the eye


The Pupil and The Iris


The eye in structure is only approximately spherical, and is layered. Outermost layers consist of the cornea and sclera. Contained within these layers are the iris, ciliary and choroid. Within again is the retina. When light reaches the retina it precipitates chemical changes and electrical activity, stimulating nerve impulses to travel along the optic nerve located at the back of the eye to specialised centres of the brain. In order to reach the retina, light enters the eye through the pupil. The pupil appears black and typically will measure, dependent upon the volume of light striking it, between 3 and 9 millimetres. The iris encircles the pupil and controls its aperture, effectively the amount of light to flow through it. The iris, which is coloured, increases the pupil’s aperture in darkness and contracts it in response to higher ambient emissions of light. When you ask someone what they see, none of this is apparent.