. On the other hand, in the brain, the muscles, and the glands, the cells are the impor¬ tant part of the tissue. Only a little intercellular fibrous structure is needed to support the cells, since it is the func¬ tion, or duty, of these tissues to work, not to serve as sup¬ porting or binding material for other tissues.

What is the characteristic structure of connective tissue ? What is its work? Why are there intercellular fibers in other tissues? What makes bones and tendons so strong? Why do the brain cells, and glands need but little connecting fiber in their make-up ?

Lymph. — In little spaces which are distributed through all tissue there is a watery fluid called lymph. The lymph is the liquid part of the blood which has oozed through the walls of the smallest blood tubes, which are called cap'- illaries. As soon as this liquid has passed from the cap-

WHAT LIFE IS illaries into the intercellular spaces it is called lymph. As it flows slowly about among the cells, it brings them food and oxygen, and gathers up their wastes. Then with its load of wastes it goes back into the blood.

What is lymph? De¬ scribe its movements.

Summary. — You will find the following ways of thinking of the body convenient and helpful:

The body is a sack of moving liquids (blood, lymph) encased in a skin almost water-tight. It is shaped over a frame¬ work of connective tis¬ sues (bones and their binding straps). On this support the softer and more active tissues are fastened. It is every¬ where alive because in all its parts is protoplasm in cells which are ar¬ ranged either side by side in compact masses or with intercellular sub¬ stances between them.

These seven views represent a cell dur¬ ing the process of division. The star-like body seems to be a center around which the activities proceed. The granules of the nucleus in A have in B arranged them¬ selves in lines, — a certain definite num¬ ber for each plant or animal. In cell division each line of the nucleus divides, thus preserving the characteristic number.

The body is an aggregation of mu¬ tually dependent living beings (cells) immersed in a salt sea (lymph, blood).

What Life Is. — We all have a hazy notion of what life is, but we can get a more definite idea of it by learning the activities common to all life. In the first place motion — movement arising from a power within'—is a character-

LIFE istic of life. We distinguish a living animal from a dead one by its power to move. In plants and in animals which grow fixed, with no power of locomotion, there is nevertheless movement wdthin the cells. Movement is characteristic of all life.

Another distinguishing feature of life is growth. Growth consists of several processes. Every cell at some time as¬ similates food, that is, it takes various food substances, breaks up the molecules of which they are composed, and combines the atoms so obtained into molecules of proto¬ plasm, thus increasing the quantity of live protoplasm in the cell. Another process in growth is cell division. Cells do not increase in size indefinitely. When a cell reaches a certain limit it divides into two cells, each complete but of small size. Each small cell assimilates food until it reaches its limit, then divides. A third process in the growth of tissue consists in the production of intercellular material. The cells take from the food the substances required and build them into intercellular fibers or mineral.

Another characteristic of life is the cell activity called oxidation. In the inanimate world the burning of fuel is an example of oxidation. The oxygen of the air combines with certain elements of the fuel, producing heat. Heat is a form of energy —which is another way of saying that heat has the ability to do work.

In the animate world the process of oxidation in the cell is practically like the burning of fuel in a furnace. Oxygen is brought into the cell and combines with some of the mole¬ cules, burning them up and so transforming their energy into heat and activity. Without oxidation in the cell we could have no muscle work, no brain work. Oxidation in the cell is as necessary to any bodily activity as oxidation in the fire-box of an engine is necessary to mechanical motion.

Life, then, consists of certain activities. Those discussed

THE CYCLE OF LIFE above motion, growth, and oxidation — are common to all cells. The peculiar duties of various kinds of cells you will learn as you study the functions of the different parts of the body.

Name the activities common to all cells. What, is meant by assimilation? What is energy? What is oxidation? Name three processes involved in growth.

Figure 7. — A Municipal Playground.

These children are experiencing a healthful breaking down of muscle cells. Every city should provide its children a safe place to play in the fresh air. It is a matter of health, safety, and education.

The Cycle of Life. — The chief activities of all cells may be restated as an alternation of processes, building up fol¬ lowed by breaking down. A muscle cell builds up by as¬ similating food. Then it breaks down by oxidation of many of its molecules in order to supply the energy for muscular contraction. A brain cell while you are sleeping assimilates food till it becomes plump and full; then while you are thinking or working it oxidizes, molecule after molecule,

LIFE until it becomes ragged and nearly worn out. Cells build up that they may break down. The breaking down is the end for which the cell exists. Thus life is a cycle of chemi¬ cal changes.

What is meant by the cell’s cycle of life? Why is it desirable that a cell break down?

