THE LOOKOUT DEPARTMENT
Why the Eyes, Ears, and Nose are Near the Mouth. If you had no eyes,
ears, or nose, you might just as well be dead; and you soon would be, if
you had no one to feed you and guide you about and take care of you.
Naturally, all three of these scouts and spies of the body, which warn
us of danger and guide us to food and shelter, are near the mouth, at
the head-end of the body. The nose by means of which we smell food, to
see whether it is sweet and good or not, is directly above the mouth;
the eyes are above and on each side, like the lamps of an automobile,
but swinging in sockets like search-lights; while the ears are a couple
of inches behind, on each side of us, for catching from the sea of air
the waves that we call sound.
You could almost guess what each of these is for, just by looking at it.
The nose and the ears are open and hollow because air must pass into
them in order to bring us odors or sounds; while the eyes are solid,
somewhat like big glass marbles, to receive light--because light can go
right through anything that is transparent. Eyes, ears, and nose all
began on the surface, and sank gradually into the head, so as to be
surrounded and protected, leaving just opening enough at the surface to
allow smells, light-rays, and sound-waves to enter; and all of them have
at their bottom, or deepest part, a sensitive patch of surface, which
catches the light, or the smells, or the sounds, and sends them by a
special nerve to the brain.
These three sets of organs have gradually and slowly grown into the
shape in which we now find them, in order to do the particular kind of
smelling, seeing, and hearing that will be most useful to us. Every kind
of animal has a slightly different shape and arrangement of eye, of ear,
and of nose to fit his particular "business"; but in all animals they
are built upon the same simple, general plan.
THE NOSE
How the Nose is Made. The nose began as a pair of little puckers, or
dimples, just above the mouth, containing cells that were particularly
good smellers, in order to test the food before it was eaten. All smells
rise, so these cells were right on the spot for their particular
"business."
The original way of breathing, before the nose-dimples or pits opened
through into the throat, was through the mouth; and that is one
reason why it is so easy to fall into the bad habit of mouth-breathing
whenever the nose gets blocked by _adenoids_ or _catarrh_. Some
creatures--fishes, for instance,--breathe through their mouths entirely;
if you watch one in an aquarium or a clear stream, you will easily see
that it is going "gulp, gulp, gulp" constantly. The saying "to drink
like a fish" is a slander upon an innocent creature; for what it is
really doing is breathing, not drinking. Even a frog, which has nostrils
opening into its throat, still has to swallow its air in gulps, as you
can see by watching its throat when it is sitting quietly. And, strange
as it may seem, if you prop its mouth open, it will suffocate, because
it can no longer gulp down air.[28]
Our noses are nine-tenths for breathing, and only about one-tenth for
smelling; so that by far the greater part of the nose is built on
breathing lines. But the smelling part of it, though small, is very
important, because it now has to decide, not merely upon the goodness or
badness of the food, but also upon the purity or foulness of the air we
breathe. The _nostrils_ lie, as you can see, side by side, separated
from each other by a thin, straight plate of gristle and bone known as
the _septum_. This should be perfectly straight and flat; but very often
when the nose does not grow properly in childhood, it becomes crumpled
upon itself, or bulged over to one side or the other, and so blocks up
one of the nostrils. This is a very common cause of catarrh, and
requires, for its cure, a slight operation, a cutting away of the
bulging or projecting part of the septum. The rims of the openings of
the nose, known as the _wings_, have little muscles fastened to them
which pull them upward and backward, thus widening the air openings or,
as we say, dilating the nostrils. If you will watch any one who has been
running fast, or a horse that has been galloping, you will see that his
nostrils enlarge with every breath; and these same movements occur in
sick people who are suffering from disease of the lungs or the heart,
which makes it difficult for them to get breath enough.
Each nostril opens into a short and rather narrow, but high, passage,
known as the _nasal passage_, through which the air pours into the back
of the throat, or _pharynx_, and so down into the windpipe and lungs.
Instead of having smooth walls, however, the passage is divided into
three almost separate tubes, by little shelves of bone that stick out
from the outer wall. These are covered with thick coils of tiny blood
vessels, through which hot blood is being constantly pumped, like steam
through the coils of a radiator, so that the air, as it is being drawn
into the lungs, is warmed and moistened. The passage is lined with a
soft, moist "skin," called mucous membrane, very much like that which
lines the stomach and bowels, except that it is covered with tiny little
microscopic hairs, called _cilia_, and that its glands pour out a thin,
sticky _mucus_, instead of a digestive juice. This thick network of
blood vessels just under the thin mucous "skin" is easily scratched into
or broken, and then we have "nose-bleed."
