Practical physiology;

Front matter

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Preface

I

Every human being is appointed to take the charge of his own body. He must supply its wants, direct its powers, regulate its actions, and thus sustain his life. This responsibility for the care of health takes precedence of all others, and requires the earliest attention to prepare to meet it. Before any one can have any use for other knowledge, he must know how to live. He should, there¬ fore, learn the nature and the wants of his frame, and of its various organs, even earlier than he studies the features of the earth, the science of numbers, or the structure of language ; for, before he can put these to any practical use, he must eat, and breathe, and move, and think. Physiology should, then, be included among the subjects of all school education, and thus preparation be first made for the fulfilment of the first responsibility of life.

This book has been written to aid youth and others in this prepa¬ ration. It is intended to be exclusively a practical work. It de¬ scribes only those organs, and teaches only those principles, which are necessary to be known for the correct management of our organs, the maintenance of health, and the preservation of life. All other anatomical and physiological matters, which, however in¬ teresting to the naturalist, or useful to the medical student, are not necessary for the government of our daily actions, are here omitted.

With the hope that the knowledge which may be gained from these pages will not only excite both youth and adults to feel their responsibility for the care of their health and lives, but guide them in the discharge of it, this work is respectfully offered to the schools and the public.

In order to make the principles of this book more easily under¬ stood, and the work more acceptable and useful, fifty-five new en¬ gravings are added to those which were in the former editions. These illustrations of the various parts and positions of the human body have been selected with great care, and designed by the best artists, with nice adaptation to the text. Several pages of an¬ atomical descriptions are also added to explain them.

In doing this, the great purpose before adopted in writing this book has still been kept in view, — of making Anatomy entirely sub¬ servient to the study of Physiology, and of making Physiology sub¬ servient to the knowledge of its uses and applications to the affair? and the circumstances of life, and of holding these up as the end of this study.

The great and sole object of this work is to teach the laws of life, the powers of the several organs, the limit of their strength, the way in which they are to be developed and sustained, their proper uses, and the certain and evil consequences that follow their mis¬ application.

For these purposes, it is necessary to learn, first, the general structure of the organs and parts which are submitted to our man¬ agement ; second, the law of their being and action ; and third, their application to, and connection with, the common affairs of life.

The first is limited, and is only preparatory to the second. The second also is limited, and secondary to the third. The last is almost infinite in extent and variety, and requires much study.

In sustaining the body with food, and drink, and air, — in defend¬ ing it with clothing and shelter, — in the use of our muscles and brain, — in applying the body and the mind to whatever purpose, we use some or all of our organs ; and the health and strength, or pain and weakness, the good or the evil consequences, must be in accordance with the wisdom and faithfulness with which we govern ourselves in these matters. Therefore, the knowledge of the Phys¬ iological Laws, and of their requirements, becomes of practical im¬ portance in every moment, and in all the circumstances of our being.

To all Teachers of Youth, this view of this study is respect¬ fully commended ; and they are urged not to be led away by the vain hope of making the subjects of their instruction mere anatom¬ ical scholars, to the neglect of that practical wisdom which alone will avail them in the government of their lives.

EDWARD JARVIS.

Dorchester, Mass., November 1851.

Part I. Digestion And Food

Chap. Page.

1. Food supplies the Growth and the Changes of Particles of the

Body. — Digestive Apparatus. — Mouth . . . 9

2. Saliva. —Mastication. — Pharynx. — (Esophagus. — Swallowing. 13

3. Stomach. — Gastric Juice. — Digestion. — Beaumont’s Observations. 18

4. Gastric Juice flows during Eating. — Measure of Food to be eaten.

— Hunger . 24

5. Motions of Stomach. — Heat during Digestion. — Pyloric Valve. 29

6. Time required for Digestion of various Kinds of Food . 33

7. Drink with Food impedes Digestion.— Effect of Bulk on Diges¬

tion. — Light and heavy Bread . 36

8. Chyme. — Alimentary Canal. — Lacteals. — Chyle differs with Food. 39

9. Man selects his Food. — Healthy Digestion comfortable. — Hun¬

ger needs the Attention of the Brain . 43

10. Appetite affected by State of Mind — Not identical with digestive

Power or Taste . 47

11. Great Eaters. — Hunger recurs when Blood wants more Chyle. —

Intervals between Meals. — Regular Hours of Eating best . 50

12. Breakfast should be before Labor or Exposure. — Dinner. —

Lunch. — Supper, injurious if late . 55

13. Quantity of Food should be larger for the Active and Laborious. . 60

14. Quantity of Food determined by Wants of System and digestive

Power. — Measured thus when we eat slowly. — Excessive Food oppressive. — Rapid Eating . 64

