the iris of a deep brown; but whether this difference in colour
occasions any difference in the sense, is not yet discovered. In the
human eye the whole choroid coat, and even the interior surface of
the iris or uvea, is lined with a black mucus; this mucus, or as it
is called, pigmentum, is darkest in young persons, and becomes more
light coloured as we advance in years. In many animals, but more
particularly those which catch their prey in the night, this
pigmentum is of a bright colour: its use will appear afterwards.

The last, and innermost coat of the eye, is the retina, it differs
much from the above mentioned coats, being very delicate and tender.
It is nothing but an expansion of the medullary part of the optic
nerve, which is inserted into each eye, nearer the nose, and a little
higher, than the axis. This coat has been thought by many to end
where the choroides, going inwards, towards the axis of the eye,
forms the ciliary ligament; Dr. Monro thinks that it is not continued
so far, and we cannot see with what advantage it could have been
continued to the ciliary ligament, since none of the rays of light,
passing through the pupil, could fall upon that part of it. In the
middle of the optic nerve is found the branch of an artery, from the
internal carotid, which is diffused and ramified in a beautiful
manner along the retina. From this artery, a small branch goes
through the middle of the vitreous humour, and giving off branches on
every side, expands itself upon the capsule of the crystalline lens.

We shall now consider the humours of the eye, which are three in
number, the aqueous, the crystalline, and the vitreous; all
transparent, and in general colourless; but of different densities.

The aqueous humour, so called from its resemblance to water, fills up
all the space between the cornea and the crystalline humour. It is
partly before and partly behind the uvea, and is divided by that
membrane into two parts, which are called the chambers of the aqueous
humour; which chambers communicate with each other by means of the
pupil.

The next humour is the crystalline; it is situated between the
aqueous and vitreous humours, and is connected to the choroid coat by
the ciliary ligament: it is not the least of all the humours, as has
been generally supposed, the aqueous and it being of equal weights;
but its substance is more firm and solid than that of the other
humours: its figure is that of a double convex l