MYOPIA, OPHTHALMOSCOPIC APPEARANCE. 39
nerve-sheaths recurs at once to the mind. The elongation of the sagittal diameter stretches the sclera equally with the choroid, and a traction must necessarily be brought to bear upon the outer nerve- sheath, which is continuous with the sclera. The outer nerve- sheath yields to this traction, separates from the inner sheath, the lamellas of the sclera nearest the disc (Fig. 9, a'h') lose their sup- port and become stretched and thinned in proportion as the space between the sheaths becomes greater. The same happens with the choroid, which maintains its normal attachments to the inner surface of the sclera, the border of the optic nerve and the lamina cribrosa. It is easy to understand why this exceedingly thin place should yield to the normal intraocular pressure, and thus form an ectasia, which, to be sure, is generally slight. Exter- nally this is not perceptible, since it is too small and is covered by the external nerve-sheath. These conditions, however, are no obstacle to the ophthalmoscopic examination. For the recognition of this condition the choroid need not be fully atrophied; with a very slight atrophy of the choroid about the optic nerve the course of the retinal vessels is often sufficient to determine the existence of a slight ectasia.
If in a high degree of myopia the greatest diameter of the eye coincide nearly with the visual axis, the outer optic nerve-sheath appears to separate on all sides from the inner one, and the cho- roidal atrophy assumes the annular form. Donders states that such was the case in the eye whose optic nerve is represented in Fig. 9.
Of course in high degrees of myopia the diffuse atrophy of the choroid caused by the stretching of the ocular membranes may easily overstep the limits bounded by tlie outer nerve-sheath. Nevertheless, one often sees in these cases upon the white scleral background which has been laid bare by the choroidal atrophy, a distinct curved line, which may be regarded as probably the line of junction of the external nerve-sheath with the outer surface of the sclera,
CAUSES AND COURSE.
In the majority of cases myopia, or at least a predisposition to it, is to be regarded as congenital or inherited. On the other hand it is certain, and one hears it often enough confirmed by the