Cell Needs. — Out of the activities of the cells arise cer¬ tain needs. If the cell is to grow or to replace its brokendown parts it must have food. In order to break down by the process of oxidation the cell must be supplied with oxygen. In this process of oxidation certain wastes are pro¬ duced just as ashes and gases are produced in a furnace. The chief of these wastes are carbon diox'id (a gas resulting from the oxidation of carbon), water (produced by the oxi¬ dation of hydrogen), and certain substances containing nitrogen. Just as ashes must be removed to prevent their choking the fire and putting it out, so these wastes must be removed from the cell. This is done by the blood and lymph which move past the cell.

Another need of the cell is suitable temperature. Pro¬ toplasm can live only within certain temperature limits. These limits differ much for different cells. Our skin cells can endure a temperature of freezing for a short time and also a temperature of hot water, perhaps 120 degrees. The cells of the brain, liver, and other internal organs, when the body is in a healthy state, do not vary more than a degree or two from the normal, 98.6 degrees in the mouth. In sickness their temperature may go up or down ten or twelve degrees, but such changes are very serious.

List under four heads the needs of the cell. Why is food needed ? Why is oxygen needed? Why must waste be removed from the cell? Discuss the range of temperature tolerable to protoplasm.

Supplying the Needs.— Every cell in the body has the needs named above. The body as a whole provides for the

SUPPLYING THE NEEDS needs of its own cells. It supplies the food and the oxygen, removes the wastes, and adjusts the temperature for the various cells. How it supplies its own needs is the problem, or rather series of problems, that will occupy our attention in this course.

The body differs from man-made machines in that it is self-managing and self-repairing. The parts all work in harmony to provide just the conditions necessary for the work of the whole human machine. If we are to be our own engineers we shall have to study the machine carefully, that our treatment of it may keep it working smoothly and not upset its delicate adjustments.

How does the body differ from man-made machines? Why do we need to study the human mechanism carefully?

Chapter II The Body At Work

A sound mind in a sound body is a short but full description of a happy state in this world.

— John Locke.

The Activity of the Body. — In preparing to study this machine, our body, we ought first to get a general idea of how it works. To give such an idea is the aim of this chap¬ ter. Any plant or animal is called an organism. Its parts all cooperate to supply its own needs. The human organism is the body working as a unit, all parts cooperating to pro¬ vide for the needs of every part.

As we go on in the succeeding chapters to take up in de¬ tail various studies in individual and community health, we shall understand them the better for being able to relate them to the organic activities of the body. We can not get the best out of the practical studies that follow unless we first learn thoroughly the organic activities in this chapter.

To make the chapter simpler and clearer it is divided into nine sections, which illustrate the various functions of the body. These are: (1) movements, (2) food, (3) oxygen, (4) transportation, (5) elimination of wastes, (6) regula¬ tion of temperature, (7) control of the body, (8) sensation, and (9) protection.

Section 1. Movements

Transformation of Energy. — The muscles and bones compose most of the body. They have for their function the movements of the entire body or of its parts. This

THE FRAMEWORK OF MAN

Figure 8.— The Framework of Man.

means the transforming of energy, which is supplied in the form of food and oxygen. The food must be obtained, taken in, prepared for the cells (digested), and transported by the blood along with oxygen to the muscle cells. There it is oxidized to liberate energy, and this energy the cells transform into motion.

Muscles and Tendons. — The muscle is composed of long cells or fibers. To produce motion the fibers contract.

Figure 9. — Arm Muscles.

These muscles are used in bending and in straightening the elbow.

This draws on the bone to which the muscle is fastened and so produces movement in the skeleton. For example, when the arm is held out straight the forearm may be drawn up¬ ward and backward toward the head by the contraction of the muscle called bl' ceps acting on the bones of the arm as levers. It is thrust out again by the contraction of an¬ other muscle called trl'ceps. Muscles exert force only in contraction.

The muscles are fastened to the bones sometimes directly and sometimes by means of tendons. The tendons, as has been said, are composed chiefly of long parallel threads of

INVOLUNTARY MOVEMENT intercellular fibers, the toughest structure in the body. By means of long tendons, muscles can be situated at some con¬ venient place and yet produce movement in a part of the body at a distance. For example, the muscles that bend the fingers are in the arm near the elbow, |

and the muscles that lift the toes are in the shin.

Bones and Ligaments. — To serve as strong levers for the movements of the body the bones are made stiff by stony material like marble or limestone, which is deposited between the cells all through the bone. Stony matter alone would be brittle. To make the bone tough there is mingled everywhere with the stone be¬ tween the cells an animal or gristly ma¬ terial. Where two bones meet at a mov¬ able joint they are fastened together by strips of tissue like tendon called liga¬ ments. The joint cavity is lined with smooth membrane, the synovial sac, always kept slippery by a watery fluid, so that there is practically no friction in the movement.