MYOPIA, ANATOMICAL CHANGES. 85
According to Dondcrs, the place of entrance of tlie optic nerve
on the outer surface of the sclera may attain by this widening a
diameter of 8 millimetres or more, so as to present an appearance
as if the optic nerve Avere set upon a second terrace of the staphy-
loma. This widening of the space between the outer and inner
sheath of the optic nerve is mentioned by Von Ammon.*
The investigations of E. v. Jaeger f show that the same thing occurs in myopia of a low grade.
As a rule the choroid remains normal in its anterior part, but the more it approaches the expanded part the thinner, paler and more atrophied does it become. These changes are greatest close to the optic disc, so that here nothing remains of the choroid but a thin, structureless, transparent membrane, containing no trace of blood-vessels or even capillaries. Beyond this fully-atrophied part, however, the anatomical elements of the choroid are but little changed. The pigmented epithelium forms a uniform layer ; the individual cells lose more or less of their pigment molecules in the greatly distended part, and may even become colorless ; in places they are noticeably enlarged and flattened. The branching pigmented cells of the choroidal stroma behave in the same way ; anteriorly, where the choroid retains its normal thickness, they remain entirely normal ; further back they con- tain less pigment, and finally none at all.
A further consequence of the distention suffered by the choroid is that its anterior portions, the ciliary body, the iris and at the same time the lens, move backward ; this accounts for the very frequent deepening of the anterior chamber in myopic eyes. The ciliary muscle having a tendinous attachment on the edge of the cornea, experiences from this process a traction which may induce atrophy.
In uncomplicated cases of myopia the retina shows no recog- nizable changes.
OPHTHALMOSCOPIC APPEARANCE.
In myopia, changes are almost always observable about the optic disc; as a rule there is a sharply-defined sickle-shaped or crescentic spot on the temporal side and close to the disc, where
- Von Ammon, Zeitsclirift fiir die Ophthalinologie, Bd. ii. pag. 250.
f Einstellungen des dioptr. Apparates, Taf. ii. Fig. 18 bin 29.
