SYSTEMS AND ORGANS OF THE BODY

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PRINCIPAL BONES OF THE BODY

1. Collar Bone. (Clavicle)

2. Breast Bone. (Sternum)

3. Ribs.

4. Arm Bone. (Humerus)

5. Lumbar Vertebra.

6. Haunch Bone. (Pelvis)

7. Ulna.

8. Radius.

9. Wrist. (Carpus)

10. Metacarpus.

11. Phalanges.

12. Thigh Bone. (Femur)

13. Knee Cap. (Patella)

14. Brooch Bone. (Fibula)

15. Shin Bone. (Tibia)

16. Tarsus.

17. Metatarsus.

18. Phalanges.

PRINCIPAL MUSCLES OF THE BODY

1. Sternoclidomastoid (the muscle that bends the head).

2. Trapezius.

3. Pectoralis (chest muscle).

4. Deltoid (arm lifting muscle).

5. Coraco brachialis (rudimentary arm muscle).

6. Triceps (forearm extension).

7. Pronator radii teres (turns forearm and hand).

8. Annular ligament of wrist.

9. External oblique of abdomen.

10. Muscular sheath of abdominal erectus muscle.

11. Tensor fasciæ latæ (fibrous muscle covering thigh muscles).

12. Gluteus (controls thigh and helps to keep body erect).

13. Sartorius, or tailor, muscle (enables legs to be crossed).

14. One of quadriceps extensor cruris muscles.

15. Gastroenemius (bends the knee).

16. Long extensor of toes.

17. Peroneus longus (helps to keep foot arched).

18. Annular ligament of ankle.

19. Platyama.

20. Brachialis (moves elbow joint).

21. Biceps (flexor of arm).

22. Supinator longus (turns hand).

23. Extensor carpi radialis (extensor of forearm and wrist).

24. Flexor carpi radialis (bends wrist and turns hand).

25. Rectus abdominis (retracts abdominal wall).

26 and 27. Vastus externus and internus. These, with 14 and an abductor muscle, together make up the quadriceps extensor, the largest muscle in the body. It extends the leg.

28. Tibialis (extends the ankle).

29. Extensors of the toes.

The bones which make up the framework of the body are held together by joints of different kinds which allow of widely varying ranges of motion. The skull, which contains twenty-two bones in all, includes the cranium which contains the brain, and the bones which form the framework of the face. The vertebral column, which acts as a hinged and pliable tube down the center of which runs the spinal cord, is made up of twenty-four true vertebræ and the sacrum and the coccyx. The thorax, the bony box or cage protecting the heart and lungs, is made up of the twelve dorsal vertebræ with the twelve ribs on each side and the sternum or breast bone in front. The upper extremities consist of the shoulder-blade or scapula, the collar-bone or clavicle, the humerus or upper arm bone, the two fore-arm bones (radius and ulna), and the twenty-seven bones of the hand and wrist. The pelvis is composed of the two hip bones, together with the sacrum and coccyx. The female pelvis is larger in all diameters than the male. The bones of the lower extremity, which is joined to the pelvis by the head of the thigh bone (the femur), making a ball and socket joint at the acetabulum, are the two bones of the leg, the tibia and fibula; the patella or knee-cap; and the twenty-six bones of the ankle and foot.

BOOK OF THE HUMAN BODY

The study of the Human Body involves numerous other branches of science, and, as a whole, is the most complex and intricate of all the sciences. To explain its structure and workings we apply the principles of Biology, Physiology, Chemistry, Physics, Psychology, and Metaphysics.

The individual man, as a whole, is frequently forgotten both in physiology and in medicine, owing to the extraordinary minuteness and exactness with which each part and organ is examined and described. At the outset, then, it should be remembered that the human body is an organic whole, and what makes itoneis not the similarity or unity of the machines and processes, for they are unlike and many; but it is the unity of the one governing force, themind, and especially theunconscious mind, which presides over the body.

Nothing in the body is merely mechanical, although there is much mechanism; all is vital, all is united in one great aim—the health and well-being of the individual.

All organs and systems are held together and formed into one body by means of a framework, partly fixed and partly movable, partly rigid and partly flexible, partly hard and partly soft.

Theskeletonpart of the framework is made ofbone; flexibility is given to certain parts by means of joints, which are simply smoothed and rounded ends of bone covered with gristle to avoid friction, and joined together by fiber and ligament for strength. This forms the rigid and hard parts of the framework.

The flexible and soft part, which everywhere covers organs and muscles, is composed of a layer of fat to preserve the warmth, as fat is a non-conductor, and an outer covering of skin.

This framework is exquisitely adapted to give strong protection to the vital parts so that they cannot readily be injured; and the whole of the organs are so arranged and stowed away that a perfect human body is a beautiful object full of symmetry and graceful curves and lines.

