sclerotic (sc.), within which is a highly vascular and internally
pigmented layer, the choroid, upon which the percipient nervous layer,
the retina (r.) rests. The chief chamber of the eye is filled with
a transparent jelly, the vitreous humour (v.h.). In front of the eye, the
white sclerotic passes into the transparent cornea (c.). The
epidermis is continued over the outer face of this as a thin,
transparent epithelium. The choroid coat is continued in front by a
ring-shaped muscle, the iris (ir.) the coloured portion of the eyes. This
iris enlarges or contracts its central aperture (the black pupil)
by reflex action, as the amount of light diminishes or increases.
Immediately behind this curtain is the crystalline lens (l.), the
curvature of the anterior face or which is controlled by the ciliary
muscle (c.m.). In front of the lens is the aqueous humour (a.h.). The
description of the action of this apparatus involves the explanation of
several of the elementary principles of optics, and will be found by
the student in any text-book of that subject. Here it would have no
very instructive bearing, either on general physiological considerations
or upon anatomical fact.