A handy-book of ophthalmic surgery for the use of practitioners.

  • Laurence, John Zachariah.
Date:
1869
Catalogue details

Licence: Public Domain Mark

Credit: A handy-book of ophthalmic surgery for the use of practitioners. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

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    dots exhibit in such a case not only a difference of height, but, in addition, a lateral crossed separation. The images then appear thus :— A second prism held with its angle outwards before the second eye destroys this lateral separation, leaving the difference of height unchanged. The strength (= number of de- grees) of the prism necessary for this purpose affords a direct measure of the degree of weakness of the internal rectus. It has been thought that weakness of the recti interni is con- fined to myopic patients, but it occurs also, although more rarely, in hypermetropic ones. Asthenopia from this cause, occurring in a myopic patient may be sometimes relieved by the use of glasses, which enable him to read at a sufficient distance from the eyes to moderate their convergence to a degree that entails but a very slight ac- tion of the affected muscles ; or, by the systematic application of prismatic glasses with the bases turned outwards, the internal recti may be gradually strengthened in the effort they will in- stinctively make by convergence of the optic axes to overcome the crossed double vision the prisms would otherwise cause. Prismatic glasses with the bases turned inwards have no effect in radically curing the disease—on the contrary, rather en- courage it, but render vision more distinct for the time, and relieve the asthenopia by supplementing the action of the in- ternal recti. The last resource is total or partial division of the external rectus muscle. Before undertaking the division of the external rectus, two points must be accurately determined. 1st. What is the amount of insufficiency for near objects, say, for a foot off, measured both in degrees by prisms and linearly by the strabismometer ? and 2d. What is the amount of abducting power for distant and near objects ? The following general considerations flow from these two observations. (a) If the amount of insufficiency is large and the abducting power fairly so, the external rectus of the affected eye may be divided. As regards the amount of effect admissible on one eye, we should observe, in a general way, the same rules as laid down at page 84 in reference to the operation for squint, whether we should operate on one, or both eyes, excepting that the external rectus admits of freer division without the fear of an