Editing Pupil (section)

==Function==
{{See also|Iris (anatomy)}}
The [[iris (anatomy)|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. This is known as the [[pupillary light reflex]].

The iris contains two groups of smooth muscles; a circular group called the [[Iris sphincter muscle|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 pupil to dilate when they contract. These muscles are sometimes referred to as intrinsic eye muscles.

The sensory pathway (rod or cone, bipolar, ganglion) is linked with its counterpart in the other eye by a partial crossover of each eye's fibers.  This causes the effect in one eye to carry over to the other.

===Effect of light===
[[File:Pupillary light reflex.jpg|thumb|300x300px|The pupil diameter can vary greatly due to various factors (primarily the [[pupillary light reflex]]), from constriction to as small as 2 mm, to dilation larger than 8 mm in some individuals, though the maximal dilation also varies substantially by individual and decreases with age]]
The pupil gets wider in the dark and narrower in light.  When narrow, the diameter be 1.5 to 4 millimeters.<ref name=":1" />  In the dark it will be the same at first, but will approach the maximum distance for a wide pupil 3 to 8&nbsp;mm.<ref name=":1" /> However, in any human age group there is considerable variation in maximal pupil size. For example, at the peak age of 15, the dark-adapted pupil can vary from 4&nbsp;mm to 9&nbsp;mm with different individuals. After 25 years of age, the average pupil size decreases, though not at a steady rate.<ref>{{cite web |url=http://amateurastronomy.org/Events/EH361.html |title=Aging Eyes and Pupil Size |publisher=Amateurastronomy.org |access-date=2013-08-28 |url-status=dead |archive-url=https://web.archive.org/web/20131023134125/http://amateurastronomy.org/Events/EH361.html |archive-date=2013-10-23 }}</ref><ref>{{cite journal|url=http://www.iovs.org/content/35/3/1132.full.pdf |title=Factors Affecting Light-Adapted Pupil Size in Normal Human Subjects |journal=Investigative Ophthalmology & Visual Science |date=March 1994 |volume=35 |issue=3 |pages=1132–1137 |access-date=2013-08-28|last1=Winn |first1=B. |last2=Whitaker |first2=D. |last3=Elliott |first3=D. B. |last4=Phillips |first4=N. J. |pmid=8125724 }}</ref>  At this stage the pupils do not remain completely still, therefore may lead to oscillation, which may intensify and become known as [[hippus]].  The constriction of the pupil and near vision are closely tied.  In bright light, the pupils constrict to prevent aberrations of light rays and thus attain their expected acuity; in the dark, this is not necessary, so it is chiefly concerned with admitting sufficient light into the eye.<ref>"Sensory Reception:  Human Vision:  Structure and Function of the Eye"  Encyclopædia Brtiannicam Chicago, 1987</ref>

When bright light is shone on the eye, light-sensitive cells in the retina, including rod and cone photoreceptors and [[melanopsin]] [[retinal ganglion cell|ganglion cells]], will send signals to the [[oculomotor nerve]], specifically the [[parasympathetic]] part coming from the [[Edinger-Westphal nucleus]], which terminates on the circular iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil.  This is the [[pupillary light reflex]], which is an important test of [[brainstem]] function. Furthermore, the pupil will dilate if a person sees an object of interest.{{citation needed|date=May 2020}}