Gambar halaman
PDF
ePub

though strengthened by a number of decussating fibrous bands, is the weakest point in the eyeball. The cornea, crystalline lens, and aqueous and vitreous humors are all transparent, and they constitute what are known as the dioptric media of the eye.

When rays of light enter the normal eye from a distance of twenty feet or more, they are practically parallel, and, passing through the dioptric media, are brought to a focus, not only on the retina, but at its most sensitive part, which is a minute, circumscribed foramen, known as the fovea centralis, or the yellow spot of Sömmerring. This is absolutely essential to perfect vision; and anything which interferes with or prevents parallel rays from being so focused must necessarily render the visual impression more or less indistinct. In the three errors of refraction above mentioned, we have three distinct ways in which this is brought about. These differ from each other in their modus operandi, but they produce a common result; namely, defective vision.

In myopia, or, as it is commonly called, near-sightedness, we find an abnormal condition due to elongation of the eyeball antero-posteriorly; and this change of shape is not caused by the bulging forward of the cornea, as is generally supposed, except in rare cases of what is known as conical cornea, but it is due to the posterior portion being lengthened or forced backward. Myopia may be acquired or congenital; congenital in the sense, at least, that weak lungs and throats and brain-affections are transmissible from parents to children.

Carefully prepared statistics, compiled in Europe and more recently in our own country, show a steady increase in the congenital form of this affection with each succeeding generation. The acquired form, however, is by far the more common and dangerous one, and its pathology should be carefully considered and clearly understood.

During childhood, the eye, like other organs of the human body, does not attain maturity. The sclerotic, which, later in life, becomes firm and unyielding, is at this period capable of being distorted by undue “pressure; and this change of shape may be brought about by any over-exercise of the muscles which

control the movements of the

eye.
Whenever the

eye

is accommodated for near work, as in reading or study, a corresponding amount of convergence takes place, due to the contraction of the internal rectus muscle, which, through its nervous center, acts in unison with the ciliary muscle by which the change in shape of the lens is effected. As the internal rectus muscle is attached to the sclerotic, it must exert, whenever it contracts, a certain amount of pressure upon the eyeball. This, if too long continued, causes the sclerotic to yield at its weakest point; namely, the optic-nerve entrance. Owing to their close relationship, the choroid is likewise forced back, dragging with it the delicate retina. A clear image can now be obtained only by bringing the object to be examined still closer to the eye; whereupon the increased convergence necessarily causes additional pressure upon the globe. Thus, the very attempt we naturally make to overcome this defect only tends to increase and confirm it. Distant or parallel rays of light ought to be focused on the retina without any effort on our part; but, if the retina be too far back, this, of course, will be impossible, as only divergent rays can then be so focused. Hence it is at once apparent that the rational treatment would be to place a minus, or double-concave glass of proper focal strength before the eye, which will render the parallel rays divergent. The internal rectus, and with it the ciliary muscle, will then be relieved from extra duty, the increased convexity of the crystalline lens will disappear, and undue pressure upon the eyeball will immediately

If this treatment be adopted sufficiently early in life, it is even possible that the natural shape of the organ may be in part restored ; although, as a rule, to check further deterioration of sight, is all we may hope for. It should, however, he always borne in mind, that myopia in the young shows a tendency to increase, and that, when the inner coats have undergone a certain amount of stretching, permanent and irremediable damage has been done. The moment that acquired myopia is developed, the eye becomes a diseased organ, liable at any time, under excessive or improper use, to suffer, through retinal detachment, almost instantaneous blindness.

cease.

In hypermetropia, we find the eyeball too short in its antero-posterior diameter; and consequently the parallel rays are focused behind the percipient layer of the retina, and thus a blurred image is formed, as in myopia; but there exists a marked difference between the two defects. If the hypermetropia be not of too high a degree, the individual is able to increase the convexity of his lens sufficiently to focus, not only parallel rays, but even the divergent rays from a near object. Hence, in early life, while the lens is elastic and the muscles of accommodation are active, no special inconvenience may be experienced. So soon, however, as the child applies himself to his books or the system becomes enfeebled by disease, the defect becomes at once apparent. The ciliary muscles, which up to this time have acceded without demur to the demands made upon them, now refuse longer to respond to this extra task. If the demand be persisted in, indistinct vision and painful and irritable eyes necessarily result. When the normal eye is directed toward distant objects, the muscles of accommodation are relaxed and the eye is at rest. Not so with the hypermetropic eye. This eye must bring into play these muscles, in order clearly to discern remote objects, and must still further make use of them sharply to define near ones. It is constructed to focus convergent rays only, and convergent rays are nowhere naturally found except as coming from the interior of a myopic eye. Hence, as Jäger tersely puts it, “ the fundus of a myopic eye is the only object under heaven which a hypermetrope is able to see without the correction of his ametropia.” When this increased degree of accommodation necessary for close work is continued beyond a certain point, we frequently find that these muscles become spasmodically contracted and refuse to relax at all. Then results the condition known as ciliary spasm, which, inasmuch as distant objects are indistinctly seen, so closely simulates myopia that a careless observer may easily be misled.

