[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anatomy](https://durgical.com/en/anatomy/) › [Head and neck](https://durgical.com/en/anatomy/region/head-and-neck/) › Iris [Sense organs](https://durgical.com/en/anatomy/system/sense-organs/) · [Head and neck](https://durgical.com/en/anatomy/region/head-and-neck/) # Iris The iris is the coloured ring of the eye; its two tiny muscles change the size of the pupil to control how much light gets in. [See in 3D →](https://durgical.app/?from=durgipedia&lang=en#region=head_neck&select=iris&side=r) Opens in the Durgical app, no sign-up needed. ## What it is The coloured ring of the eye, with two tiny muscles (sphincter and dilator). ## What it does Changes the size of the pupil to control how much light gets in. ## Key points - Sphincter pupillae (circular): Edinger–Westphal nucleus, [oculomotor nerve](https://durgical.com/en/anatomy/oculomotor-nerve/), ciliary ganglion, short ciliary nerves; dilator pupillae (radial): sympathetic, via the long ciliary nerves - A compressive oculomotor nerve palsy ([posterior communicating artery](https://durgical.com/en/anatomy/posterior-communicating-artery/) aneurysm, uncal herniation) gives a large unreactive pupil, because the pupil fibres run on the outside of the nerve - Horner syndrome: small pupil that dilates slowly in the dark, mild ptosis and anhidrosis, from a lesion anywhere on the sympathetic path ([hypothalamus](https://durgical.com/en/anatomy/hypothalamus/), cord, [sympathetic trunk](https://durgical.com/en/anatomy/sympathetic-trunk/), [internal carotid artery](https://durgical.com/en/anatomy/internal-carotid-artery/)) - A pupil difference of about 1 mm or less with normal reactions (physiological anisocoria) is common in healthy people ## Key facts **Where** Between the cornea and the lens, part of the uvea **Blood supply** Long posterior and anterior ciliary arteries (major arterial circle) **Nerve** Sphincter: parasympathetic (Edinger-Westphal nucleus, III, ciliary ganglion, short ciliary nerves); dilator: sympathetic (ciliospinal centre C8–T2, superior cervical ganglion, internal carotid plexus, long ciliary nerves) **Function** Controls pupil size ## Relations **In front** [Anterior chamber](https://durgical.com/en/anatomy/anterior-chamber-of-eyeball/) and, beyond it, the [cornea](https://durgical.com/en/anatomy/cornea/) **Behind** Posterior chamber; the front of the [lens](https://durgical.com/en/anatomy/lens/) supports its pupillary margin, and the ciliary processes lie behind its root **Contents** Sphincter and dilator muscles, a stroma rich in melanocytes, and the major arterial circle at its root, from which radial vessels run towards the pupil ## How to find it The coloured ring seen through the cornea, with the pupil at its centre; its root joins the [ciliary body](https://durgical.com/en/anatomy/ciliary-body/) at the [drainage angle](https://durgical.com/en/anatomy/iridocorneal-angle/). At the slit lamp its front surface shows crypts and the collarette, a ridge a short way from the pupil margin. The pupil is roughly 2–4 mm in bright light and up to about 8 mm in the dark in young adults, and it narrows with age. ## Examination - Pupil light reflex, direct and consensual: an efferent defect (oculomotor nerve or iris) leaves that pupil sluggish to both; an afferent defect ([optic nerve](https://durgical.com/en/anatomy/optic-nerve/), [retina](https://durgical.com/en/anatomy/retina/)) shows as a relative afferent pupillary defect on the swinging flashlight test - Near reflex: better constriction to near than to light (light-near dissociation) suggests Argyll Robertson pupils of neurosyphilis or an Adie tonic pupil; dilute pilocarpine constricts an Adie pupil, while a pupil blocked by atropine stays wide even with the standard strength - Anisocoria in dim and bright light: larger difference in the dark means the small pupil is abnormal (Horner); larger in bright light means the large pupil is abnormal - Apraclonidine or cocaine eye drops confirm Horner syndrome: the small pupil dilates with apraclonidine and fails to dilate with cocaine ## On imaging Seen directly at the slit lamp, with gonioscopy for the angle at its root. Anterior segment OCT and ultrasound biomicroscopy show its profile in cross section: a forward-bowed iris (iris bombé) or a narrow angle in angle-closure glaucoma, and the size and depth of an iris cyst or tumour. On CT or MRI of the orbit it is only a thin line in front of the lens. ## Common problems - Iritis (anterior uveitis): a painful red eye with light sensitivity. - Unequal pupils (anisocoria): often harmless, but new unequal pupils with a droopy lid or headache need urgent care. ## How it is diagnosed Slit-lamp exam and checking