a groove in the front part of the sclerotic
coat in the same manner that a watch-glass is received into its case.
But for this arrangement light could not gain admission to the eye.

The _choroid coat_, which constitutes the second investing membrane of
the eye, is of a dark brown color upon its outer surface, and of a deep
black within. The internal surface of this membrane secretes a dark
substance resembling black paint, upon which the retina is spread out,
and which is of great importance in the function of vision, as it seems
to absorb the rays of light immediately after they have struck upon the
sensible surface of the retina.

The _retina_, which is the third and innermost membrane of the eye, is
the expansion of the optic nerve, and constitutes the immediate seat of
vision. Such is the arrangement of the humors of the eye, and so
perfectly are they adapted to the functions they are called upon to
perform, that in the healthy state of this organ, the light entering the
pupil is so refracted as to paint upon the retina an exact image of the
objects from which it proceeds. The optic nerve, whose expansion forms
the retina, receives this image and transmits it to the mind.

Arnott has well remarked, that "a whole printed sheet of a newspaper may
be represented on the retina on less surface than that of a finger nail;
and yet not only shall every word and letter be separately perceivable,
but even any imperfection of a single letter. Or, more wonderful still,
when at night an eye is turned up to the blue vault of heaven, there is
portrayed on the little concave of the retina the boundless concave of
the sky, with every object in its just proportions. There a moon in
beautiful miniature may be sailing among her white-edged clouds, and
surrounded by a thousand twinkling stars, so that to an animalcule
supposed to be within and near the pupil, the retina might appear
another starry firmament with all its glory."

Besides these three coats, and the cornea which constitutes about one
fifth of the anterior portion of the outer coat, it is necessary to
notice the _iris_, so called from its variety of color in different
persons, and upon which alone the color of the eye depends. The iris is
a circular membrane situated just behind the cornea, and is attached to
one of the coats at its circumference. In its center is a small round
hole, called the _pupil_; and sometimes spoken of familiarly as the
sight of the eye, as no