On the nature of myopia and the mechanism of accommodation

Me&A Hypotheses (1989) 28, 197-211 0 Longman Group UK Ltd 1989

On the Nature Accommodation

of Myopia

and the Mechanism

of

R. J. McCOLLlM 10666 North Torrey Pines Road, La Jolla, CA 92037, USA

Abstact - An experiment in which pressure was applied to the globe of the eye by artificially-induced contraction of the superior oblique muscles produced two effects: 1) Dual vision, consisting of two separate, superimposed, retinal images, one blurred and one clear; and 2) A 5 diopter increase in myopia. It was concluded that the pressure, transmitted.through the sclera to the vitreous, forced the vitreous against the back of the lens, flattening the periphery but not the axial (central) region, resulting in a high degree of negative spherical aberration, combined with accommodation. This suggests that accommodation can be actuated by contraction of the extraocular muscles. When the pressure was released, it took several years for the dual image to subside. This suggests that when the lens is released after a long period of accommodation, the return to the unaccommodated state is extremely slow, and that a significant factor in the etiology of myopia is repeated long periods of accommodation in which periods of rest are insufficient to allow the lens to return completely to the unaccommodated state.

Introduction Is it possible for a longstanding

and universally accepted scientific theory to be proven wrong? The answer, of course, is yes, for the simple reason that it has happened. An example is the theory of nervous system plasticity in neurology (see section Erroneous Theories), an error which lasted for decades. I pose the question because of the surprising result of an experiment in vision which suggests the possibility of a similar case in ophthalmology. This result was the creation of two separate modes of vision in a human subject: emmetropia and myopia, simultaneously in each eye. This

condition, which was an accidental result of an experiment designed for a different purpose, suggests that the Helmholtz-Fincham theory of accommodation is in error, and that this in turn has misled researchers about the role of the crystalline lens and the extraocular muscles in the etiology of myopia. The reaction of eye researchers will no doubt be that the suggestion is preposterous. The Helmholtz-Fincham theory is extremely well documented and for more than forty years has been considered the only acceptable explanation of how the eye accommodates for near vision. The possibility that large numbers of researchers in many different coun-

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198 tries could be wrong about such’ a basic mechanism of the eye will certainly not be taken seriously. The experiment in question was an attempt to produce myopia by artificial means. Ophthalmology seems to be as subject to fashions in research as any other branch of medicine, and nowadays an eye researcher would be about as eager to study myopia as a physiologist to study phrenology. This was not always the case. For almost a hundred years, myopia was one of the major subjects of ophthalmological speculation and research. By the nineteen thirties, almost every conceivable aspect of this condition had been studied exhaustively and every possible cause had been suggested. As Curtin put it incisively, “It would appear at one point toward the close of the 19th century that any ophthalmologist who experienced a night of insomnia arose in the morning with a new, and usually more bizarre, theory” (1, p. 61). With new research techniques, and therapies such as radial keratotomy, there seems to be a mild resurgence of interest in myopia, but the emphasis has been almost exclusively on therapeutic ,measures, not on etiology. Apparently this is considered a field that has been worked over so thoroughly that there is little possibility of further significant progress.

An explanation of the experiment requires, first, a brief description of myopia and the reasoning behind the attempt to produce this condition artificially. Myopia can be described briefly as that condition in which vision is blurred because parallel rays of light from infinity come to a focus in front of the retina instead of on it. This could occur because of excessive refractive power of the cornea or the crystalline lens, or both, or if the eye is too long. For many years much of the research on myopia was directed to the question of axial length. This was due in part to the demonstration that while in genera1 there was little variation in cornea1 power, and the crystalline lens was assumed to be a constant, there was considerable variation in the size of the globe, with a tendency for myopic eyes to be elongated; thus there was little alternative but to regard myopia as due simply to excessive axial length. This in turn led to an enormous amount of work on the question of how myopic eyes became elongated. Was the eye stretched by the

MEDICAL HYPOTHESES

pull of the optic nerve, perhaps when the eyes are directed downward? Was the sclera, the outer coating of the eye, weakened in some manner by hormonal or dietary deficiency? Did the action of the extraocular muscles mold the eye into an elongated shape? This whole approach is now seen to have been misguided, perhaps influenced by the overlymechanistic views of nature prevalent in the last half of the nineteenth century. It was later realized that the axial length theory was a gross oversimplification, that it failed to explain, for example, cases of normal vision in elongated eyes and myopia in relatively short eyes. With the investigations of Steiger (2) and others, there was a shift in emphasis to the question of variability of the different ocular components and how they interact with each other to produce emmetropia (absence of a refractive error) or ametropia (presence of a refractive error). In seeking answers to problems in biology and medicine, it is in some cases instructive to contrast present living conditions of an organism with those that obtained in the distant past, since the slowness of the body’s adaptation to new conditions sometimes disposes it to various forms of malfunction. In applying this concept to myopia, one of the most obvious examples of a changed condition or function is the vastly increased amount of nearwork done by civilized men as compared with primitive and non-literate peoples. Because the most common form of nearwork is reading, a most “unnatural” use of the eyes, and because the continuous horizontal scanning movements of the eyes in reading with downward gaze require alternate contraction and relaxation of the oblique muscles, it was thought that an interesting experiment would be to simulate this condition in enhanced form to determine if such contraction is capable of elongating the globe. Another aspect of the shape of the globe in myopia that seemed to implicate the obliques was the finding that in addition to axial elongation, eyes with a refraction of greater than -5.5 diopters had a vertical diameter/horizontal diameter ratio of .8 instead of the normal 1.0 (3). This suggests the possibility that the globe is compressed between the superior and inferior obliques. The idea that the action of the extraocular muscles could be a cause of myopia is an old one, proposed by numerous investigators over the years, but as far as 1 could determine,

