The Eye in its Relation to Health ================================= By Chalmer Prentice, M.D. Chicago, A.C. McClurg & Company, 1895 Transcription (c) A. Wik, 2004 +--------------+ | Chapter VIII | pages 129-136 +--------------+ 129 WHEN the optic axes deviate or have a tendency so to do, the fault has com- monly been attributed to muscular weakness or paresis. If the left eye deviated above its mate, this theory assumed that the inferior mus- cle of that eye was insufficient in strength to hold it down. If it deviated inward, the outer muscle was said to be too weak; if outward, the inner muscle was too weak. This reasoning nat- urally assumes that to maintain a balance, there is a constant contention in the various eye mus- cles, and whenever a muscle becomes weaker than its fellow, there will be a deviation in a direction opposite to the weak one. That this can hardly be true is evident from the fact that the internal rectus is known to have generally fully five or six times the strength of its opposite, the exter- nal rectus; and if it were a relative difference in strength of the muscles that caused the eyes to deviate, all eyes having muscles of such compar- ative strength would be constantly turned in to- ward each other. The fact is, it is not a question as to the relative strength of the muscles. Nor- mal eyes should, under absolute relaxation of all 9 130 the muscles, be in a perfectly parallel plane for an infinite distance; although some muscle might be weak, an eye would present perfect balance with absolute relaxation of the muscle opposite the weak one. WHEN THE OPTIC AXES ARE NOT PARALLEL, THE CONDITION IS DUE TO ONE OF TWO CAUSES--EITHER AN ANATOMICALLY SHORT MUSCLE, OR ONE CONTRACTED BY SPASM. This rule does not hold in paralysis, which can not be determined by a relative position of the optic axes alone, but by individual movements of one eye. Neither are the tendencies to relative devi- ation generally due to paresis, a term sometimes used in lieu of muscular asthenopia. The almost universal deviation of the eyes of the dead, the blind, the new born babe; of those who have been rendered unconscious by the use of an‘sthetics, alcohol or narcotic poisons; and of those who are in a state of suspended anima- tion from other causes, such as fainting, apo- plexy, or coma, is due to a suspension of inner- vation more or less complete, during which the eyes fall into those positions in which the ana- tomical length of the muscles would naturally hold them. Deviations under the above condi- tions are certainly not due to paresis or muscular weakness. True, we do have weak muscles, but they are generally on the side of the eye oppo- site to that in which the theory of muscular as- 131 thenopia would have them. For instance; when the deviation of an eye is due to a short external muscle, tending to draw the eye out- ward, it is the external muscle which is really in- sufficient, asthenopic, or underdeveloped, for never has it had normal action. It has always been drawn upon and stretched by the opposite mus- cle, in the effort of the latter to pull the eyes into line. In such cases, gymnastic exercise of the ocular muscle in which there is a manifest deviation will sometimes restore the physical bal- ance, but it does so by making the defect latent again. It does its work by once more increasing the already excessive innervation to a hiding standard. For the muscle that we increase in strength by gymnastic exercise, is the over-de- veloped one. This exercise simply stretches the short muscle more, and increases the abnormal innervation in the strong one; hence the results of these gymnastics are the opposite of what we should seek to obtain. Such exercise will often restore a balance of the optic axes, but it does so at the expense of the nerve-centers. In that class of cases where there is a mani- fest deviation of the optic axes, and where such deviation is due to a spasm, which usually turns the eye in a direction opposite the short muscle, gymnastic exercise will bring some relief, for it represses in a small measure the abnormal inner- 132 vation, as it brings the eyes back into line. This relief might possibly be increased by crossing the line of balance and forcing the eyes in the oppo- site direction to a considerable extent by repres- sion. There is always a physical comfort in seeing easily, but the disturbances arising from eye- strains are not always local; they do not always affect the vision. Whenever the shortness of a muscle begins to manifest itself, we may infer that the nerve- centers have wearied of holding the eyes in place and begin to suspend their labor. There is an effort of the nerve-centers to rest from an over-taxed condition, and we should always aid them as much as possible in this effort. This is done by placing prisms before the eye in the direction opposite to that prescribed in gymnas- tic exercise, increasing our prism in this direction as long as fusion will take place. We establish a repressive strain in the opposite direction; that is, a strain in which there is an effort to suspend nerve-impulse which has become excessive and more or less fixed in the muscle opposite the short one. By exercising this repressive strain