d through the conjunctiva, forms what is called the
white of the eye. Some anatomists have supposed that this coat is a
continuation of the dura mater, which surrounds the optic nerve; but
later observations have shown this opinion to be ill founded. The
tunica sclerotica consists of two layers, which are with difficulty
separated.

The next coat is the cornea, so called from its resemblance to
transparent horn; it arises where the sclerotic coat ends, and forms
the fore part of the eye. The cornea is a segment of a lesser sphere
than the rest of the eye, and consequently makes it more prominent on
the fore part: it is transparent, and firmly connected by its edges
to the sclerotica.

Immediately adherent to the sclerotica, within, is the choroides,
which takes its rise from that part of the eye where the optic nerve
enters, and accompanies the sclerotica to the place where it is
joined to the cornea; here it is very closely connected to the
sclerotica, where it forms that annulus, called ligamentum ciliare;
then leaving the sclerotic coat, it is turned inwards, and surrounds
the crystalline lens; but as this circle, where it embraces the
crystalline, is much narrower than where the membrane leaves the
sclerotic coat, it becomes beautifully corrugated, which folds or
corrugations have been, by the more ancient anatomists, improperly
called ciliary processes.

To the same part of the choroid coat, where the ciliary ligament
begins, is fixed a moveable and curious membrane, called the iris;
this membrane has a perforation in the middle, called the pupil, for
the admission of the rays of light. The iris is composed of two kinds
of fibres: those of the one sort tend, like the radii of a circle,
towards its centre, and the others form a number of concentric
circles round the same centre. The pupil is of no constant magnitude,
for when a very luminous object is viewed, the circular fibres of the
iris contract, and diminish its orifice; and, on the contrary, when
objects are dark and obscure, those fibres relax, and suffer the
pupil to enlarge, in order to admit a greater quantity of light into
the eye: it is thought that the radial fibres also assist in
enlarging the pupil. The iris is variously coloured in different
persons, but according to no certain rule; though in general, they
who have light hair, and a fair complexion, have the iris blue or
grey; and, on the contrary, they whose hair and complexion are dark,
have