Divisions of the Body.—If we divide the body into six parts—four limbs, trunk, and head and neck—we find each part contains about thirty bones (counting the ribs in pairs) there being abouttwo hundredin the entire body.

The height of the body depends mainly on the length of the bones of the lower limbs.

Everything in Pairs.—In the body almost everything is paired, right and left, giving it symmetry. There are but five central bones: two in the head, one in the throat, and the breastbone and backbone (or spine); and there are but five single muscles, all the rest—out of many hundreds—being in pairs. In the interior, where economy rather than symmetry is required, it is not so; there being as many single organs as there are double.

The Body Viewed as a Machine.—A favorite way of looking at the body as a whole is to regard it as an anatomical machine. In this view the body has an internal skeleton, of which the chief feature is the central axis or backbone.

Considering the skull and backbone as one, the body may be said to be built up of two tubes. The smaller posterior or neural tube includes the cavity of the skull and the vertebral canal. Within this tube is lodged the nervous center, or engine, of the body. The anterior, or body, tube is much larger, consisting of the face above, and the neck and trunk below, and it contains thefour nutritive systemsof life, so that the whole body in section is like an eight with the lower circle immensely exaggerated. The limbs, of course, are not tubular, and merely form part of the machinery.

Adopting the simile of the human engine and boiler and machinery, we see that the limbs, etc., are the machinery; the posterior tube the engines and force that move them; and the anterior tube the human boiler that generates the force. This boiler, like one in a steam engine, has an upper and lower part. The upper part is where the steam is generated (in lungs) and sent to the engine (the brain) by the heart. The lower part is where the fuel is burned (the stomach) and the ashes and refuse drop through (the intestines). So that the analogy between the two is close and striking.

Centers of Control.—There are two distinct seats of government in the human body: the one in theupper brain, or cortex, the other principally in the very center of the human body. That in the upper brain, or cortex, is the human will and the conscious mind. It has absolute control given to it over the animal part of the human life—that is, over the part that consists in the using of force, which includes the nervous and locomotor systems, and the special senses.

Nutritive Systems.—The other government, situated in the lower part of the brain and spinal cord and in the center of the body—in front of the spine and behind the stomach—is of an entirely different order. To put this more plainly: The four systems that lie in the body—digestive,circulatory,respiratory, andexcretory—may be termed the nutritive systems, being designed for the maintenance and storage of life-forces. They are almost entirely under the control of the involuntary nerve centers, and have full and undisputed sway over life itself—that is, over the generating and storing of vital force, rather than over its usage.

How the Body is Built.—In a building such as the body it is well to begin with theunit—the building unit. In a house this is a brick or a stone; in all living structures, animal and vegetable, it is acell.

All living structures, whether animal or vegetable, are built up of cells (which we shall consider in due course), and these cells are grouped together for different purposes to form different tissues. Thetissuesare the different materials of which the body is made. There are eight principal tissues in the body:bone,gristle,muscle,nerve,skin,fat,fiber, andconnecting tissue.

ORGANS OF CHEST CAVITY IN RELATION TO STOMACH

THE BRONCHIAL TUBESORGANS INVOLVED IN FIRST STAGES OF DIGESTION

THE BRONCHIAL TUBES

ORGANS INVOLVED IN FIRST STAGES OF DIGESTION

DIAGRAMS DISCLOSING HEART AND CONNECTIONS, RIBS AND LUNGS

(1) TheOsseous, or bone tissue, is the framework of the body. This material is found, of course, in every part of the body and forms the skeleton.

(2) TheCartilaginous, or gristle, forms the joints of the body. This tissue covers the ends of the bones to form the joints; it unites the ribs with the breastbone; it forms the rings of the windpipe and the lid of the larynx at the back of the tongue; the lower part of the nose, the upper eyelid, and the ear.

(3) TheMuscular, or muscle, forms the machinery of the body. This tissue covers all the bones with flesh, which is muscle, and is the chief part of a number of machines by which every movement is performed. It is also an important tissue in the wall of the abdomen and the floor of the chest.

(4) TheNervous, or nerve tissue, is the moving power of the body. It is the chief constituent of the brain and the spinal cord, inside the backbone or spine. It also forms the nerves, which run like white threads from the brain to all the muscles, and give them power to move.

(5) TheEpithelial, or skin, forms the outer covering of the body. This tissue is the skin that covers the body outside, and lines it as mucous membrane inside, and also forms the teeth and nails.

(6)The Adipose, or fat, forms the under covering of the body. This tissue is the inner protective sheathing and padding of the body, beneath the skin, and round the internal organs. It consists of drops of oil, enclosed in separate cells.

