Pupil
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225702793
2008-07-15T00:00:33Z
Jwinius
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/* Comparative anatomy */ Edit to boa wlink.
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[[Image:030608 Pupil.jpg|thumb|159px|'''The human eye'''<br>The pupil is the central transparent area (showing as black). The greenish-brown area surrounding it is the [[iris (anatomy)|iris]]. The white outer area is the [[sclera]], the central transparent part of which is the [[cornea]].'']]
[[Image:Schematic diagram of the human eye en.svg|thumb|200px|Schematic diagram of the human eye.]]
The pupil is the hole that is located in the center of the [[Iris (anatomy)|iris]] of the eye and that controls the amount of light that enters the [[eye]].<ref name="Cassin">Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainsville, Florida: Triad Publishing Company, 1990.</ref> It appears black because most of the light entering the pupil is absorbed by the [[biological tissue|tissues]] inside the eye. In optical terms, the anatomical pupil is the eye's [[aperture]] and the iris is the [[aperture stop]]. The image of the pupil as seen from outside the eye is the [[entrance pupil]], which does not exactly correspond to the location and size of the physical pupil because it is magnified by the [[cornea]].
==Comparative anatomy==
In [[humans]] and many [[animal]]s (but few [[fish]]), the size of the pupil is controlled by involuntary constriction and dilation of the [[iris (anatomy)|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#Maximum and minimum apertures|aperture]] for a more detailed explanation)
The shape of the pupil varies between [[species]]. Common shapes are [[circular]] or slit-shaped, although more convoluted shapes can be found in [[aquatic animal|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.<ref>[http://www.madsci.org/posts/archives/mar98/889495842.Gb.r.html Re: Why do sheep and goats have horizontally positioned pupils?<!-- Bot generated title -->]</ref> Animals like goats and sheep may have evolved horizontal pupils because better vision in the vertical plane may be beneficial in mountainous environments.<ref>[http://www.madsci.org/posts/archives/mar98/889495842.Gb.r.html Re: Why do sheep and goats have horizontally positioned pupils?<!-- Bot generated title -->]</ref>
Many [[snake]]s, such as [[Boidae|boa]]s, [[Pythonidae|python]]s and [[viperidae|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.<ref>[http://www.madsci.org/posts/archives/dec96/840904233.Zo.r.html Re: The shape of the eye pupil<!-- Bot generated title -->]</ref>
When an eye is [[photograph]]ed with a [[Flash (photo)|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==
{{contradict-other|miosis}}
[[Image:The number of the beast.jpg|thumb|The pupils of [[sheep]] [[goat]]s and [[octopus|octopi]] are horizontal, almost rectangular, ovals.]]
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 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 the sphincter pupillae contract, the iris decreases or constricts the size of the pupil. The dilator pupillae, innervated by sympathetic nerves from the superior cervical ganglion, cause the iris to dilate when they contract. These muscles are sometimes referred to as intrinsic eye muscles.
Certain drugs cause constriction of the pupils, such as [[alcohol]] and [[opiates]]. Other drugs, such as [[atropine]] and [[amphetamines]] cause pupil dilation.
Another term for the constriction of the pupil is [[miosis]]. Substances that cause miosis are described as [[miotic]].
==See also==
*[[Eye examination#Pupil function|Pupil function (Eye examination)]]
*[[Dilated fundus examination]]
*[[Eye contact]]
*[[Horner's syndrome]]
*[[Mydriasis]]
*[[Synechia]]
*[[Anisocoria]]
*[[Adie's pupil]]
*[[Argyll Robertson pupil]]
*[[Marcus Gunn pupil]]
*[[Light Near dissociation]]
==Additional images==
<gallery>
Image:Gray878.png|Iris, front view.
</gallery>
==References==
{{reflist}}
1
==External links==
* {{UMichAtlas|eye_1}} - "Sagittal Section Through the Eyeball"
* {{UMichAtlas|eye_2}} - "Sagittal Section Through the Eyeball"
*[http://cim.ucdavis.edu/EyeRelease/Interface/TopFrame.htm A pupil examination simulator], demonstrating the changes in pupil reactions for various nerve lesions.
{{Eye}}
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