The purpose of this mucus is to catch and hold, just as flypaper catches
flies, all specks of dust, lint, or germs that may be floating in the
air we breathe, and to keep them from going on into the lungs. As these
are caught upon the lining of the nose, they are washed down by the flow
of mucus or wafted by the movement of the tiny hairs back into the
throat, and swallowed into the stomach, where they are digested. Or, if
they are very irritating, they are blown out of the nostrils, or sneezed
out, and in that way got rid of.
If the dust is too irritating, or the air is foul and contains disease
germs, these set up an inflammation in the nose, and we "catch cold," as
we say. If we keep on breathing bad or dusty air, the walls of the nasal
passages become permanently thickened and swollen; the mucus, instead of
being thin and clear, becomes thick and sticky and yellowish, and we
have a catarrh.
Catarrh is the result of a succession of neglected "bad colds," caused,
not by fresh, cold air, but by hot, stuffy, foul air containing dust and
germs. The best and only sure way to avoid catarrh is by breathing
nothing but fresh, pure air, day and night, keeping your skin clean and
vigorous by cool bathing every day, and taking plenty of play in the
open air.
So perfect is this heating, warming, and dust-cleansing apparatus in the
nose, that by the time quite cold air has passed through the nostrils,
and got down into the back of the throat, it has been warmed almost to
the temperature of the body, or blood-heat, and has been moistened and
purified of three-fourths of its dust or disease germs. When you go out
of doors on a cold, frosty morning, your nose is very likely to block
up, because so much hot blood is pumped into these little steam-coils of
blood vessels, in order to warm the air properly, that they swell until
they almost block up the nostrils.
The Sense of Smell. The lower three-fourths of the nasal passages have
nothing whatever to do with the sense of smell; this is found only in
the highest, or third, division of the passages, right up under the root
of the nose, where odors can readily rise to it. Here can be found a
little patch of mucous membrane of a deep yellowish color, which is very
sensitive to smells, and from which a number of tiny little nerve twigs
run up to form the nerve of smell (_olfactory nerve_), which goes
directly to the brain. The position of the smell area at the highest and
narrowest part of the nose passage explains why when you have a very bad
cold, you almost lose your sense of smell; the lining of the lower part
of the nose has become so inflamed and swollen as to block up the way to
the highest part where the smelling is done.
[Illustration: ADENOIDS
A section through the nose and mouth: _A_, adenoid growth; _P_, soft
palate; _T_, right tonsil.]
Adenoids. If colds are neglected and allowed to run on, the
inflammation spreads through the nose back into the upper part of the
throat, or pharynx. Here it attacks a spongy group of glands, like a
third tonsil, which swells up until it almost blocks up the nose and
makes you breathe through your mouth. These swollen glands are called
adenoids, and cause not only mouth-breathing, but deafness, loss of
appetite, indigestion, headache, and a stupid, tired condition; so that
children that are _mouth-breathers_ are often two or more grades behind
in school, poor students, and even stunted and undersized. You can often
tell them at sight by their open mouths and vacant, stupid look. A very
simple and harmless scraping operation will remove these adenoids
entirely, and what a wonderful improvement the mouth-breather will make!
He will often catch up two grades, and gain two inches in height and ten
pounds in weight within a year.
[Illustration: MOUTH-BREATHERS
Note how swollen the face is under the eyes and how tired and dull the
whole expression.]
Adenoids not only cause deafness by blocking up the tube (_Eustachian_)
that runs from the throat to the ear,--the tube through which the air
passes when your ear "goes pop,"--but are also the commonest cause of
ear-ache and gatherings in the ear, which may burst the drum.
THE TONGUE
The Tongue is not Used chiefly for Tasting. If you will notice the
next time that you have a bad cold, you will find that you have almost
lost your sense of taste, as well as of smell, so that everything tastes
"flat" to you. This illustrates what scientists have known for a long
time, but which seems very hard to believe, that two-thirds of what we
call taste is really smell. If you carefully block up your nostrils with
cotton or wax, so that no air can possibly reach the smell region at the
top of them, and blindfold your eyes, and have some one cut a raw
potato, an apple, and a raw onion into little pieces of the same size
and shape, and put them into your mouth one after the other, you will
find that it is difficult to tell which is which.
The only tastes that are really perceived in the mouth are bitter,
sweet, sour, and salty; and even these are perceived quite as much by
the roof and back of the mouth, especially the soft palate, as they are
by the tongue. All the delicate flavors of our food, such as those of
coffee or of roast meat or of freshly baked bread, are really smells.
The tongue, which is usually described as the organ of taste, is really
a sort of fingerless hand grown up from the floor of the mouth--to help
suck in or lap up water or milk, push the food in between the teeth for
chewing, and, when it has been chewed, roll it into a ball and push it
backward down the throat. It is not even the chief organ of speech; for
people who have had their tongues removed on account of cancer, or some
other disease, can talk fairly well, although not so clearly as with the
whole tongue.