(7) TheFibrous, or fiber or sinew, is the tissue that forms the cords and bands of the body. This tissue makes the strong tendons that fasten the muscles to the bones, and forms the covering or sheath of the bone itself, and the various organs.

(8) TheConnective, or cementing tissue, joins all the parts and cells of the body together. This substance is found everywhere, all over the body, and is like the mortar in a house, fastening all the bricks together. It is a sort of network of cells and long fibers.

DIAGRAM ILLUSTRATING THE ALIMENTARY CANAL

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Special Systems.—These eight tissues are combined together into various groups oforgansorsystemsfor special purposes. These groups are six in number, and include: thecirculatory,respiratory,digestive,excretoryorsecretory,locomotor, andnervous systems. There is also thereproductivesystem, which has to do with the propagation of the race, and involves many important and vital questions.

We may divide these six into three groups:

There are two in the chest:

(1) TheCirculatory systemis that by which the blood or liquid food is distributed throughout the body to all the tiny cells. This system includes theheartor force-pump, and thearteries,capillaries, andveinsor the three kinds of pipes through which the blood travels.

(2) TheRespiratorysystem is that by which we breathe, and by which the body is fed with oxygen, which gives the blood its bright red color. This system includes the nostrils and mouth, the windpipe and the lungs.

Then there are two in the abdomen, or stomach:

(3) TheDigestivesystem, by which all the food is made into liquid and changed so as to nourish the body and pass into the blood. This system includes the mouth, gullet, stomach, liver, pancreas, intestines, and other organs.

(4) TheSecretory, or excretory, system (for they are best grouped as one) manufactures the various fluids of the body, such as bile, urine, sweat, saliva, gastric juice, etc. It consists of various glands or secretory organs in different parts of the body, such as those in the skin, the kidneys, the lymphatic glands, the spleen, etc. It also gets rid of the refuse of the body.

Lastly, there are two in the head and limbs:

(5) TheLocomotorsystem, by which all movement is effected. This includes the bones, joints, and muscles.

(6) TheNervoussystem, by which all the body is controlled, directed, and regulated. This system includes the brain, spinal cord, and the special senses, such as the ear, the eye, and all the nerves.

The Human Chest, or Thorax.—In it, the blood is purified and circulated. Thethoraxis closed above and below: above, by the neck, through which the windpipe enters it in front, conveying air to the lungs; and by thegulletbehind, conveying food to the stomach. Below, the floor, dividing it from the abdomen beneath, is formed by a very large muscle stretching right across the body, called theDiaphragm, or partition wall; also called theMidriff. The thorax is walled in at the sides by the ribs, and behind by the backbone in which is the other tube that contains the spinal cord. The thorax contains the two organs ofrespirationandcirculation.

Thelungsare the organs of respiration. They are like two sponges filling the right and left halves of the chest. Wherever you can feel a rib there is part of the lung underneath. Each of these lungs is contained in a bag, like a skin, that separates it from the ribs, and is called thepleura(frompleuron= arib), but the lung is notinsidethe bag.

Theouterlayer of the pleura is fixed to the side of the chest, theinnerlayer to the lung, and the two layers move on each other like a joint when we breathe.

The lungs are full of small air-cells with minute tubes leading from them. These gradually increase in size as they join together, till at last they unite in one large tube, or bronchus, for each lung. These two bronchi join together, and form thewindpipe, ortrachea, which conveys the air through the larynx into the mouth.

The windpipe is kept stretched widely open by a series of elastic rings of gristle. Behind the windpipe is the gullet, leading to the stomach.

PERICARDIUM OF THE HEARTLEFT AURICLE AND LEFT VENTRICLEThe heart, the main pump of the circulatory system, rests on the diaphragm between the two lungs. The heart is enclosed in a smooth, moist membrane or sac, the pericardium, which allows it to dilate and contract without friction against the adjoining parts. There are four cavities in the heart, the right and left auricle, and the right and left ventricle. The auricles, which are thinner walled, collect blood from the veins, while the thicker and stronger walled ventricles force the blood into the arteries. The left auricle pumps the purified blood into the left ventricle, the valve between the auricle and ventricle opening to allow this passage. When the left ventricle is full the valve between its chamber and that of the auricle closes, the ventricle itself contracts down, and the blood is pumped out through the aorta to supply all the tissues of the body.After leaving the left ventricle through the aorta the purified blood is carried to the head, arms, trunk, and lower limbs, etc. Finally, after being deprived of its oxygen as it passes through the tiny end-arteries, or capillaries, of the tissues it has to nourish, it is collected in the veins and is emptied into the right auricle. Passing from the right auricle to the right ventricle, this impure blood, which is of a dull purplish color, is pumped into the lungs, where it is deprived of its waste gases and once more takes up a fresh supply of oxygen. Bright scarlet in color again, it now is collected and carried to the left auricle by the pulmonary veins. From the auricle it passes through the mitral valve to the left ventricle, whence it is once more pumped out through the aorta to supply the tissues.

