On the etiology and treatment of convergent squint.

  • Snell, Simeon, 1851-1909.
Date:
[1887]
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Licence: Public Domain Mark

Credit: On the etiology and treatment of convergent squint. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    the efi'ort of accommodation pertaining to a certain position of con» vergence for any length of time, or even to attempt it at all. The faculty of convergence is difficult to maintain whenever accommod a- tion makes default, and therefore one of the eyes or both pass into the position of repose. By sacrificing binocular vision the hypermetrope frees himself from the laws of relative accommodation ; the absolute accommodation becomes free for monocular vision and asthenopia dis- appears partly or entirely. It appears as if Stilling required to explain more the factors in- fluential in determining the position of rest, besides the refraction and muscular condition. The shape of the orbital cavity, the typo- graphical relations of the fibrous structures and soft parts surround- ing the globe, the position of the optic nerve entrances are all given. These may have some influences. We know from Donders, that in hypermetropes, the macula is further from the temporal border of the papilla than it is in either emmetropia or myopia. Landolt's researches support this. In a case, however, of complete ophthalmo- plegia, as under observation at the present time, on uncovering the globe, the eyeball is not found to be convergent or divergent but placed in the centre. Such a case seems to show that the other causes mentioned by Stilling play a very unimportant part, and it throws back on the muscular apparatus the production of the position of repose. He has not found that the relative powers of adduction or abduction bore any constant relation to the position of rest; he thinks the squint may be occasioned by fatigue of the external recti. It thus approaches nearer to Schweigger's and others views as to mus- cular influence. It appears to me a tair criticism of Stilling's work, to say as Mr. Frost4 has done, that throughout he lays too littlo stress upon the simultaneous innervation of the two eyes ; he speaks repeatedly of one eye passing into its position of rest. Stilling adopts the opinion that the amblyopia is precedent to the squint, but then it is well known that the position assumed by am- blyopic eyes quite independently of their state of refraction is diver- gent. If the position 01 rest in hypermetropes and emmetropes be convergent, and the grounds on which Stilling argues be correct, the form of amblyopic squint should be convergent, and divergent in myopes. Stilling makes use of the position as made by Donders, up to the time when the eye turns it, when Stilling asserts it assumes its position of rest, and Donders that it does so to avoid conflicting images, and in consequence of convergence not keeping pace with the requirements of accommodation. Gardiner, in the recent March number of Knapp's Archives, under the title of Is the Position of Rest the Cause of Strabismus, has an article in which the author controverts Stilling's views. He believes that the generally accepted theory, that holds that one eye by exces- sive convergence actively aids the other to attain and to maintain a tension of accommodation much higher thau what its angle of con- vergence would warrant, is correct. This is proved by covering one eye of a hypermetrope of 2 or 3 D. While he fixes a distant object, the covered eye will deviate inwards, not because it assumes a position of rest, but because the accommodation, operating to the extent of 2 or 3 D, calls for convergence of 2 or 3 AM. The inherent desire for binocular fixation and a consequent shifting of the relative range of convergence prevented this deviation before one eye was covered. That the posi- tion of rest is not the cause here is obvious, because emmetropic and even myopic eyes, with parallel or slightly divergent axes, if rendered hypermetropic, will respond in like manner to the experiment. Again, if the absolute near point of hypermetropia or emmetropia is found, by placing an object in the median line while one eye is covered, on * Medical Record, June 15th, 18S7.