how each pupil reacts to light and to near focus. ## Treatment - Steroid and pupil-dilating drops for iritis. - Urgent scans when unequal pupils come with other nerve signs. ## Clinical notes - Anterior uveitis can stick the iris to the lens (posterior synechiae) and make the pupil irregular; if the whole margin sticks, the iris bows forward (iris bombé) and closes the angle - Heterochromia: a lighter iris on the side of a congenital Horner syndrome; also Waardenburg syndrome, Fuchs heterochromic iridocyclitis, and darkening from prostaglandin eye drops - Lisch nodules (pigmented iris hamartomas) in neurofibromatosis type 1; Brushfield spots in Down syndrome - New vessels on the iris (rubeosis) in proliferative diabetic retinopathy or retinal vein occlusion herald neovascular glaucoma - Acute angle closure leaves a mid-dilated, sluggish, often oval pupil in a red, painful eye; dilating drops for a fundus exam can rarely set it off when the angle is narrow ## Anatomical variants - Persistent pupillary membrane: fine strands across the pupil left from that fetal membrane, common and usually harmless ## Development The iris muscles come from the neuroectoderm of the optic cup, an exception among muscles; the stroma comes from neural crest. Failed closure of the optic fissure leaves an inferonasal (keyhole) coloboma. A thin vascular membrane closes the fetal pupil until shortly before birth. Aniridia (PAX6) can be part of WAGR syndrome with Wilms tumour. ## Memory aid Parasympathetic makes the pupil Pinpoint; sympathetic opens it wide for fight or flight. ## Related conditions - [Oculocutaneous albinism](https://durgical.com/en/conditions/oculocutaneous-albinism/) - [Acute angle-closure glaucoma](https://durgical.com/en/conditions/acute-angle-closure-glaucoma/) - [Uveitis](https://durgical.com/en/conditions/uveitis/) - [Ocular trauma: hyphema and open globe injury](https://durgical.com/en/conditions/ocular-trauma-hyphema-and-open-globe-injury/) - [Chédiak–Higashi syndrome](https://durgical.com/en/conditions/chediak-higashi-syndrome/) - [Cataract](https://durgical.com/en/conditions/cataract/) ## Test yourself Think of the answer, then tap the question to see it. ## What is the nerve supply of the sphincter pupillae and the dilator pupillae? Sphincter: parasympathetic (Edinger–Westphal, oculomotor nerve, ciliary ganglion, short ciliary nerves); dilator: sympathetic, via the long ciliary nerves. ## Why does a compressive oculomotor nerve palsy give a large unreactive pupil? The pupil fibres run on the outside of the oculomotor nerve, so compression (posterior communicating artery aneurysm, uncal herniation) hits them first. ## What are the features of Horner syndrome? A small pupil that dilates slowly in the dark, mild ptosis and anhidrosis, from a lesion anywhere on the sympathetic path. ## If anisocoria is greater in dim light, which pupil is abnormal? The small pupil, which fails to dilate, as in Horner syndrome; a larger difference in bright light means the large pupil is abnormal. ## What do new vessels on the iris (rubeosis) herald? Neovascular glaucoma, typically from proliferative diabetic retinopathy or retinal vein occlusion. [Add to my cards (5)](https://durgical.app/estudiar/?add=/en/anatomy/iris/) Educational information, not medical advice. For any symptom, see a health professional. ## Its names **Official Latin (TA2)** iris TA2 6753 **Official Spanish (DeCS)** Iris DeCS D007498 **English (TA2)** Iris ## Related - [Anterior chamber](https://durgical.com/en/anatomy/anterior-chamber-of-eyeball/) - [Lens](https://durgical.com/en/anatomy/lens/) - [Drainage angle](https://durgical.com/en/anatomy/iridocorneal-angle/) - [Oculomotor nerve (III)](https://durgical.com/en/anatomy/oculomotor-nerve/) Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=28111); [Anatomy, Head and Neck: Eye Iris Sphincter Muscle](https://www.ncbi.nlm.nih.gov/books/NBK532252/); [Anatomy, Head and Neck, Eye](https://www.ncbi.nlm.nih.gov/books/NBK482428/); [14.1 Sensory Perception](https://openstax.org/books/anatomy-and-physiology-2e/pages/14-1-sensory-perception) (OpenStax (Rice University), 2022) · adapted, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/); [15.2 Autonomic Reflexes and Homeostasis](https://openstax.org/books/anatomy-and-physiology-2e/pages/15-2-autonomic-reflexes-and-homeostasis) (OpenStax (Rice University), 2022) · adapted, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/); [16.3 The Cranial Nerve Exam](https://openstax.org/books/anatomy-and-physiology-2e/pages/16-3-the-cranial-nerve-exam) (OpenStax (Rice University), 2022) · adapted, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/anatomy/iris/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/iris/