that this circumstance forces the person to procure very large retinal images. He succeeds in so doing by bringing objects very close to his eyes, thereby inducing an exaggeration of convergence and accommodation. Others attribute this form of strabismus to conditions which facilitate convergence, such as a stronger development, or more favourable insertion of the internal rectus, or a more powerful innervation of the muscles of convergence; but no positive arguments have been furnished in support of this hypothesis, which, however, seems very plausible. Mauthner thinks that, if all hyperopes do not squint, it is because only a small number of them come to possess this artifice calculated to procure, for them, distinct images. In order that they shall discover how to do this, certain favourable conditions are necessary, as, for instance, paresis of the accommodation (Javal), or the example given by a condisciple, expert in the art of squinting, who excites in them the desire of imitation. In this way the strabismus would be the punishment for imitating the infirmity of a companion. The hypothesis of our learned confrère of Vienna may have much about it that is true, but neither this theory, nor any of those that we have enumerated, fully satisfies us, and we prefer to acknowledge that many cases of convergent strabismus are still beyond our comprehension. This avowal does not cost us much, and certainly less than the building up of theories without foundation, and far-fetched attempts at explanation. We shall encounter still other obscure points in the course of these studies. We must know how to look them calmly in the face, in order to undertake with courage the task of elucidating them. General medicine, which has for ages been the object of investigation, affords many more unfilled gaps than does ophthalmology, which was, so to say, born yesterday. Hence let us not despair, but seek diligently, analyse the many facts that practice presents every day, and light will eventually dawn upon us. The convergent strabismus of hyperopes is generally developed in early infancy. It is not rarely met with at the age of from two to four years. Periodical and alternating at the outset, i.e., manifesting itself only under the impulsion of attentive fixation, and sometimes in one eye, sometimes in the other, it becomes constant, as the child grows older, and localises itself upon one eye. This is generally the poorer of the two, if a difference between them exists. In this connection, it has been asserted that the inferiority of the deviated eye, in point of visual acuteness, was not the cause, but the consequence of strabismus. The eye which was excluded from binocular vision, it has been supposed, either does not acquire, or else loses, its normal acuteness of vision, from want of exercise. We do not participate in this way of thinking. An eye may for years be excluded from vision without experiencing any alteration in visual acuteness. Hence the amblyopia of a squinting eye seems to us to be primitive and not secondary. It is interesting, on the other hand, to observe with what facility the individual learns to ignore the retinal impression of the deviated eye, and the small value he seems to set upon binocular vision. Indeed, one never hears complaints of diplopia from such cases. The latter may, however, almost always be produced, though this is contrary to the generally entertained opinion. It is a question merely of facilitating this. Choosing some small object which vividly impresses the eye, such as a candle-flame, the eye that is in the habit of fixing is covered with a coloured glass, and the attention of the deviated eye is attracted to its own retinal image by the successive covering and uncovering of its fellow. It is, however, curious to notice that, when once having succeeded in seeing double, these persons have the greatest difficulty in projecting the retinal image of the squinting eye. They can scarcely tell on which side the flame, seen with that eye, is located; most frequently it is impossible for them to tell how far apart the images are from each other, and so to give the element of a subjective determination of the degree of strabismus. This measurement is, however, easily made by the objective method, i.e., by perimetric measurement.1 The impossibility of obtaining it by the first method is not due to inconstancy of the deviation, or to a vacillation of the eye causing an analogous variation in the reciprocal position of the two images, but rather to the loss, by the squinting eye, of the faculty of locating retinal impressions. It is this important fact which distinguishes this form of strabismus, which has been called concomitant, from paralytic strabismus, in which the diplopia is so marked, so distinct, so troublesome and so persistent. The difference is, moreover, susceptible of a very natural explanation. Paralytic strabismus with diplopia is nearly always produced rapidly, and at an age when the eyes have for a long time been accustomed to binocular fixation. The convergent strabismus of hyperopes, on the contrary, is almost always developed slowly and gradually in childhood, when binocular vision has not become firmly 1 See Landolt in Traité complet d'Ophthalmologic, De Wecker et Landolt, vol. i., p. 910. established, and especially in an eye whose retinal impressions are less distinct, and perhaps less vividly perceived than those of its fellow. Convergent strabismus is, however, occasionally produced rapidly in hyperopic children, especially when consequent upon general diseases, or even very slight affections of the eye, such as phlyctenular conjunctivitis and keratitis. In the first case, the result has been ascribed to a weakening of the power of accommodation; in the second, to a direct irritation of the internal rectus muscle, or, what seems more probable, to the influence of the occlusion or exclusion of one eye. The latter would, so to say, constitute an apprenticeship to strabismus. It suppresses binocular vision and gives to the excluded eye its freedom of direction. If the individual finds an excess of convergence advantageous, it is not surprising that he brings it about with that eye, which is of no use to vision properly so called. Even in cases of concomitant strabismus of rapid development, spontaneous diplopia is lacking. But we must not conclude from this that young hyperopes attach so little value to binocular vision that they renounce it for ever on the slightest, and even transient, occasion. On the contrary, the great number of hyperopes who do not squint and who, nevertheless, have passed through the same incidents, have resisted the temptation and have been saved from strabismus by the stability of their binocular vision. It must be admitted that there is, with the minority of those who squint, a predisposition to strabismus, with whose exact nature we are not yet entirely acquainted. C. Strong Hyperopia.