PERICARDIUM OF THE HEARTLEFT AURICLE AND LEFT VENTRICLE

The heart, the main pump of the circulatory system, rests on the diaphragm between the two lungs. The heart is enclosed in a smooth, moist membrane or sac, the pericardium, which allows it to dilate and contract without friction against the adjoining parts. There are four cavities in the heart, the right and left auricle, and the right and left ventricle. The auricles, which are thinner walled, collect blood from the veins, while the thicker and stronger walled ventricles force the blood into the arteries. The left auricle pumps the purified blood into the left ventricle, the valve between the auricle and ventricle opening to allow this passage. When the left ventricle is full the valve between its chamber and that of the auricle closes, the ventricle itself contracts down, and the blood is pumped out through the aorta to supply all the tissues of the body.

After leaving the left ventricle through the aorta the purified blood is carried to the head, arms, trunk, and lower limbs, etc. Finally, after being deprived of its oxygen as it passes through the tiny end-arteries, or capillaries, of the tissues it has to nourish, it is collected in the veins and is emptied into the right auricle. Passing from the right auricle to the right ventricle, this impure blood, which is of a dull purplish color, is pumped into the lungs, where it is deprived of its waste gases and once more takes up a fresh supply of oxygen. Bright scarlet in color again, it now is collected and carried to the left auricle by the pulmonary veins. From the auricle it passes through the mitral valve to the left ventricle, whence it is once more pumped out through the aorta to supply the tissues.

RESPIRATORY SYSTEM OR AIR PASSAGES OF THE BODY

Left: larynx from behind. Middle: cross-section of the pharynx. Right: section through larynx.VIEWS OF THE LARYNX, SHOWING HOW THE AIR REACHES THE LUNGS

Left: larynx from behind. Middle: cross-section of the pharynx. Right: section through larynx.

VIEWS OF THE LARYNX, SHOWING HOW THE AIR REACHES THE LUNGS

The organs of respiration are the nose, throat, larynx, windpipe or trachea, and the two lungs. On the outer walls of the nasal cavities are three shelves known as the turbinated bones, the surfaces of which contain blood-vessels to heat the air as it passes through the nose. The mucus which constantly forms on the lining membrane of the nose and the little hairs in the nostrils, act as screens, preventing dust being breathed into the lungs. The pharynx is the cavity behind the nose, mouth and larynx. The larynx forms a prominence in the throat known as the “Adam’s Apple.” It contains the vocal cords, the vibrations of which, as air from the lungs passes through them, give rise to voice sounds. The epiglottis is a cartilaginous curtain above the larynx which blocks up its entrance when food is being swallowed. The trachea or windpipe is a continuation of the larynx. Shortly after entering the chest it divides into two main branches, the right and left branches, which lead to all parts of the lungs. The lungs, two spongy, air-filled organs, take up most of the space in the chest-box or thorax. The smallest end-branches of the bronchial tubes open into numerous tiny sacs known as the air vesicles, in the walls of which the end-branches of the capillaries ramify. Here the impure gases in the blood escape through the vessel walls into the air vesicles, while the oxygen breathed into the lungs is taken up the same way by the blood in the vessels.

HOW THE HUMAN BODY IS CONTROLLED BY THE BRAIN

CORD WITHDURA MATERTHE ARRANGEMENT OFTHE DURA MATERLarge illustration(375 kB)The nervous system consists of (1) the brain; (2) the spinal cord; (3) the nerves which run off from these structures; and (4) the sympathetic system. The chief mass of the brain is known as the cerebrum, or fore-brain, the small mass at the lower part being termed the cerebellum, or little brain. From the brain, which is contained within the bony skull, twelve pairs of cranial nerves proceed. The most important of these are the first or nerve of smell, the second (sight), eighth (hearing), and twelfth (taste). The fifth, one of the most important nerves of sensation, has three main branches running to the orbit and forehead, the jaws and teeth, and the skin of the face. Six of the twelve pairs of cranial nerves govern the movements of different parts (motor nerves), others have to do with the special sense organs, taste, smell, hearing, and sight (sensory nerves), and others are a combination of motor and sensory nerves. The spinal cord is a continuation of the brain, and is contained in the hollow canal running through the vertebræ of the spine. From it thirty-one pairs of nerves originate. The nerves which run to the arm are collected in a network called the brachial plexus. In the same way the great nerves to the leg come together in the lumbar plexus. The sympathetic nervous system consists of a main nerve trunk running downward along the spine from the skull to the coccyx. This sympathetic system communicates indirectly with the brain and spinal cord, and also with all the great arteries and other important structures in the abdomen.The dura mater is the strong external cranial membrane which adheres to the skull and also penetrates into the cavities of the brain, dividing it into partially separate compartments. These dividing portions of the dura mater may be seen at A, A, in the diagram above. B marks the various venous blood sinuses of the brain, which receive blood from veins in the different parts of the brain, and, merging into one large sinus (seen at lower right of diagram), afterwards become the jugular vein. C is the great cerebral vein. The Roman numerals mark the great cranial nerves.

