Eye

The human eyeball is about 0.9 in (24 mm) in diameter and is not perfectly round, being slightly flattened in the front and back. The eye consists of three layers: the outer fibrous or sclera, the middle uveal or choroid layer, and the inner nervous layer or retina. Internally the eye is divided into two cavities—the anterior cavity filled with the watery aqueous fluid, and the posterior cavity filled with gel-like vitreous fluid. The internal pressure inside the eye (the intraocular pressure) exerted by the aqueous fluid supports the shape of the anterior cavity, while the vitreous fluid holds the shape of the posterior chamber. An irregularly shaped eyeball results in ineffective focusing of light onto the retina and is usually correctable with glasses or contact lenses. An abnormally high intraocular pressure, due to overproduction of aqueous fluid or to the reduction in its outflow through a duct called the canal of Schlemm, produces glaucoma, a usually painless and readily treatable condition, which may lead to irreversible blindness if left untreated. Elevated intraocular pressure is easily detectable with a simple, sight-saving, pressure test during routine eye examinations. The ophthalmic arteries provide the blood supply to the eyes, and the movement of the eyeballs is facilitated by six extraocular muscles which run from the bony orbit which insert the sclera, part of the fibrous tunic.

The outer fibrous layer encasing and protecting the eyeball consists of two parts—the cornea and the sclera. The front one-sixth of the fibrous layer is the transparent cornea, which bends incoming light onto the lens inside the eye. A fine mucus membrane, the conjunctiva, covers the cornea, and also lines the eyelid. Blinking lubricates the cornea with tears, providing the moisture necessary for its health. The cornea's outside surface is protected by a thin film of tears produced in the lacrimal glands located in the lateral part of orbit below the eyebrow. Tears flow through ducts from this gland to the eyelid and eyeball, and drain from the inner corner of the eye into the nasal cavity. A clear watery liquid, the aqueous humor, separates the cornea from the iris and lens. The cornea contains no blood-vessels or pigment and gets its nutrients from the aqueous humor. The remaining five-sixths of the fibrous layer of the eye is the sclera, a dense, tough, opaque coat visible as the white of the eye. Its outer layer contains blood vessels which produce a "blood-shot eye" when the eye is irritated. The middle or uveal layers of the eye is densely pigmented, well supplied with blood, and includes three major structures—the iris, the ciliary body, and the choroid. The iris is a circular, adjustable diaphragm with a central hole (the pupil), sited in the anterior chamber behind the cornea. The iris gives the eye its color, which varies depending on the amount of pigment present. If the pigment is dense, the iris is brown, if there is little pigment the iris is blue, if there is no pigment the iris is pink, as in the eye of a white rabbit. In bright light, muscles in the iris constrict the pupil, reducing the amount of light entering the eye. Conversely, the pupil dilates (enlarges) in dim light, so increasing the amount of incoming light. Extreme fear, head injuries, and certain drugs can also dilate the pupil.

The iris is the anterior extension of the ciliary body, a large, smooth muscle which also connects to the lens via suspensory ligaments. The muscles of the ciliary body continually expand and contract, putting on suspensory ligaments changing the shape of the lens, thereby adjusting the focus of light onto the retina facilitating clear vision. The choroid is a thin membrane lying beneath the sclera, and is connected the posterior section of the ciliary body. It is the largest portion of the uveal tract. Along with the sclera the choroid provides a light-tight environment for the inside of the eye, preventing stray light from confusing visual images on the retina. The choroid has a good blood supply and provides oxygen and nutrients to the retina.

The front of the eye houses the anterior cavity which is subdivided by the iris into the anterior and posterior chambers. The anterior chamber is the bowl-shaped cavity immediately behind the cornea and in front of the iris which contains aqueous humor. This is a clear watery fluid which facilitates good vision by helping maintain eye shape, regulating the intraocular pressure, providing support for the internal structures, supplying nutrients to the lens and cornea, and disposing of the eye's metabolic waste. The posterior chamber of the anterior cavity lies behind the iris and in front of the lens. The aqueous humor forms in this chamber and flows forward to the anterior chamber through the pupil.

The posterior cavity is lined entirely by the retina, occupies 60% of the human eye, and is filled with a clear gel-like substance called vitreous humor. Light passing through the lens on its way to the retina passes through the vitreous humor. The vitreous humor consists of 99% water, contains no cells, and helps to maintain the shape of the eye and support its internal components.

The lens is a crystal-clear, transparent body which is biconvex (curving outward on both surfaces), semi-solid, and flexible, shaped like an ellipse or elongated sphere. The entire surface of the lens is smooth and shiny, contains no blood vessels, and is encased in an elastic membrane. The lens is sited in the posterior chamber behind the iris and in front of the vitreous humor. The lens is held in place by suspensory ligaments that run from the ciliary muscles to the external circumference of the lens. The continual relaxation and contraction of the ciliary muscles cause the lens to either fatten or became thin, changing its focal length, and allowing it to focus light on the retina. With age, the lens hardens and becomes less flexible, resulting in far-sighted vision that necessitates glasses, bifocals, or contact lenses to restore clear, close-up vision. Clouding of the lens also often occurs with age, creating a cataract that interferes with vision. Clear vision is restored by a relatively simple surgical procedure in which the entire lens is removed and an artificial lens implanted.