A quick look at the eyeball

Have you ever wondered about the structures that make up the eyeball and allow us to see? Let’s start at the front of the eye and work our way back.

The Cornea: 

The clear layer covering the front of the eye that lets the light get through. It has 5 layers, which can be susceptible to disease. An example is Keratoconus, when the cornea becomes “cone-shaped”. Contact lenses are placed over the cornea to help with guiding light towards the retina to allow you to see. 

The Iris

The colourful, circular muscle that expands and contracts to control the amount of light that gets in. The iris has two layers – at the front, some fibrous cells collectively called the stroma, and at the back, pigmented cells. The pigment is called “melanin” which makes the iris brown and opaque, allowing it to control the amount of light passing through. This is the same pigment found in our skin, and the amount that each person has varies according to their genetics. 

People with blue eyes have no pigment at all in this front layer – this causes the fibres in the iris to scatter and absorb some of the longer wavelengths of light (i.e. red, yellow, green) that come in. More blue light is reflected back out and the eyes appear to be blue. For people with hazel or green eyes, at least one of the layers of the iris contains light brown pigment. The light brown pigment interacts with the blue light and the eye can look green or speckled. 

The Lens:

The transparent disc that changes its shape to focus on objects at different distances. It can be thought of as a magnifying glass that bends and adjusts the trajectory of light coming into the eye so that it can hit the retina and be converted to an image. Cataracts happen when the lens loses its transparency, preventing light from entering the eye. Cataract surgery involves replacing the cloudy part of the lens with a new, transparent lens. 

The Retina: 

The thin layer of tissue at the back of the eye where the photoreceptors (light-sensitive cells called rods and cones) are. It receives adjusted light from the lens. The retina has lots of blood vessels and connections with the brain through the optic nerve. It is important to keep your blood pressure and blood sugar levels controlled so that the blood vessels inside the retina do not get damaged to make you lose your vision. 

The Optic Nerve: 

This collects visual information from the retina and rapidly transmits it to the brain for processing. Each of your eyes transfers information to your brain at about the same speed as a fast ethernet connection cable. 

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