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phenomenon in ophthalmology, incapable of explanation by any existing theory. How could it have happened? I was 35 years old at the time, far beyond the age at which Experiment developmental changes occur, and in fact my myopia had remained unchanged for the My method for compressing the eye was a device preceding 16 years. Thus it seemed obvious that that made the superior oblique muscles contract the visuai change had been caused by something while maintaining relative relaxation of the other extraocular muscles (for a description of the to do with the experiment, yet the objective of the experiment had been to increase the degree device, see Appendix A). Because of their unique orientation, these muscles wrap part way of myopia. In some way I had gotten the exact around the globe, so that when contracted they opposite, but how? It was only some time later, as I began to compress the globe in the general area of the observe more carefully, that I noted something equator. I reasoned that if the superior obliques that had escaped me initially. Along with the could be made to contract sufficiently to distort the globe, it would probably take the form of near-perfect visual acuity there was also a degree of blur. I had been so entranced by the sharpness elongation. Thus the retina would be displaced posteriorly, beyond the focal point of light rays of vision that I had overlooked this component entering the eye, with the result that vision of acuity. What I had. then, was a bizarre phenomenon, dual vision: simultaneous emmewould become blurred, i.e. myopic. Since I could hardly ask someone to be the tropia and myopia. (A note on terminology: dual subject of an experiment designed to make him vision is used instead of diplopia because of the a myope. the subject necessarily had to be special characteristics of the condition described here. The use of the term emmetropia is, strictly myself. This complicated matters somewhat, speaking, incorrect; for want of a better word, since I was already a myope, with a visual acuity I am using it to indicate a visual acuity very close of O.D. -7.75 -1.25; O.S. -5.50 -1.50. The i.e. in the range of 20/20 to objective of the experiment then became to to emmetropia, 20/25). increase an existing myopia. The subjective experience of this effect was if Because I was unable to make axial length transparencies, one blurred measurements. I had to rely on changes in visual two photographic acuity to determine if there were any changes in and one sharp, were superimposed one on the axial length. Thus, if my visual acuity began to other. For example, an object viewed at a was seen as highly blurred. yet deteriorate in the course of the experiment, this distance could be an indication that the globe had elon- appearing through the blur was a pefectly clear gated, presumably due to compression of the image of the same object. The crucial question globe by the superior obliques. (It was only later was, how could squeezing the eye have produced that I realized how fortunate it was that I was dual vision? I was initially unable to even guess at a plausible mechanism for this result. A search unable to use a subject with normal vision). of the literature turned up only a few reports of a double focal point resulting from cataracts, Results which was clearly unrelated to this case. Compression of the eyes had been exerted for However, there was one well-established finding three to four hours a day for several weeks when that did provide a clue: the normal spherical I got the tirst indication that the experiment was aberration of the eye. not turning out as planned. I was out walking Spherical aberration one day when I felt a change in my visual acuity. I took off my glasses and was astonished to Spherical aberration is that condition in which the rays passing through a convex lens do not all discover that I had suddenly acquired perfectly clear vision. I stopped and stared at the razor come to a focus at a single point. Ivanoff (4) and others have shown that spherical aberration is sharp edges of buildings, windows, lamp posts. I found that I could read signs, large and small, normal in the human eye. When the eye is at rest blocks away. To a lifelong myope, this was the spherical aberration is positive, which means astonishing. It seemed even more so as I began that the rays passing through the periphery of to realize that this was apparently a new the lens come to a focus in front of rays passing

no one had ever attempted to produce long-term extraocular muscle compression in human eyes.

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MEDICAL HYPOTHESES

produced the primary image, the source of the blurred component of vision (Fig. 2). Induced hyperopia

Fig 1. The negative spherical aberration that occurs when accommodation exceeds approximately 3 diopters. Rays passing through the flattened periphery of the lens come to a focus behind axial rays.

through the axial region of the lens. As the lens accommodates to view a near object and begins to change its shape, the spherical aberration decreases, and at around 3 diopters of accommodation there is almost no aberration at all, i.e. all the rays come to a focus at the same point. If the eye accommodates further, the aberration begins to reverse, becoming negative, in which case the peripheral rays come to a focus at a point behind the axial rays (Fig. 1). I eventually reached a tentative conclusion: that the dual vision resulted from spherical aberration carried to extremes, a remarkable deformation of the lens far beyond that which occurs normally in accommodation. But if this were true, what had caused such a marked change in the shape of the lens? I hypothesized that contraction of the superior oblique muscles had exerted pressure on the globe, which was transmitted through the sclera to the vitreous, forcing the vitreous against the back of the lens and flattening its periphery. Rays passing through this region of the lens came to a focus at a point very close to the retina, which produced the secondary image (clear vision), while the rays passing through the axial region came to a focus in front of the retina, which

Fig 2. Peripheral rays displaced posteriorly now come to a focus on the retina, producing the clear secondary image. With further displacement of the axial-ray focal point away from the retina, there is increased blur of the primary image.

T. H. Huxley once said that the great tragedies of science are the slaying of beautiful hypotheses by ugly facts. I was in precisely the opposite situation. I had what, to me, was a beautiful fact, the case of a myope who also had normal visual acuity, to which were opposed all the ugly hypotheses of ophthalmology. In fact, the weight of conventional theory was so opposed both to this anomaly and to what I felt were the most feasible explanations that I began to have doubts. I decided that the best way to resolve these doubts was to repeat the experiment. The hypothesis would predict that if in fact the vitreous had flattened the lens periphery, further flattening might move the focal point of the peripheral rays on to the retina. Since I was so close to being an emmetrope, with a visual acuity of 20/2.5 in the best eye, I thought there might be a further improvement to 20/20, as well as improved acuity in the other eye. The second experiment was different only in the fact that there was a greater degree of tilt of the images and the wearing time was increased. The results were a major disappointment. My expectation of additional improvement of visual acuity (without corrective lenses) not only was not realized, but I began to notice my acuity was deteriorating. The best visual acuity of 20 25 soon was no longer attainable. It became 20I 40, then 20/60 and progressively worse. The cause of the decreased acuity was found by testing with plus lenses. After testing with various powers, I found that with a lens of +14 diopters I had a visual acuity of 20/25. The only explanation. I find reasonable is that further pressure had flattened the periphery of the lens to such an extreme degree that the focal point of the peripheral rays had been pushed to a point located behind the retina. In other words, I had

Fig 3. With further flattening of the lens periphery, the peripheral-ray focal point is now located behind the retina.

ON THE NATURE

OF MYOPIA

become a high hyperope, myope (Fig. 3).