CORD WITHDURA MATERTHE ARRANGEMENT OFTHE DURA MATER

CORD WITHDURA MATER

THE ARRANGEMENT OFTHE DURA MATER

Large illustration(375 kB)

The nervous system consists of (1) the brain; (2) the spinal cord; (3) the nerves which run off from these structures; and (4) the sympathetic system. The chief mass of the brain is known as the cerebrum, or fore-brain, the small mass at the lower part being termed the cerebellum, or little brain. From the brain, which is contained within the bony skull, twelve pairs of cranial nerves proceed. The most important of these are the first or nerve of smell, the second (sight), eighth (hearing), and twelfth (taste). The fifth, one of the most important nerves of sensation, has three main branches running to the orbit and forehead, the jaws and teeth, and the skin of the face. Six of the twelve pairs of cranial nerves govern the movements of different parts (motor nerves), others have to do with the special sense organs, taste, smell, hearing, and sight (sensory nerves), and others are a combination of motor and sensory nerves. The spinal cord is a continuation of the brain, and is contained in the hollow canal running through the vertebræ of the spine. From it thirty-one pairs of nerves originate. The nerves which run to the arm are collected in a network called the brachial plexus. In the same way the great nerves to the leg come together in the lumbar plexus. The sympathetic nervous system consists of a main nerve trunk running downward along the spine from the skull to the coccyx. This sympathetic system communicates indirectly with the brain and spinal cord, and also with all the great arteries and other important structures in the abdomen.

The dura mater is the strong external cranial membrane which adheres to the skull and also penetrates into the cavities of the brain, dividing it into partially separate compartments. These dividing portions of the dura mater may be seen at A, A, in the diagram above. B marks the various venous blood sinuses of the brain, which receive blood from veins in the different parts of the brain, and, merging into one large sinus (seen at lower right of diagram), afterwards become the jugular vein. C is the great cerebral vein. The Roman numerals mark the great cranial nerves.

We take air into the lungs to pass thence into the blood, and thus be carried to all the cells of the body to enable them to live and breathe.

The Heart.—Theheartis at the lower part of the chest, between the two lungs. It is afleshyor muscular organ, about the size of the fist—flat above, and pointed below like a sugar-loaf. It lies in a slanting direction behind the breastbone—the broad part, or thebase, of the heart being upwards and partly to therightof the breast-bone; the point, or apex of the heart, being downwards and to theleft, where it can often be seen beating against the chestwall.

The heart is hollow, and acts like a pump, forcing the blood all over the body through the great vessel that leaves the heart at the upper part. The heart, like the lungs, is enclosed in a double layer of folded bag, called thepericardium, because it is round the heart.

Thegulletruns right down the back of the thorax, and passes out through the diaphragm, which forms the floor, into the abdomen.

Theabdomenforms the lower half of the trunk, and is often called thestomach. It is full of organs belonging to thedigestive systemandsecretory system, by which the fuel or food is rendered fit for use in the blood and the body.

Thewallsof the abdomen are not protected by ribs like the thorax, but are all formed of flesh or muscle. The principal organs they contain are thestomach, theliver, thepancreas, orsweetbread, thespleenormilt, thekidneys, theintestines, and thebladder.

The Human Brain.—Theheadandspinecontain the principal nervous systems of the body and four organs of special sense—sight,hearing,smelling, andtasting.

Thebrain, which fills the head, consists of two parts: theCerebrum, or greater brain, and theCerebellum, or lesser brain, placed behind and below the larger one. From this brain, nerves run to every muscle of the body, enabling them to move the limbs and body as the mind directs; and another set of nerves run from every part of the body and skin to the brain, enabling the mind to know and feel all that goes on.

The brain is connected with the spinal cord by a flat band of brain matter, that lies on the inside of the occipital bone, called theMedulla Oblongata, or the Oblong Marrow. Thespinal cordruns through a large hole in the occipital bone and right down the open tube formed by the spinal vertebræ, to the bottom of the backbone, and, all along its course, nerves leave it and enter it, as in the brain.

