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Glossary of Terms

Aqueous Humour and the Vitreous Humour


The aqueous humour and the vitreous humour are jelly-like substances, which fill the central part of the eye.



The choroid layer is located at the back of the eye between the retina and the sclera. It contains a pigment with absorbs excess light and radiation to prevent vision becoming blurred. 

Ciliary Muscle


The ciliary muscle is a ring-shaped muscle attached to the iris. It is important because its contraction and relaxation controls the shape of the lens.



The cornea forms part of the eye’s protective outer layer. It is a transparent circular bulge at the front of the eye that refracts light onto the lens, which focuses it onto the retina. The cornea contains no blood vessels but is extremely sensitive to pain.



The iris is the visible coloured part at the front of the eye. Its function is to alter the size of the pupil, in order to control the amount of light that passes into the eye.



The lens is a transparent structure situated behind the pupil of the eye, suspended by the ciliary muscles and enclosed in a thin transparent capsule. It refracts incoming light and varies in shape to focus light onto the retina.

Optic Nerve


The optic nerve transmits all visual information in the form of electrical impulses from the retina to the brain. It is made up of approximately one million fibres, which pass on the information from the light-sensitive cells at the back of the retina.

Blind Spot


The blind spot is the point at the back of the eye where the optic nerve leaves the retina. There are no light-sensitive cells on this part of the eye, therefore any light that falls on this part of the retina is not converted into an electrical impulse, and leaves a gap in the image. Normally, your brain compensates for this gap.



The pupil is the circular opening through which light enters the eye. The iris controls widening and narrowing (dilation and constriction) of the pupil to regulate the amount of light entering the eye.



The macula is a yellow spot on the retina. It is the area with the greatest concentration of cone cells, meaning that the part of the image that falls on the macula is most accurately registered by the brain.

Rod cells


Rod cells are one of the two types of light-sensitive cells that make up the retina. There are about 125 million rods, which are necessary for seeing in dim light.

Cone cells


Cone cells are the second type of light sensitive cells found in the retina. The human retina contains 6-7 million cones; they function best in bright light and are essential both for acute vision andcolour vision. It is thought that there are three types of cones, each sensitive to the wavelength of a different primary colour, all othercolours are seen as combinations of primary colours.



The retina is the light sensitive layer at the back of the eye that images are focused onto. It contains many light-sensitive cells (rods and cones) that convert the light they detect into nerve impulses that are then sent onto the brain.



The sclera is a tough white covering around the outside of the eye. Its primary function, along with the cornea, is to protect the eye.

Zonule Fibres


The zonule fibres attach the lens to the ciliary muscles, and form part of the mechanism that alters the shape of the lens.