AND THE MECHANISM

OF ACCOMMODATION

yet I was also a high

The primary image

Up to this point I have dealt exclusively with the most dramatic effect of the experiment, the clear secondary image, produced by peripheral rays. No less significant was the blurred primary image, produced by the rays passing through the axial region of the lens. This blurred image normally became clear when the primary focal point was displaced toward the retina by the use of corrective lenses, but during the second experiment I noted that this was no longer true. In other words, my visual acuity, which with corrective lenses had always been normal, was now deteriorating. I suspected that I was becoming more myopic. This was confirmed when a refraction showed that by the end of the second experiment my myopia had increased by an astonishing 5 diopters (to O.D. -11.75 -2.25; O.S. -9.00 -2.00). Thus the original objective of the experiment had been achieved after all: compression of the globe did produce myopia. The crucial question was: Did the myopia come from elongation of the globe or from increased lens power? I believe the answer is both, but that the major proportion of it was due to increased lens power. I reason that the only feasible source of dual images was spherical aberration, and since spherical aberration always accompanies accommodation. this strongly suggests that the lens was accommodated. If the lens remained accommodated (more on this later), a natural consequence would be decreased acuity for distance, i.e. myopia, and this is precisely what occurred. If vitreous compression of the lens is limited to the periphery, flattening the periphery might increase the curvature of the unaffected central region of the lens. This could explain the increase in the degree of myopia. The extraocular muscles and accommodation

All this suggests an astonishing conclusion: compression of the globe by the extraocular muscles can cause the lens to accommodate. Further. because I was careful to prevent any stimulus to accommodation, by the use of undercorrected lenses and avoiding near fixation and convergence. this suggests that accommodation can be actuated without the intervention of the ciliary muscle. But this is only one side of the equation; the

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other is axial elongation. It would appear that a single factor, external pressure on the globe, produces two separate effects, in opposite directions: anteriorly it accommodates the lens, and posteriorly it elongates the globe. I believe that one consequence of this dual effect is that axial elongation has masked the role of the lens. With such a logical and easily demonstrated explanation available, there has been little incentive to look for an additional factor, and thus the lens has been practically ignored. The indications that external pressure had produced accommodation by forcing the vitreous against the lens suggests the possibility that the vitreous is involved in normal accommodation, i.e. that the experiment in part mimicked what actually happens in accommodation, It may be that in normal accommodation, contraction of the ciliary muscle pulls the vitreous forward against the lens (as suggested oy Cramer in 1851 and later by Tscherning), whereas in this experiment the vitreous was pushed forward by external pressure exerted on the globe. The Helmholtz-Fincham accommodation

theory of

The vitreous/lens hypothesis is in direct contradiction with the Helmholtz-Fincham theory, which postulates that the lens accommodates by means of relaxation of the zonular fibers. “Accommodation results from decreased tension; the driving force - the motor - is the lens capsule. This decreased tension theory is attributed to Helmholtz. Considering the evidence. there is little reason to still call it a theory. Its only serious rival, proposed by Tscherning at the turn of the century, just survives by textbook repetition” (5. p. 87). Objections

The oblique muscle/vitreous/lens hypothesis will probably be characterized as untenable, and with good reason: the evidence against it is overwhelming. There are three exceptionally strong arguments: 1) The vitreous is unimportant in accommodation. This is proven by experiments in which accommodation occurs even without the presence of the vitreous (in cases of vitrectomized eyes). 2) When the lens is freed from zonular tension it assumes a more spherical shape. i.e. increases its power.

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MEDICAL HYPOTHESES

3) The zonular fibers relax during accommoposterior surface of the lens was increased by dation. When accommodation is observed in an forward movement of the ciliary body and eye in which the iris is absent, thereby exposing . consequently it resulted in pressure to the the zonules to view, they are clearly seen to relax posterior peripheral part of the lens . . . the their tension. increase in pressure of the vitreous body due to the contraction of the accommodative muscle is The role of the vitreous considered to be the most important factor for The first objection, that the vitreous is not the transformation of the lens” (italics added) required in order for the eye to accommodate, (IO). Suzuki performed an experiment in which he would probably be supported by most eye injected radiopaque material into the vitreous of researchers. Fisher states that “The vitreous plays a negli- a cat’s eye, which during accommodation moved in a direction indicating that the vitreous was gible role during accommodation in modifying the position or shape of the lens” (6). His forced against the back of the lens and also assertion is based on measurements of the lens somewhat toward the posterior pole of the lens which showed almost the same amount of move(II). A similar result was obtained by Koke, who ment of the anterior and posterior poles of the lens whether with the vitreous or without the injected cat eyes with radiopaque material and vitreous. In another context he states that “it is took X-rays during miosis and mydriasis, which clear that the vitreous is not essential either for showed that during accommodation the vitreous the human eye to accommodate effectively or for moved toward the lens and inward toward the the anterior pole to move forwards” (7). This is optic axis (12). The experiment that is perhaps most closely based on the observation of two eyes in one ’ related to the author’s, because it involved subject, one with the vitreous present and one external pressure on the globe, is that of Von without (the result of vitrectomy); no great Pflugk. He cut windows in the equatorial region difference in the amplitude of accommodation of bovine eyes and injected a drop dye of into was found. the anterior vitreous, midway between the ciliary Burian and Allen state that “our new obserbody and the posterior pole of the lens; pressing vations on the periphery of the vitreous surface strongly suggested that the vitreous body. far against the ciliary body from the outside in a radial direction made the dye move toward the from pressing upon the periphery of the lens. lens capsule (13). was actually under reduced tension during accommodation” (8). They believe that evidence of this is the bowing back of the vitreous, which Accommodation mimicked? they believe creates “an optically empty space in The second objection to the vitreous/lens front of the vitreous.” They fail to explain how hypothesis is perhaps the strongest: the demonthis optically empty space could occur, but a stration that when freed from the tension of the possible explanation is that this space, apparzonule, the lens assumes a more spherical form. ently the canal of Petit (the space between the Probably more than any other factor, the zonule and the vitreous), has expanded from an experiments of Fincham tilted opinion away inflow of aqueous under pressure. Johnson from Tscherning’s theory and toward that of von demonstrated such an inflow by the use of dyes. Helmholtz. (For a more detailed discussion of He believed that accommodation was actuated the theory of accommodation, see Appendix B). by hydraulic pressure exerted around the In one of the most convincing of these experperiphery of the lens (9). Duke-Elder dismissed iments, he showed conclusively that without the this as a “bizarre hydraulic theory,” but the tension of the zonule, the lens becomes more opening and closing of the trabecular meshwork spherical. An eye was made to accommodate for by the action of the ciliary muscle does suggest distance viewing by the instillation of atropine a hydraulic component of accommodation. and then removed from the orbit and pointed A completely different view of the role of the upward after dissection of the cornea and iris. vitreous is taken by other eye researchers. The profile of the lens can then be photographed According to Araki, in experiments on pig, dog and in this condition it demonstrates the characand cat eyes, “. . . it is suggested that tension of teristic shape of the lens when the eye is looking the ciliary muscle/zonules stretching from the at a distance. However, when the fibers of the

