eye [AS, eage] , one of a pair of organs of sight, contained in a bony orbit at the front of the skull, with retrobulbar fat, and innervated by four cranial nerves: optic, oculomotor, trochlear, and abducens. Associated with the eye are certain accessory structures, such as the muscles, the fasciae, the eyebrow, the eyelids, the conjunctiva, and the lacrimal gland. The bulb of the eye is composed of segments of two spheres with nearly parallel axes that constitute the outside tunic and one of three fibrous layers enclosing two internal cavities separated by the crystalline lens. The smaller cavity anterior to the lens is divided by the iris into two chambers, both filled with aqueous humor. The posterior cavity is larger than the anterior cavity and contains the jellylike vitreous body that is divided by the hyaloid canal. The outside tunic of the bulb consists of the transparent cornea anteriorly, constituting one fifth of the tunic, and the opaque sclera posteriorly, constituting five sixths of the tunic. The intermediate vascular, pigmented tunic consists of the choroid, the ciliary body, and the iris. The internal tunic of nervous tissue is the retina. Light waves passing through the lens strike a layer of rods and cones in the retina, creating impulses that are transmitted by the optic nerve to the brain. The transverse and the anteroposterior diameters of the eye bulb are slightly greater than the vertical diameter; the bulb in women is usually smaller than the bulb in men. Eye movement is controlled by six muscles: the superior and inferior oblique muscles and the superior, inferior, medial, and lateral rectus muscles. Also called bulbus oculi, eyeball.
