Editing Iris (anatomy) (section)

== Function ==
[[File:Iris structure.png|thumb|Structure of the iris and surrounding parts showing the dilator and sphincter muscles (''dilator pupillae'' and ''sphincter pupillae'').]]
The iris controls the size of the [[pupil]] by means of contracting the [[iris sphincter muscle]] and/or the [[iris dilator muscle]]. The size of the pupils is dependent on many factors (including light, emotional state, cognitive load, arousal, stimulation), and can range from less than 2&nbsp;mm in diameter, to as large as 9&nbsp;mm in diameter. However, there is considerable variation in maximal pupil diameter by individual humans, and decreases with age.<ref>{{cite web |title=Aging Eyes and Pupil Size |url=http://amateurastronomy.org/Events/EH361.html |url-status=dead |archive-url=https://web.archive.org/web/20131023134125/http://amateurastronomy.org/Events/EH361.html |archive-date=2013-10-23 |access-date=2013-08-28 |publisher=Amateurastronomy.org}}</ref><ref>{{cite journal |last1=Winn |first1=B. |last2=Whitaker |first2=D. |last3=Elliott |first3=D. B. |last4=Phillips |first4=N. J. |date=March 1994 |title=Factors Affecting Light-Adapted Pupil Size in Normal Human Subjects |url=http://www.iovs.org/content/35/3/1132.full.pdf |journal=Investigative Ophthalmology & Visual Science |volume=35 |issue=3 |pages=1132–1137 |pmid=8125724 |access-date=2013-08-28}}</ref> The irises also contract the pupils when [[Accommodation reflex|accommodation]] is initiated, to increase the [[depth of field]].

Very few humans possess the ability to exert direct voluntary control over their iris muscles, which grants them the ability to dilate and constrict their pupils on command.<ref>{{Cite journal |last1=Eberhardt |first1=Lisa V. |last2=Grön |first2=Georg |last3=Ulrich |first3=Martin |last4=Huckauf |first4=Anke |last5=Strauch |first5=Christoph |date=2021-10-01 |title=Direct voluntary control of pupil constriction and dilation: Exploratory evidence from pupillometry, optometry, skin conductance, perception, and functional MRI |url=https://www.sciencedirect.com/science/article/pii/S0167876021008448 |journal=International Journal of Psychophysiology |volume=168 |pages=33–42 |doi=10.1016/j.ijpsycho.2021.08.001 |pmid=34391820 |issn=0167-8760}}</ref> However, there is no clear purpose or advantage to this.