Pupil
- The term pupil can also mean student.
In the eye, the pupil is the hole in the middle of the iris. It appears black because most of the light entering it is absorbed by the tissues inside the eye. In humans and many animals (but few fish), the size of the pupil is controlled by involuntary contraction and dilation of the iris, in order to regulate the intensity of light entering the eye. This is known as the pupillary reflex.
The shape of the pupil varies between species. Common shapes are circular or slit-shaped, although more convoluted shapes can be found in aquatic species. The reasons for the variation in shapes are complex; the shape is closely related to the optical characteristics of the lens, the shape and sensitivity of the retina, and the visual requirements of the species.
Slit-shaped pupils are found in species which are active in a wide range of light levels. In strong light, the pupil constricts and is small, but still allows light to be cast over a large part of the retina.
The orientation of the slit may be related to the direction of motions the eye is required to notice most sensitively (so a vertical pupil would increase the sensitivity of the eyes of a small cat to the horizontal scurrying of mice).
In snakes, slit-shaped pupils are associated with venomous species, while non-venomous snakes have round pupils.
When an eye is photographed with a flash, the iris cannot close the pupil fast enough and the blood-rich retina is illuminated, resulting in the red-eye effect.
The human pupil widens slightly if the person sees something of interest. Studies have shown that humans (especially females) are judged as more attractive if their pupils are wide open and more dilated than normal. The name Belladonna (beautiful lady) comes from the fabled use of the juices of the Nightshade plant by Italian women who would use eyedrops made from an extract of the plant in their eyes in order to enlarge their pupils and make their eyes appear more beautiful. In some cases gradual blindness has occurred through the overuse and abuse of belladonna to enhance the beauty of the eyes. Atropine, a chemical found in belladonna, is used by modern eye doctors to dilate the pupils so that they can examine the retina.
Our understanding of how the eye works is still developing.
See Also
Sensory system - Visual system - Eye
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Retina -
Cornea -
Iris -
Pupil -
Lens -
Macula -
Sclera -
Optic fovea -
Blind spot -
Vitreous humour -
Aqueous humour -
Choroid -
Ciliary body -
Conjunctiva -
Angle structure -
Tapetum lucidum
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