15. Appetite allowable, but not a Guide ; injurious if it governs Eating. 68

16. Exercise before or after Eating impedes Digestion . . 74

17. Cheerfulness at Meals. — Consequences of Abuse of digestive

Organs . 78

18. Animal and vegetable Food. — Climate and Season should affect

Diet . 80

19. Temperaments — should affect Diet . . . 83

20. Food according to Difference of Constitution, Age, and Habit ... 87

21. Digestibility and Nutritiousness of Food not identical. — Condi¬

ments and Alcohol exhaust Sensibility of Stomach. — No Rule of Diet for all . 89

Part II. Circulation Of The Blood, And Nutrition

1. Apparatus of Circulation. — Heart. — Arteries . 93

2. Veins. — Capillaries. — Pulmonary Arteries and Veins. — Double

Circulation . 100

3. Power of Heart. — Valves. — Arteries. — Quantity and Flow of

Blood varies with Circumstances, Exercise, States of Mind, and

Feelings . . . 105

1*

CONTENTS.

4. Object of Eating. — All Parts of Body formed out of Blood. —

Composition of Blood and Flesh. — Atoms of Body enjoy short Life, and are removed. — Absorbents . HO

5. Nutrients and Absorbents, active in Laborer, who needs more

Food. — Wens and Swellings . . . . • • 113

6. Young and Active have new Atoms of Flesh. — Old and Inactive

have old Atoms. — Unerring Precision of Nutrition . 117

Part III. Respiration

1. Wasted Particles carried out of the Body. — Composition of Blood

in right Side of the Heart. — Chest. — Ribs. — Spine. . . 120

2. Movement of Ribs. — Diaphragm. — Inspiration. — Expiration. —

Size of expanded and contracted Chest . 123

3. Lungs. — Organ of Voice. — Air-Vessels. — Coughing. — Respi¬

ration . 127

4. Waste Particles. — Carbon. — Air. — Oxygen. — Nitrogen. — Car¬

bonic Acid. — Blood absorbs Oxygen from Air and gives Car¬ bonic Acid . 132

5. Venous changed to arterial Blood. — Oxygen consumed, and Car¬

bonic Acid and other Matter given out, in Respiration. — Of¬ fensive Breath . 135

6. Respired Air unfit to be breathed again. — Air should have full

Proportion of Oxygen. — Air loaded with Carbonic Acid or Water can take no more from Lungs . 138

7. More Oxygen consumed, and Carbonic Acid given out in cold,

than warm Air ; by strong and cheerful than by consumptive and melancholy Persons . 141

8. Lungs must have Room to breathe . 114

9. Natural Shape of Chest most beautiful. — Corresponds to Size of

Body . 147

10. Frequency of Respiration. — Size of Chest and Quantity of Air in¬

haled, correspond to the Waste to be carried away . 150

11. Quantity of Air spoiled by Loss of Oxygen, by Carbonic Acid Gas,

and Vapor of Lungs and of Skin . 152

12. Want of fresh Air in Houses, Parlors, Sleeping-Rooms, Cabins.. 154

13. Foul Air in Workshops, Churches, Public Halls and School-

houses. — V entilation . 157

14. Connection between Fulness of Life and Respiration. . . . . 162

15. Breathing foul Air creates Weariness, depreciates Life, impairs

Constitution. — Drowning. — ■ Consumption . 165

16. Privation of Air. — Supply of Air for all Animals. — Plants give

Oxygen, and consume Carbonic Acid Gas . 168

Part IV. Animal Heat

1. Animal Heat not changed by Temperature of surrounding Matter.

— Blagden’s Experiments . ° . 171

2. Animal Heat generated within. — Warm and cold blooded Ani-

mals. Animal Heat connected with Respiration . 174

3. Latent and sensible Heat. — Internal Combustion . 177

4. Animal Heat increased by Exercise. — Dependent on good* Air

and Food. — Well-fed warmer than ill-fed. — Effects of Diet and Alcohol on internal Heat .

CONTENTS.