Theorgan of sightconsists of thetwo eyes, which receive every image that we see, and transmit it to the brain. Theorgan of hearingconsists of thetwo ears, by which we receive all the waves of sound that we hear, and transmit them to the brain. Theorgan of smellis in the upper part of thenose; theorgan of tasteat the hinder part of thetongue.

Theorgan of the voiceis contained in thelarynxin the neck, which joins the head to the body. Just under the chin in front of the neck you can feel what is called theAdam’s Apple, which is the front of the larynx, or voice-box, by which the air coming out of the lungs is formed into sounds.

Thesoundsare formed into words by themouth,tongue, andteeth.

PERMANENT TEETH AND THEIR NAMESUpper Jaw: 1, 2, incisors; 3, canine; 4, 5, premolars; 6, 7, 8, molars.Lower Jaw: 1, 2, incisors; 3, canine; 4, 5, premolars; 6, 7, 8, molars.

PERMANENT TEETH AND THEIR NAMES

Upper Jaw: 1, 2, incisors; 3, canine; 4, 5, premolars; 6, 7, 8, molars.

Lower Jaw: 1, 2, incisors; 3, canine; 4, 5, premolars; 6, 7, 8, molars.

These gateways—which we otherwise name the Organs of the Senses, and call in our mother speech, the Eye, the Ear, the Nose, the Mouth, and the Skin—are instruments by which wesee, andhear, andsmell, andtaste, andtouch: at once loopholes through which the soul gazes out upon the world, and the world gazes in upon the soul.

The ear is divided into three parts:

(1) The external ear, made up of the outer portion and passage-way which leads up to the drum.

(2) The middle ear or drum, the continuation of the ear passage internal to the drum membrane, and

(3) The internal ear containing the labyrinth and the nerve of hearing.

The outermost part, the skin-coveredauricle, contains no bone, being simply a mass of cartilage covered by skin. It acts as a sound catcher and improves the hearing by directing sound-waves into the opening or externalmeatus. This meatus or passage-way runs directly inward for an inch and a half. The inner half of the passage-way runs through solid bone, ending abruptly at the membrane or sounding-board of the ear.

THE JOURNEY OF SOUND WAVES TO THE BRAIN

This diagram shows the marvelous structure of the ear, and how sound reaches the brain. There is marked similarity between the ear and a telephone receiver by which we are able to receive messages from the outside world. Hearing is simply the result of sound-waves striking the drum of the ear which set in vibration the bones of the middle ear, and they in turn vibrate the drum of the inner ear. This sets in motion a fluid, and the wave motions are conveyed along the spiral staircase to the wires, or nerves of hearing, and from there to the telephone exchange, or brain.

Large illustration(398 kB)

This part begins at the inner surface of the membrane, and extends inward for about a quarter of an inch. The outer surface of the membrane can be seen by the observer on pulling the top of the auricle or fleshy part of the ear a little upward, so as to straighten out the somewhat curved passageway or meatus. The membrane which is placed transversely across the meatus is whitish-pink or yellowish color.

The chief contents of the cavity of the middle ear are three tiny bones called themalleusor hammer bone, theincusor anvil bone, and thestapesor stirrup bone. In addition, an important nerve called thechorda tympanipasses across the middle ear chamber. The three little bones contained in the middle ear may be looked upon as the connecting link between the outer ear, which gathers the sounds, and the internal ear, which transmits the effect of the sound waves to the brain, where they are translated into what we call hearing.

From without inward the three little bones lie touching each other, end to end, the outer end of the first bone being implanted between the layers of the drum membrane and the inner end of the innermost bone, fitting into a tiny opening which connects the middle ear with the internal ear. As the result of their lying touching each other, any movement of the ear drum caused by a sound wave striking against its outer aspect, moves the malleus bone; this, in turn, moves the middle incus, and this passes the movement on to the innermost part of the stirrup. This, in turn, passes the movement onward to the fluid orperilymphin the outermost part of the internal ear, and here the endings of the nerve of hearing receive the stimuli which we recognize as “sounds.” (SeePlates.)

In addition to these contents of the middle ear there are also two tiny openings which, very necessary for health, are nevertheless sometimes a pathway by which serious disease may attack the ear and destroy the hearing. The first is a small passage-way leading from the upper part of the middle ear cavity through the bone to themastoid antrum, a hollow space in the prominent mass of bone to be felt immediately behind the ear projecting outward and downward from the skull.

The second passage-way opening into the middle ear cavity is that of the Eustachian tube which leads directly to the back of the throat. The importance of this tube is that through it air can find its way directly into the middle ear, so that the air pressure on the two sides of the drum is always kept the same. If it were not for some such arrangement the pressure on the outer side of the drum would become greater than that on its inner surface. This would, of course, push the drum inward, and greatly reduce its mobility.

