The Eye

Categories: THE LOOKOUT DEPARTMENT
Sources: 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, in tead 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.