A Handbook of Health by Woods Hutchinson (learn to read books .txt) 📖
- Author: Woods Hutchinson
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We have just seen how the eye deals with rays of light coming from a distance, which are practically parallel. When, however, books or other objects are brought near the eye, the rays of light coming from them do not remain parallel, but begin to spread apart, or diverge; and a stronger lens is required to bring them to a focus upon the retina. To provide for this, there is in the middle of the eyeball a firm, elastic, little globular body about the size and shape of a lemon-drop, called the crystalline lens. Around this is a ring of muscle, which is so arranged that when it contracts it causes the lens to change its shape and become more bulging, or thicker in the middle. This makes the eyeball a "stronger" lens so that the rays of light can be brought to a focus upon the retina.
This action is known as accommodation, or adjustment; and you can sometimes feel it going on in your own eye, as when you pick up a book or a piece of sewing and bring it up quickly, close to the eye, in order to see clearly.
If this little muscle is worked too hard, as when we try to read in a bad light, it becomes tired and we get what is called "eye-strain"; and if the strain be kept up too long, it will give us headache and may even make us sick at the stomach. The commonest cases of eye-strain are in eyes that are too flat (hyperopic) where this little muscle has to "bulge" the lens enough to make good the defect and bring the rays to a focus. This, however, of course keeps it on a constant strain; and the eye is continually giving out, and its owner suffering from headache, neuralgia, dyspepsia, sleeplessness, and other forms of nervous trouble, until the proper lens or spectacle is fitted.[30]
A surface as delicate and sensitive to light as the retina, would, of course, be damaged by too bright a glare; so in the front of the eye, just behind the cornea, a curtain has grown up, with an opening or "peep-hole" in its centre, which can be enlarged or made smaller by little muscles. This opening is the pupil; the curtain, which is colored so as to shut out the rays of light, is known as the iris, for the quaint, but rather picturesque, reason that Iris in Greek means "rainbow," and this part of the eye may be any one of its colors.
It is the iris which, according to the amount of coloring matter (pigment) in it, makes the eye, as we say, blue, gray, green, brown, or black. Blue eyes have the least; black, the most.[31]
The Care of the Eyes. The most dangerous diseases of the eye are caused by infectious germs, which get into them either from the outside, as in dust, or by touching them with dirty fingers; or through the blood, as in measles, smallpox, tuberculosis, and rheumatism. The more completely we can prevent these diseases, the less blindness we shall have in the nation. About one-sixth of all cases of blindness in our asylums is caused by a germ that gets into babies' eyes at birth, but can be done away with by proper washing and cleansing of the eyes.
THE EAR
Structure of the Ear. Next after sight, hearing is our most important sense; without it, speaking, and consequently reading and writing, would be impossible. Man learned to speak by hearing the sounds made by other people and things, and then by listening to his own voice and practicing until he could imitate them. Children who are unfortunate enough to be born deaf also become dumb, not because there is anything the matter with their voice organs, but simply because, as they cannot hear the sounds they make, they do not form them by practice into words and sentences. By proper training, deaf mutes can now be taught to speak, though their voices sound flat and "tinny," like a phonograph.
As in the nose and the eye, the important part of the ear is the nerve spot that can "feel" the air waves that we call sound, just as the retina "feels" light. It is from this sensitive spot that the auditory nerve carries the sound to the brain. This spot has grown into quite an elaborate structure, buried, for safety, deeply in the bones of the skull, close to the base of the brain. It is made up of a long row of tiny little nerve rods, laid side by side like the keys of a piano, only there are about three thousand of them. Each one of these is supposed to respond, or vibrate, to a particular tone, or sound. This keyboard, from the fact that, to save space, it is coiled upon itself like a sea-shell, instead of running straight, is called the cochlea (Greek for "snail-shell"); it is also called, because it is the deepest, or innermost, part of the hearing apparatus, the internal ear.
Just as the retina has a lens and a vitreous humor in front of it to act upon the light, so the internal ear has an apparatus in front of it to act upon the sound waves. This is called the drum (tympanum). It consists of a fold of thin, delicate skin stretched tightly across the bottom of the outer ear canal, as parchment is stretched across the head of a drum. If you should take a hand-mirror—best a hollow, or concave, one—and throw a bright ray of light deep into some one's ear, you would be able, after a little trying, to see this drum-skin stretched across the bottom of it and about an inch and a quarter in from the surface of the head.
