A treatise on physiology and hygiene : for schools, families, and colleges

Front matter

Open Knowledge Commons and Harvard Medical School

„y

CIECULATION THEOUGH THE HEAET AND LUNGS.

Preface

This book is intended as a means of instruction in Physiology and Hygiene for pupils and general readers who have no previous knowledge on medical subjects. The writer has endeavored to arrange the statements and descriptions in such a way that no anatomical or physiological term should be employed, the meaning of which has not been already explained in the text. For convenience of reference, however, in cases where this explanation may have been overlooked or forgotten, a Glossary is added in the latter part of the book, which contains the meaning of all professional terms employed in the body of the work. A large proportion of the complicated names and phrases required for the complete study of Anatomy and Physiology are entirely unnecessary for those who pursue it simply as a part of their general education— that is, in the same way as they study Geography, Astronomy, or Mathematics. The most important, and, at the same time, the most interesting facts of Physiology may be taught with success in a perfectly simple manner, provided they be given in the proper order and in their natural relation to each other. It has been the aim of the present work to accomplish this object ; and if it be found to have succeeded in that respect, it is hoped that it will be an assistance to the teachers, parents, and pupils for whose use it was designed.

New York, 18G8.

Chapter I. General Structure And Mechanism Op The Animal Frame

The Bones. — Their Composition. — Their Structure. — The Skeleton, — Spinal Column. — The Skull. —Pelvis. — Tibia. — Bones of the Foot. — Curvatures of the Skeleton. — Elastic Ligaments.

The Muscles. — Their Contraction. — Relaxation. — Flexor and Extensor Muscles. — Tendons. — Movement of Joints. — Walking. — Running. — Leaping. — Elasticity of the Skeleton. — Exercise of the Muscular System. — Its Repose Page 19

Chapter II. The Food And Its Ingredients

Inorganic Substances. — Water. — Salt. — Lime. — Other Inorganic Matters. — Starch. — Its different Varieties. — Properties of Starch. — Its conversion into Sugar. — Sugar. — Whence obtained. — Proportion in the Food. — Fermentation of Sugar. — Fats. — Stearine. — Margarine. — Oleine. — Crystallization of Fat. — Emulsions. — Condition of Fat in Animal and Vegetable Tissues. — How extracted. — Adipose Tissue. — Proportion of Fat in the Food. — Albuminous Matters. — Different kinds. — Coagulation. — Ferments. — Putrefaction.— Proportion of Albuminous Matters in Food. — All the nutritious substances necessary to Life *.. ...... 45

Chapter III. The Different Kinds Of Food, And Their Mode Of Prepara- Tion

Meat — its Composition. — Effect of Cooking. — OEggs. — Composition of the White and Yolk. — Milk — its Composition. — Butter — how obtained. — Cheese — its Preparation. — Bread — how made. — Yeast. — Fermentation of Bread. — Baking. — Wine — its Fermentation.— Beer. — Vegetables. — Effect of Cooking on Vegetables. — Essential Qualities of Food. — Necessary Quantity 72

Vlll CONTENTS.

Chapter IV. Digestion

Necessity for Food. — Nature of Digestion. — Alimentary Canal — its different Parts. — Digestive Fluids. — Mastication. — The Teeth. — Incisors. — Canines. — Molars. — Their different Functions. — Saliva.— Salivary Glands. — Composition of Saliva — its double Function.— Action of the Tongue. — (Esophagus — its Peristaltic Action. — Deglutition. — The Stomach — its lining Membrane. — Gastric Tubules.— Gastric Juice — its Secretion. — Peristaltic Movements of the Stomach. — Composition of the Gastric Juice. — Pepsine. — Lactic Acid. — Action of Gastric Juice on the Food. — It is a Ferment. — Digestibility of Food. — Food should be properly cooked — should be taken in moderate quantity — with regularity. — The small Intestine. — Follicles of Lieberkuhn. — Intestinal Juice — its action on Starch. — Pancreatic Juice — its Composition — its action on Fat. — The Chyle. — Peristaltic Movement of Intestine. — Changes of Food in the Alimentary Canal. — End of Digestion Page 83

