THE form of the eye, its structure, and its powers, call forth fresh admiration as they successively claim our attention. Its form affords the greatest resistance to external violence, and is most perfectly adapted for the fulfilment of the function of vision ; as it allows of the continued exercise of its powers during the act of turning the eye, which could have been accomplished in no other way. Had its shape been different it would have been less strong, unless made up of heavier materials ; and its glance could not, as now, have strayed at will from heaven to earth, from earth to heaven, reading, near and far, the ever-present lessons of light and beauty.
Even bone would have been a less perfect protection for the delicate internal parts than is the light tissue which forms the framework of the globe, so yielding in its elasticity, yet so firm in its tough resistance. In front, we find the strong white membrane, the sclera, modified to a transparent structure, the cornea, an equal safeguard against intruding enemies, and giving unimpeded entrance to light alone.
The nerves and blood-vessels, which if placed in it would have weakened the fibrous envelope, are included in another interior membrane, the choroid, and within these is spread out the delicate nervous perceptive tissue, the retina, to which all the other parts are auxiliary and subservient. Upon this images of all visible nature are pictured ; from it the fleeting impressions are telegraphed to the brain, and we are conscious that we see.
But to this function of seeing other conditions are essential. The eyeball, the most perfect of optical instruments, must have its focal powers completed by the presence of refracting substances, the aqueous and vitreous humors and crystalline lens ; which, at the same time, give form to the globe, and by their quiet pressure keep the nervous tissue expanded and in readiness for its appropriate work. By a combination of these refracting agents the rays emanating from objects are transmitted and concentrated so as to render the image upon the retina distinct and well defined.
Furthermore, the passage of rays through these transparent media is regulated by a curtain, the iris, of which the circular opening, the pupil, expands and contracts according to the amount of light, the movements of the curtain being self-impelled, without any effort of the will. Thus, contracting in a strong light, it protects the retina from being injured by the glare, and expanding where the light is dim it allows a larger number of rays to enter, so as to form a clear image. Like the choroid, this curtain is lined with dark coloring matter, not only to absorb any irregularly refracted rays and prevent reflections from side to side within the globe, but to exclude the entrance of light through the coats of the eye, except in the direction most suitable for refraction, through the cornea and pupil.
To complete the noble endowment of capacities, the eye has yet another power. It is a self-regulating optical instrument. We may turn our eyes from the printed page to gaze at a distance, or withdraw them from space to fix them upon a minute atom, and the eye adapts itself instantly to each of these uses. By means of a circle of delicate fibres, so small that till lately their existence and uses were unknown, — the ciliary muscle, — the convexity of the crystalline lens can be increased and its focal power varied ; and thus, without conscious effort, the eye may contemplate the glories of the firmament, or catch the first flitting expression of an infant’s love, or explore the mysteries of microscopic existences.
Finally, we have two eyes. Not merely that we may be doubly provided against the danger of accidental loss of the inestimable privileges of vision, but to enable us to estimate the size, form, and distance of objects with more correctness than we could with one eye alone. To a single eye everything would have appeared as a plane surface, and it would have been difficult to determine whether objects had solid or only superficial dimensions, or whether they were near or far. With two eyes the impressions made on each mutually correct one another, and combine to convey to the brain perceptions of the properties of things which could otherwise have been gained by the touch alone.
How TO USE THE EYES.
Every normal eye is capable of a great variety and amount of use. It sees near or far with the same ease and with equal clearness. But these powers, extensive as they are, may be overtasked. Because the eyes can see minute objects without difficulty, it does not follow that they should be kept almost constantly looking at small objects. They were intended for varying use, and, like any other organ of the body, they may be enfeebled or injured by having their most delicate powers continually and exclusively employed in one manner.
