An endeavour to show that insufficiency of the internal recti muscles and myopia have been erroneously associated : and that the muscular asthenopia of myopia is not the result of such insufficiency, but of the anomaly of refraction / by Samuel Theobald.
- Samuel Theobald
- Date:
- 1874
Licence: Public Domain Mark
Credit: An endeavour to show that insufficiency of the internal recti muscles and myopia have been erroneously associated : and that the muscular asthenopia of myopia is not the result of such insufficiency, but of the anomaly of refraction / by Samuel Theobald. Source: Wellcome Collection.
Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.
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![recti interni.” Speaking of asthenopia muscularis in connection with myopia, he says :— “ Cases have occurred to me, in which there was at first vision [in near work] with both eyes, but where, on fatigue, the one eye gave way, and the work was then attended with less difficulty ; others where precisely that giving way caused trouble and was complained of. 1'his latter state I met with when the degree of the myopia was comparatively slight, and where, therefore, be- sides resistance of the eye, a certain weakness of the muscles (not only insuffi- ciency of movement, but true infinfficiency of the musculi rectiinterm, must be understood); a condition which, in moderate degrees of myopia, I have found to be hereditary, with the phenomena just described.” This assumption of Donders, together with the test of Yon Graefe, which I have described, are chiefly responsible for the generally adopted belief in the association of insufficiency of the internal recti muscles with myopia. To account, however, for the symptoms, which Donders has de- scribed, no such assumption is necessary. They are, indeed, exactly such as we should anticipate from the disturbed relation between convergence and accommodation, which, as I have before stated, exists in myopia as well as in hypermetropia. In hypermetropia, accommodation is in excess of convergence. When the eyes are used, especially for near work, asthe- nopia ensues ; the ciliary muscles soon become tired and give way, and the sight grows indistinct. A convergent squint, by neutralizing the excess, may do away with these symptoms. In myopia, convergence is in excess of accommodation. Asthenopia, attended by giving way of the internal recti, is the result. As, in hypermetropia, there is no actual weak- ness of the ciliary muscles, so, in myopia, there is no real insufficiency of the internal recti. It is, in each case, the muscles which work in excess, that evince such signs, and, as in the former condition an effort is made to neutralize this excess by a convergent squint, so, in the latter is it accomplished by a divergent squint. There is this difference, however, between the tw'O. In hypermetropia, as there is excessive action of the ciliary muscles, the parallelism is restored by excessive action upon the part of the internal recti; the repeated efforts of the ciliary muscles to do so, by relaxing their tension, being incompatible with useful vision. In myopia, on the contrary, as there is diminished action of the ciliary mus- cles, it is restored by diminished action upon the part of the internal recti; its restoration by increased action of the former being of course impossible, although, as is well known, the ciliary muscles do act unnecessarily, and even to the detriment of vision, in myopia, causing the binocular far point to lie nearer the eyes than the monocular. This view of the origin of di- vergent strabismus affords us a most satisfactory explanation of the fact of relative divergent strabismus being so much more common than rela- tive convergent strabismus. In hypermetropia accommodation is always in excess of convergence, not only for near work, but for distant vision as well ; therefore absolute soon follows upon relative convergent squint. In myopia, on the contrary, convergence is in excess of accommodation](https://iiif.wellcomecollection.org/image/b22450051_0006.jp2/full/800%2C/0/default.jpg)