5. Effects of Disease, Fatigue, Age, Sleep, on Heat. — Amount of Heat prepared in a Day. — More in warm than cold Seasons. Winter and Summer Constitution . . . 185

Part V. The Skin

1. Skin, Cuticle, thickened by Friction. — Blisters. — Corns . 191

2. Cuticle. — Nails. — Hoof. — Horns. — Seat of Color. — True Skin

contains perspiratory Apparatus . 194

3. Perspiration, sensible and insensible ; Quantity . 198

4. Perspiration affected by State of Air. — Oily Excretion of Skin. —

Effect of tight Clothing . 201

5. Connection between Skin and Stomach, Lungs and Muscles.—

Effect of Cold . 204

6. Skin absorbs Food, Liquids, Medicine, Contagion, Poison . 207

7. Sense of Touch; impaired if Skin is foul; acute in the Blind.... 210

8. Skin regulates Heat by Perspiration.' — Sensation of Cold and

9. Quantity of Clothing affected by Habit . 216

10. Clothing, Need of, affected by Food, Digestion, Health, Air, Hab¬

its, Age . . 220

11. Clothing should be loose. — Linen, Cotton, Silk, Wool . 224

12. Flannel. — Cutaneous Excretions. — Foul Clothing. — Airing Cloth¬

ing and Beds . 226

13. Bathing makes Skin soft and healthy . 230

14. Cold Bathing. — Laborers need Bath . 233

15. Effects of Cold Bathing on Health. — Time and Conditions of

Bathing . 235

16. Sense of Touch made more acute by Bathing and Friction . 238

Part VI. Bones, Muscles, Exercise, And Rest

1. Bones, Composition of, in Childhood ; in Old Age ; have Blood-

Vessels and Nerves; subject to Growth and Decay; grow strong by Use . 241

2. Skeleton. — Bones. — Head. — Chest. — Spine. — Pelvis . 245

3. Arms. — Wrist. — Hand. — Leg. — Foot. — Arch. — Shape . 250

4. Joints. — Elbow. — Knee. — Shoulder. — Hip. — Structure. — Lig¬

aments. — Sprains. — Dislocations . 255

5. Muscles, Number, Action, Description, Use . 260

6. Muscles, Shape, Attachments, Arrangement, Action . 268

7. Muscles, Situation, sacrifice Power, Cooperation . 274

8. Muscles, Strength increased by Exercise . 278

9. Muscular Strength unequal ; Action increases Power in other

Organs . 282

10. Muscular Action strengthens Body and Mind. — Kinds of Exer¬

cise . 2S6

11. Exercise adapted to Strength, Effect of, on Weak ; on Dyspeptics.

— Gymnastic Exercises . 289

12. Kinds of Exercise. — Walking. — Exercise of Boys; of Girls. —

Effects of. — Habits of English and American Women. — Time for Exercise . 292

13. Exercise, Place for. — Should be regular. — Needed by All . 295

14. Labor, Limit of Power. — Proper Expenditure of Strength . 298

CONTENTS.

15 Effect of great Efforts. — Proper Exercise of Children and Youth. 302

16. Laborer needs healthy Organs of Digestion, Nutrition, Res¬

piration, and Skin. — Connection of Exercise with Brain . 304

17. Effect of Hope and Confidence, Doubt and Fear, Cheerfulness

and Melancholy, of Passion and Alcohol, on Strength . 307

18. Attitudes. — Spine, supported by Muscles, very strong. — Porters.

— Pedlers. — Burdens on Head. — Spine erect. — Centre of Gravity over Line of Support . 309

19. Erect Attitude best for Walking ; for Labor ; for Mechanics ;

Farmers. — Spine curved from side to side ; Bent by stooping.

— Students, Writers have curved Spine ; and Girls more than Boys . 315

20. Time for Labor. — Experiments. — Sleep, Time for; Effect of

Loss. — Disturbed by difficult Digestion . 321

Part VII. Brain And Nervous System

1. Brain. — Nervous System. — Spinal Cord. — Nerves . 325

2. Brain connected with all Parts by Nerves; receives Impressions

through them. — Sensations in Brain. — If Nerve is cut, Sensa¬ tion and Power cease . 330

3. If Nerve is touched or diseased, Sensation is excited in the Brain. 333

4. Pain in amputated Limbs. — Voluntary and involuntary Motion. 336

5. All Organs and Functions impaired when Brain is impaired. —

Brain not sensitive; subject to Growth and Decay; has large Supply of Blood; fatigued with Labor . 340

6. Day the Time for mental Labor. — Brain gains Power by Exer¬

cise ; weakened by Over- Action ; connected with other Organs — Effect of Alcohol . 344

