Iris (eye)
The iris (plural: irises or irides) is a pigmented muscular structure which modifies the amount of light entering the eye, by controlling the size of the pupil, its central aperture.
On this page:
Summary
- location: between the anterior and posterior chambers of the globe
- function: controls the amount of light entering the eye
- blood supply and drainage: anterior ciliary vessels
-
relations
- anterior: anterior chamber
- posterior: posterior chamber
- medial: pupil
- lateral: ciliary body
Gross anatomy
Location
The iris forms the anterior most component of the uvea, which is the vascular layer of the globe. Posteriorly the ciliary body and, most posteriorly, the choroid complete the uvea.
The iris is a ring-shaped structure, secured to the inner aspect of the ciliary body, which encircles it. The iris has a central aperture, the pupil, the variable diameter of which modifies the quantity of light reaching into the eye.
Most of the iris comprises vascularized stroma with scattered melanocytes. The coloration of the iris varies with its pigment content. At birth the iris is blue, due to absence of pigment. As the infant grows, pigment cells may proliferate, resulting in darker iridal colors.
Function
The iris has radially-arranged dilator pupillae muscles, and concentrically-arranged sphincter pupillae muscles. These muscles dilate and constrict the pupil, in response to sympathetic and parasympathetic nerve impulses, from the long ciliary nerve and oculomotor nerve (III), respectively.
Relations
- anterior: anterior chamber and cornea
- posterior: posterior chamber and lens
- in vivo, the margin of the pupil is in contact with the lens
- lateral: ciliary body
- medial: pupil
Arterial supply
Venous drainage
- anterior ciliary veins
Lymphatic drainage
- there is no lymphatic drainage of the globe or its parts
Histology
Most of the iris comprises markedly vascularized loosely-compacted stroma, with melanocytes dispersed throughout the tissue. The anterior aspect of the iris is uneven, formed by an incomplete layer of fibroblasts and melanocytes. Conversely its posterior aspect is fairly even, formed by an epithelial layer, which originates embryologically in continuity with the double layer of cuboidal epithelium lining the ciliary body.
The superficial layer of this iridal epithelium is so heavily pigmented (cf. non-pigmented in the ciliary body) that the cells can no longer be distinguished from one another. The deeper layer consists of radially-oriented myoepithelial cells which are only lightly colored and constitute the dilator pupillae muscle.
The constrictor pupillae muscle is formed by intrastromal muscle fibers located near the inner margin of the iris, which are laid down concentrically.
The coloration of the iris relates to the quantity of pigment in the anterior stroma, which shows individual variation across the population. The melanocytes of the posterior epithelium exhibit little variation in pigmentation between individuals. People lacking stromal pigment have blue eyes, conversely those whose stroma is hyperpigmented have brown eyes.
Related pathology
Related Radiopaedia articles
Anatomy: Head and neck
- skeleton of the head and neck
-
cranial vault
- scalp (mnemonic)
- fontanelle
-
sutures
- calvarial
- facial
- frontozygomatic suture
- frontomaxillary suture
- frontolacrimal suture
- frontonasal suture
- temporozygomatic suture
- zygomaticomaxillary suture
- parietotemporal suture (parietomastoid suture)
- occipitotemporal suture (occipitomastoid suture)
- sphenofrontal suture
- sphenozygomatic suture
- spheno-occipital suture (not a true suture)
- lacrimomaxillary suture
- nasomaxillary suture
- internasal suture
- basal/internal
- skull landmarks
- frontal bone
- temporal bone
- parietal bone
- occipital bone
- skull base (foramina)
-
facial bones
- midline single bones
- paired bilateral bones
- cervical spine
- hyoid bone
- laryngeal cartilages
-
cranial vault
- muscles of the head and neck
- muscles of the tongue (mnemonic)
- muscles of mastication
-
muscles of facial expression
- occipitalis muscle
- circumorbital and palpebral muscles
- nasal muscles
- buccolabial muscles
- elevators, retractors and evertors of the upper lip
- levator labii superioris alaeque nasalis muscle
- levator labii superioris muscle
- zygomaticus major muscle
- zygomaticus minor muscle
- malaris muscle
- levator anguli oris muscle
- risorius muscle
- depressors, retractors and evertors of the lower lip
- depressor labii inferioris muscle
- depressor anguli oris muscle
- mentalis muscle
- compound sphincter
- orbicularis oris muscle
- incisivus superior muscle
- incisivus inferior muscle
- muscle of mastication:
- elevators, retractors and evertors of the upper lip
- muscles of the middle ear
- orbital muscles
- muscles of the soft palate
- pharyngeal muscles
- suprahyoid muscles
- infrahyoid muscles
- intrinsic muscles of the larynx
- muscles of the neck
- accessory muscles of the neck
- deep cervical fascia
-
deep spaces of the neck
- anterior cervical space
- buccal space
- carotid space
- danger space
- deep cervical fascia
- infratemporal fossa
- masticator space
- parapharyngeal space
- stylomandibular tunnel
- parotid space
- pharyngeal (superficial) mucosal space
- perivertebral space
- posterior cervical space
- pterygopalatine fossa
- retropharyngeal space
- suprasternal space (of Burns)
- visceral space
- surgical triangles of the neck
- orbit
- ear
- paranasal sinuses
- nose
- oral cavity
- pharynx
- larynx
- viscera of the neck
- blood supply of the head and neck
-
arterial supply
-
common carotid artery
- carotid body
- carotid bifurcation
- subclavian artery
- variants
-
common carotid artery
- venous drainage
-
arterial supply
- innervation of the head and neck
-
cranial nerves
- olfactory nerve (CN I)
- optic nerve (CN II)
- oculomotor nerve (CN III)
- trochlear nerve (CN IV)
-
trigeminal nerve (CN V) (mnemonic)
- trigeminal ganglion
- ophthalmic division
- maxillary division
- mandibular division
- abducens nerve (CN VI)
- facial nerve (CN VII)
-
vestibulocochlear nerve (CN VIII)
- vestibular ganglion (Scarpa's ganglion)
- glossopharyngeal nerve (CN IX)
-
vagus nerve (CN X)
- superior laryngeal nerve
- recurrent laryngeal nerve (inferior laryngeal nerve)
- (spinal) accessory nerve (CN XI)
- hypoglossal nerve (CN XII)
- parasympathetic ganglia of the head and neck
- greater occipital nerve
- third occipital nerve
-
cervical plexus
- muscular branches
- longus capitis
- longus colli
- scalenes
- geniohyoid
- thyrohyoid
-
ansa cervicalis
- omohyoid (superior and inferior bellies separately)
- sternothyroid
- sternohyoid
- phrenic nerve
- contribution to the accessory nerve (CN XI)
- cutaneous branches
- muscular branches
- brachial plexus
- pharyngeal plexus
-
cranial nerves
- lymphatic drainage of the head and neck
- embryological development of the head and neck