Chapter V. Absorption

Lining Membrane of Intestine. — Valvulae Conniventes. — Villi. — Endosmosis. — Absorption by Living Membranes. — Influence of the Circulation. — Blood-vessels of the Intestine. — Portal Vein. — Blood of the Portal Vein during Digestion contains Albuminose, Sugar, and Chyle. — Change in its appearance owing to Chyle. — Distribution of Portal Vein in the Liver. — Reabsorption of the Digestive Fluids. — Ljmphatic Vessels. — Lacteal Vessels. — Appearance of Lacteals during Digestion. — Receptaculum Chyli. — Thoracic Duct. — Discharge of Digested Matters into the Blood. — Their Transformation and Disappearance. — Nutrition of the Blood 119

Chapter VI. The Liver And Its Functions

Situation and Structure of the Liver— its Lobules.— Biliary Ducts.— Secretion of the Bile. — Gall-bladder. — Accumulation of Bile in the Gall-bladder — its discharge into the Intestine. — Appearance and Composition of the Bile. — Biliary Salts. — Mode of extracting them from the Bile. — Their Crystallization. — Changes of the Bile in the Intestine — its absorption by the Blood. — Function of the Bile. — Formation of Sugar in the Liver — its absorption by the Blood-vessels.— Sugar finally decomposed in the Circulation 134

CONTENTS. IX

Chapter VII. The Blood

Ingredients of the Blood. — Water. — Salt. — Lime. — Albumen. — Properties of Albumen. — Fibrine — its Properties. — Blood Globules — their Form — Size — Color — Consistency. — White Globules. — Quantity of different Ingredients in the Blood. — Coagulation of the Blood. — Clot. — Serum. — Coagulation dependent on the Fibrine. — Use of Coagulation in stoppage of Bleeding. — Why Blood does not Coagulate in the Vessels. — Daily Production and Decomposition of Fibrine. — Entire Quantity of Blood in the Body — its Variation. — Variation in Composition of the Blood. — Two different Kinds of Blood in the Body Page 145

Chapter VIII. Respiration

The Oxygen of the Air — its Necessity to Life. — Nature of Respiration. — The Lungs — their Structure. — Larynx. — Trachea. — Bronchial Tubes. — Lobules. — Air Vesicles. — Movement of Inspiration. — The Diaphragm — its Contraction. — Entrance of the Air. — Intercostal Muscles. — Motion of the Ribs. — Movement of Expiration. — Elasticity of the Lungs. — Quantity of Air used in Respiration. — Movements of Respiration involuntary. — Effect of Respiration on the Blood. — Its Change of Color. — Venous Blood. — Arterial Blood. — Absorption of Oxygen in the Lungs. — Loss of Oxygen in the Tissues. — Carbonic Acid — where formed. — Discharged with the Breath. — Animal Vapor. — Watery Vapor. — Ventilation. — Ventilation by Doors and Windows — by Fires and Chimneys — by other Means. — Necessity for complete Ventilation 158

Chapter IX. The Circulation

The Organs of Circulation. —The Heart— its Muscular Fibres.— Auricles. — Ventricles. — Pulmonary Artery. — Aorta. — Movement of the Blood through the Heart. — Contraction of the Heart — its Relaxation. — Valves of the Heart. — Ventricular Valves. — Semilunar Valves. — Action of the Heart involuntary. — The Arteries. — Arch of the Aorta. — Distribution of the Arteries — their Elasticity. — The Pulse — how felt. — Rapidity of the Pulse. — Pressure on the Blood in the Arteries. — The Capillaries. — Capillary Network. — Circulation of the Blood in the Capillaries. — The Veins — their Valves. — Movement of the Blood in the Veins. — Obstruction of the A 2

X CONTENTS.