7. Brain Seat of Mind and Affections. — Power of Mind limited by

Power of Brain. — Mind impaired by Indigestion, Hunger, Ex¬ cess of Food. — Cheerfulness favors Respiration, Digestion, and muscular Action . 349

8. Brain sustains physical and mental Actions. — Mind wrorks best

when Body is easy. — Uncomfortable Sensations interrupt Study . 353

9. Mental Action interrupted by moral Feelings, Anxiety, Fear. —

Proper and improper Motives for Study . 357

10. Various mental Powers affected by Education. — Precocious Chil¬

dren. — Effect of Study on Health. — Ill Health of literary Men. 361

11. Inequality of Powers ; affected by Education . 364

12. Habitual Actions easy. — Perfect Education. — Concentration of

Mind. — Vacations . 368

13. If Inequality of Powers be disregarded, Mind may be deranged.

— Insanity from Misuse of Mind, Dyspepsia, Cold, Over- Action . 373

14. Insanity from misdirected Education, religious Anxiety, per¬

verse Habits . 375

15. Day-Dreaming, Passion, Intoxication, Fright may cause Insanity.

— Various Grades of mental Health. — Sound Mind in sound Body . 377

16. Eye. — Composition. — Tumors. — Pupil. — Effect of Light. —

Tears . 380

17- Near-sightedness. — Spectacles. — Far-sightedness. — Diseases of

the Eye. — Use and Care of the Eye . 385

18. Ear. — Structure. — Hearing. — Deafness .. . 387

Conclusion . 391

PRACTICAL PHYSIOLOGY.

Chapter I

Giowthof the animal Body, and the Changes of its Particles supplied by Food. — Food converted into Flesh by the Digestive Apparatus and the Blood-Vessels. — Digestive Apparatus. — Mouth. — Teeth.

1. The animal body increases in size and weight from birth to manhood. The material which supplies this growth is the food which we eat. Beside this, there is another de¬ mand for food. During the whole of life, there is an inces¬ sant change going on in the particles that compose the body. It is a law established by the Creator from the beginning, that life cannot continue long in animal matter. The atoms, which compose the living body, receive the principle of life when they enter their appointed places, and become apart of the animal frame. They retain this principle but a short period, and, while they retain it, they perform their part of the work of life. But soon their work is finished, and then they yield their vitality, and give up their places to other atoms, that come to enjoy life, and work a while, and yield and de¬ part, as those that went before them had done.

2. This law of unceasing change is impressed upon all animal beings. In all, the particles are constantly going out through the skin and the lungs, and an equal quantity of matter must, therefore, be coming in through the mouth. If

PRACTICAL PHYSIOLOGY.

this be not djne, and the supply be unequal to the waste, the animal loses flesh, and the body diminishes in weight. This change of particles and waste of matter differ in different persons, and in different circumstances, as will be shown in the course of this book.

3. Here is a double necessity for the addition of matter to the body from without — the growth or increase of the animal body in its earlier years, and the waste consequent upon the changes of the particles through the whole period of exist¬ ence. The food supplies both these demands, and is there¬ fore necessary from the beginning to the end of life.

4. Food is not living flesh: much that we eat — bread, vegetables, fruit — has not even any resemblance, in its appearance or character, to flesh. Yet these matters — the vegetables and the lifeless meats — are converted into living flesh; and not only so, but into many and various kinds of flesh, as many and as various as enter into the composition of the human body. All this is done in part by the digestive apparatus, and in part by the blood-vessels.

5. The digestive apparatus effects the first change in the food ; it grinds it in the mouth, dissolves it and converts it in the stomach into a pulpy material fit to supply the wants of the blood, and sends the nutritious portions to the heart. The blood-vessels carry this blood to all the portions of the body, and with this they supply the growth and the waste of all the organs and textures.

6. This apparatus consists of the mouth and pharynx, the oesophagus or gullet, the stomach, and the alimentary canal. In the mouth are the lips; the teeth, the tongue, and the sali¬ vary glands. The pharynx lies back of the palate, between the mouth and the gullet. The oesophagus is a tube, that connects the mouth with the stomach. The stomach is a large sack, in which the digestive process is mostly performed. From the alimentary canal, the lacteals or absorbent vessels open. These take up the nutritious portion of the digested food, and carry it to the veins. Beside these organs, there are some others, such as the liver and the pancreas, which

DIGESTION AND FOOD.

render some assistance in the work of digestion. All these organs together form the digestive apparatus, which is com¬ plete and perfect in itself ; nothing is wanting for the work, and there is nothing unnecessary. Each one of these organs, or parts of organs, has a separate and distinct part to perform in the work of converting food into the nutriment of the blood.