One of the first rules laid down by a teacher to his pupils should be, not to keep their eyes fixed upon their books. Apart from the probable injury to the eye itself by too close application, I am satisfied that lessons, especially those requiring thought, cannot be as well committed to memory when the eyes are fixed upon the page, as if they are permitted to wander. The eyes must of course look at the book often and long enough to take in the idea, but if they are too steadily kept there the perceptive power seems to occupy itself with the visible objects to an extent which is unfavorable to other mental processes. A distinguished engraver once said to me, “ I know now how to make a face think.” And he explained that the secret lay in giving a certain expression to the eyes by causing their axes to have a very slight divergence from each other. This corresponds with my observation ; and this position of thought is exactly the opposite of that assumed by the eyes when looking at a book.
For the sake of even normal eyes, it would be most desirable that education should be simplified; that children should not be required to learn an infinity of details which they are sure to forget, and which could be of no possible use to them if retained ; that they should be taught to think as well as to remember, — and in fact as a means of remembering, — instead of giving all their time in school, and often out of school, and by artificial light, to acquiring a parrot-like facility of repeating lessons which they do not comprehend. It might require more pains, but it would certainly be a great advantage if teachers would teach children what they know, rather than content themselves with being mere hearers of lessons which may have been learned by the eye, but often not by the understanding.
It would scarcely seem to be necessary to say a word of warning in regard to imprudent testing of the power of the eyes ; but instances are not rare where children or adults have done their eyes serious harm by trying to look at the sun, or by observing an eclipse without using a smoked glass. The direct solar light and heat seems in these cases to destroy the perceptive power in a greater or less portion of the retina. Injury may also result from using the eyes for looking at small objects by moonlight, which does not give sufficient illumination for such purposes.
SOME POPULAR ERRORS.
There comes a time when normal eyes find their powers grown limited, and require more light, or assistance from glasses, when looking at small near objects. When this period arrives it is an error to persist in endeavors to do as formerly with the eyes ; but much use must be avoided except in a clear light or with the required auxiliaries. It is also a mistake, as will hereafter be shown, to suppose that glasses should not be worn while it is possible to avoid doing so. On the contrary, they serve to prevent straining of the eyes, and preserve rather than injure vision.
Certain defects of refractive power are due to malformation of the eye, either existing from birth or acquired afterwards, and are not to be removed by remedies or by manipulation. It is a mischievous error to suppose that the form of an elastic globe, filled with fluid or semi-fluid substances, can be changed, except for the moment, by pressing upon it with the fingers, as has been recommended by charlatans. All the theories that the eye can have its form favorably modified by rubbing it always in one direction, or by any other manipulation, have no foundation in facts. But while persistent squeezing, according to these methods, can never do any permanent good, it involves great risks. It may lead to congestion and hemorrhage within the eyes ; or give rise to destructive inflammation or the formation of cataract by dislocating the crystalline lens ; or cause almost immediate loss of sight by separation of the retina from its neighboring parts ; or may increase the giving way of the back part of the globe, which is already often begun in near-sighted eyes.
The same warnings will apply with equal force against the use of the eyecups fitted with rubber bulbs, to alter the form of the eyeball, as is asserted, by suction. Valueless and dangerous as they are, persons are often persuaded to purchase and try them, — sometimes to their sorrow.
VARIATIONS FROM THE NORMAL STANDARD OF SIGHT.
Every eye ought to see distant objects clearly. If it cannot do so, its refractive power is at fault or it is the subject of disease. The eye ought also to have clear perception of small objects, such as print, etc., when held at a proper distance. If it has not, the fault may lie in either the refractive power or the accommodative function, or there may be disease of some of its parts.
Myopia, or “ near-sight,” is by far the most important, as it is also one of the most common of the refractive defects of the eye. In the other forms of abnormal refraction we have merely a defect of construction, giving rise, it is true, to annoying disabilities, but having no tendency to further changes of structure or function. Near-sightedness, on the contrary, where it exists in a high degree, is not simply an infirmity, as is usually supposed, but is in many cases associated with grave disease of the posterior parts of the eyeball, having progressive tendencies, and not seldom resulting in loss of all useful vision. It has, furthermore, a strong disposition to hereditary descent, reappearing in the children or grandchildren of myopes.
