Page:Handbook of Ophthalmology (3rd edition).djvu/39

Rh ments, Donders attained the unexpected result that the radius of curvature of the cornea is generally greater in the case of myopes than with emmetropes. Of course a too great curvature of the cornea, when it exists, will cause myopia. The same is true of the lens. Changes in it may cause myopia, but such cases do not often occur. A forward displacement of the lens must, for optical reasons, increase the refractive condition; this seems, however, to be an infrequent cause of myopia. Shortsightedness seems to be oftener caused by an increase of the index of refraction, particularly in the nucleus of the lens. The condition may be easily recognized ophthalmoscopically, if the pupil be large enough, or has been dilated by mydriatica. The reflection from the nucleus of the lens is generally stronger than normal, as well upon examination by daylight with the naked eye as by focal illumination; upon ophthalmoscopic illumination, especially with a weak mirror, the nucleus appears evidently differentiated from the cortical part of the lens; this can be best seen by making slight movements of the mirror. These changes occur as the prodroma of nuclear cataract, both in eyes previously healthy and in those recovered from glaucoma. They often occur, also, as one of the many complications of myopia depending upon an elongation of the axis of the eye. Under the title "Diseases of the Accommodation" it will be shown that apparent myopia may be caused by a continuous tension of the accommodation.

The most frequent and important cause of myopia is the elongation of the axis of the eye. The other diameters are not unfrequently somewhat increased. As a rule, however, the axis is most elongated, and the eye assumes therefore an ellipsoid form. The distention of the membranes occurs mostly at the posterior part of the eye, involving generally the region of the optic nerve. The greatest diameter of the eye, therefore, very nearly coincides with its axis; it may, however, deviate laterally, generally toward the median side. According to E. v. Jaeger, the greatest expansion may occur even on the median side of the optic nerve.

This anomaly of form was first described by Scarpa. Its designation as "staphyloma posticum" is not exactly appropriate, inas-