About the eye

The eye is our window on the world. Through it we experience the beauty of nature, see the faces of our loved ones, and take in all the information that allows us to lead the lives we know and enjoy. Housed in a sphere smaller than a ping-pong ball, this delicate evolutionary wonder works with the brain to transform light into the images we see. Each part of the eye performs a unique function that, if interrupted by injury or disease, disrupts the visual system. How do changes lead to visual impairment? To find out, let’s learn about how the healthy eye works.

The human eye is an opaque ball that rests in the eye socket of the skull.  The front side of the eye has a clear window that allows light to enter. At the back, the optic nerve connects the eye to the brain, allowing it to transmit the images that the brain then processes as vision. In between this front window and the “output” to the brain is an intricate arrangement of cells, tissues, fluids and nerves that each have their own role to play.

The cornea is the clear, dome-shaped “front window” of the eye. The cornea is a lens that bends (refracts) light rays as they pass through. This accounts for about 70 percent of the eye’s total focusing power.

The iris is the ring of pigmented tissue surrounding the pupil that varies in color. The iris opens and closes to control the amount of light entering the eye through the pupil.

The pupil is the opening in the center of the iris where light enters the eye. When looking at the eye, the pupil appears black. In dim light, the opening in the center of the iris expands to allow more light to enter the pupil. In bright light, this opening  constricts to decrease the amount of light entering the eye to protect it from damage.  

The sclera is also known as the “white of the eye”. Continuous with the cornea, the sclera forms a smooth, fibrous protective coating around the eye globe and over the optic nerve as it joins with the brain. 

The lens is the transparent, biconvex-shaped tissue that further focuses light and directs it to the retina. The lens provides about 30 percent of the eye’s total focusing power. 

The conjunctiva is the transparent mucous membrane lining the inside of the eyelids and the sclera. The conjunctiva, along with the tear film that covers it, protects the eye from foreign particles and some viruses and bacteria.

The aqueous humor is the clear, watery fluid filling the space behind the cornea and forward of the lens. It provides nutrients to the lens, cornea and iris and maintains pressure within the eye.

The retina is the tissue-thin layer of light-sensitive nerve cells lining the rear two-thirds of the eye.  The retina contains two varieties of receptor cells, called rods and cones, that convert light into electrochemical impulses to be sent to the brain. Rods are very light sensitive and aid vision in dim light and night vision, while cones provide sharp visual acuity and color perception.

The macula is the small area in the middle of the retina responsible for distinguishing well-resolved, acute details and colors. At its center is the fovea, a tiny pit containing the highest concentration of cones and provides the sharpest vision. It contains no retinal blood vessels.

The optic nerve is the conduit of nerve fibers that carries electrochemical impulses from the retina to the brain. It is the largest sensory nerve in the eye.

The vitreous humor is the clear, jelly-like substance filling the otherwise empty space between the lens and the retina.  The vitreous provides the pressure needed to keep the retina pressed against the inside wall of the eye.

The choroid is the major blood vessel layer of the eye, positioned between the sclera and retina. It supplies most of the retina’s nourishment.