The Marvellous Mechanism of the Human Eye - Eye-strain a Frequent Cause of Nervous Disorder - Structure of the Eye - Myopia - Hypermetropia - Weak Sight - Rules for the Care of the Eyesight he eye, both from the physical and psychical T aspects, is a marvellous piece of mechanism. Blessed with perfect sight, we have an asset which makes for greater efficiency and increased enjoyment of life. Imperfect vision, unless we take steps to remedy it, affects our business or professional success for the worse. Most people realise this truth in a vague sort of way.
A very large number of men and women suffer from preventible ill-health of mind, body, and temper all through their lives from sheer ignorance of how to take care of their eyesight. The nagging wife is, in a certain number of cases, the victim of astigmatism. Indeed, eye-strain is one of the chief causes of the nervous disorder of which nagging is a symptom. The business man who cannot keep on pleasant and profitable terms with his colleagues might become a perfect administrator by proper attention to an unsuspected defect of vision.
Indeed, the faculty of sight is of the greatest importance to everybody. Blindness is almost the worst physical calamity which can befall man or woman. Success in many walks of life, from sport to warfare, from art to manual work, depends considerably upon the perfect adjustment and efficiency of the sense of vision. There are very few types of work which are not affected by vision, good or bad. And the remarkable thing is that so many people are indifferent to eye troubles which could easily be put right. More precious than gold or worldly possessions, we jeopardise our eyesight in all sorts of ways by overwork, by working at the wrong time and in the wrong place every day of our lives.
Before dealing with the causes and prevention of eye mischief let us first of all describe, simply and briefly, the structure of the eye.
The eyes are two globe-shaped bodies which lie in the orbits, the bony walls of which effectively protect them. At the back of each orbit there is a little opening through which the optic nerve, surrounded by its blood-vessels, passes from the back of the eye to the brain.
The eye consists of various coats, enclosing in the centre a clear, jelly-like material, which will be described later. The outermost coat, the sclerotic, is commonly called the white of the eye. It is a strong, protective membrane round the eye like the rind round an orange, but bulging in front where it becomes transparent to the light. Rays of light pass through the transparent part, or "cornea," to the interior of the eye.
Lying underneath the sclerotic coat is the choroid, which really consists of a layer of bloodvessels with dark pigmented cells. This is continued in front as the iris, which gives the colour - grey, black, brown, or blue - to the eye. The iris is a circular curtain with a hole in the centre called the pupil. The iris, by means of minute muscles, can dilate or contract so that the opening, or pupil, is small for near vision and large for distant vision or in a dim light.
The retina is the most important structure of the eye, and is the innermost coat. It lines the choroid coat just as the choroid in its turn lines the inside of the sclerotic, and it consists of nervous material. The optic nerve passes from the brain, penetrates the sclerotic and choroid coats, and then spreads out to form the retina, which is really the sensitive plate of the eye. On this retina images of the objects viewed by the eye are formed.
The normal eye, in which the rays of light pass through the lens and are focussed on the retina
Now, what fills up the space inside these coats? The greater part is called the posterior chamber of the eye and is filled with "vitreous humour," a clear, jelly-like material. In front of the posterior chamber lies the lens of the eye, which is transparent and doubly convex. It is held in place by a fine ligament. In front of the lens is the anterior chamber of the eye, filled with the "aqueous humour," which is a colourless liquid.
The eye is very properly compared to a camera, which is, indeed, modelled upon the eye. In the camera there is a sheet of ground glass behind in place of the retina and a doubly convex lens in front. Rays of light from an object are refracted by the lens so as to form an inverted image upon the plate behind. A ray of light is "refracted" when it enters a different medium from the air, as in the case of water, or the lens of the eye, or the glass lens in the camera. (See drawing.) In the same way the crystalline lens of the eye refracts rays of light from objects to form images upon the sensitive retina behind.
The eye, although resembling the camera, is far more complex. Also the eye can see, which, of course, the camera cannot do, having no optic nerve and no brain behind it. In the case of the eye, impressions upon the sensitive retina are carried by the optic nerve to that part of the brain which has to do with sight.
And now we come to the question of accommodation of the eye, and again we can compare it with the camera. The photographer knows that if he wants to get a clear picture he has to focus by altering the position of the screen, or plate, forwards or backwards. The eye "accommodates," but in this case it is not the sensitive plate, or retina, that is altered in position. The focussing power lies in the lens. In near vision the lens becomes more convex by bulging forwards in response to the action of ligaments and muscles. In distant vision the lens is flattened or less convex, and so it refracts less directly.
Now, the healthy, perfect eye can focus without difficulty and without effort. An eye accommodates itself to objects near or at a distance so as to focus clearly and sharply upon the retina,; but few eyes are perfect, and thus we have various errors of refraction due to the fact that the eye is anatomically incapable, from its shape, perhaps, of focussing clearly and satisfactorily. Thus, instead of objects being focussed clearly and sharply on the retina, they are blurred and indistinct, unless artificial lenses or glasses are worn to counteract the deficiency. The diagrams give (a) a picture of a perfect eye; (b) one that is myopic, or shortsighted; (c) an eye that is long-sighted, or hypermetropic.