CHAPTER VI

Errors of Refraction: Their Cause

ERRORS of refraction are responsible for most cases of defective vision and often lead to actual disease of the eye. These errors fall into four classes: Myopia, Hypermetropia, Astigmatism and Presbyopia.

In myopia, commonly called short-sight or near-sight, rays of light coming from a distance are focused in front of the retina.

In hypermetropia they are focused behind the retina. This condition is usually called long-sight or far-sight; but in reality the sight is defective both for near and for distant vision.

In astigmatism the rays are not brought to a single focus, because the curvature of the refracting surfaces is greater along certain meridians than along others. There are six different kinds of astigmatism.

When one meridian is correct and the one at right angles to it myopic or hypermetropic, the condition is called simple myopic or hypermetropic astigmatism; when both meridians are hypermetropic or myopic, but one more so than the other, we get compound hypermetropic or myopic astigmatism; while a combination of myopia and hypermetropia is known as mixed astigmatism. Simple hypermetropia or myopia, without any astigmatism, is rare.

Presbyopia is that condition of the eyes which comes on usually between forty and fifty, and compels the subject to wear glasses for reading or sewing, the vision at the distance being at first apparently unaffected.

As already stated these conditions are generally supposed to be both incurable and largely unavoidable; but abundant evidence is available to show that they are purely functional troubles, and therefore both curable and preventable.

It can be and has been demonstrated, both clinically and by means of experiments on the eyes of rabbits, fish and other animals, that the lens and ciliary muscle have nothing to do with accommodation; and that, on the contrary, the shape of the whole eyeball is changed when the focus is changed, through the agency of the external muscles. When the lens have been removed from the eyes of experimental animals, or pushed out of the line of vision (as on p. 24, Chapter III), they have continued to accommodate just as before. So long as certain muscles, known as the obliques, were intact, electrical stimulation of the eyeball, or of the nerves of accommodation, always produced accommodation, but when one of them was cut accommodation could not be produced. When the severed muscle was sewed together again, however, accommodation took place as before.

These observations are in harmony with records of accommodation in the lensless human eye which may be found scattered through the literature of the subject for over a hundred years. Many persons, unfortunately, lose their lenses through the operation for cataract, and usually they are supplied afterward with two sets of glasses, one for reading, and one for distance; but occasionally such a person is able to see at both distances without any change of glasses. The correctness of these observations used to be disputed but it is no longer possible to do so, and in consequence the idea that the lens cannot be the only agent of accommodation is creeping into the orthodox literature.

From these facts it would appear that whatever may be the cause of that failure of sight which comes to most people living under civilized conditions during their later years, it cannot be the hardening of the lens; and since the change of focus in the eye depends upon the action of the external muscles, we would naturally expect that failure to focus properly would be due to failure in the action of these muscles. The accuracy of this conclusion has been demonstrated by numerous experiments upon the eye muscles of animals.

These muscles form an almost complete band around the eyeball and lengthen it when they contract, as the camera is lengthened to take pictures at the near-point.

In these experiments myopia was produced by operations increasing the pull of the obliques, leading to a lengthening of the eyeball; hypermetropia by increasing the pull of a set of muscles known as the recti, thereby shortening the eyeball; and astigmatism by operations causing an unsymmetrical change in the shape of the eyeball (as on page 25, Chapter III). Cutting one or more of the obliques, moreover, prevented the production of myopia, while hypermetropia was prevented by the cutting of one or more of the recti.

These observations leave no room for doubt that when errors of refraction exist in any eye it is because the outside muscles are squeezing it out of shape, making it, for the time being, too long or too short, or lengthening it or shortening it unevenly. Myopia is evidently due to an abnormal contraction of the oblique muscles, hypermetropia to the abnormal contraction of the recti, and astigmatism to the unequal contraction of these two sets of muscles, causing a greater elongation or shortening in one part than in another. In presbyopia the abnormal action of the recti is evidently confined at first to those periods during which the subject is looking at near objects, leaving the distant vision but slightly affected, but later on the distant vision fails also.

That these conditions cannot be due to any permanent change in the shape of the eyeball is further proven by the fact that they can be produced at will in a moment of time, as demonstrated by the retinoscope, and that they have been cured in thousands of cases, while they often disappear without treatment.

The cause of this abnormal action of the muscles is a strain, conscious or unconscious, to see, and as such is both preventable and curable.

The underlying causes of this strain are, obviously, those factors which have marked the change from primitive savage life to ultra-civilization. Improper dietetic habits, bad air and shallow breathing, insufficient exercise, too little bathing and sunlight, constipation, the excessive use of stimulants, city noises, hurry, worry, rivalry—all the physical and mental abuses ordinarily associated with civilized life—result in tension and strain of the whole body, including the eyes. Hearing and the other senses suffer as the sight does.

Of course some of the evils of civilization can not be completely overcome. The average man is likely to succumb to the powerful combination of its adverse influences if he follows the line of least resistance and makes no effort to rise above the example of his associates. But a person of average intelligence, if he understands the problem that confronts him, can, as a rule, live in the most congested centers of civilization with very little or no detriment to his health and sense organs.

• Previous Chapter
• Next Chapter
• Up To Contents Page