The tongue is simply a "tongue-shaped" bundle of muscles, covered with a
thick, tough skin of mucous membrane, dotted all over with little
knob-like processes called _papillæ_, which are of various shapes, but
of no particular utility, except to roughen the surface of the tongue
and give it a good grip on the food. If the mucous "skin" covering the
tongue does not shed off properly, the dead cells on its surface become
thickened and whitish, and the germs of the mouth begin to breed and
grow in them, forming a sort of mat over the surface. Then we say that
the tongue is badly coated. This coating is in part due to unhealthy
conditions of the stomach and bowels, and in part to lack of proper
cleaning of the mouth and teeth.
The Sense of Taste can usually be Trusted. Since the nose and the
tongue have had about five million years' experience in picking out what
is good and refusing what is bad, their judgment is pretty reliable, and
their opinion entitled to the greatest respect. As a general thing,
those things that taste good are wholesome and nutritious; the finest
and most enjoyable flavors known are those of our commonest and most
wholesome foods, such as good bread, fresh butter, roast meats, apples,
cheese, sugar, fruit, etc.; while, on the other hand, those things that
taste bad or bitter or salty or sour, or that we have to learn to like,
like beer or pickles or strong cheese or tea or coffee, are more often
unwholesome or have little nutritive value. Very few real foods taste
bad when we first try them. If we used our noses to test every piece of
food that went into our mouths, and refused to eat it if it "smelt bad,"
we should avoid many an attack of indigestion and ptomaine poisoning. It
is really a great pity that it is not considered polite to "sniff" at
the table.
THE EYE
How the Eye is Made. Next in importance after the smell and the taste
of our food comes the appearance of it; hence, our need of eyes to help
us in choosing what to eat, as well as how to avoid the dangers about
us.
The eyes began as little sensitive spots on the surface of the head.
Like the nose pits, as they became more sensitive, they too sank in
beneath the surface; but with this difference, that, instead of
remaining open, the rims or edges of the eye-pit grew together and
became transparent, forming a cover, or eye-glass, which became the
clear part of the eye, called the _cornea_. At the same time, the little
sensitive spot at the bottom of the eye-pit spread out into the shape of
the bottom of a cup, called the _retina_; and then the hollow of that
cup between the retina and the cornea filled up with a clear, soft,
animal jelly called the _vitreous humor_, and we have the eye as it is
in our heads to-day.
The sensitive retina, spreading out, as it does, to form the back of the
eyeball, is the nerve-coat of the eye; and from its centre a thick round
bundle of nerve fibres, known as the _optic nerve_, runs back to the
brain.
[Illustration: THE APPARATUS OF VISION
A cross-section diagram, showing eye and optic nerve, the bones forming
the orbit or socket, and the front lobes of the brain.]
The bones of the head, grown out in a ring in order to protect the eyes,
are called the _orbit_ or _socket_.
To protect the delicate glass (cornea) of the eye, there are two folds
of skin, one above and one below, known as the eyelids. The eyelids
carry a row of extra long hairs at their edges, called the eyelashes,
and a number of little glands, somewhat like those of the stomach, to
pour out a fluid, which makes the lids glide smoothly over the eyeball
and keeps them from sticking together. Underneath the upper lid a number
of these glands become gathered together and "grow in," after the
fashion of the salivary glands, to form a larger gland about the size
of a small almond, which pours out large amounts of this fluid as tears.
It is called the tear gland (_lachrymal_ gland).
Whenever a cinder or a grain of sand or a tiny insect or any other
irritating thing gets into the eye, this gland pours out a flood of
tears, which washes the intruder down into the inner corner of the eye
where it can be wiped out; or, if it be small enough, carries it down
through a little tube in the edge of each eyelid, through a little
passage known as the _nasal_, or _tear, duct_, into the nose. So, if you
get anything into your eye, much the best and safest thing to do is to
hold the lids half shut, but as loose, or relaxed, as possible, and
allow the tears to wash the speck of dust down into the inner corner of
the eye. If you squeeze down too hard with the lids, and particularly if
you rub the eye, you will be very likely to scratch the cornea with the
speck of dust or sand, or, if the speck be sharp-edged, to drive it
right into the cornea and give yourself a great deal of unnecessary pain
and trouble, or even seriously damage the eye. If the cinder or dust
doesn't wash down quickly, pull the upper lid gently away from the
eyeball by the lashes and hold it there a minute or so, when often the
cinder will drop or wash out.
As the light rays cannot be bent, or drawn into the eyes as smells can
into the nostrils, it is necessary that the eyes should be able to roll
about so as to turn in different directions; and so nature has made them
round, or globular, attaching to their outer coat or shell (the
_sclerotic_ coat) little bands of muscle, each of which pulls the
eyeball in its particular direction. There are four straight bands--one
for each point of the compass: one fastened to the upper surface of the
eye to roll it upward; another to the lower to roll it downward; another
to the outer to roll it outward; and another to the inner side to roll
it inward for near vision.[29]
There is another reason for the rounded shape of the eye--that it may
act as a lens in condensing the rays of light. In order that we may see
things clearly, the rays of light must be brought to a focus upon or
close to the retina, at the back of the eye; and our eyes are so shaped
that they form a lens of proper thickness, or strength, to do this.
