Patient Education

Eye Anatomy

The human eye is the organ which gives us the sense of sight, allowing us to learn more about the surrounding world than any of the other five senses. We use our eyes in almost everything we do, whether reading, working, watching television, writing a letter, driving a car, and countless other activities. Sight is the most precious of the five senses, and many people fear blindness more than any other disability. The eye allows us to see and interpret the shapes, colors, and dimensions of objects in the world by processing the light they reflect or give off.
When you look at an object, light rays are reflected from the object to the cornea, which is where the miracle begins. The light rays are bent, refracted and focused by the cornea, lens, and vitreous. The lens job is to make sure the rays come to a sharp focus on the retina. The resulting image on the retina is upsidedown. Here at the retina, the light rays are converted to electrical impulses which are then transmitted through the optic nerve, to the brain, where the image is translated and perceived in an upright position!

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Think of the eye as a camera. A camera needs a lens and a film to produce an image. In the same way, the eyeball needs a lens (cornea, crystalline lens, vitreous) to refract, or focus the light and a film (retina) on which to focus the rays. If any one or more of these components is not functioning correctly, the result is a poor picture. The retina represents the film in our camera. It captures the image and sends it to the brain to be developed. The macula is the highly sensitive area of the retina. The macula is responsible for our critical focusing vision. It is the part of the retina most used. We use our macula to read or to stare intently at an object.

The eye changes light rays into electrical signals then sends them to the brain, which interprets these electrical signals as visual images. The eyeball is set in a protective coneshaped cavity in the skull called the orbit or socket and measures approximately one inch in diameter. The orbit is surrounded by layers of soft, fatty tissue which protect the eye and enable it to turn easily. Six muscles regulate the motion of the eye. Among the more important parts of the human eye are the iris, cornea, lens, retina, conjunctiva, the macula, and the optic nerve.

The wall of the eye. These include: (1) the sclera and cornea, (2) the uveal tract, and (3) the retina. The sclera and cornea consist of tough tissues that make up the outer layer of the eyeball and give it strength as well as protect it. The sclera covers about 85 percent of the eyeball and the cornea the remaining 15 percent. The sclera is the white part of the eye and has the strength and feel of soft leather. Although the sclera appears to have many blood vessels on its surface, most of these vessels are part of the conjunctiva. In contrast, the cornea contains no blood vessels and is relatively dehydrated; as a result, it is transparent. The cornea lies in front of the colored part of the eye and resembles the crystal of a wrist watch. The cornea allows light rays to enter the eyeball.

The lens lies directly behind the iris and is connected by strong, microscopic fibers to the ciliary body, which encircles the iris. The lens is a flexible structure about the size and shape of an aspirin tablet. Like the cornea, the lens is transparent because it contains no blood vessels and is relatively dehydrated. The muscles of the ciliary body make constant adjustments in the shape of the lens. The adjustments produce a sharp visual image at all times as the eye shifts focus between nearby and distant objects. Both the cornea and lens are kept nourished and lubricated by a clear, watery fluid produced continuously by the ciliary body called aqueous humor. Aqueous humor fills the area between the lens and cornea. Fluid that has already nourished both these parts of the eye flows into a drainage system at a spongy, circular groove where the cornea and the sclera meet. It then travels through the veins of the eyeball into the veins of the neck.

The iris is the colored disk that lies behind the cornea and is what people refer to when they speak of the color of their eyes. Melanin, a dark brownishblack substance inside the cornea, determines eye color: the more melanin there is and the closer it is to the surface of the tissue, the darker the color of the iris. For example, there is more melanin in brown eyes-and it lies closer to the tissue surface-than in blue eyes. In addition to giving the iris color, melanin absorbs strong or bright light that might otherwise be too overwhelming or cause blurred vision. Melanin is the same substance that give skin and hair their color. People called albinos have little or no melanin and appear to have milkywhite skin, white hair, and pinkishgray irises. Quite often they have very poor vision and their eyes are extremely sensitive to light.

The conjunctiva is a membrane that lines the inside of the eyelids and extends over the front of the white part of the eye. It produces mucus, a clear viscous fluid that lubricates the eyeball, as well as some tears, which help keep the eye clean. Most tears, however, are produced by the lacrimal glands. A lacrimal gland lies at the upper outer corner of each orbit and spreads a smooth layer of mucus and tears over the eye each time a person blinks. After a person blinks, the fluid spreads evenly over the eye then flows into tiny canals in the lids. These canals lead to the lacrimal sac, a pouch at the lower inner corner of each orbit. From the lacrimal sac, the mucus and tears drain through a passage into the nose.

The optic nerve is formed by rods and cones joined by nerve fibers at the center of the retina. This nerve consists of about a million fibers and serves as a flexible cable that connects the eyeball to the brain. The optic nerve carries the electrical signals produced in the retina to the brain, which interprets them as visual images. The point at which the optic nerve enters the eye is known as the blind spot. It has no rods or cones and as a consequence cannot respond to light. Normally, a person does not notice the blind spot because it covers such a small area and the eye makes so many rapid movements.

The retina makes up the innermost layer of the wall of the eyeball. It is only as strong as a wet piece of tissue paper. Lightsensitive cells in the retina absorb light rays and change them into electrical signals which are passed to the brain and interpreted as visual images. There are two types of these lightsensitive cells-rods and cones. The retina has about 120 million rods and about 6 million cones. Bits of pigment or colored material fill the rods and cones and absorb even the minutest particles of light that strike the retina. The pigment in the rods is called rhodopsin or visual purple, and enables the eye to discern shades of gray and see in dim light. There are three types of pigment in the cones that enable the eye to see colors and to see sharp images in bright light. Cyanolabe absorbs blue light. Chlorabe absorbs green light. Erythrolabe absorbs red light. These pigments enable us to see and distinguish more than 200 colors.

The macula is a round area that lies near the center of the retina. The macula consists primarily of cones and produces a sharp image of scenes at which the eyes are directly aimed, especially in bright light. The rest of the retina provides peripheral vision, which enables the eyes to see objects off to the side while looking straight ahead. Most of the rods lie in this part of the retina. Because rods are more sensitive in the dark than cones, faint objects can often be seen more clearly if the eyes are not aimed directly at them. For example, looking to the side of a dim star makes its image fall on the part of the retina that has the most rods and provides the best vision in dim light. In contrast, the cones in the macula often give a sharper image of an object in bright light if the eyes are aimed directly at it.

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