ens; but the fore
part next the pupil is not so convex as its other side, which is
contiguous to the vitreous humour; the diameter of the sphere, of
which its anterior segment is a part, being in general about seven or
eight lines, whereas the diameter of the sphere, of which its
posterior segment forms a portion, is commonly only about five or six
lines. It is covered with a fine transparent capsule, which is called
arachnoides. This humour is situated exactly behind the pupil, but
not in the centre of the eye, as was supposed by Vesalius, being a
good deal nearer its forepart. The convexity of its posterior surface
is received into an equal concavity of the vitreous humour. It is not
of an equal density throughout, but is much more hard and dense
towards its centre than externally, the reason of which will appear
hereafter. Till we arrive at about our thirtieth year, this humour
continues perfectly transparent, and colourless; about that time it
generally has a little tinge of yellow, and this colour increases
with age.

The third humour of the eye, is the vitreous; it is the largest of
all the humours, filling up the whole of that part of the eye which
lies behind the crystalline humour. It is thicker than the aqueous,
but thinner than the crystalline humour; on its back part is spread
the retina, and in the middle of its fore part is a small cavity, in
which the whole posterior surface of the crystalline lens lies; this
humour is also enclosed in a very fine capsule, called tunica vitrea;
this capsule at the edge of the crystalline humour is divided into
two membranes, of which the one is continued over the whole anterior
surface of the vitreous humour, and lines that cavity into which the
back part of the crystalline is received; the other passes over the
crystalline humour, and covers all its fore part, by which means
these two humours are closely connected together. The weights of the
aqueous, crystalline, and vitreous humours in a human eye, are,
according to the accurate Petit, at a medium, to each other, as 1, 1,
and 25.

It was thought necessary to premise this general description of the
structure of the eye, in order that what we are going to add in the
remaining part of this Lecture may be the more easily comprehended. A
more distinct idea will perhaps be had from a contemplation of the
following figure, which represents the section of an eye by a
vertical plane passing through its centre.

[FIGURE]