Circulation by compressing the Veins. — Rapidity of the Circulation.— Its local Variations Page 182

Chapter X. Animal Heat

Warmth of Living Bodies — how preserved. — Production of Animal Heat. — Warm-blooded Animals. — Cold-blooded Animals. — Temperature of the Human Body. — Source of Animal Heat. — Difference in Temperature of the Internal Organs. — Animal Heat necessary to Life. — Effects of cooling the Blood and Internal Organs. — Regulation of Heat by Perspiration. — Perspiratory Glands. — Composition of the Perspiration. — Insensible Transpiration. — Increase of Perspiration by Heat. — Effect of its Evaporation. — Effect of heating the Blood above 100° Eahrenheit. — Solid Ingredients of the Perspiration. — Sebaceous Secretion. — Necessity for frequent Bathing 200

Chapter XI. Nutrition

Definition of Nutrition. — Its three different Parts. — Assimilation. — Absorption of Inorganic Ingredients. — Their different Proportions in -the Tissues and the Blood. — Different kinds of Albuminous Ingredients.— Transformation of the Albumen. — Secretion. — Structure of a Gland. — Follicles. — Lobules. — Ducts. — Inorganic Ingredients of a Secretion. — Its Albuminous Ingredient. —Increase of Secretion with excitement of the Circulation. — Excretion. — Alteration of Animal Tissues during Life. — A necessary consequence of Activity. — New Substances so produced. — Carbonic Acid. — Animal Vapors. — Urea. — Creatine. — Creatinine. — Urate of Soda. — Double Movement of Nutrition.— Supply and Discharge. — Quantity of Material absorbed and discharged in twenty-four hours... 210

Chapter XII. General Structure And Functions Of The Nervous System

United Action of the Organs. — System of Telegraphs. — Nervous Filaments.— Nerves. — Spinal Cord. — Spinal Nerves. — Brain. — Cerebrum. — Cerebellum. — Medulla Oblongata. — Cranial Nerves. — Nervous Fibres are Organs of Communication. — Irritability of Nerves. — Sensitive Fibres. — Motor Fibres. — Nerve Cells. — Nervous Centres. — Reflex Action of the Nervous System 227

CONTENTS. Xi

Chapter XIII. The Spinal Nerves And Spinal Cord

Function of the Spinal Nerves. — Sensation and Motion. — Paralysis from injury of Nerve. — Restoration of a divided Nerve. — Anterior and Posterior Roots of Spinal Nerves. — Anterior Roots motor. — Posterior Roots sensitive. — Anterior and Posterior Columns of the Cord. — Anterior Columns motor. — Posterior Columns sensitive. — Connection of Spinal Cord with Brain. — Paralysis from injury of Spinal Cord. — Paraplegia. — Crossed Action of the Spinal Cord. — Hemiplegia. — Intercostal Nerves. — Phrenic Nerve. — Sensibility of the Skin — its Variation in different Parts. — Ordinary Sensibility. — Sensibility to Pain. — Sensibility to Heat and Cold. — Spinal Cord as a Nervous Centre. — Its Gray Matter. — Reflex Action of the Spinal Cord. — Its Character and Function Page 238

Chapter XIV. The Cranial Nerves

Character of the Cranial Nerves. — Sensibility of the Face. — Fifth Pair of Cranial Nerves — its three Branches — their Distribution — their Sensibility. — Neuralgia. — Tic Douloureux. — Motor Branch of the Fifth Pair. — Muscles of Mastication.— Temporal. — Masseter. — Pterygoids. — Movements of Expression. — Facial Nerve — its Distribution — its Motive Power. — Paralysis of the Face. — Hypoglossal Nerve. — Movements of the Tongue. — Pneumogastric Nerve — its Origin and Distribution. — Pharyngeal Branch. — Act of Deglutition.— Sensibility of the Pharynx. — Protection of the Nostrils in Deglutition. — Laryngeal Branches of the Pneumogastric. — The Glottis — its Movements in Respiration. — Protection of the Glottis in Deglutition. — Formation of the Voice. — Action of the Larynx 256

