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Eye Anatomy

Internal and External Structures of the Eye

Placing and clicking your cursor over the "G", "I" and "J" in the external
view and the text overlying the eyeballs will take you to descriptions
of those items. Just click on the back arrow to return to the graphic.

 

Glossary of Eye Anatomy

Aqueous

A water-like fluid which fills the front part of the eye between the lens and cornea. This fluid is produced by the ciliary body and drains back into the blood circulation through channels in the chamber angle. It is turned over every 100 minutes.

Chamber Angle

Located at the junction of the cornea, iris, and sclera, the anterior chamber angle extends 360 degrees at the perimeter of the iris. A permeable membrane here allows aqueous fluid to be passed back into the blood circulation from the eye. This filtration system is less functionalin glaucoma.

Choroid

A very vascular layer between the sclera and retina which serves to nourish the outer portions of the retina. Has one of the highest blood flows in the body.

Ciliary Body

A structure located behind the iris (rarely visible) which produces aqueous fluid that fills the front part of the eye and thus maintains the eye pressure. It also allows focusing of the lens. Return to top

Conjunctiva

A thin lining over the sclera, or white part of the eye. This also lines the inside of the eyelids. Cell in the conjunctiva produce mucous, which helps to lubricate the eye.

Cornea

The clear window through which we see. Actually, this is a very vital part of the eye's focusing, and the curvature of the cornea itself accomplishes about 80% of the focusing of the eye.

Episclera

A fibrous layer between the conjunctiva and sclera. Sometimes lumps (pingueculum) will form in this layer on the surface of the eye near the inside or outside corners.

Extraocular Muscles

Six muscles control eye movement. Five of these originate from the back of the orbit and wrap around the eye to attach within millimeters of the cornea. Four of these move the eye roughly up, down, left and right. Two muscles (one originating from the lower rim of the orbit) control the twisting motion of the eye (when the head is tilted).

Fovea Centralis

See rods and cones below. Return to top

Iris

This is the part of the eye which gives it color. It contains muscles which open or close the pupil in response to the brightness of surrounding light. A blue iris actually has a lack of pigment.

Lens

This is located just behind the iris, and helps to focus light. A "capsule" surrounds the lens "nucleus". The nucleus can become cloudy and this is termed cataract.

Macula

The part of the retina which is most sensitive, and is responsible for the central (or reading) vision. It is located near the optic nerve directly at the back of the eye (on the inside). There are no overlying blood vessels in the macula. This area is also responsible for color vision.

Optic Nerve

This contains visual information from the eye, and has about 1.2 million nerve fibers. The optic disc is visible on the inside of the eye, where the nerve is viewed "end on." The sheath around the optic nerve is continuous with that of the brain, and the nerve connects directly into the brain. Return to top

Orbit

The boney socket containing the eye, fat, extraocular muscles, nerves, and blood vessels. The floor and inside walls of the orbit are paper thin, and are easily fractured by trauma.

Pupil

Essentially, a hole in the iris. This is the black opening in the center of the eye. Its size is controlled by the iris muscles.

Retina

This thin layer lines the inside of the eye and receives light rays, processes them, and sends signals to the brain via the optic nerve. The retina is like the "film of a camera". It is separated from the very vascular choroid by the "retinal pigment epithelium". Sometimes breakdowns in this pigmented layer allow macular degeneration.

Rods and Cones

The retina contains two types of photoreceptors, rods and cones. The rods are more numerous, some 120 million, and are more sensitive than the cones. However, they are not sensitive to color. The 6 to 7 million cones provide the eye's color sensitivity and they are much more concentrated in the central yellow spot known as the macula. In the center of that region is the "fovea centralis," a 0.3 mm diameter rod-free area with very thin, densely packed cones. This is where 20/20 vision occurs. Return to top

Sclera

The white, tough wall of the eye. Few diseases affect this layer. It is covered by the episclera and conjunctiva, and eye muscles are connected to this.

Uveal tract

A group of similar eye structures including the choroid, ciliary body and iris. May be prone to inflammatory conditions (uveitis or iritis).

Vitreous

A jelly-like, clear fluid which fills most of the eye (from the lens back). This tends to liquefy with age, and its separation from the retina can lead to retinal tears and detachment.

Zonules

Slender but very strong suspensory ligaments, also known as zonules or zonules of Zinn, attach at one end to the lens capsule and at the other end to the ciliary processes of the circular ciliary body, around the inside of the eye.  These thin ligaments or zonules hold the lens in place. Return to top

 

 

 

 

 
     
 
Vitreous Zonules vitreous chamber angle retina optic nerve fovea cornea conjunctiva sclera iris lens ciliary body choroid sclera optic nerve macula macula retina optic nerve retina macula iris pupil