The defect in form, in short-sighted eyes, does not consist, as was formerly supposed, in an undue prominence of the front part of the eye, but in an elongation of the whole globe from before backwards, so that it assumes an olive or egg shape, instead of being round. This lengthening mostly occurs at the back part of the eyeball, and is not to be observed at first sight ; but in many cases we may see that the eye has this altered form, and extends back farther than usual in the socket, by drawing the lids apart at the side next the temple, the eye being at the same time turned towards the nose.
All the coats of the eye are implicated in these changes, which take place, sometimes by gradual expansion at every point, but usually by a more considerable giving way around the entrance of the optic nerve. In examination of such eyes after death, a positive bulging of the sclera is seen at this point. During life we can observe these changes, and watch their progress, by means of the ophthalmoscope. This instrument, by which we are enabled to illuminate and explore the interior of the eye, has thrown new light upon the whole subject of near-sightedness. By its aid we are able to follow the morbid changes as they are successively developed. We can distinctly observe the progressive giving way of the retina, optic nerve, choroid, and sclera, to form the bulging which is termed posterior staphyloma ; can notice the congestion and other changes following imprudence ; and, too often, can see the advent of separation of the retina from the choroid, with its accompanying loss of sight.
As the retina expands with the general enlargement, the nerve tissue, in that layer of the retina which is the seat of its especial function, is of course extended over a larger surface and its perceptive power proportionally weakened. Many such eyes are therefore unable to see distant objects with normal clearness, even with the glasses which most completely correct their myopia, although they see small near things perfectly well. It seems to be necessary that a larger number of rays should fall upon a given area of the retina in order to produce a distinct impression. This lack of acuteness of vision is often much greater in the evening, so that persons thus affected cannot see to drive a horse safely or distinguish the outlines of objects.
Eyes which are but slightly myopic often see nearly as well as others at a distance by the aid of suitable glasses, and they have almost microscopic vision of near objects, and can read in a dim light ; these facts have given rise to the popular belief that near-sighted eyes are stronger than others, and able to bear every kind of use and abuse ; and the delusion is encouraged by the disposition shown by myopic persons to choose occupations requiring close sight, and by their ability to read at an advanced period of life without glasses. This belief would be well founded, but for the tendency to the gradual changes already described.
The progress of the alterations in the posterior parts of the eye is favored by the stooping position of the head, which allows the blood to accumulate in the vessels of the eyeball, and by too long-continued use of the eyes upon minute objects, which requires such action of the external muscles that the globe is compressed from side to side, and is thus made to yield still further at that part where the already thinned tissues offer but slight resistance. With each degree of change the process becomes easier, the eyeball grows misshaped to a degree which limits its motions in the socket, and the eye most affected no longer acts with its fellow, but is disposed to turn outwards, and to give up attempts at vision.
With increased implication of the retina in the morbid changes, its perceptive acuteness is more or less reduced, especially as regards distant objects, and glasses no longer give them the same clear outlines.
The morbid processes may be arrested at the early stages of their development, and by good fortune and prudent management the eyes may retain through life nearly the normal powers ; or if even considerable changes have taken place, these may remain stationary and give rise to little inconvenience. But if they are not recognized, and means taken to avert their progress, they may go on till the retina becomes useless, being separated from the choroid by fluid which collects between these membranes.
The changes I have described are insidious in their course. Slight warnings are sometimes felt, but, as a rule, the eyes, unless examined with the ophthalmoscope, exhibit and feel no symptoms calculated to excite alarm, except, perhaps, an increase of the myopia, which frequently, but not invariably, occurs, often unnoticed by its subject. After reaching a certain degree, there is little hope that further changes will be averted by any care or skill. The conditions have become so unfavorable that the morbid tendencies can no longer be successfully opposed, and each year sees a downward progress. Even where the staphylomatous enlargement has not been excessive, separation of the retina will sometimes suddenly occur. I have repeatedly seen cases where this had taken place within a single twenty-four hours, after some unusual exertion of the eyes, and where blindness was already nearly complete.