Chapter XV. The Brain

Form of the Brain — its Anatomical Divisions. —Medulla Oblongata. — Cerebellum. — Pons Varolii. — Tuber Annulare. — Cerebrum.— Functions of the Cerebrum. — Memory. — Judgment. — Reason. — Effects of Injury to the Cerebrum. — Functions of the Tuber Annulare.— Sensation and Volition. — Instinctive Movements. — Functions of the Medulla Oblongata. — Movements of Respiration — how performed. — Reflex Action of Medulla Oblongata. — Effects of Injury to Medulla Oblongata. — Different kinds of Reflex Action in the Brain 282

Xll CONTENTS.

Chapter XVI. System Op The Great Sympathetic

General Structure of the great Sympathetic — its Ganglia — its Nerves. — Arterial Plexus — their Distribution — connection with the Cerebrospinal System.— Slow operation of Sympathetic Nerves. — Effect of Cold and Wet. — Inflammation of the Internal Organs. — How to avoid the effects of Exposure. — Different kinds of Reflex Action through Sympathetic and Cerebro-spinal Systems Page 297

Chapter XVII. The Special Senses

Definition of the Special Senses. — Nerves of Special Sense. — Organs of Special Sense. — The Sense op Sight. — Optic Nerve. — The Eyeball— its different Parts. — Field of Vision. — Line of distinct Vision. — Estimation of Distance. — Solidity and Projection. — Stereoscope. — Thaumatrope. — Internal Impressions. — Muscles of the Eyeball. — Eyelids. — The Tears. — Winking. — Meibomean Glands. — Lachrymal Canal. — Practices injurious to the Sight. — Sense op Hearing.— Auditory Nerve. — Labyrinth. — Tympanum or Drum. — Chain of Bones. — Eustachian Tube. — External Ear. — Direction, Contrast, and Pitch of Sounds. — Sense op Smell. — Olfactory Nerves. — Nasal Passages. — Turbinated Bones. — Two kinds of Sensibility in Nose. — Uses of the Sense of Smell. — Sense op Taste. — Papillae of Tongue. — Two kinds of Sensibility in Tongue.- — Glossopharyngeal Nerve. — Uses of the Sense of Taste 305

Chapter XVIII. Development

Definition of Development. — Newly-born Infant — its Weight — condition of its Skeleton. — Ossification of the Skeleton. — Spinal Column.— Skull. — Fontanelles. — Pelvis. — Long Bones. — Respiration of the Infant — its Digestion and Nutrition — its Nervous System. — Reflex Actions. — Formation of the Teeth. — First Set. — Childhood. — Muscular System of the Child. — Nervous and Mental Functions. — Preponderance of the Instincts. — Formation of new Set of Teeth. — Youth. — Continued Ossification of the Skeleton. — Union of the Bones. — Completion of Development 359

Introduction

Physiology is the science which teaches us the natural actions of the living body, and the manner in which they are performed. It is very important for us to know what these actions are, and how they take place, for two reasons :

First, because we learn in this way what kind of work the living body is capable of accomplishing, and to what uses it is adapted, so that we may be enabled to employ our natural powers to the best advantage, and not waste them by attempting what is useless or impracticable.

Secondly, by understanding the natural actions of our bodies, and how they are maintained in a healthy condition, we are enabled to avoid injuring them by improper treatment, and thus actually diminishing our vital powers, or exposing them to debility and'disease. Nothing can be more important than this either to our comfort, our usefulness, or even our mental improvement. For the bodily frame is the organization by which our animal life is maintained, through which all our knowledge of the world about us is acquired, and by means of which alone all our designs are carried into execution. The knowledge of the mode in which health is to be maintained, and the bodily and mental powers kept unimpaired, constitutes the science of Hygiene.