This is a complicated structure of bony passages curled on themselves, roughly as in a snail shell, and lined with a delicate membrane. This membrane is, so to speak, floating in fluid. The layer of fluid between it and the bone is called theperilymph, while the two layers of the membrane enclose a similar fluid termed theendolymph. The internal ear or membranous labyrinth may be divided roughly into three chief parts: (1) the cochlea, the true organ of hearing; (2) the semi-circular canals, which control the act of balancing; and (3) the vestibule, or introductory chamber to the semi-circular canals.

The cochlea is a collection of three tubes curled up on themselves in snail-shell fashion.

The central canal of these three is the connecting link by which the sound waves, passed along over the three tiny bones—the malleus, incus, and stapes—finally reach the endings of the main nerve of hearing, the auditory nerve. (SeePlate.)

Working of the eye

The human eye is a hollow globe containing fluids and the crystalline lens. Surrounded by its muscles it lies embedded in a cushion of fat in a conical bony hollow called the orbit. Through an opening in the bones making up the back of the orbit, the optic nerve leads from the back of the eye to the brain.

The eyelids are made of layers of muscle and cartilage with an outer surface of skin and an inner surface which is a continuation of the conjunctiva that covers the eyeball. In the edge of the eyelid a series of tiny glands are embedded. The mouths of these open on the margin of the lids. The eye-lashes, whose duty it is to act as a screen, preventing foreign bodies such as dust or other air-born objects getting into the eye, are also inserted in the edge of the lid.

About one-eighth of an inch from the internal angle of the eye, a small projection is to be seen on the margin of the lid. In the center of this is a tiny opening through which the tears as they collect in the eye are led away through two small canals to the lachrymal sac in the upper part of the nose. The lachrymal gland, which secretes the tears, or water, of the eye, is situated above on the outer side of the eyeball, between it and the bones of the orbit. The lachrymal gland is constantly secreting tears, which are carried by narrow ducts to the upper surface of the eyeball, whence they flow down over the eye, finally being collected at the inner corner of the eye and passing into the nose through the lachrymal punctures described above. Under certain circumstances, as from emotion, a blow, or the irritation of a cold wind, the tear fluid is secreted faster than it can escape through the punctures, and so flows over the lids and down the cheeks.

The eye is held in its socket or orbit by (1) the optic nerve, (2) by its six muscles attached to various points of its circumference, (3) by the conjunctiva, which is reflected off from its attachments to the outer coat of the eye directly on to the lids, and (4) by the eyelids themselves. (SeeColor Plate.)

Thecorneais the transparent, bulging, central portion of the eye covering the pupil and the colored iris. Made of tiny transparent cells closely packed together, the cornea is not nourished by blood carried to it by the blood-vessels but by lymph which permeates through it in the tiny channels between the cells. By its curved surface it plays a part in focusing rays of light on to the lens situated just behind the iris.

PICTURE DIAGRAMS SHOWING THE DELICATE STRUCTURE OF THE EYE AND EAR

Large illustration(441 kB)

Directly behind the cornea come theirisandpupil. The latter is nothing more than a hole in the center of the iris through which light enters the eye.

The iris is the screen of the eye. Just as the photographer uses a screen with a large opening when he wants more light to enter his camera and a small opening when he requires less, so Nature arranges that the iris automatically contracts or dilates to make a larger or smaller pupil opening, according to the amount of light needed within the eye for purposes of vision. When the light is very bright less is needed in the eye. Thus in brilliant artificial light at night one’s pupil is small. On the other hand, when the light is waning, as in the dusk or semi-darkness, the pupil is enlarged by the iris contracting down to a narrow ring under the outer circumference of the cornea.

The color of the eye depends on the position and amount of pigment cells in the iris. In the dark brown eye there is an abundance of pigment scattered through the substance of the iris as well as in the front layers nearest the surface. In the blue eye the pigment cells are buried deep in the iris and are fairly plentiful in amount. The colorless eye of the albino is the result of a deficiency of pigment in the iris.

The iris is fixed at its outer circumference, but its inner rim, which makes the border line of the pupil, is free, so that when the iris contracts the pupil becomes larger, since its inner free margin is drawn outwards toward the fixed outer margin. Close up against the deeper surface of the iris comes the crystalline lens.

The lens is a compact body of transparent cells, concave in form, and closely similar to the glass lens of a camera. The lens of the eye, however, differs from the camera’s glass lens because it changes its shape in focusing for objects at different distances. This focusing, which takes place automatically, is known as “accommodation.”