7. The mouth is the first and only visible organ of digestion, and first receives the food from our hands and our tables. It is composed of several parts, all of wh;ch are employed in the preparatory work of digestion. The lips and the cheeks form the outward walls of the mouth, and retain the food after it is received. The teeth serve to divide and break down the morsel to a fineness suitable to the stomach. The tongue rolls the morsel about, and keeps it in its place be¬ tween the teeth, while it is undergoing the process of masti¬ cation or chewing.

8. The teeth are the most prominent organs in the mouth. They differ in various animals, according to the food upon which they live. The carnivorous or flesh-eating animals have teeth fitted for seizing upon their prey, and for cutting up flesh. Hence they have sharp cutting or front teeth; and long, sharp and pointed canine or stomach and eyeteeth; and grindefs, with high and sharpened points, by which they chew or masticate their fleshy food with facility.

9- The vegetable-eating animals have short, blunt, and strong front or incisor teeth, by which they break off their food, either grass or foliage. As they seize no prey, they have no use for the sharp canine or stomach and eye teeth ; therefore these teeth are very small, and in some scarcely seen ; but their molar or grinding teeth are very large, broad, and flat. Their surface is covered with slightly-raised lines, to enable them to grind down their food, which requires more crushing than cutting.

10. Man is neither herbivorous nor carnivorous exclu¬ sively, but is either or both, as occasion requires. Com¬ monly he is both. His food, for the most part, is a mixture of vegetables and flesh. He is therefore fitted with a set of

PRACTICAI PHYSIOLOGY.

teeth partaking of the nature of each of these classes. He has sharp incisor teeth, but they are not so long and pointed as those of the dog. His grinders are not covered with points as prominent and sharp as those of the lion, nor are they so flat on the surface as those of the ox ; yet they partake some¬ what of the character of both. He can chew either meat or grains, as he may desire.

Fig. I. Teeth of one Side.

11. Man has sixteen teeth in each jaw. The four incisor teeth (Fig. I. a, a) stand in front ; these are broad, flat, and somewhat sharp : with these he can bite or cut off his morsel of bread, meat, &,c. Next to these are the canine teeth, (Fig. I. 6,) one on each side ; these are commonly called the stomach teeth in the lower jaw, and eye teeth in the upper jaw. Next to these are the bicuspid teeth , (Fig. I. c, c,) two on each side, with two fangs ; and behind these are the molar teeth , (Fig. I. d, d , c?, c, c, c,) or grinders , three on each side : these have three fangs, as shown in Fig. I., in the upper jaw, d, d , c?, and only two in the lower jaw, c, c, c. These have pointed elevations sufficiently sharp to cut off meat, and sufficiently flat to grind vegetables and grains.

12. The teeth are firmly set in each jaw, with long fangs,

DIGESTION AND FOOD.

so that they are not easily started from their places. They are composed of soft bone within, but are covered on the outside with an enamel of very hard texture, which admits of an exquisitely fine polish. This enamel comes in contact with the food, the drinks, and the air. It will bear great variety of exposure, and resist the wear of great friction. Yet, when the mouth is neglected, the enamel is apt to decay. If any of the food, which has been masticated and mixed with the saliva, be suffered to remain about the teeth, this mixture undergoes a corrupting fermentation, and acts upon the hard enamel. After a while, a hole is eaten through this cover¬ ing ; and then, when this decay reaches the softer sub¬ stance of the body of the teeth, it acts much more rapidly.

13. This decay is assisted also by the gathering of the secretions of the mouth. The salts and acids of the saliva combine with the food, and form tartar, which covers parts of the teeth with a hard crust. This can be easily prevented by washing the teeth frequently, and completely removing all the food and other gatherings of the mouth, after each meal, and also in the morning, after an interval of sleep.

14. The teeth have blood-vessels and nerves; they are endowed with life; and, as most people have occasion to know, they also have an exquisite sensibility. When they are sound, they seem to have little or no sensation ; but when they are decayed, and exposed to extremes of temperature, to very hot or very cold matters, or even to the air, they suffer acute pain.

