Licence: Public Domain Mark
Credit: Bilateral deviations of the eyes / by Priestley Smith. Source: Wellcome Collection.
Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.
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![[Reprinted from The Birmingham Medical Review, October, 1875.] Bilateral Deviations of the Eyes. By PRIESTLEY SMITH, M.R.C.S., Ophthalmic Surgeon to the Queen's Hospital, Birmingham. The action of each one of the six muscles which move the eye, has been ascertained with great precision ; the symptoms pi'oduced by paralysis of the motor nerves of these muscles are accurately known, and it is possible in every case of such paralysis, however slight may be the deviation of the corresponding eye, to determine which is the nerve affected. Many investigators have contributed to the present state of knowledge on this subject, but it is to von Graefe that we chiefly owe the ingenious and accurate methods of studying the causes of double vision, by means of the relative positions of the true and false images, and by the application of prisms. English readers are indebted to Dr. Soelberg Wells, for a complete exposition of the whole subject in the Royal London Ophthalmic Hospital Reports, Vol. H. Notwithstanding the precision of these means of diagnosis, cases of deviation of the eyes occur, which cannot, by any ingenuity, be referred to lesion of any individual motor nerve or nerves. Two such cases I have to record. The impairment of movement in each case was due, I believe, to lesion of the brain-centre which normally presides over the movement in question,—a cause of ocular deviations which has hitherto received very little attention from ophthalmic surgeons. In investigating cases of this kind, it is necessary to remember not only the action of each muscle upon the eye, but also the various ways in which movements of one eye are associated with movements of the other. This association of symmetrical voluntary movements on the two sides of the body is probably more complete, and more beyond our control, in the case of the eyes than in any other instance. For example, the muscles of the mouth, though usually acting bilaterally, can, with attention, be brought into independent action; most persons can acquire the habit of closing one eye to some extent while keeping the other open ; but no person with healthy eyes can follow the movement of an object with one eye, and keep the other eye motionless. The advantage of this close association is obvious. Perfect vision occurs only when both optic axes are accurately directed towards the object looked at, the slightest deviation of either produces double vision, and hence the power of moving the eyes independently of each other, did we possess it, could bring us nothing but confusion.](https://iiif.wellcomecollection.org/image/b2148238x_0005.jp2/full/800%2C/0/default.jpg)
