The Eye : Part 1

A Handbook of Health
by Woods Hutchinson

(Page 26 of 32)

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.

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.

Houghton Mifflin Company
Copyright, 1911, By Woods Hutchinson
All Rights Reserved
Tenth Impression

About the Author

Woods Hutchinson (1862-1930) was an American physician, born at Selby, Yorkshire, England. He graduated from Penn College, Oskaloosa, Iowa, in 1880 and received his medical degree from the University of Michigan four years later. He worked as a professor of anatomy at the State University of Iowa and then became a professor of comparative pathology at the University of Buffalo.