ON THE NATURE

OF MYOPIA

AND THE MECHANISM

OF ACCOMMODATION

zonule are severed all around by the sharp edge of a knife, the curvature of the anterior surface increases markedly and “assumes the shape that it has under maximum accommodation,” i.e. the lens becomes thicker, and this is clearly seen in the photographs taken by Fincham (14). This is, apparently, an invincible argument. To recapitulate: when the zonules that hold the lens in place are cut, the lens immediately becomes more spherical, which obviously increases its power. This would seem to be conclusive proof of the relaxation theory of accommodation, yet there is another possibility. There is no doubt that Fincham’s experiment is extremely convincing, yet 1 believed that my own findings were equally convincing. There is a possible way out of the dilemma, however. This requires rejection, not of Fincham’s observation (the photographic evidence is too strong for that), but of his interpretation. When the zonules were cut and the lens became more spherical. he assumed that the consequent change in shape was the same as that which occurs in accommodation. Could this be a non sequitur? I believe it is at least possible. It is not inconceivable that the spherical shape that he observed was not the shape that occurs in accommodation, but merely looked like it. It is possible that the lens could assume a more spherical shape under two different sets of conditions: 1) When released from the tension of the zonule. and 2) When molded by vitreous pressure, with only the latter being true accommodation. Admittedly, this is pure speculation; I am merely proposing a hypothesis that would encompass two contradictory findings. Accommodation und the zonule The third objection is based on well documented evidence that when the lens accommodates, the zonular fibers relax their tension. The vitreous/lens hypothesis, however, requires that there be some means to counteract vitreous pressure. Obviously. the zonular fibers could not fullfil this function if they are relaxed. Although most of the standard textbooks of ophthalmology state simply that in accommodation the zonules relax their tension. this is not the whole story. Several investigators have shown that there are two sets of fibers. and that while the anterior fibers relax in accommodation, the posterior fibers either remain tensed or increase their tension. I suspect that acceptance of the relaxation theory was due i_npart to the fact that the anterior fibers, being

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the most easily observed, were the first to be discovered and studied. Evidence for the existence of two sets of zonular fibers has been reported by several investigators: According to Suzuki, “During accommodation the posterior portion of the valley became swollen toward the inner direction of the eyeball. This could account for the relaxation of the zonules attached to the anterior surface of the ciliary muscle. “During more advanced accommodation, the anterior portion of the valley sank toward the outer direction of the eyeball. This could account for the contraction of the zonules attached to the posterior surface of the lens” (italics added) (11). An experiment by Araki showed that “electric recordings of the changes in tension of the ciliary zonules suggested relaxation of the zonules which was (sic) stretched to the anterior surface of the lens and on the contrary. increased tension of that stretching to the posterior surface (cat and dog eyes)” (10). Koke reports that “traction of the posterior fibers of the zonules changes in direction but not in tension. That is, the zonular fibers are shifted so that their traction is directed more toward the posterior pole of the eye and less toward the equator” ( 12). The iris Although tension of the posterior zonular fibers during accommodation might be sufficient to withstand the pressure of the vitreous against the lens, I find this unconvincing. The second compression experiment, especially, suggests that the zonular fibers would be unable to resist such extreme pressure from the vitreous. What other mechanism could hold the lens in place? An obvious candidate would be the iris, except that it was proven by von Graefe more than a hundred years ago that the lens can accommodate perfectly well even when the iris is not present. His experiment was prompted by earlier investigators who proposed that in accommodation the conoidal form of the lens resulted from constriction by the iris, so that the lens was molded by being forced through the pupil. Apparently, von Graefe’s demonstration was all that was needed to disprove the iris hypothesis, yet it would seem unwise to base such an important conclusion on a single case. Furthermore, as I suggested above, when the lens is

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released from traction, the more spherical form that it assumes may not be true accommodation. If this is correct, and if the full amplitude of accommodation seen by von Graefe was not true accommodation, then his conclusion that the iris is not required for accommodation may not be correct. It is highly improbable that with the vast amount of research done on the iris, a function as significant as counterpressure on the lens could remain undetected. Yet a number of reports do suggest an iris/lens connection. It is interesting that the researchers themselves seem to be surprised at their findings. For example, Burian and Allen state that “The most remarkable change was seen in the middle one-third of the body of the iris. This part of the iris bowed backward during active accommodation, forming a deep hollow, and returned to its normal position when the eye was relaxed” (8). Lowe reported that “During examination of a large series of eyes that had pupils dilated after peripheral iridectomy . . . I was struck by the marked curvature of the anterior lens surface within the enlarged pupil. The lens frequently appeared as though it were herniating through the enlarged pupil, with the pupillary margin of the iris seeming to grip the lens” (15). Suzuki states that “Concerning the iris, its silhouette was a slightly curved line, being convex anteriorly in the form of a physiological ‘iris bombe’. On stimulation, the iris showed a peculiar change. That is, besides the change of the contraction of the pupil, the iris was bent reversely to the posterior chamber, so that the central half of the iris was held in contact with the anterior surface of the lens and the iris-lens apposition became tighter over a much larger area” (16). Jampel and Mindel, in a report on stimulation of the oculomotor nucleus in monkeys, observed changes “. . . characterized by a conspicuous forward bulging of the pupillary or central portion of the iris which produced a marked convexity of the iris diaphragm and a marked increase in the depth of the anterior chamber . _ . On observation of the eye from the side during iris-bulge, the central portion of the lens appeared to become conoidal and to move forward into the anterior chamber” (17). All four of these reports describe the iris as being pressed firmly against the lens, and two of them note that the conoid form of the lens appears to be the result of bulging through the

MEDICAL HYPOTHESES

pupil. Could the iris play a major role in accommodation after all? This question will be considered too speculative to warrant serious consideration, yet the iris/lens mechanism is a well documented fact in certain birds and mammals. According to Walls, “The avian iris is always of material assistance during accommodation in holding back the lens against which it presses, and in inhibiting the peripheral part of the anterior surface of the lens from bulging, thus concentrating the change-of-curvature in the part of the surface opposite the pupil” (18). Studies of the rhesus monkey show a similar mechanism involving the iris and sphincter muscle, although it is not clear which of these is of greater importance in molding the lens. The persistence of accommodation etiology of myopia

and the

The creation of a dual mode of vision was quite remarkable, but another intriguing finding was that the dual vision persisted even after the experiment was stopped. If in fact the cause of dual vision was spherical aberration resulting from deformation of the lens, i.e. accommodation, then the persistence of dual vision indicated that accommodation also persisted. An eye in which the lens remains accommodated is a myopic eye - due to increased lens power. But the lens is not supposed to remain accommodated. According to orthodox theory, accommodation is maintained only as long as the gaze is directed at a near object; when the stimulus to accommodation ceases, e.g. when the gaze is shifted to a distant object, the lens reverts almost immediately to the unaccommodated form required for viewing at infinity. The consensus of opinion is that such accommodative changes take about one second. I believe that this is too restrictive, a result of excessive reliance on laboratory studies that deal only with momentary accommodation, and that there is a crucial difference between momentary and prolonged accommodation. The persistence of dual vision in my own experiment, as well as the findings of a number of investigators on the slowness of lens changes, leads to the conclusion that as a general rule the longer the lens is maintained in a particular form, the longer it takes to return to its original form when released. Further, with high degrees of deformation the lens does not return completely to its original form. In fact, in my case the sharp image persisted for more than