XIV INTRODUCTION.

The study of physiology, therefore, leads directly to that of hygiene, and the two are necessarily associated with each other.

In examining the bodily frame, we easily see that it is composed of various parts, which differ from each other in size, appearance, texture, and location. In one region, for example, there is the heart, in another the liver, in another the brain ; while the surface is every where enveloped by the sensitive expansion of the skin. These different parts or portions of the body are called its Organs. They are connected in such a way as to give to each other a mutual support, and to form by their union a complete bodily organization.

Now each one of these organs is intended for a particular use, which is necessary in some way to the maintenance of life. Thus the heart circulates the blood, the. lungs breathe, the liver produces the bile, the stomach digests the food, and the brain directs the movement of the limbs. The particular act or duty which each organ performs in this way is called its Function. Each function is different from everv other, just as the organ which performs it is different from the rest; but they are all necessary to the healthy action of the whole body. For the different functions are associated together like the different organs that perform them, and are in the same way mutually dependent. Thus the liver can not produce bile unless it be properly supplied with blood, and the blood will soon become poor and useless unless the stomach continues to digest the food.

INTRODUCTION. XV

The whole body, accordingly, is made up of different organs, united with each other, and performing at the same time a variety of functions. It is a kind of workshop in which various duties are performed by different workmen, while all combine to produce the final result. This result is the complete and healthy activity of the entire frame.

While studying, accordingly, in their regular order, the various animal functions, we find that they are naturally arranged in certain groups, which are distinguished from each other by the nature of the acts performed and the object to be accomplished by them. Some of these acts are comparatively simple, others more peculiar and complicated. We shall find that their study is facilitated by taking up first the more simple of the animal functions, and afterward in succession those which are more complex.

These functions will accordingly be described in four different groups or sections.

Section I. — The animal functions belonging to the first group are entirely Mechanical in their nature. They are those by which the body .is held together by the articulation of its various parts ; by which the internal organs are protected from mechanical injury; and by which the voluntary movements of the limbs and trunk are performed. This group comprises the functions of the bones, the cartilages, the ligaments, the tendons, and the muscles.

Section II. — The second group of functions are those which are physical or chemical in their nature, and which provide for the nourishment or Nutrition

XVI INTRODUCTION-.

of the body. This group includes a great variety of functions, which all tend, however; to the accomplishment of a single object, viz., the maintenance of the animal frame in its proper condition of flesh, strength, and activity. We shall study, in this division of the subject, the ingredients and qualities of the food, its preparation, digestion, and absorption, the blood and its circulation, respiration, secretion, and the nutrition of the animal tissues.

Section III. — The third group includes the actions of the Nervous System. These actions are quite different in their nature from any of the preceding, and have for their ultimate object the guidance or regulation of the other functions. It is in the study of this group that we shall learn the action of the senses, of the will, of the instincts, of many involuntary movements, and of the various operations of the mind.

Section IY. — Finally, an important division of the subject is that which relates to the changes in the functions of the body at different ages, to its growth, adolescence, maturity, and Development. For the history of the animal functions is not entirely the same at different ages, but undergoes various modifications, like the external appearance of the frame, and the capacity of the mental and bodily powers. The entire organization, therefore, is adapted to different purposes at different ages, somewhat in the same way as the various organs are permanently devoted to particular functions.

Chapter I. General Structure And Mechanism Op The Animal Frame

The Bones. — Their Composition. — Their Structure. — The Skeleton. — Spinal Column. — The Skull. — Pelvis. —Tibia. — Bones of the Foot. — Curvatures of the Skeleton. — Elastic Ligaments.

