Eye, the organ of the special sense of vision, lodged in man in a cavity on each side of the upper portion of the face, called the orbit. The orbits have the form of a quadrangular pyramid of which the base is in front and the summit behind; their direction is horizontal, and their axes, directed backward and inward, would cross at or near the sella tursica of the sphenoid bone in the cranial cavity. They have four triangular surfaces, the upper formed by the orbital plate of the frontal and the lesser wing of the sphenoid bone; the lower by the palate behind, the upper maxillary in the middle, and the malar in front; the external by the sphenoid behind and the malar in front; the internal by the sphenoid behind, the ethmoid in the middle, and the lachrymal bone in front. The cavity has at its upper external portion a depression for the gland which secretes the tears, at its inner portion the commencement of the bony passage to the nose; at the summit is the round opening for the entrance of the optic nerve, the union of the sphenoidal, spheno-maxillary, and pterygo-maxillary fissures, and the commencement of the suborbital canal.

Besides these bony enclosing cavities, the eyes are protected from dust and foreign bodies by the hairs of the eyebrows above, and in front by the movable lids, fringed with the eyelashes. The globe of the eye is of a generally spherical shape, the anterior fifth being the segment of a circle smaller than that of the rest of the organ; the anteroposterior diameter, greater than the transverse, is 10 or 11 lines; differing from the axes of the orbits, the axes of the eyes are parallel. In front, the globe of the eye is in relation with the reflection of the mucous membrane of the lids; behind and all around,with the muscles, vessels, nerves, and a cushion of soft fat. The eye is composed of membranes and humors. Of the membranes of the eye, the cornea has already been described under its own title; the others are the sclerotic, choroid, ciliary processes, iris, and retina. The sclerotic is the external membrane, forming: the posterior four fifths, the anterior fifth being formed by the cornea; it is white, firm, and resisting, opaque, thick, and composed of interlaced fibres.

Beneath the sclerotic is the choroid, composed of small arteries and veins united by delicate areolar tissue; it extends from the entrance of the optic nerve forward to the ciliary circle; both its surfaces are covered with a dark pigment, which gives the deep color seen in the interior of the eye. The ciliary circle or ligament is a grayish ring, a line or two wide, united by its larger circumference to the choroid, and by its lesser to the iris; the ciliary processes are membranous folds, 60 to 80 in number, extending from the choroid to the neighborhood of the opening of the pupil; they form by their union a ring behind the iris and in front of the vitreous humor, surrounding the crystalline lens like a crown. At a short distance behind the cornea is the circular, vertical, membranous curtain, the iris, pierced in the middle by the pupil; this curtain hangs in the aqueous humor, separating it into the anterior and posterior chambers of the eye; it presents anteriorly a great number of radiations converging toward the pupil, the muscular fibres for the dilatation of this opening, and is variously colored in differ-ent individuals; the posterior surface has a number of circular fibres for contracting the pupil, and is covered with a thick dark pigment layer called uvea; both surfaces are lined with the delicate membrane of the aqueous humor; the greater circumference is connected with the ciliary ligament and processes; its movements are doubtless partly owing to its erectile and vascular tissue.

Beneath the choroid is the retina, a thin soft expansion of the optic nerve, surrounding the vitreous humor and extending forward as far as the ciliary processes and crystalline lens; about two lines to the outside of the tubercle of the nerve it presents a circular dark spot and a small perforation discovered by Sommering. . The retina is the immediate organ of vision, which receives the rays of light and transmits the visual impressions by the optic nerve to the sensorium. Of the humors of the eye, the crystalline lens has been described under that head; the others are the aqueous and vitreous humors. The aqueous humor is a limpid transparent fluid, varying in quantity from four to six grains, occupying the space in front of the lens which is divided into anterior and posterior chambers by the iris; it contains in solution a little albumen and the salts usually found in such secretions; when lost by accident or in the operation for cataract by extraction, it is speedily formed again. The vitreous humor occupies the posterior three fourths of the globe of the eye, having the lens encased in its anterior portion; it consists of a transparent, gelatinous fluid enclosed in a great number of cells formed by the partitions of the hyaloid membrane, communicating with each other; in the operation for cataract by depression the lens is pushed backward and downward into this humor.

The optic nerves are the second pair of cerebral nerves. The globe of the eye is moved by six muscles arising from the contour of the optic foramen and its vicinity, and attached to the sclerotic coat; of these muscles four are straight, called the external, internal, superior, and inferior recti muscles, moving the eye respectively outward, inward, upward, and downward. The first two muscles are often permanently contracted, producing divergent or convergent strabismus, a deformity curable by the division of the contracted muscles, a simple and comparatively painless and bloodless operation; the superior oblique muscle passes through a pulley in the inner portion of the orbital process of the frontal bone, from which it extends to the posterior and external part of the globe, rotating the organ inward and forward; the inferior oblique passes from the internal and anterior part of the floor of the orbit to the external and posterior surface of the globe, rotating the eye outward and upward. The conjunctiva, the mucous membrane of the eye, is reflected from the lids and covers the anterior portion of the globe; it is in this membrane that the redness and swelling of ordinary ophthalmia have their seat.