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by an apparently invincible argument. If in myopic eyes the lenses are permanently accommodated, they would tend to be thicker than the lenses of emmetropes. Not only is this not true, but, in general, myopic eyes have thin lenses. Thus, as far as myopia is concerned, there is a very clear consensus of opinion regarding the importance of the lens: It ranks very low: “Three variables, then, the axial length, the shape of the cornea, and the power of the crystalline lens, exert the greatest effect upon refraction. There is good agreement among authors as to the relative influence which each of these exerts, the axial length being greatest, followed by the cornea and lens in that order. There are minor disagreements among investigators as to the relative importance of the last of these three elements, the crystalline lens: Van Alphen’s work suggests perhaps the lowest estimate of the importance of the lens. However, all investigators arrive at the same order of importance, and at relative values not too different from those obtained by others” (22). Sorsby apparently was puzzled by the existence of thin lenses in myopes and tried to find a way out of the difficulty by speculating about the tension of the zonule. He stated that, “Obviously a large fairly spherical eye will have not only a long anteroposterior axis but also a flatter cornea. Flattening of the lens in a large eye is more difficult to understand, but a more marked tension on the suspensory ligaments may be a possible factor” (23, pp. 15-18). The barrier to a resolution of the contradiction seems to be the belief that a thin lens can not be an accommodated lens, But if the slowness of lens changes is correct, and apparently no one disputes this, then in myopes the lens must be accommodated. Consider the case of a myope with a history of nearwork, e.g. with hour after hour of reading over a period of months and years. It is not inconceivable that with repeated periods of prolonged accommodation the lens would never have sufficient time to return completely to the unaccommodated state. How can it be asserted that such lenses are not just the same power, but of even lower power than those of emmetropes? Not only has there been no explanation of this, but the question has not even been asked. Apparently eye research has become so compartmentalized that two such The thin lens paradox contradictory facts - thin lenses in myopes and The idea that myopia could be the result of the slowness of lens changes - can go unnoticed increased lens power has always been countered and unresolved.

four years before it gradually began to fade. I assume that this was due to a gradual decrease in the degree of flattening of the periphery. Since the blurred image remained largely unchanged, apparently this was because the degree of curvature of the central region of the lens changed very little. Because the decrease in lens power was going so slowly, I attempted to accelerate it by means of a non-invasive technique, for which I constructed a centrifuge (described in Appendix C). Fortunately, I am not alone on the question of the slowness of lens changes: Lancaster states that “. . . if the accommodation is maintained a few minutes at the maximum, the near point does get nearer and the eye may become accommodated 20% to 30% or more, nearer than at first. If the near point at the start was 6 D. it may become 7, 8, or 9 D. This . . . is due to the viscosity of the lens substance. An immediate rapid (about one second) change takes place when the lens adjusts itself for a near object, but if a maximum effort of accommodation continues to be made, the lens slowly (5 to 10 minutes) goes on changing its shape and becoming more strongly refractive. “Commonly, when the eve, after such an intense effort of accommodation, is shifted to a distant object, although the ciliary muscle may promptly relax, it takes time (a few seconds to a few minutes depending on how long the near effort was continued) for the lens to regain its normal shape adapted to a distance. This is due to the viscosity which makes a change in the shape slow” (19, pp. 115-116). Other investigators have also demonstrated the slowness of lens changes. According to Kikkawa and Sato, “Application of an external force to the lens caused rapid deformation followed by a second phase of slow deformation. On removal of the force, a rapid partial reversal of the deformation occurred and was followed by a gradual restoration; complete recovery was not achieved” (italics added) (20). A similar result was obtained by Kabe, who showed that when accommodation is increasing, the change in the apparent curvature of the anterior surface of the lens is slow and continuous, but when accommodation is decreasing, there is a prompt followed by a slow phase (21).

206

The possibility that a lens subjected to frequent prolonged accommodation for months and years may not have the same shape as a lens that is accommodated momentarily apparently has not been considered. The hypothesis of vitreous pressure suggests that prolonged vitreous pressure might produce a lens with a flattened periphery but a high degree of curvature of the axial area, i.e. a lens that is thin yet accommodated. I have been able to find only one reference that even indirectly supports this hypothesis. Otsuka, in a study of accommodation, states that “the thicker the lens became during accommodation, the thinner the lens became annually” (24). He did not elaborate. Theory versus observation

Curiously enough, there is a case of photographic evidence that the lens becomes thinner with accommodation, even with momentary accommodation. A paper by Burian and Allen (8) shows photographs of the lens during three stages: 1) Relaxation of accommodation; 2) Active accommodation; and 3) Active accommodation (apparently further). Yet instead of showing that the lens thickens, it shows precisely the opposite: in each photograph it is clearly seen that the lens becomes progressively thinner, at least in the peripheral area. These photographs are reproduced in DukeElder’s System of Ophthalmology (25, p. 163) and in a more recent work, Visual Optics and Refraction (5, p. 184), yet except for noting the flatness of the posterior surface of the lens, none of these authors comment on the fact that the lens clearly becomes thinner as it accommodates. In fact, in the text preceding the photographs, Duke-Elder states: “All are agreed that the lens increases in thickness during accommodation.” This seems to be a case of a theory being more potent than direct observation. The lens capsule

When Helmholtz first proposed his relaxation theory of accommodation, it was criticized on the ground that relaxation of the zonule failed to explain how the anterior central region of the lens assumes a conoidal shape. Tscherning claimed that th.is could only be produced by pressure from the vitreous, which he believed molded the softer cortex of the lens around the harder nucleus. Fincham thought he found an answer in evidence that the thickness of the lens capsule varies, and he believed that these minute