The Muscles. — Their Contraction.— Kelaxation. — Flexor and Extensor Muscles. — Tendons. — Movement of Joints. — Walking. — Running. — Leaping. — Elasticity of the Skeleton. — Exercise of the Muscular System. — Its Repose.

l. Structure and Composition of the Bones.— The body depends for its general form and solidity upon the bones. The bones are of various sizes and shapes in different parts of the frame, but they are distinguished from the other organs by being hard and rigid, instead of soft and yielding ; so that they are adapted to sustain the other tissues which are attached to them, and to protect the internal organs which they inclose more or less perfectly in interior cavities. Now the bones have this important quality of hardness and rigidity because they contain, among other substances, a large proportion of a mineral ingredient. This ingredient is Lime, which, in certain forms of combination, is united with the animal matter of the bones, and makes them firm and resisting. If the bones were composed entirely of this mineral ingredient, they would be stony and brittle, like limestone or marble ; but, in reality, they consist partly of animal matter and partly of lime. The lime gives them their stiffness and resistance, while the animal matter makes them, at the same time, somewhat tough and elastic ; so that they combine, to a certain extent, the two qualities of stiffness and elasticity. They are hard enough to sustain the weight of the body, and yet are not so brittle as to be easily broken. As a general rule, the bones are composed of about one half animal matter and one half mineral substance.

Another important quality of the bones is their lightness, which is combined in a proper degree with firm- Fig. 1. n©ss and resistance. If they were entirely dense and solid, they would be too heavy for motion, and would be rather a load and impediment to the animal body than a support and assistance. But, in point of fact, the bones are hollow (Figs. 1 and 2). They are covered every where on their exterior with a layer of very compact bony tissue, which acts as a firm and resisting shell, while in their interior there are cavities occupying a large portion of the bony mass. In the middle of the bones of the limbs these cavities are elongated and cylindrical, and are filled with a soft vascular substance termed the Marrow / but their rounded ends open lengthwise, contain a multitude of small bony needles and filaments, which branch and unite in various directions, dividing the principal cavity into a great

Thigh-bone, sawn

THE BONES. 21

number of smaller spaces, and giving to the bone in this situation a very finely reticulated or honey-combed

Fig. %

Lower end of the thigh-bone, sawn across, showing its central cavity.

structure. By this means the bones are made lighter, while they also preserve their firmness and resistance, and at the same time give a secure fastening to the muscles and tendons which are attached to their surface.

2. General arrangement of the Skeleton. — The bones are united to each other, by strong fibres or ligaments, into a connected series or system, which is called the Skeleton. The skeleton is therefore a bony scaffolding or frame-work, which sustains by its physical solidity the other organs, and which gives its general configuration to the entire body. It consists, first of all, of the " spinal column," or Back-bone (Fig. 3), which runs up and down along the middle line of the back, and which can be readily felt in that situation, like a bony ridge, extending from the back of the neck to the region of the hips. This ridge is formed by a line of sharp prominences or " spines," each of which belongs to a separate bone ; and as these bones are placed one upon

PHYSIOLOGY AND HYGIENE.

Fig. :

another, like a pile or column, the entire series has accordingly received the name of the " spinal column."

The bones of the spinal column, twenty-six in number, are articulated and locked together by various projections, and fastened by ligaments in such a way that it is capable of being maintained with security in the erect position. It stands, therefore, as a 0^f&\ kind of central upright pillar, around

JyHM ) which the other organs of the body

are grouped, and upon which they depend for their support. It is the most important part of the whole skeleton.

The spinal column sustains upon its upper extremity the " cranium," or /Skull, which is a hollow bony case containing the brain, and having attached to it the jaws and other parts of the frame - work of the face. Its lower extremity rests upon the " pelvis," or Hip -bone, between the two halves of which it is fastened like a wedge, and secured by strong ligaments. The pelvis is a bony expansion, very much like a basin in shape, and is intended to receive and support the organs of the lower part of the abdomen. By placing the hands upon the hips, we can easily feel the flaring edges of this basin -like bone, and can readily understand how it serves to support the intestines and other organs placed above it.

Diagram of the skeleton, etc., iri profile.