-The highest degrees of hyperopia bear, most of all, the imprint of the arrest of development which characterises this form of ametropia and best present certain consequences of it. Here a certain kinship between the conformation of the cranium and that of the eyes is oftenest recognisable. The hyperopic type of head is more pronounced. The eye itself is reduced in all its dimensions. The cornea is visibly smaller, and this smallness seems to us to explain the diminution, in its radius of curvature, which has been found to exist in such cases. It seems, indeed, that this membrane is more curved only because the diameters have been shortened by the same cause to which the smallness of the entire eyeball is due. One would say, moreover, that this phenomenon of constriction of the circumference of the cornea was effected in an unequal way, according to the various diameters or even in a totally irregular manner. Indeed, the cornea often presents considerable regular astigmatism, with which is frequently associated an incorrigible irregular astig matism. The smallness of the eye might seem a condition favourable to extended excursions. But, in reality, the field of fixation of hyperopes of this category is restricted and shows a degree of mobility inferior to the normal. The cause of this is, without any doubt, the weakness of the extrinsic muscular apparatus, which has also suffered from the general dystropy of the visual organ.1 The ciliary muscle, even, does not escape this influence. Contrary to what we might expect, it is less vigorous than that of a normal eye, or of one affected by hyperopia of moderate degree, and the range of accommodation in such cases is generally restricted, so that this function is almost always insufficient for the correction of the imperfect retinal images produced by an insufficient dioptric apparatus. This would, indeed, of itself be a satisfactory explanation of the incapacity of strongly hyperopic eyes for sustained vision. The principal cause of it must, however, be sought in the inferiority of the nervous apparatus, which also participates in the incomplete formation of the eye. In spite of perfect correction of the refractive error, not only is the visual acuteness often considerably reduced, but the eyes are unable to work long at a time without experiencing a sensation of unbearable fatigue which soon obliges them to cease work. It would seem that the provision of nervous substance, requisite for prolonged vision, is less than in others and is exhausted. with greater rapidity. The ophthalmoscope, moreover, often furnishes strong indications of the imperfections of the nervous apparatus. The papilla presents itself in the form of a disc, sometimes of abnormal pallor, sometimes, on the contrary, of a deeper colour than is normal, and which scarcely differs from that of its surroundings. Its limits lack distinctness and are often irregular. Moreover, we may here add a peculiar disposition of the retinal vessels, which are extraordinarily sinuous, so that they may, in certain cases, suggest the appearances of optic neuritis. This appearance may be explained, however, according to Horner, in the following manner: -The retinal vessels are developed in a cavity previously formed, that of the ocular globe. The latter being remarkably small in high degrees of hyperopia, the vessels are hindered from extending themselves to their full length and, therefore, become twisted. We may mention, finally, the peculiar lustre of the retina, which at times resembles asbestos. That which specially characterises high degrees of hyperopia, and authorises us to make of them a separate group, is the peculiar way in which their vision is accomplished. 1 Landolt, "Etudes sur les mouvements des yeux, &c. ;” Archives d'opht., i., p. 587, 1881. The vision of hyperopes is variable, according to the degree of their refractive defect. When the latter is not very pronounced, the only inconvenience resulting from it is, as we have seen, the precocious appearance of symptoms of presbyopia, from insufficiency of the accommodation, which, in this case, is compelled to perform the double task of supplying the lack in static refraction, and of adapting the eyes to near work. We remember that moderately hyperopic eyes do not differ much from the normal, and that it is only with advancing age that a necessity arises for holding the book farther away from the eye, or for having recourse to glasses, while distant vision is still for a long time perfect. We have noticed that, in the medium degrees, the characteristic tendency to hold the object farther away is still more marked, unless the hyperope renounces binocular vision, giving way to a no less pathognomonic convergent strabismus. The phenomena presented by an individual affected with strong hyperopia are quite different. Far from moving the book away from him, he brings it, on the contrary, very close to his eyes-much nearer than his punctum proximum-in such a way as to simulate true myopia. This error is soon dissipated by an examination of the eyeballs, which present a noticeable smallness, or, at all events, a form very different from that which myopic eyes, reading at so near a punctum remotum, would have. The hyperope, indeed, so to say, applies his eyes to the paper; being unable to obtain distinct retinal images, he seeks to obtain as large ones as possible. His circles of diffusion are not an obstacle for him, for, in the first place, their diameter, generally limited by the narrowness of the pupils, increases only in proportion to the square root of the decrease in distance, while the size of the image increases proportionally to this decrease itself. In the second place, there exists, in his case, a singular aptitude for analysing imperfect retinal images, and making out the form of the object in spite of the diffusion-circles of which its image is composed. That does not prevent his having recourse to all possible means of reducing them. Thus, he will seek, by preference, a very strong light, which contracts his pupils; he will limit the size of the latter still more by partially closing the lids, or, when he uses only one eye, by carrying the object to the opposite side of the median line. In this way, he sees over his nose, the bridge of which intercepts a part of the luminous rays and thus restricts the diffusion-circles. A second capital difference, which exists between the second and the third class of hyperopes, is that convergent strabismus is exceptional in cases belonging to the latter. |