Definition:
The structures within the spherical eyeball provide the sense of vision as activated photoreceptors transmit nerve impulses to the visual cortex of the occipital lobes where interpretation occurs.

Principal structures, descriptions, and functions of the eye

Structure	Description and Function
Sclera	A vascular, white, outer layer of eyeball; provides shape
Cornea	Anterior, convex surface of eyeball; transmits and refracts light
Choroid	Vascular, middle layer of eyeball; supplies blood to eyeball
Ciliary body	Smooth muscle fibers; supports the lens through suspensory ligament and determines its shape; secretes aqueous humor
Iris	Pigment cells and smooth muscle fibers; regulates diameter of pupil
Retina	Inner layer, contains rods and cones; photoreception, transmits impulses
Lens	Transparent, protein fibers; refracts light and focuses onto fovea centralis
Anterior and posterior chambers	Aqueous-filled chambers between cornea and lens; aqueous humor provides intraocular pressure and refracts light
Vitreous chamber	Vitreous humor-filled chamber between the lens and retina; vitreous humor provides intraocular pressure, holds retina against the choroid, and refracts light

Clinical Information
Astigmatism. A visual impairment in which an irregular curvature of the cornea or lens of the eye distorts the refraction of light waves.
Hypermetropia (hyperopia). Farsightedness. The eyeball is too short, and thus the image is focused at a point behind the retina.
Myopia. Nearsightedness. The eyeball is elongated, and thus the image is focused at a point in the vitreous humor in front of the retina.