The eyes

The wall of the eye is the outer part of the eyeball that surrounds the chambers inside the eye. Itʼs made up of 3 layers (called tunics).

• The sclera and cornea make up the outermost layer.
• The uvea is the middle layer.
• The retina is the innermost layer.

The sclera is the white part of the eye. It covers most of the outside of the eyeball. The sclera is made up of tough connective tissue and acts as a protective coating for the eye.

The cornea is the clear, dome-shaped part at the front of the eye. It covers the pupil (the black, circular opening in the centre of the eye) and the iris (the coloured circle around the pupil). The cornea helps to direct and bend (refract) the light coming into the eye before it reaches the lens. The lens and cornea work together to allow you to see.

The uvea is made up of 3 parts: the iris, the choroid and the ciliary body.

The uvea has many blood and lymph vessels that help bring nutrients to the eye. The muscles in the uvea help control how much light enters the eye before it reaches the lens. They also help direct the light before it reaches the retina.

The iris is the coloured circle that surrounds the pupil (the black dot in the middle of the eye). The iris is made of connective tissue and muscle. The muscles of the iris change the size of the pupil to control how much light enters the eye.

The colour of your iris comes from the amount of melanin it contains. The more melanin in your iris, the darker it is.

The choroid is a thin layer of tissue between the sclera and retina. It contains many tiny blood vessels that bring oxygen and nutrients to the retina. The choroid also contains melanin that absorbs extra light entering the eye. This prevents reflections inside the eye that can impair vision.

The ciliary body is a ring of muscle that extends out from the choroid at the point where the choroid meets the retina. It helps hold the lens in place.

The ciliary body is divided into 2 parts: the ciliary muscle and the ciliary process.

The ciliary muscle changes the shape of the lens to focus on objects near or far.

The ciliary process makes and secretes aqueous humour.

The retina is the innermost part of the wall of the eye. It’s made up of many layers of nervous system cells, including neurons and light-sensitive cells called photoreceptors. There are 2 types of photoreceptors: rods and cones.

When we look at something, light enters the eye, focuses on the retina and activates the rods and cones. They send signals to the layers of neurons below them, which pass the signals along the optic nerve to the brain. The brain then uses the signals to create an image of what you are looking at.

The lens is a transparent disc-shaped structure inside the eye. It’s held in place behind the iris by the ciliary body. The shape of the lens changes to allow the eye to focus on objects near and far. Before light reaches the retina, it first passes through the lens.

The adnexal structures of the eye include the eyelid, conjunctiva and tear (lacrimal) gland. These structures protect, lubricate and support the eyeball. The adnexal structures of the eye are also called accessory structures of the eye.

The eyelids are folds of skin above and below each eye. They cover and protect the eye. Muscles around each eye control how the eyelids open and close.

The sweeping motion of the eyelid across the eyeball when it opens and closes helps remove any particles that have gotten into the eye. When we open and close our eyes, it also helps keep the eyes moist because the motion spreads tears across the exposed parts of the eyeball. The eyelids also have glands that produce an oily layer on the surface of the eyeball to stop the layer of tears from evaporating and the eye drying.

Eyelashes grow out of the edge of the eyelids and help to keep sweat, dust and other particles from getting in the eyes.

The conjunctiva is a clear mucous membrane that covers the inner surface of the eyelid and the outer surface of the sclera. The conjunctiva protects the eye, and the mucus it secretes lubricates the eye and helps keep it moist.

The tear gland (also called the lacrimal gland) is at the upper, outer corner of the eye. It secretes tears. When we blink, tears are swept across the surface of the eye. Tears lubricate the conjunctiva covering the surface of the eye and inner eyelid. Tears also remove dust and other particles from the eye and help to prevent infection.

Tears drain from the eye through very small openings in the inner corners of the upper and lower eyelids. They then travel down through the tear ducts and out the nose.

The eye socket, or orbit, is the bowl-shaped structure in the skull where the eyeball sits. It also holds the surrounding muscles, nerves, fat, blood vessels, connective tissues and tear gland.

The eye socket is made up of the bone, muscle and fat around the eyeball. The bone and other tissues of the eye socket help to cushion and protect the eyeball.

The eye muscles in the eye socket attach to the bones on 1 end and to the sclera at the other. These muscles control the movement of the eyeball, allowing you to look around.

The eye and the brain work together to allow us to see. How they do this is very complex and depends on many parts and processes. One of those parts, the eyeʼs retina, uses its photoreceptor cells (called rods and cones) to process light, collecting visual information about what we’re looking at. What the light illuminates for rods and cones is then converted to a form of information we can use (signals). These signals are sent to the neuron cells, also in the retina, before travelling to the brain along the optic nerve. The brain then processes the information in these signals and creates the image that we see.

The eyes work together to allow us to see in 3D (called binocular vision). Binocular vision gives us the ability to determine depth and speed. If the vision is lost in 1 eye because the eyeball is removed or because a part of the eye is severely damaged, the other eye allows you to see most of what you could see before (though this depends on the vision in your remaining eye). But you will be less able to judge depth and speed.