The eye

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Greg Foot demonstrates how the eye works and what happens if someone is long- or short-sighted

The eye is a sense organ containing sensitive to light intensity and colour. There are a range of tissues in the eye that work together to allow it to carry out its function.

Part	Structure	Function
Cornea	Tough, transparent covering over the front part of the eye. Convex in shape.	Refracts light as it enters the eye (by a fixed amount).
Iris	Coloured part of the eye that contains muscles. These relax or contract to adjust the size of the pupil.	Controls how much light enters the pupil.
Pupil	Hole in the middle of the iris.	Allows light to pass through as it enters the eye.
Lens	Transparent, bi-convex, flexible disc behind the iris. It is attached to the ciliary muscles by the suspensory ligaments.	Refracts light to focus it onto the retina. The amount of refraction can be adjusted by altering the thickness and curvature of the lens.
Ciliary muscles	Muscles connected to the lens by suspensory ligaments.	Adjust the shape of the lens to make it more or less curved, so as to increase or decrease the refraction of light.
Suspensory ligaments	Connect the ciliary muscles to the lens and hold the lens in place.	Slacken or stretch as the ciliary muscles contract or relax, to adjust the thickness and curvature of the lens.
Retina	The lining of the back of eye containing two types of light receptor cells (rods and cones).	Contains the light receptors, which trigger electrical impulses to be sent to the brain when light is detected.

Part	Cornea
Structure	Tough, transparent covering over the front part of the eye. Convex in shape.
Function	Refracts light as it enters the eye (by a fixed amount).

Part	Iris
Structure	Coloured part of the eye that contains muscles. These relax or contract to adjust the size of the pupil.
Function	Controls how much light enters the pupil.

Part	Pupil
Structure	Hole in the middle of the iris.
Function	Allows light to pass through as it enters the eye.

Part	Lens
Structure	Transparent, bi-convex, flexible disc behind the iris. It is attached to the ciliary muscles by the suspensory ligaments.
Function	Refracts light to focus it onto the retina. The amount of refraction can be adjusted by altering the thickness and curvature of the lens.

Part	Ciliary muscles
Structure	Muscles connected to the lens by suspensory ligaments.
Function	Adjust the shape of the lens to make it more or less curved, so as to increase or decrease the refraction of light.

Part	Suspensory ligaments
Structure	Connect the ciliary muscles to the lens and hold the lens in place.
Function	Slacken or stretch as the ciliary muscles contract or relax, to adjust the thickness and curvature of the lens.

Part	Retina
Structure	The lining of the back of eye containing two types of light receptor cells (rods and cones).
Function	Contains the light receptors, which trigger electrical impulses to be sent to the brain when light is detected.

The retina

Image caption,
A photograph of a human retina seen through the eye

The pupil reflex

The amount of light entering the eye is controlled by a . The size of the pupil changes in response to bright or dim light. This is controlled by the muscles of the iris.

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