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The pupil (Latin pupilla "little doll" > pupa "doll") is the variable-sized, black circular or slit shaped opening in the center of the iris that regulates the amount of light that enters the eye.[1] It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye.


Comparative anatomy

In humans and many animals (but few fish), the size of the pupil is controlled by involuntary constriction and dilation of the iris in order to regulate the intensity of light entering the eye. This is known as the pupillary reflex. In normal room light, a healthy human pupil has a diameter of about 3-4 millimeters, in bright light, the pupil has a diameter of about 1.5 millimeters, and in dim light the diameter is enlarged to about 8 millimeters. The narrowing of the pupil results in a greater focal range. (see aperture for a more detailed explanation)

The shape of the pupil varies betweens 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). The narrower the pupil, the more accurate the depth perception of peripheral vision is, so narrowing it in one direction would increase depth perception in that plane.[2] Animals like goats and sheep may have evolved horizontal pupils because better vision in the vertical plane may be benficial in mountainous environments.[3]

Many snakes, such as boas, pythons and vipers, have vertical, slit-shaped pupils that help them to hunt prey under a wide range of light conditions. Small cats and foxes also have slit shaped pupils while lions and wolves have round pupils even though they are in the same respective families. Some hypothesize that this is because slit pupils are more beneficial for animals that hunt small prey rather than large prey.[4]

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.

Constriction of the pupil


When bright light is shone on the eye, it will automatically constrict. This is the pupillary reflex, which is an important test of brainstem function. Furthermore, the pupil will dilate if a person sees an object of interest.

The oculomotor nerve, specifically the parasympathetic part coming from the Edinger-Westphal nucleus, terminates on the circular iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil.

The Iris is a contractile structure, consisting mainly of smooth muscle, surrounding an opening called the pupil. Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil. The iris contains two groups of smooth muscles; a circular group called the sphincter pupillae, and a radial group called the dilator pupillae. When they contract, the iris decreases or constricts the size of the pupil. These muscles are sometimes referred to as intrinsic eye muscles.

Certain drugs cause constriction of the pupils, such as alcohol. Other drugs, such as psychedelics, cause pupil dilation.

Another term for the constriction of the pupil is miosis. Substances that cause miosis are described as miotic. The pupil opens wide when dark and small when light.


Emotion is well known to effect the size of pupils, some animals (in individuals just as much as in species) more than others. One well known example is that in domestic cats - when they become excited by prey or by playing, their eyes dilate, sometimes to extent that their condition resemblesMydriasis, when they are actually fine. Cats are only one of the many examples.

In most cases, the irises in an animal will contract(constrict) and relax (dilate) in situations which the rest of their muscles contract and relax, such as fear, sadness, pain, on the other side there's happiness,sleepiness, relaxation ect.

Bear in mind there are numberous exceptions to the rule, some being certain animals and some being certain emotions, sometimes both.

In humans the difference that moods cause is usually slight (depending on individual), and not obvious unless the one in question is experiencing an emotion on a high scale.

See also

Additional images


  1. ^ Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990.
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  3. ^
  4. ^
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pupil". A list of authors is available in Wikipedia.
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