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The Eye

Category: THE LOOKOUT DEPARTMENT
Source: A Handbook Of Health

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.



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.


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.





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Previous: The Tongue

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