MEDICAL HYPOTHESES

differences were sufficient to impose a conoidal shape on the anterior surface of the lens (14). While it is conceivable that the capsule could mold the lens to a slight degree in this manner, the evidence of my own experiment indicates that this explanation is insufficient. Because the degree of spherical aberration was so extreme, which indicated extreme flattening of the periphery of the lens, it is difficult to believe that it could have been produced by such minute differences in capsule thickness. In fact, the contrary could probably be argued just as persuasively: that the differences in capsule thickness are the result of accommodative forces. The thin segment of the capsule in the anterior axial area could be caused by stretching of the capsule, while the thin posterior segment could be caused by the vitreous squeezing the capsule against the lens. Posterior and interior lens changes

Although it is generally believed that the changes in lens shape in accommodation occur principally in the anterior surface of the lens, the hypothesis proposed here suggests that the posterior surface might undergo equal or greater changes due to its direct contact with the vitreous. The conventional wisdom that the principal changes occur in the anterior lens was challenged by Patnaik, who wrote that “. . . the often stated and commonly accepted statement, that it is the anterior lens surface which moves forward while the posterior surface remains stationary and that it is only the anterior surface which changes its curvature during accommodation seems not to be correct. “Our observations strongly indicate that during accommodation the increase in the thickness of the anterior cortex is minimal, that the change in the posterior cortex is greater, and that in the nuclear thickness change is greatest” (26). This last may be particularly significant, since it raises the possibility that the principal source of increased lens power in myopes could be the nucleus. Young also commented on the importance of the posterior surface: “The pressure changes in the vitreous chamber may also play a role in the process of accommodation, since the back lens surface could be molded by the increase in pressure more effectively than the front lens surface. Unpublished phakometric studies now indicate that the back lens surface contributes

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ONTHENATUREOFMYOPIAANDTHEMECHANISMOFACCOMM~DATION

almost twice as much to the total vergence of light and is second only to the cornea in its refractive power. The attachment of the hyaloid membrane to the back lens surface may play a major role in the development of the greater lens power of the back lens surface. There is some evidence from children (sic) phakometry that the back lens may have several curvatures rather than the simple, monotonic curve of the front lens surface” (27). It is interesting that if significant lens changes do occur in the posterior surface of the lens, this would be just one more example of how the clues to lens involvement in myopia are hidden. Consider the case of a myope undergoing a routine refraction. With increased power the posterior surface of the lens could be permanently flattened to the extent that the peripheral rays have formed a secondary focal point. However, the examiner would never discover its presence for two reasons: He will not look for something whose existence he is unaware of; and because the secondary focal point does not reach all the way to the retina. no secondary image is formed. The only way to detect it would be to find the precise negative lens power that would move the secondary focal point posteriorly until it contacted the retina. However, even if the correct lens power were chosen, this would be difficult if the subject were a low power myope, in which peripheral lens flattening has just begun. Because the primary and secondary focal points would be very close together, the two images would be seen as mixed, so that the subject would have difficulty distinguishing between the two. Thus far I have limited the discussion to the primary and secondary images, i.e. focal points, but testing with various lens powers suggests that there are other images, fainter and more difficult to detect. which indicates the presence of other focal points situated between the primary and secondary focal points. The origin of these could be the various isoindicial surfaces within the lens. High myopes. when tested with various lens powers (undercorrected or plus lenses), describe not a smooth. uniform blur, but rather several superimposed blurred images. I stated above that it was fortunate that the first subject for the experiment, myself, was a myope. What if I had found a willing emmetrope? I believe the outcome would have been very different. The experiment probably would

have produced a small degree of myopia, partly from axial elongation and partly from lenticular changes (just as I believe occurs in normal myopia). The important point, however, is that I would never have suspected the lens, but would have attributed the myopia to axial elongation alone. I believe that in my case. because I was a myope of fairly high degree, the flattening of the lens periphery was fairly well advanced, so that the secondary focal point was already situated very close to the retina. It then required very little additional flattening of the lens for it to reach the retina, at which time I became aware of the secondary image. The failure of rheraperltic measures

The lens/vitreous hypothesis provides an explanation for the failure of two therapeutic measures aimed at preventing or slowing the progress of myopia: cycloplegics, and base-in prisms. In the case of cycloplegics, they relax the ciliary muscle for only a few hours, while the lens requires many months for a significant reduction of accommodation. In addition, in all these regi mens the subjects were allowed to continue doing nearwork, so that accommodation could still have been largely maintained by vitreous pressure alone. The use of base-in prisms to prevent convergence (and consequently, accommodation) has largely failed for an unsuspected reason: the distorsion inherent in such prisms. I used basein prisms extensively in various experiments aimed at reducing accommodation and was surprised to find that in some cases the degree of myopia increased. Strong base-in prisms produce considerable image distorsion. A possible explanation is that in trying to fuse the distorted images the eyes were forced to cyclorotate in antagonism to each other, so that the oblique muscles were forced to maintain contraction as long as the image was fused, thereby exerting pressure on the globe and maintaining accommodation. The vitreous/lens hypothesis also provides a potential solution to the question of potentiation of accommodation when the eyes are converged. The contraction of the obliques that occurs with convergence could increase vitreous pressure on the lens and thus enhance the degree of accommodation. The hypothesis proposed here might also explain another fact about myopic eyes. If the

208

MEDICAL HYPOTHESES

lens maintains accommodation, there would be little need for the action of the ciliary muscle, which in myopes tends to be poorly developed.

experiments seemed to support discounted opinion . . .” (28).

the

now-

Questioning basic assumptions Extraocular

muscle/myopia

theories

Acceptance of the extraocular muscle/vitreous/lens hypothesis would of course require rejection of the Helmholtz-Fincham theory. In regard to myopia, the hypothesis that the extraocular muscles could play a role is not new; this has been suggested by numerous investigators over the years. A major difference, however, is that in none of these hypotheses has it been proposed that there is any effect on the lens. All are limited to the concept of elongation of the globe, produced by the action of the extraocular muscles in elevating the intraocular pressure. The case is similar with regard to the numerous hypotheses that propose contraction of the ciliary muscle as a cause of myopia. These hypotheses propose that such contraction produces elongation of the globe, and do not suggest any effect on the lens. Erroneous

theories

It is highly unlikely

that such well-established concepts as the theory of accommodation and the role of the crystalline lens in myopia could be wrong. Nevertheless, there have been cases of major reversals in scientific opinion. For example, for some fifty years it was universally believed, and confirmed by hundreds of experiments by reputable scientists throughout the world, that the plasticity of the central nervous system allowed any muscle nerve to be reconnected to any other muscle and, with training, achieve full restoration of function. It is now known that this is not true. According to R. W. Sperry, “During the past 15 years, however, scientific and medical opinion has undergone a major shift, amounting to an almost complete about-face . . . The evidence for this vew, which comes from new experiments and exacting clinical observations, is so persuasive that it is difficult to understand how the opposite view could have prevailed for so long. It appears that most of the earlier reports of the high functional plasticity of the nervous system will go down in the record as unfortunate examples of how an erroneous medical or scientific opinion, once implanted, can snowball until it biases experimental observations and crushes dissenting interpretations . . Hundreds of

