The most frequent cause of myopia is an
abnormal increase in the length of the eyeball in
its antero-posterior axis. This extension chiefly
occm-s at the posterior portion of the globe, and
may here produce a more or less considerable ovoid
bulging (posterior staphyloma), which is accom-
panied by thinning and atrophy of the choroid and
sclerotic. This proJongation of the visual axis of
course varies greatly in extent in different cases,



48 MYOPIA.
and thus gives rise to very varying degrees of short
sight. In cases of considerable myopia, we find
that this peculiar atrophy of the choroid (posterior
staphyloma, sclerotico- choroiditis posterior) is
almost always present. Von Graefe lays it down as
a general rule that, when the far point lies nearer,
than 5 inches fi:om the eye (the myopia exceeding
3^), we may almost with certainty foretell the
presence of sclerotico-choroiditis posterior. But
we very frequently meet with it in much slighter
degrees of myopia ; indeed, I have often seen it in
cases where the myopia did not exceed ^^ or ^.