It is quite time that the attention of the community was drawn to a matter of so much importance. At least in some classes of society, the possibility of blindness at or near middle life from changes incident to excessive nearsightedness, as well as the predisposition to transmit the same infirmities and liabilities, ought to be taken into account in forming matrimonial alliances, like any other impending disability from incurable ailment. The fact of its being frequently inherited once understood, parents should watch for any early manifestations of its presence in their children, and take measures to prevent its progressive increase. Teachers should impose upon myopic eyes as little as possible of studies requiring close application, even though at the time the child makes no complaint. It is questionable if our system of education, augmenting as it does the frequency and degree of near-sightedness, is an advance in civilization. It would be better to go back at once to the oral teachings of the schools of Athens, than to go on creating our favorite type of educated men and women, at the expense of their own and their children’s eyesight.
No medical skill can bring back these delicate tissues, once distended, to their former healthy condition, or even in some cases prevent the steady onward march of the disease. But prevention is in a measure within our power. Myopic eyes should not be used continuously for small objects, and especially with the head bent forward ; fine and bad print should be a fatal objection to a school-book ; the use of lexicons, or close mathematical work, should be limited and interrupted; written exercises should be almost dispensed with ; and the child should be spared search upon the map for unimportant places. The book should be held up when possible, and the pupil should not keep his head leaned over his desk, nor be allowed to study by a feeble light.
If by these precautions the child reaches adult age Without any considerable development of his myopia, he will thenceforth be comparatively safe, as changes are less likely to occur after this period. But if, from thoughtless mismanagement, large and progressive structural alterations of his eyes have been brought on during his years of study, he may not only find himself disabled from pursuing such other occupations as he may desire, but may be in a condition foreboding further misfortune.
Except when slight, myopia lessens little if at all with age ; but it sometimes happens with those who are only a little near-sighted that, while still requiring concave glasses for clear vision of distant objects, they will, after middle life, also need convex glasses for reading.
There are a few cases of apparent myopia where this does not really exist ; as sometimes in children or aged persons affected with cataract, or where ulceration or other disease has existed. But it is sufficient here to refer to these as possible, without attempting their description.
The axis being too long in myopic eyes, parallel rays, such as proceed from distant objects, are brought to a focus at a point so far in front of the retina, that only confused images are formed upon it. Such a malformation, constituting an excess of refractive power, can only be neutralized by concave glasses, which give such a direction to rays entering the eye as will allow of their being brought to a focus at the proper point for distinct perception. It is therefore irrational and useless to attempt a substitution of other means instead of resorting to these glasses, which, by rendering parallel rays divergent, adapt them for the excessive refraction of the myopic eye, thus relieving an infirmity which is not to be removed.
The use of glasses for distant vision is often objected to by parents and friends, from an idea that the shortsightedness will thus be increased, or in the expectation that the eyes will become of normal power at a later period if glasses are not worn. Both of these opinions are erroneous. Myopic eyes are not injured by wearing suitable glasses ; but, on the contrary, are often preserved from injurious pressure on the globe in the indulgence of the habit of nearly closing the lids in order to obtain a clearer impression of the images of distant objects, as is commonly done when glasses are not worn. Nor will the myopia be appreciably lessened by abstinence from glasses. It is best, therefore, not to deprive young people of the many pleasures arising from distinct vision of things around them, in the illusive hope that the great sacrifice thus made will be compensated by any benefit.
Such glasses should be selected as make distant objects clear without lessening their size and giving them an unnatural brilliancy. If no glass gives this clearness, the acuteness of perception may have already become impaired, or there may be a complication of the myopia with astigmatism. Many myopes use the same glasses for reading or music which they wear for distant vision. It is best, however, when the myopia is but slight, to dispense with these in reading, sewing, etc., or to wear a lower number, such as will allow of distinct sight at the distance where the book or music would ordinarily be placed.
Henry W. Williams, M. D.