Textbooks of ophthalmology give an impression of certitude about our knowledge of the eye that may not be warranted. Ludlam suggests that at least some of the most basic facts about the eye are based on faulty data and should be re-evaluated. These include invalid mathematical assumptions, mixed sampling, inadequate experimental technique, and oversimplified models of the refractive system of the eye, some of these dating from the nineteenth century. “Nevertheless, the analyses and conclusions drawn from such studies can be no better than either the methods of acquisition of the basic data or the validity of the assumptions underlying the mathematical formulation of the ocular model. “It is well to note that in all of these studies the model of the ocular refractive system utilized has consisted of: 1. Spherical refracting surfaces, causing a systematic under-estimation of the paraxial refracting power of each surface. 5. A homogeneous monoindicial lens. This places a high order of importance on the accuracy and precision of the measures of curvature of both the anterior and posterior surfaces of the lens and concomitantly increases the potential effects of the spherical assumption. “In addition, in none of these studies have all the refractive components of any given eye been measured. There has always been at least one component whose value was calculated from the other measured elements, so that the measurement errors would all tend to accumulate in the non-measured element. Since the measurement errors have not always been stated wth sufficient clarity to enable the effects of these errors to be assessed, the probability exists that measurement errors have contributed substantially to spurious correlations of measured and calculated elements, as for example, between the lens and axial length” (29).

With respect to long-established theories, reluctance to rock the boat is understandable. As Kuhn puts it, “. . . few scientists will easily be persuaded to adopt a viewpoint that again opens to question many problems that had previously been solved” (30, p. 169), and thus scattered reports in the literature that cast doubt on the conventional wisdom, for just one example, a case described by Luedde (31), are simply ignored.

ON THE NATURE OF MYOPlA AND THE MECHANISM OF ACCOMMODATlON

209

This study can be criticized (and un{oubtedly against the crystalline lens, which produced will be) on a number of grounds, e.g. the fact spherical aberration and accommodation. When that it is based on a single case. Nevertheless, I the compression of the globe was stopped, it have no reason to believe that it would have took several years for the sharp component of the dual vision to subside. turned out differently with any other myopic subject. One serious deficiency is the failure to These findings suggest the following: make before and after ultrasound measurements 1. Accommodation is actuated internally by of axial length. One reason is that the study was contraction of the ciliary muscle molding the begun rather casually, and by the time it had lens, in part by forcing the vitreous against its posterior surface. evolved into a more serious long-term project, it was too late. In addition, much of the initial 2. This vitreous-mediated accommodation can be enhanced by additional compression by the work was done in the isolation of a third-world country with the consequent difficulty of, vitreous from an external source, the action of obtaining the proper equipment. The study also the extraocular muscles. 3. Because of the slowness of recovery from fails to resolve certain difficult points, e.g. the accommodation, long periods of accommodation effect of gravity on accommodation, described by Hess, and the question of sustained accom- with insufficient intervals of rest result in a lens modation in hyperopes, but as Kuhn points out, that is permanently accommodated. “no paradigm ever solves all the problems it 4. The argument that myopic lenses are not because they tend to be thin defines and since no two paradigms leave all the accommodated could be explained by long-term compression. same problems unsolved, paradigm debates always involve the question: Which problems is This could produce an accommodated lens with it more significant to have solved?” (30, p. 110). either a flattened periphery and convex axial It is interesting to speculate on the extent to region, or a thin lens with accommodative changes in the nucleus. which an erroneous theory constitutes a barrier It would be curious indeed if, after the enorto the resolution of a problem, and, conversely, mous amount of work and speculation on the how a new viewpoint could open up previously unconsidered possibilities. Such possibilities are causes of myopia, the answer turned out to be not necessarily limited to finally determining the a simple one, the kind of answer that might be given by a layman with a superficial application etiolgy of myopia. but could include prevention. of logic. Suppose that this imaginary layman is The pessimistic view was expressed by Donders, who over a hundred years ago stated: “The more given a brief explanation of the mechanism of our knowledge of the basis of this anomaly has accommodation. He is told that when looking at been established, the more certainly does any distant objects the lens is relatively flat, but that in order to see near objects clearly it becomes expectation (of a cure) appear to be destroyed, even with respect to the future” (32, p. 415). If more spherical, i.e., it accommodates. While the they thought about it at all, probably most lens has this more spherical form, near objects are seen clearly while distant objects are blurred. researchers today would share this view. If the hypothesis of the oblique He is then given a brief description of vision in muscle/vitreous/lens connection is confirmed, it myopia: the myope sees nearby objects clearly could open the way to new techniques to prevent but distant objects are blurred. He could then be or slow the progression of myopia. It is even forgiven if he naively made the logical connecconceivable that a cure for myopia could be tion and exclaimed, “Oh, I get it. The eye got devised, for example, invasive techniques to adjusted for near vision and then sort of got reshape the curvature of the lens. Because the ‘stuck’ so that it can’t re-adjust for distant vision.” No comment. lens substance is thixotropic, such reshaping could be aided by vibration. Appendix Conclusions

Compression of the globe of the eye by contraction of the superior oblique muscles produced dual vision and increased myopia. It is hypothesized that the cause was vitreous pressure

A

Because the most common of all “unnatural” use of the eyes is reading, and because the continuous horizontal scanning movements of the eyes in reading with downward gaze require alternate contraction and relaxation of the oblique muscles, it was proposed to simulate this condi-

