Chapter LXIX: Section 111: The eye (Figure 8) has a tough, dense, outer coat, the

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.