Still another complication may now present itself to add to the patient's discomfort. When speaking of myopia, I mentioned the fact that increased accommodation was attended by a corresponding degree of convergence; but, inasmuch as in a hypermetropic eye there is always present a certain degree of accommodation even when looking at distant objects, it follows that, when the hypermetrope wishes to examine anything close to him, an undue amount of convergence will direct the axis of vision to a point nearer than the object looked at. The image, therefore, will be received upon the retina at points which do not correspond, and diplopia, or a double image, will result. Any attempt to lessen this excessive convergence must be attended by a relaxation of the ciliary muscle, in which case the lens becomes less convex and no longer focuses sharply. In order, therefore, to avoid this duplicate image, one eye must be directed to the object; but in doing this, another nerve-center is brought into play. This center regulates the lateral movements of the eye and causes the external rectus of one and the internal rectus of the other, which heretofore have been antagonistic, to work in unison; and, therefore, as one eye moves outward, the other simultaneously turns inward to a corresponding degree, producing a condition known as internal squint. Persons commonly look upon squint as simply an ugly deformity; but it is more than this. As a general rule, an eye that has squinted for any length of time is practically a worthless eye; for by its non-use, especially in early life, the retina has lost its sensitiveness; and, even though the deformity be removed by operative interference, the amblyopia will persist. How important is it, therefore, for the future comfort of the child, that this refractive error receive early attention; and how easy, in this instance at least, is it to remedy nature's shortcomings and the attendant train of evils ! If we give our patient a plus, or double-convex glass, of suitable focal strength, we at once enable him to relax his accommodation for distant objects. His ciliary muscles, no longer constantly contracted, regain their normal tone. The pain and irritability vanish, for the cause has been removed ; and the eye becomes practically a normal one, capable of doing its full share of work, without danger or discomfort.

The correction of the remaining refractive error, astigmatism, is somewhat more difficult, presenting, as it often does,

many complications which require great care and thoughtful study before we can thoroughly master them, but which, when once understood, afford a most satisfactory proof of the great advance that the science of ophthalmology has made in modern times.

Astigmatism, as ordinarily met with, is due to variations in the shape of the cornea from its normal form. It may be elongated in one diameter and shortened in another; or it may be longer or shorter in both, but in a different degree; or normal in one, and longer or shorter in the other. In any case, the rays of light emanating from a given point and passing through the cornea are unequally refracted and are not reunited into one point upon the retina. The image must consequently be blurred; and, in the involuntary effort that the individual makes with his apparatus for accommodation to focus for each of these different meridians, the ciliary muscles are so constantly called into play that they become fatigued from excessive action, and asthenopia is added to the patient's discomfort. Strictly speaking, astigmatism is not a separate error of refraction, but rather a combination of the two errors before described. If the cornea project more than is normal, the eye is just as truly myopic as if the posterior portion was forced backward; and hypermetropia in the same way results, whether the eyeball be flattened anteriorly or posteriorly. The glass, however, which will correct the simple hypermetropia or myopia will not answer for hypermetropic or myopic astigmatism; for it must be remembered that only a part of the cornea requires its refraction to be changed. Take, for instance, a patient who is myopic in one meridian, but hypermetropic in the other; now place before such a one a minus glass capable of correcting the myopia, and what is the result? The myopic meridian becomes normal, but the hypermetropic one is rendered still more hypermetropic, and nothing is gained. For astigmatism, then, the glass required will be one that represents a section of a cylinder and not of a sphere. The rays of light, in passing through such a glass, will be refracted only in a plane at right angles to its axis, and, by a combination of these cylindrical lenses, we may use both a plus and a minus glass for the same eye. One pecu

« SebelumnyaLanjutkan »