The object of the change in the shape of the lens is that no matter at what angle the rays of light reflected from the object looked at fall on the outer surface of the lens (through the opening in the iris), they may be accurately focused on the surface of the retina, or lining membrane at the back of the eye. When looking at a distant object the lens is fairly flat, because when in this position the rays of light will be accurately focused on the retina. If the eye is now turned to an object near at hand the rays of light from the object are more divergent than in the previous case, and if the lens retained its previous shape they would fail to be focused accurately on the surface of the retina. Hence Nature has arranged that the lens of the eye is elastic, automatically becoming flatter by the action of the ciliary muscle when distant objects are looked at and rounder or deeper when nearer objects are looked at.

Up till middle age the eye retains in full this power of automatic accommodation. From middle age onward, however, the lens becomes less and less elastic. As a result the lens constantly remains more or less flattened. Although vision for objects at some distant from the eyes remains perfect, oldish people very frequently have to wear glasses (to correct the too great flatness of the natural lens) to obtain clear vision of objects close at hand.

The lens is slung in a ligament that is a part of the “ciliary body,” which is a continuation of the choroid coat of the eyeball. This ciliary body is a ring of tissue lying behind the iris connected with the anterior portion of the choroid coat of the eye.

Between the iris and the underlying lens on the one hand and the inner surface of the bulging cornea on the other is a small space or cavity filled with a clear transparent fluid called theaqueous humor.

Looking at the white of the eye, the first coat is the transparent conjunctiva,which is reflected back on to the eyeball from the eyelids. Next comes the sclerotic coat, formed of dense whitish tissue, which seen through the transparent conjunctiva makes up the “white of the eye.” The sclerotic coat covers the whole globe of the eyeball with the exception of the transparent bulging cornea in front (which, however, is practically a continuation of the sclerotic), and the back of the eye where the optic nerve enters. The sclerotic is the thickest and densest coat of the eye.

Within the sclerotic coat, and so to speak lining it, comes the choroid coat. Countless blood vessels run through this coat, supplying both the one above it and that beneath it. As this coat approaches the front of the eye under the circumference of the cornea, it thickens into the ciliary body, forming a dense ring of tissues underneath the junction of the cornea and the sclerotic coat.

The innermost coat of the eye is called the retina. This coat contains the nerve endings of the optic nerve which, coming through the opening in the bony orbit, passes through the sclerotic and choroid coats. After entering the eye, the optic nerve divides into myriads of fibers, which, spreading from the point of entrance at the back of the eye, form a fibrous network all over its inner surface. In addition to this network of nerve fibers and highly specialized nerve cells, tiny blood vessels entering with the optic nerve branch out on all sides over the retina.

The retina is a comparatively thick membrane composed of eight layers of different kinds of nervous tissue. The essential layer, that of the “rods and cones,” is the seventh from within outward. Thus a ray of light on entering the eye must pass through six superficial layers before it reaches the “rods and cones.”

The “rods and cones” are lying on a layer of colored or pigment cells whose duty it is to prevent diffusion of light within the eye. The eyeball, therefore, is to all intents a camera obscura, the iris representing the shutter, the crystalline lens the camera lens, and the layer of “rods and cones” the sensitive plate. When a ray of light falls on the layer of the “rods and cones,” this layer receives a nervous stimulus which is conveyed by the optic nerve to the brain.It is these sensations which the brain translates into what we term sight.

Where the optic nerve enters the back of the eye, there are no “rods and cones,” hence rays of light falling on this portion of the retina send no stimulus to the brain; in other words, images falling on the “blind spot” are not visible.

The “yellow spot” is a small area at the center at the back of the eye where the retina is very thin, consisting of little more than a single layer of “cones.” Images which fall upon this region are seen with the greatest distinctness.

Sight is a nervous sensation due to the translation by the brain of the effects caused by rays of light being reflected from some object in front of the eye on to the innermost layer of the eye, the retina.

When an object is looked at, rays of light which reach the object from some source of light (such as the sun, a lamp, etc.) fall on the transparent outer part of the eye, the cornea. On account of its curved surface these rays of light are more or less bent inward so as to fall more or less perpendicularly on the forward anterior convex surface of the lens. If the light is weak or dim, the iris, which lies in front of the lens, will automatically contract down so as to make the opening by which the rays can enter the posterior chamber of the eye (the part behind the lens) as large as possible.

If the light is very bright the muscle fibers in the iris will relax so that the iris itself gets larger, and its central opening smaller, so that too much light may not enter. Passing through the lens the rays are focused by the lens so that they are brought together to a point exactly on the surface of the retina.

Here their presence has a certain effect on the rod and cone layer of the retina, the result of which is conducted along the optic nerve to the brain, where it is transformed into what we know as sight.

HOW WE ARE ABLE TO TASTE, SMELL AND FEEL

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