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Internal Parts and Functions of the Eye

Updated on February 1, 2013
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Parts and Functions


Sclera

The sclera is the outermost layer of the eye, and is commonly known as being ‘the white part of the eye’. It is basically the tough, opaque and leathery tissues on the exterior of the eye that serve as a protective coat to the eye. It extends from the cornea to the optic nerve at the rear of the eye. Six minuscule muscles connect to the sclera around the eye, controlling its movements. Whenever we look up, down, left or right, it is actually the sclera moving. The sclera enables us to look at different places whilst keeping our head in the same position. The sclera also holds the internal organs of the eye in place, so that they can actually function correctly. They eye is also full of liquid, which is under pressure. The sclera contains this pressure and simultaneously keeps the iris, the pupil, the retina and the other parts of the eye in position so that the correct focal length is always maintained. The sclera is perhaps the most important part of the eye as without it, we wouldn’t be able to see.

Choroid

The choroid is the middle layer of the eye, it is behind the sclera. The choroid is responsible for absorbing light and preventing light reflection within the eye. It also supplies nutrients to the different organs within the eye. The choroid contains a substance known as melanin; this is the same substance that gives the colour to our skin and hair. In the eye, the melanin colours the choroid black and helps to trap stray light. Without melanin, the images we see would be faded and the lighting would be different. The choroid also contains the Ciliary body, which is a muscle that alters the shape of the eye lens so we can focus correctly.

Cornea

The cornea is the transparent outer surface of the eye. It allows light to enter the eye and performs a large majority of the focusing tasks for the eye. The cornea is often compared to a camera lens, but this is not entirely accurate. The cornea’s convex shape allows it to focus light towards the pupil and the lens. The cornea provides us with vision, but the pupil and lens intensify the image we see. The inner layers of the cornea provide the eye protection from infection and also give the eye some rigidity.

Ciliary Muscle

The Ciliary muscles are ‘sphincter’ muscles which control the thickness of the eye lens. Whenever the Ciliary muscle moves, the lens moves, this could be forwards, backwards or sideways. Each eye has a Ciliary muscle around the lens, when the Ciliary muscles contracts, the lens compresses shortening the focal distance. The same works in reverse. As objects get closer or move further away, the Ciliary muscles contract or expand providing us with a clear image of objects from varied distances.

Optic Nerve

The optic nerve is one of the most important parts of the eye as it transmits what we see with our eyes to other parts of the body so that this information can be processed. It is responsible for sending the image we see in our eyes to the appropriate place for visual processing, which is most commonly the brain.

Blind Spot

The human eye, as perfect as it seems, does have its defects. The blind spot doesn’t actually have a set function; it’s more like one of the eyes side effects. It is connects the optic nerve from the eye to the brain and allows messages from the optic nerve to be transmitted to the brain. The blind-spot is not capable of sensing light but this is not a problem as the brain puts the image from each eye together.

Fovea

The fovea is the central part of the macula that is essential in providing us with a sharp vision which is necessary for reading, driving, watching Television and any other tasks that require high visual detail. The fovea is only made up of cones; it is the absolute centre of vision in the human eye. The light that lands on your macula and fovea form the central vision, so when reading this, the words are landing on your fovea, all this information is then sent to the optic nerve.

Lens

The lens of the eye helps to focus the image we see by bending the light to strike the retina correctly, this is known as refraction. The lens is positioned behind the outer cornea and the coloured iris. The Ciliary muscles open and close the lens. The lens focuses light, which bends when it enters the lens material. Without the lens, the image we saw would have inaccurate lighting.

Iris

The iris is responsible for controlling the size of the pupil which controls the amount of light entering the eye. So, in effect, the iris is also responsible for the amount of light in the eye. The iris is situated between the cornea and crystalline lens. The iris the coloured part of the eye, the colour varies on the amount of pigment in the iris. It can act as a protective measure by ensuring the eye does not get damaged by an excess amount of light.

Pupil

The pupil is the sphere located in the middle of the iris. It controls the amount and brightness of light entering the eye and passing through to the retina. When you are in bright light, your pupil will become smaller and vice versa. The pupil can be thought of as getting wider whenever it needs light.

Retina

The retina is a light-sensitive area that receives the image seen through the eye, only upside down. It converts this image to into nerve signals and then transmits this image to your brain via the optical nerve which inverts the image. The retina does the basics of the ‘optical processing’ such as edge enhancement and colour separation.


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