You can see how this is done with an ordinary magnifying glass, or
burning-glass. The little sharply lighted and heated point to which the
light-rays can be brought is the focus of the lens, and the distance it
lies behind the lens is called the focal distance. The thicker the lens,
or burning-glass, is in the middle, the shorter its focal distance, and
the more strongly it will magnify.
A healthy, or normal, eye is of just such shape and "bulge" that rays of
light entering the eye are brought to a focus on, or close to, the
retina at the back of the eyeball. Some people, however, are
unfortunately born with eyes that are too small and flat, or do not
"bulge" enough; and then the rays of light are focused behind the retina
instead of upon it, and the image is blurred. This is known as "long
sight" (_hyperopia_), and can be corrected by putting in front of the
eyes lenses of glass, called spectacles, which bulge sufficiently to
bring the rays to focus on the retina.
An eye that is too large and round and bulging brings the rays to a
focus in front of the retina, and this also blurs the image. This form
of poor sight is called "short sight" (_myopia_), and can be relieved by
putting in front of the eye a glass that is concave, or thinnest in the
middle and thickest at the edges, in the right proportions to focus the
image where it belongs, right on the retina. This kind of glass is
sometimes called a "minifying" glass, from the fact that it makes
objects seen through it look smaller. It is also called a "minus" glass,
while the magnifying glass is called a "plus" glass. The shape of the
glasses or spectacles prescribed for an eye is just the opposite of that
of the eye. If the eye is too flat (_long-sighted_), you put on a
bulging, or convex, glass; and if the eye is too bulging
(_short-sighted_), a hollow, or concave, glass. Other eyes are
irregularly shaped in front and bulge more in one direction than
another, like an orange. This defect is called _astigmatism_ and is very
troublesome, making it hard to fit the eye with glasses, as the glasses
have to be ground irregular in shape.
[Illustration: A SCHOOL EYE-TEST
A normal eye should be able to read the smaller type easily at a
distance of twenty feet.]
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.
[Illustration: DISINFECTING A BABY'S EYES AT BIRTH]
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.
[Illustration: THE APPARATUS OF HEARING
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-LEVELS
The 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
long or too high, or from riding on the cars.
FOOTNOTES:
[28] To show in how many different ways nature may carry out the same
purpose, the smelling organs in insects, lobsters, and crabs are on the
ends and sides of tiny feelers, which they wave about; and the eyes in
lobsters, crawfish, and snails, are on the ends of stalks, which they
thrust about in all directions as a burglar handles a bull's-eye
lantern. Snakes "hear," or catch the sound-waves, with their flickering,
forked tongues; and grasshoppers and locusts have "ear-drums" on the
sides of their chests.
[29] These are called the _recti_ or "straight" muscles, upper, lower,
inner, and outer, according to their position. Then, to roll the eye
round and round, there are two little muscles, one above and one below,
which run "crosswise" of the orbit, called the upper and lower _oblique_
muscles.
[30] The retina is chiefly made up of a great number of fine little
nerve cells called, from their shape, the _rods_ and _cones_. These are
kept soaked in a colored fluid called the _retinal purple_, which
changes under the influence of light, somewhat in the same way that the
film on a photographic plate does, thus forming pictures, which are
translated by the rods and cones and telegraphed along the fibres of the
optic nerve to the brain. Naturally, all parts of the retina are not
equally sensitive to light; its centre, which is directly opposite the
pupil of the eye, is far the most so, while those around the rim of the
cup are dull. This is why, when you are looking, say at some one's face
across the room, only the face and a few inches around it are seen
perfectly clear and sharp, while the rest of the room is seen only
vaguely.
[31] As the inside of the eye is dark, or comparatively so, the pupil,
or little opening in the centre of the iris, looks black, and was at one
time supposed to be a solid body instead of a hole. You can easily watch
the pupil changing in size, according to the brightness of the light,
from a mere pin-point in very bright sunlight or gaslight, up to the
size of the butt-end of a lead pencil in the dark or in a dim light.
This change in size is very simply but ingeniously carried out by two
sets of tiny muscles. One set of these muscles runs in a ring right
around the pupil; and when they shorten, the opening is contracted or
narrowed. The other set runs outward through the iris like the spokes of
a wheel; and when they shorten, they pull the pupil open. If anyone has
had "drops" (_atropin_) put into his eyes in order to have them fitted
with glasses, he will know what a disagreeably dazzling thing it is to
have the pupil permanently enlarged, so that it cannot _contract_ in a
bright light.