A cross-section diagram from the outer ear to the lobes of the brain.
When the sound waves go into the ear canal and strike upon this tiny drum, which is about two-thirds the size of a silver dime and really more like a tambourine or the disk of a telephone or phonograph than a drum, they start it thrilling, or vibrating, just as a guitar string vibrates when you thrum it. These little vibrations are carried across the hollow behind the drum by a chain of tiny bones, known as the ear-bones (called from their shapes, the hammer, the anvil, and the stirrup), and passed on to the keyboard of the cochlea.
Here comes in one of the most curious things about this ingenious hearing-apparatus. This little hollow behind the drum-skin has to be kept full of air in order to let the drum vibrate properly, and this is arranged for by a little tube (the Eustachian tube) which runs down from the bottom of it and opens into the back of the throat just behind the nasal passages, and above the soft palate. When you blow your nose very hard, you will sometimes feel one of your ears go "pop"; and that means that you have blown a bubble of air out through this tube into your drum cavity.
If your nose and throat become inflamed, then the mouth of this little tube may become blocked up; the drum can no longer thrill, or vibrate, properly; and, for the time being, you are deaf. This tube is of great importance, because nearly all the diseases that attack the ear start in at the throat and travel up the tube until they reach the drum cavity. This is why one so often has earache after an attack of the grip or after a bad cold. The drum cavity, with its chain of bones and its tube down to the throat, is called, from its position, the middle ear.
The outer, or external, ear, though far the largest of the three parts, and quite imposing in appearance, is really of little use or importance. It is simply a sort of receiving trumpet for catching sounds, with a very wide and curiously curved and crumpled mouth, or bell. The large, expanded mouth of the trumpet, called the concha ("conch shell"), was at one time capable of being "pricked up" and turned in the direction of sounds, just as horses' or dogs' ears are now; and in our own ears there are still for this purpose three pairs of tiny unused muscles running from them to the side of the head. But the concha is now motionless and almost useless, except for its beauty; and it is very troublesome to wash.
The Care of the Ear. The tube of the trumpet leading down from the surface of the ear to the drum is lined with skin; and this skin is supplied with glands, which pour out a sticky, yellowish fluid called ear wax, which catches the bits of dust or insects that get into the ear and, flowing slowly outward, carries them with it. If it is let alone, it will keep the ear canal clean and healthy; but some people imagine that, because it looks yellowish, it must be dirt; and consequently, from mistaken ideas of cleanliness, they work at it with the end of the finger, the corner of a towel, or even with a hairpin, an ear-spoon, or an ear-pick, and in this way stop the proper flow of the wax and make it dry and block up the ear.
Remember, you should not wash too deeply into your ears; (as the old German proverb puts it, "Never pick your ear with anything smaller than your elbow"). And if you don't, you will seldom have trouble with wax in the ear. Scarcely one case of deafness in a hundred is caused by wax. When your ear does become blocked up with wax, it is best to go to a doctor and let him syringe it out. Picking at it, or even syringing too hard, may do serious damage to the ear.
If an earache is neglected, the inflammation may spread into some air-cells in the bony lump behind the ear (the mastoid) and thus cause mastoid disease, which may spread to, and attack, the brain if not cured by a surgical operation.
OUR SPIRIT-LEVELSThe Sixth Sense. Though we usually speak of having five senses,—sight, smell, hearing, touch, and taste,—we really have also a sixth—the sense of direction, or of balance. The "machine" of this sense is comparatively simple, being made up of three tiny curved tubes, which, from their shape, are called the semi-circular canals. These are buried in the same bone of the skull as the internal ear, and so close to it that they were at one time described as part of it. These little canals are three in number, one for each of the dimensions—length, breadth, and thickness,—so that whichever way the head or body is moved,—backward and forward, up and down, or from side to side,—the fluid with which they are filled will change its level in one of them, just as the "bead" does in the carpenter's spirit-level that you can find in any tool shop. The delicate nerve twigs that run out into the fluid in these tiny canals are gathered together into a bundle, or nerve-cable, which runs back to the part of the brain known as the cerebellum or hind-brain, which has most to do with controlling the balance and movements of our bodies.
It is the disturbance set up in these spirit-level canals by the pitching and rolling of a ship, which makes us seasick. Neither the stomach, nor anything that we may have eaten, has anything to do with it. In the same way we sometimes become sick and dizzy from swinging too
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