How The Eye Works - Eye Anatomy

Conjunctiva

The conjunctiva is a thin, transparent tissue that covers the outer surface of the eye. It begins at the outer edge of the cornea, extends over the visible portion of the sclera and then over the inside of the eyelids. It is nourished by tiny blood vessels. Among its roles is the secretion of the tear film to lubricate the eye.

Sclera

The sclera is the "white of the eye". It is a tough, opaque layer that serves as the complete outer protective coat of the eyeball except for two major openings: the cornea in the front, and an opening for the optic nerve in the back. Six eye muscles attach to the sclera to control eye movements.

Cornea

The cornea is the transparent front window of the eye. It allows light to enter the eye, and provides 2/3 of the focusing power of the eye. It has no blood vessels, but has more nerve endings than any other part of the human body and so is extremely sensitive. In adults, it is approximately 1/2mm thick and is composed of five layers. The outer layer is called the epithelium; it is made up of 5-6 layers of cells which, if injured, can rapidly regenerate. The middle and thickest layer is the stroma; an injury to the stroma may result in a scar. The deepest layer is the endothelium; it is a single layer thick, serves to pump fluid out of the cornea to keep the cornea clear, and can not regenerate.

Iris

The iris is the colored part of the eye. The muscles of the iris help regulate the amount of light that enters the eye. The color, texture, and patterns of the iris make each individual's iris as unique as a fingerprint.

Pupil

The pupil is the dark center in the middle of the iris. It acts like an aperture in a camera, determining how much light enters the eye. The brighter the environment, the smaller the pupil, and the darker the environment, the large the pupil.

Lens

The lens is a transparent structure located behind the iris and is enclosed in a capsular-like bag. The lens provides 1/3 of the focusing power of the eye. By changing the curvature of the lens, we can change our plane of focus from far to near and back again.

Retina

The retina is a multi-layered sensory tissue that lines the back of the eye. It contains millions of photoreceptors that capture light rays and convert them into electrical impulses that are then sent through the optic nerve to the brain. There are two types of photoreceptors, termed cones and rods. There are approximately 6 million cones, most of which are located in the macula. Cones function best in bright light and provide our sharpest vision, as well as allow us to appreciate color. There are approximately 125 million rods, mostly spread throughout the peripheral portions of the retina. They function best in dim light, and are responsible for our peripheral and night vision.

Macula

The macula is a small area near the center of the retina that provides our fine, detailed vision. The fovea is the very center portion of the macula.

Choroid

The choroid lies between the retina and the sclera. It contains the blood vessels that nourish the back of the eye.

Optic Nerve

The optic nerve connects the eye to the brain, and is located in the back of the eye. It carries the electrical impulses generated by the retina to the brain, which interprets them as images.

Vitreous

The vitreous is the clear, jelly-like substance that fills the middle cavity of the eye.