The human eye is a single organ made up of many interconnected parts that work together to transmit visual information to the visual cortex of the brain. Each part plays an important role in visual function. Healthy eyes are instrumental in navigating the world around us, and can also provide clues to your health in general. An eye exam screens for vision problems and assesses eye health, but the doctor will also look for symptoms of glaucoma, diabetes, and high blood pressure.
Parts of the Human Eye
The outer covering of the eye is often referred to as the white of the eye. Its technical name is the sclera, and it is the large, round mass that gives the eye shape and protects the fluid within. At the front of the sclera is a transparent mound called the cornea. It protects the iris and pupil while allowing light into the eye. The total weight of these two pieces is only about 7.5 grams. The anterior to posterior diameter measures 24mm. While some people appear to have very large eyes and others relatively small ones, in reality human eyes are amazingly uniform, varying only slightly from one person to another. The eyelids, bone structure and placement of the globes within the socket can all contribute to the illusion of a difference in size.
The majority of the eye’s interior is made up of the aqueous humour, a clear fluid, and the vitreous body, a clear jelly. Together they nourish the corneal lens and maintain the pressure necessary to give the eye its structure. The lens of the eye sits in this fluid and functions much like the lens of a camera, changing form and shape to focus light onto the retina. Light passes through the pupil, the black space in the middle of the eye that can contract and expand to control the amount of light that reaches the retina. Three layers of coating surround this mass:
- Fibrous Tunic – The outermost layer of the eye, the fibrous tunic includes both the cornea and the sclera.
- Uvea – The middle layer of the eye includes the choroid, layers of blood vessels that nourish the back of the eye, and the iris, the colored ring that surrounds the pupil and is the eye’s most striking feature.
- Retina – This light sensitive layer is the innermost coating. It is the receptor for light images, translating them into an electromagnetic signal to be processed in the brain.
The entire unit of the eye is called the globe, and it is housed in a cavity in the skull called the orbit. The eye itself is very sensitive, and relies on a number of other body parts to protect it from debris and intense light, including the eyebrows, eyelids and eyelashes.
How the Eye Works
Accurate vision relies on the intricate coordination of every part of the eye. Light enters through the cornea and into the pupil, which changes in diameter depending on the light. In darkness the pupil opens wide to draw in as much light as possible. In bright sunlight it contracts to restrict the light that comes in. The lens focuses the image by changing its form and shape. Light rays imprint on the retina, converting the image to electrical signals for the brain to read.
The information gathered by the eye is processed in the visual cortex. Like a fine watch, the parts of the eye are meant to work harmoniously with one another. However, each piece is delicate and vulnerable, and a problem in one part can threaten the functionality and health of the whole. Modern diagnostic equipment increases an Optometrists ability to detect aberrations and can prevent a condition from becoming a threat.
