Refractive Media and Compartments of Eyeball

On their way to the retina, lightwaves pass through the refractive media of the eyeball: cornea, aqueous humor, lens, and vitreous humor. The cornea is the primary refractory medium of the eyeball - that is, it bends light to the greatest degree, focusing an inverted image on the light-sensitive retina of the fundus of the eyeball.

The aqueous humor (often shortened clinically to "aqueous") occupies the anterior segment of the eyeball. The anterior segment is subdivided by the iris and pupil. The anterior chamber of the eye is the space between the cornea anteriorly and the iris/pupil posteriorly. The posterior chamber of the eye is between the iris/pupil anteriorly and the lens and ciliary body posteriorly. Aqueous humor is produced in the posterior chamber by the ciliary processes of the ciliary body. This clear watery solution provides nutrients for the avascular cornea and lens. After passing through the pupil into the anterior chamber, the aqueous humor drains through a trabecular meshwork at the iridocorneal angle into the scleral venous sinus (Latin sinus venosus sclerae, canal of Schlemm). The humor is removed by the limbal plexus, a network of scleral veins close to the limbus, which drain in turn into both tributaries of the vorticose and anterior ciliary veins. Intra-ocular pressure (IOP) is a balance between production and outflow of aqueous humor.

The lens is posterior to the iris and anterior to the vitreous humor of the vitreous body. It is a transparent, biconvex structure enclosed in a capsule. The highly elastic capsule of the lens is anchored by zonular fibers (collectively constituting the suspensory ligament of the lens) to the encircling ciliary processes. Although most refraction is produced by the cornea, the convexity of the lens, particularly its anterior surface, constantly varies to fine-tune the focus of near or distant objects on the retina. The isolated unattached lens assumes a nearly spherical shape. In other words, in the absence of external attachment and stretching, it becomes nearly round. The ciliary muscle of the ciliary body changes the shape of the lens. In the absence of nerve stimulation, the diameter of the relaxed muscular ring is larger. The lens suspended within the ring is under tension as its periphery is stretched, causing it to be thinner (less convex). The less convex lens brings more distant objects into focus (far vision). Parasympathetic stimulation via the oculomotor nerve (CN III) causes sphincter-like contraction of the ciliary muscle. The ring becomes smaller, and tension on the lens is reduced. The relaxed lens thickens (becomes more convex), bringing near objects into focus (near vision). The active process of changing the shape of the lens for near vision is called accommodation. The thickness of the lens increases with aging so that the ability to accommodate typically becomes restricted after age 40.

The vitreous humor is a watery fluid enclosed in the meshes of the vitreous body, a transparent jelly-like substance in the posterior four fifths of the eyeball posterior to the lens (posterior segment of the eyeball, also called the postremal or vitreous chamber). In addition to transmitting light, the vitreous humor holds the retina in place and supports the lens.