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The human eye
The pupil is the central transparent area (showing as black). The greenish-yellow area surrounding it is the iris. The white outer area is the sclera, the central transparent part of which is the cornea.
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 aquaticAquatic (from the Latin aqua meaning water in English) is an adjective denoting a direct connection with, dependence upon or consistency of water. The term is most often applied to aquatic plants or aquatic animals. Terrestrial, meaning ground-dwelling is species. The reasons for the variation in shapes are complexA complex is a whole that comprehends a number of parts, especially one with interconnected or mutually related parts. In psychology, a complex is a group of mental factors that are unconsciously associated by the individual with a particular subject and; the shape is closely related to the optical characteristics of the lens, the shape and sensitivity of the retinaeye cross-sectional view. Courtesy NIH National Eye Institute. Many animals have eyes different from the human eye. The retina is a thin layer of cells at the back of the eyeball of vertebrates and some cephalopods; it is the part of the eye which transdu, 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 photographA photograph (often just called a photo is an image (or a representation of that on e. paper) created by collecting and focusing electromagnetic radiation. The most common photographs are those created of visible wavelengths, producing permanent records oed with a flashA photographic flash is a device that produces a flash of light required for indoor or other low light conditions. The earliest flashes consisted of a wad of magnesium powder that was ignited by hand. Later, magnesium filaments were contained in flash bul, the iris cannot close the pupil fast enough and the blood-rich retinaeye cross-sectional view. Courtesy NIH National Eye Institute. Many animals have eyes different from the human eye. The retina is a thin layer of cells at the back of the eyeball of vertebrates and some cephalopods; it is the part of the eye which transdu is illuminated, resulting in the red-eye effect.
The human pupil dilates if the person sees something (or someone) of interest or is aroused. 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.
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