Rods & Cones

The retina is made up of special cells called rods and cones to process light. How many rods and cones does your retina have? How about 120 million rods and 7 million cones…in each eye! Crazy, eh?!

Rods see in black, white, and shades of grey. They tell us the form or shape of an object and can also sense when an object is moving. Rods can’t tell the difference between colours but, they are super-sensitive, allowing us to see when it’s very dark.

Cones sense colour and they need more light than rods in order to work well. Cones are most helpful in normal or bright light. The retina has three types of cones; each one is sensitive to one of three colours – red, green, or blue—to help you see different ranges of colour. Together, these cones sense combinations of light waves so that our eyes are able to see millions of colours.

Rods and cones work in sync to piece together the total picture. These retinal cells let you see that your friend has brown skin, is wearing a blue hat, and throws a white baseball.

Sometimes, the shape of the eyeball makes it difficult for the cornea, lens, and retina to work perfectly as a team. When this happens, images will be out of focus. To correct this fuzzy vision, many people, including many kids, wear glasses. Glasses help the eyes focus images correctly on the retina and allow someone to see clearly.

To the brain!

The optic nerve is the messenger of the eye. The rods and cones of the retina gather the light information and change it into millions of nerve messages. The optic nerve then carries those messages from the eye to the brain. Think of the optic nerve as a high-speed telephone line connecting the eye to the brain. When you see an image, your eye “telephones” your brain with a report on what you are seeing so that the brain can translate that report into “cat”, “apple”, or “bicycle”, or whatever the object may be.

For crying out loud!

Above the outer corner of each eye lies the lacrimal (lak-ruh-mul) glands, which make tears. Every time you blink, tear fluid comes out of your upper eyelid. The tears help to wash away germs, dust, or other particles that don’t belong in your eye. Tears also help your eye from drying out. The tears drain out of your eye through the lacrimal duct (also called the tear duct). You can see the tear duct opening if you pull down the inside corner of your eye – when you see the tiny little hole, you’ve found the tear duct!

Your eyes sometimes make more tear fluid than normal. This may have happened to you if you’ve been poked in the eye, if you’ve been in a dusty/smoky area, if you’ve been near someone who is cutting onions, or if you felt sad or upset.

As you can see, your eyes are responsible for amazing things! Treat your eyes right by having regular eye check-ups with your optometrist.

The Eye

The eye is about the size of a golf ball and it sits in a little hollow area (the eye socket) in the skull. The eyelids protect the front part of the eye with great reflexes. These reflexes allow your eyes to automatically shut to protect the eye from possible danger. The lids also keep the eye moist and healthy by opening and shutting several times a minute. This is called blinking; an action that is both voluntary and involuntary, meaning you can blink whenever you want but, it also happens without you even thinking about it.

The white part of the eye is called the sclera (sklair-uh). The sclera is made of a super-tough material. Its job is to protect the inner delicate structures of the eye and to maintain the round shape of the eyeball. If you look very closely at the white of the eye, you will see tiny red threads. These are blood vessels that bring nutrients to the outer parts of the eye.

The cornea (kor-nee-uh) is a transparent dome at the very front of the eye. The cornea helps to focus light making its way through the eye. It is a very important structure yet you can hardly see it because it is made of totally clear tissue. Like clear glass, the cornea gives your eye a clear window through which you can view the world around you.

Behind the cornea are the anterior chamber, the iris, and the pupil. The anterior (an-teer-ee-ur) chamber is the space between the cornea and the iris. This space is filled with a special clear fluid that nourishes the eye and keeps it healthy. The iris (eye-riss) is the colourful part of the eye. When we say a person has blue eyes, we really mean the person has blue irises. The iris is made up of muscles that change its size. This allows the iris to control how much light goes through the pupil. The pupil (pyoo-pul) is the black circle in the centre of the iris, which is really an opening in the iris. The pupil allows light to enter the eye. Try this yourself: Use a small flashlight to see how your friend’s eyes react to changes in brightness. The pupils will get smaller when the light shines near them and they will open wider when the light moves away.

Lights, Camera, Action!

Important structures also lie behind the iris. You can’t see these parts with your own eye but, your eye doctor uses a special microscope to see the inner parts of the eyes. After light enters the pupil, it hits the lens. The lens sits right behind the iris and is clear and colourless. The lens’ job is to focus light rays on the back of the eyeball, specifically the retina (ret-i-nuh). The lens works much like the lens of a movie projector at the movies. Next time you sit in the dark theater, look behind you at the stream of light coming from the projection booth. This light goes through a powerful lens, which focuses the image onto the screen so that you can see the movie clearly. In the eye, the film screen is your retina.

Your retina is at the very back of the eye. It is a delicate tissue made up of millions of cells that are sensitive to light. The cells of the retina change the light energy into a nerve signal so that the brain can understand what the eye is seeing.

The lens is suspended in the eye by a group of fibres. These fibres are attached to a muscle called the ciliary (sil-ee-air-ee) muscle. The ciliary muscle changes the shape of the lens to focus the light correctly on the retina. When you look at things up-close, the lens becomes thicker; when you look at things far away, the lens becomes thinner.

The largest chamber of the eye sits behind the lens and it is called the vitreous (vih-tree-us) body. The vitreous body forms two-thirds of the eye’s volume and it gives the eyeball its shape. It’s filled with a clear jelly-like material called the vitreous humor. After light passes through the lens, it shines straight through the vitreous humor to the back of the eye.