Sense of sight.—Organ of vision.—Globe of the eye; sclerotic; cornea; choroid; ciliary ring; ciliary body; ciliary process; iris; pupil; pigment; retina; vitreous body; hyaloid membrane; crystalline; anterior and posterior chambers; aqueous humour.—Muscles of the eye.—Conjunctiva.—Eyelids, eyelashes.—Lachrymal apparatus.—Vision; functions of the retina, reversed images; functions of the iris; optic centre, visual angle, visual impressions, single or mixed, adaptation of the eye to distances, myopia, presbyopia; achromatism; single and double vision with two eyes, stereoscope; alternation in the action of the eyes; persistence of impressions on the retina; accidental images; irradiation; ocadental aureola; Daltonism; apparent motion of objects.—Optic nerve.— Movements of the eye.—Extent of vision.
The visual apparatus consists of the globe of the eye and its appendages, which are the eyelids and eyebrows, the motor muscles of the eye, and the lachrymal apparatus.
The globe of the eye is generally described as a spheroid, to which the segment of a smaller sphere is applied in front, and this definition is exact to the senses if it is not so mathematically. The walls of the globe of the eye are formed principally of two fibrous membranes; one white and opaque—the sclerotic (scleros, hard)—which envelops the two posterior thirds of the globe; and the other transparent, and resembling a horny plate, from whence its name, cornea. The sclerotic is one of the strongest fibrous membranes in the body; it is white on its external surface, and of a brownish-red colour internally; it is thicker at the posterior portion of the eye, where it opens to allow the passage of the optic nerve, than in front, where it terminates in a circular hollow or slope, into the border of which the cornea is set like a watch-glass. The two membranes are united by intimate adherence, so strongly as to seem but one. The cornea is thicker than the sclerotic, and is composed of superposed layers perfectly translucent; it is convex in front, and concave behind, and appears to be circular, although its transverse diameter is a little greater than the other.