Chapter II

Salivary Glands. — Flow of Saliva. •— Mastication necessary. — Ef¬ fect of imperfect Mastication. — Pharynx. — (Esophagus. Fpi glottis. — Swallowing.

15. The teeth can only grind the food to powder. In this condition, it can neither be swallowed, nor is it fit for the

PRACTICAL PHYSIOLOGY.

next stage of the process of digest’on in the stomach. It must be not only crushed and divided, but it must be mois¬ tened and reduced to a pulpy consistence. For the purpose of supplying the necessary moisture, there is provided in the mouth a set of glands , which prepare and throw out the saliva or spittle, sufficient, in time of health, to moisten and soften all the food. These little glands are placed in the

Fig. II. Salivary Glands.

cheeks, (Fig. II. «,) and under the tongue, (Fig. II. b ,) and under the jaws, (Fig. II. c,) and open, through very minute tubes and apertures, into the mouth, (Fig. II. d^d.) When the cheeks and tongue are still, these glands are inactive, and throw out no more liquid than enough to keep the mouth moist. The presence of any matter in the mouth, the chewing of our food, tobacco, &/C., and any motion of the jaws, cheeks, or tongue, excite these glands, and induce the flow of their fluids. All these motions are entirely needless, except when we are eating or talking. They are under our control, and consequently the flow of saliva is under our command. If, then, we use the mouth only for its intended purposes of eating and conversation, it will be always moist, but never full of saliva; and then we should have no occasion to in¬ dulge in the unnatural and offensive habit of spitting.

DIGESTION AND FOOD.

16. The saliva flows, during the process of mastication, in some measure proportionate to the dryness of the food. The salivary glands have a very active sympathy with the appe¬ tite, and will sometimes send forth a flow of saliva at the mere presence of savory food, so that the common saying, that “ one’s mouth waters” at the sight of agreeable dishes, is physiologically true. These glands are important assist¬ ants in the masticating process, when they and the body are in health ; but in fevers, and in some other diseases, these organs refuse to act, and the mouth then is dry and parched.

17. By the united operation of the teeth and the salivary glands, the food is first ground into small particles, and then made into a soft pulp, in the mouth. Both these operations are necessary, for we cannot swallow the morsels unless they are divided and moistened, either naturally or artificially. Let any one attempt to swallow a mouthful of dry bread without chewing, or powdered cracker, without saliva or other fluid to soften it, and he will find, that this process, w’hich, when proper preparation is made for it, is one of the easiest and most agreeable, is now one of the most difficult and unpleasant. If the food be not divided while in the mouth, it cannot be done afterwards ; there is no machinery nor power in the stomach to effect this division.

18. In the back part of the mouth is a second chamber called the pharynx , separated from the anterior or front chamber by a movable curtain, called the palate. This curtain, hanging between these two chambers, is easily seen when the mouth is partly opened, and seems then to be the farthest boundary of the cavity. But when the mouth is stretched widely open, this curtain is raised, and the other chamber is disclosed behind it. A little knob or tongue of flesh hangs down from the middle of the palate, nearly, and sometimes quite, to the tongue, and partially or entirely divides the passage-way from the front to the back chamber. This gives the appearance of two passages, one on the right and the other on the left side.

19. Four passages open out from the back chamber; one

PRACTICAL PHYSIOLOGY.

leads forward to the mouth, the second upward and forward to the nose, the third downward to the windpipe and lungs, and the fourth downward and backward to the stomach. The oesophagus , or gullet , that carries the food and drink to the stomach, opens from the farthest part of the mouth, and lies behind the windpipe, next to the back-bone.

20. The windpipe opens between the tongue and the gullet, in the front part of the throat. It is seen and felt in all per¬ sons, and in some it is very prominent. As the windpipe lies between the tongue and the gullet, the food, going from the mouth to the stomach, must pass over it, and would be liable to fall in if there were not an effectual protection pro¬ vided against this accident. The windpipe is made of a number of stiff cartilages, and its mouth is always open ; but there is placed over this mouth a little clapper, or valve, called the epiglottis, which is fixed by a hinge to the front edge toward the tongue, and opens toward the oesophagus behind.

21. This valve usually stands open to allow the passage of air into and out of the lungs. But it is exquisitely sensitive ; when it is touched with any other matter, it falls down at once, and covers the aperture of the windpipe, and protects it from any intrusion. Whenever we swallow food or drink, the instant the morsel or the fluid reaches this valve, it falls, and allows it to glide over it, into the gullet behind, and then it rises again to give passage to the air.