for a few weeks or even months, we cannot expect to sus- pend absolutely and entirely an abnormal nerve- impulse that has required many years to attain its 133 present intensity. We should reasonably expect long periods of time to be necessary to repress these brain leaks that have been established since birth. A tendency to deviation of a given number of degrees, say 6, will give rise to varying amounts of strain in different individuals; for, although by reason of the shortness the eyes tend to deviate 6 deg. in each case, the innervation that brings the eye back into line will vary in accordance with the size, strength and elasticity of the short muscle. A thin, weak muscle causing the axes of the eyes to deviate 6 deg., would require a certain amount of motive-force to correct the defect, whereas, a broad, tendonous, inelastic muscle that turned the eye 6 deg. would require much more motive-force to bring it into place. So, THE NUMBER OF DEGREES OF DEVIATION IN THE OPTIC AXES CAN NEVER DETERMINE THE AMOUNT OF EYE-STRAIN. It has often been observed that considerable manifest eye-strain may exist without materially inconveniencing the patient, or without inducing any noticeable nervous disturbance. From the fact that much smaller defects have apparently been the cause of excessive nervous disturbance, the conclusion drawn by some has been that these muscular defects are not responsible for the physical conditions co-existent with them. This 134 is erroneous and based on superficial investigation. A high degree of manifest defect, say 20 deg., may exist as in the case No. 1; but there is a great deal of time during which the eyes are at rest in manifest troubles. Many times during the day, and always during sleep, there is no effort of the nerve-centers to parallelize the optic axes. The same amount of anatomical defect that we have just supposed may exist in a case where it is entirely latent. In this case the innervation that sustains parallelism is constant and fixed, and the strain is continuous. It is a constant source of nerve-center drain and irritation. Even during the hours of sleep the spasm continues, as in latent hyperopia. Otherwise, we should find, on waking a person from sleep, that the spasm of latent hyperopia had ceased; but such is not the case. The same character of tonic spasm may exist in any of the long muscles as well as in the ciliary. THE MORE MANIFEST A MUSCULAR DEFECT IS, THE LESS WILL BE THE STRAIN AND DISTURBANCE OF THE NERVE-CENTERS, FOR THIS FORM HAS PERIODS OF REST; BUT NO PERIODS OF REST EVER COME, EVEN DURING SLEEP, TO NERVE-CENTERS THAT ARE SUSTAINING ABSOLUTELY LATENT EYE- STRAIN. THE HIGHEST STATE OF NERVOUS DIS- TURBANCE EXISTS WHERE THE DEFECTS ARE ABSOLUTELY LATENT, OR THEIR MANIFESTATION REVERSED. 135 The power of the eyes to fuse or overcome prisms held in various directions before the eyes, is much higher at the near point than at a dis- tance. At twenty feet it may be difficult to fuse 2 deg. or 3 deg. of prism, whereas 6 deg., 8 deg. or 10 deg. may be fused in the same direction at a distance of ten or twelve inches. Cases will vary considerably, and no general rule can be laid down, but fusion power is always much higher at the near point. It is the high fusion power of the near point that I often find very useful in treatment. Fusion power is proportionately high- er under a bright light, or one of high candle power, than under a weaker one. A plurality of objects, non-similarity of shape, variety of color and irregularity of arrangement, emphasize the fusion stimulus; on the other hand, any imperfec- tion in the refractive media of the eye would dim or lessen the acuity of vision and the fusion power. A plain, blank surface offers no stimulus for fusion; a single point offers the least. In utilizing fusion power to establish repress- ive stain, these conditions should always be kept in mind; acuity of vision, density of light, num- ber of objects, and distance. In testing with prisms, there is sometimes an objection to using a light. The field of vision is so sensitive in some eyes that the faint, doubly refracted image from the two faces of the prism 136 impresses itself on the retina of the patient, and this is mistaken for the second image of diplopia, which it is not. It is difficult, for this reason, to tell whether actual fusion has taken place. Con- sequently, in testing with prisms, it is safer to use some non-illuminated object to look at; but a light will be the most practical where the vision is foggy after an operation, or from other causes. Again, when we think we have obtained fusion, great care should be taken to make ourselves sure that the two images are not so far separated from each other that one is thrown out of the field of vision, or so far into it as to cause it to be sup- pressed and only one object appear. This pre- caution should always be taken. An eye, to accommodate itself to a prism, always turns towards its apex. For this reason, in seeking to relieve and reverse the strain of a short muscle or to establish a repressive strain with prisms, we should always point the apex toward the short muscle. +---------------------+ | End of Chapter VIII | pages 129-136 +---------------------+