The dotted lines show the direction of the principal muscles. '

THE BONES. 23

The pelvis, in its turn, rests upon the " femur," or great Thigh-bone ; the largest and strongest single bone in the whole body. It is placed nearly straight up and down, and is surrounded by the thick muscles of the thigh. It is itself supported by the Tibia, or leg-bone, upon which it is placed, end for end. The tibia is somewhat triangular in shape, and forms the sharp ridge which may be felt on the front part of the leg below the knee. Lastly, the lower end of the tibia rests upon the bones of the Foot. These bones are arranged very much in the form of an arch ; for while the foot touches the ground by the point of the heel behind and the ball of the toes in front, the bones between them rise into a curved figure, forming at this part an arch or vault, called the " hollow of the foot."

Now the bones of the foot are not fastened immovably to each other, but are so connected by elastic bands and ligaments that they yield a little when pressed upon, and again resume their usual position when the pressure is taken off. We can perceive this very distinctly by bearing down upon the foot forcibly from above, when it spreads out a little, and afterward recovers itself and returns to its original shape.

The principal internal organs, such as the heart, lungs, liver, stomach, and intestines, are contained in the cavities of the chest and abdomen, being situated in front and on the sides of the spinal column.

3. Balancing of the different parts of the Skeleton. — The entire skeleton, therefore, forms a connected series or vertical frame of bones, which is so balanced that it may be kept in an upright position.

The different parts of the skeleton are not placed exactly in a straight line, one above another, but, on the contrary, vary somewhat both in a backward and forward direction. The bones of the foot are placed very obliquely, as we have already seen, forming an arch. The tibia, which rests upon this arch, is the only bone which is exactly vertical. The femur is somewhat curved in shape, and is also tipped forward, so that its upper extremity is articulated with the pelvis a little in front of the junction of the pelvis with the spinal column above. And the spinal column itself has no less than three different curvatures, which are turned alternately in the backward and forward direction. Nevertheless, if we lopk at the diagram of the skeleton in Fig. 3, we shall see that all these variations compensate for each other ; so that, although the skeleton is thus curved in its different parts, its general direction is a straight one, and the weight of the head and upper part of the body rests almost exactly above the ankle-joint.

But, as the different parts of the skeleton are movable at the joints or articulations, they must be fixed or steadied in some way, in order to keep the body in an upright position. This is accomplished by the following means.

4. Action of the Elastic Ligaments. — First of all, the body is held erect by a series of Elastic Ligaments, which are attached to the back part of the spinal column. This jointed column is so arranged that it is capable of moving in various directions, and especially of bending forward ; and as the principal internal organs are situated in front of it, their weight would naturally cause it to bend over in this direction. But the spinal column is also provided throughout its entire length with bony eminences, projecting backward from

THE MUSCLES.

its posterior surface. Now between these eminences there are attached, from one end to the other, a succession of strong ligaments or bands, called the " elastic ligaments of the spinal column," because they are extensible and elastic like India-rubber. These ligaments are sufficient to prevent the spinal column from yielding to the weight of the organs situated in front. When the back bone is bent forward by the action of the muscles, the bony eminences on its posterior surface, of course, are opened and separated from each other like the sticks of a fan ; but afterward they are drawn together as before by the elastic force of their ligaments, and the spinal column is again straightened. 5. Structure and Action of the Muscles. — Secondly, the skeleton is maintained in its upright position by the action of the Muscles. These are the organs which are attached to the different parts of the skeleton in such a way as to control its movements. They form a large proportion of the whole mass of the body, and constitute the firm and ruddy fibrous Flesh, which is found every where underneath the skin, which clothes the bones, and which envelops in a kind of muscular sac the cavities of the chest and abdomen.

If the muscles be examined by the microscope (Fig. 4), they are seen to be composed of a great number of very small fibres, too minute to be seen by the naked eye, placed side by side, and all running in nearly the same direction.