210

MEDICAL HYPOTHESES

tion in enhanced form to determine if such contraction is a factor in the elongation of the globe associated with myopia. Since the eye muscles are not subject to individual voluntary control, it was necessary to devise some means to force the superior obliques to contract while maintaining relative relaxation of the other extraocular muscles. It was thought that the natural tendency of the eyes to fuse two disparate images could be utilized: A viewing device was constructed which contained two identical photographic transparencies depicting a visually rich pattern. When the Subject looked through the device, each eye viewed one of the transparencies; the visual cortex then fuses the two images to form a single scene. The transparencies were then incyclorotated, i.e. as seen by the Subject, the right-side image was rotated counterclockwise and the left-side image was rotated clockwise. In order to maintain fusion of the two images, each eye must then rotate in the same direction as the image it is viewing, i.e. the upper end of the vertical meridian of each eye leans nasalwards. The movement of incyclorotation is effected principally by the superior oblique muscles, but there is a limit to how far the globe can rotate, since this is opposed by the check ligaments and other fascial structures of the orbit. If an effort is made to maintain fusion, the traction of the superior obliques, which wrap part way around the globe, will exert compression in the general area of the equatorial meridian. The device was later modified for portable use to facilitate long-term viewing. Instead of viewing transparencies, the Subject looked though a system of mirrors that tilted in like manner any scene viewed. The amount of tilt (incycloration) varied between 6 and 12 degrees. This is not to say that if the images are incyclorotated, say, 8 degrees, each eye will also rotate exactly 8 degrees; eye rotation can be as much as 2 degrees less. This is because of Panum’s fusional area, which in steropsis allows the image to be pulled apart by some 2 degrees before being broken up into two separate images. The images are actually pulled apart on the retina, but a supra-retinal function maintains perception of a single image (33). In order to eliminate any stimulus to accommodation, distance fixation of at least six meters was maintained. Apppendix

B

There is no dispute that changes in the shape of

the crystalline lens occur by means of contraction of the ciliary muscle. The crucial question is how this is brought about. Briefly, the HelmholtzFincham theory of accommodation with its modern modifications can be described as follows: The crystalline lens is suspended within the ring-like ciliary muscle, located in the anterior aspect of the eye. The lens, enclosed in its capsule, is attached to the interior diameter of the ring by the zonular ligaments. When looking at a distance, the ciliary muscle is at rest and its interior diameter is at its maximum, which pulls on the zonular ligaments and maintains the lens in its flattest form. When the eye looks at a near object, the ciliary muscle contracts, making its interior diameter smaller and releasing the tension of the zonular ligaments. This allows the elasticity of the capsule to mold the lens into a more spherical form, which increases its refracting power. Appendix

C

Because the compression of the globe that increased lens power was directed approximately vertically (downward), I thought that horizontal compression might have potential as a corrective measure to reduce lens power. What I needed was a means to compress the eyeball from front to back. The logic was simplistic, but there was a possible additional advantage in that horizontal compression might produce two other effects, both of which would also reduce the degree of myopia. These were decreased axial length and flattening of the cornea. As far as I could determine, no attempt had ever been made to reduce axial length or decrease cornea1 curvature, not even by the more mechanically-minded researchers of the nineteenth century. I decided that a noninvasive technique using centrifugal force might be the answer, and to this end designed a centrifuge. This was a device consisting of a rotating steel cylinder five feet in diameter and eight feet tall, powered by a variable-speed electric motor; the device weighed more than a ton. The subject (myself) sat inside the cylinder in a steel chair facing the center of rotation of the cylinder. When the cylinder was rotated, centrifugal forces were generated that compressed the eye horizontally. After the initial problems of severe vertigo and nausea were overcome, I was able to gradually increase the rotation speed and eventually accumulated hundreds of hours on the device, often at speeds as high as 120 rpm, which generated a force of more than 5 g’s. At this rate

ON THE NATURE OF MYOPIA AND THE MECHANISM

OF ACCOMMODATION

the bodily discomfort was so extreme that sessions had to be limited to about three minutes. The force of compression probably did reduce axial length or flatten the cornea, or both, at least temporarily, as I did notice considerable improvement in acuity after each session. Since in the human eye one millimeter of axial length equals approximately 3 diopters, this is not surprising. However, it was impossible to separate out the potential effects of changes in axial length, cornea1 curvature and lens shape, and since the improvement was apparently only temporary, the effort was eventually abandoned.

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16. Suzuki H. Observations on the intraocular changes associated with accommodation: an experimental studv using radiographic technique. Exp. Eye Res. 17: 119-128: 1973. 17. Jampel R S. and Mindel J. The nucleus for accommodation in the midbrain of the macaque. Invest. Ophthalmol. Vis. Sci. 6: 40-50. 1967. 1x. Walls G. The Vertebrate Eye and its Adaptive Radiation. New York: Hafner. 1967. W. Refraction and Motility. Springfield: 19. Lancaster Charles C. Thomas. 1952. x Kikkawa Y. and Sato T. Elastic properties of the lens. Exp. Eye Res. 2: 210-215, lY63. Photo“. Kabe S. Dynamic aspects of accommodation. graphic records of the 3rd Purkinje-image showing changes in size associated with accommodation. Rinsho Ganka (Jnn J. Clin Onthalmol.) 21: 341-352, 1967. of the Eye. Public 22. Hirsch M. In Refractive Anomalies Health Scrvicc Publication No. 1687, National Institute of Neurological Diseases and Blindness. Monograph No. 5, U.S. Department of Health. Education and Welfare. 1967. Hirsch, M. Ophthalmology. 2nd Ed. Vol. 3. 23. Sorsby A. Modern London: Butterworth, 1972. 24. Otsuka J, Hirano S. Suzuki K. and Imagawa N. A new Approach to the Theory of Accommodation. Excerpta Medica International Congress Series No.222: 983.-990, 1970. S. System of Ophthalmology Vol. V. 25. Duke-Elder London: Henry Kimpton, 1970. B. A photographic study of accommodative 26. Patnaik mechanisms: Changes in the lens nucleus during accommodation. Invest. Ophthalmol. Vis. sci. 6: 601-61 I. 1967. F. Documenta Ophthalmologica Proceedings 27. Young Series, Volume 28 (Third International Conference on Myopia. Copenhagen. 1980). 28 Sperry R. W. The Growth of Nerve Circuits. Sci. Am. 68-75. Nov. lY.59. W. In In Refractive Anomalies of the Eye. 29. Ludlam Public Health Service Publication No. 1687. National Institute of Neurological Diseases and Blindness, Monograph No. 5. U.S. Department of Health, Education and Welfare, 1967. of Scientilic Revolutions. 30. Kuhn T S. The Structure Chicago: University of Chicago Press, 1970. H. What subluxated lenses reveal about the 31. Luedde mechanism of accommodation. Am. J Ophthalmol. 24: 40-45, 1941. of Accommodation and 32. Donders F C. On the Anomalies Refraction of the Eye. London: The Sydenham Society. 1864. 33. Fender D. Extension of Panum’s fusional area in binocularly stabilized vision. J. Opt. Sot. Am. 57: 819-830, lY67.