Tell me about iris in antomy

Reading File

Finding Sources

Reading File

Loading Image

Here is a detailed account of the iris in anatomy, drawn from Gray's Anatomy for Students and Histology: A Text and Atlas (Pawlina):

The Iris

Position and Overview

The iris is the most anterior part of the vascular coat (uvea) of the eye. It forms a contractile diaphragm positioned in front of the lens, with a central aperture called the pupil. The iris projects outward from the ciliary body and is attached to the sclera approximately 2 mm posterior to the corneoscleral junction.

The iris divides the space between the cornea and lens into:

Anterior chamber - between the cornea and the iris
Posterior chamber - between the iris and the lens

Ciliary body and iris anatomy diagram showing sphincter pupillae, dilator pupillae, anterior and posterior chambers

Fig. 8.113 from Gray's Anatomy for Students - showing the iris in relation to the ciliary body, cornea, and lens

Layers of the Iris (from anterior to posterior)

Anterior surface - a discontinuous layer of fibroblasts and melanocytes (not a true epithelium). It shows numerous ridges and grooves visible on clinical examination.
Stroma - highly vascularized loose connective tissue containing:
- Blood vessels
- Stromal melanocytes (whose number determines eye color)
- The sphincter pupillae muscle near the pupillary margin
Anterior pigment myoepithelium - a layer of myoepithelial cells with:
- Apical (posterior) portions laden with melanin granules
- Basal (anterior) portions with contractile processes forming the dilator pupillae muscle
Posterior pigment epithelium - intensely pigmented cells; so heavily pigmented that neither nuclei nor cytoplasmic detail is visible under light microscopy. Basal lamina faces the posterior chamber.

Muscles of the Iris

Histological structure of the iris showing sphincter, dilator, pigment layers, myoepithelial cells, and photomicrograph

Fig. 24.8 from Histology: A Text and Atlas - schematic and photomicrograph of iris layers

Muscle	Arrangement	Innervation	Action
Sphincter pupillae	Circular fibers near pupillary margin (in stroma)	Parasympathetic (CN III)	Constricts pupil (miosis)
Dilator pupillae	Radial myoepithelial cell processes	Sympathetic	Dilates pupil (mydriasis)

The pupil size at any moment reflects the balance between these two opposing muscle groups. This is the basis for the pupillary light reflex and is clinically important in assessing CN III function and sympathetic chain integrity (e.g., Horner syndrome).

Eye Color

The color of the iris is determined by the number and density of melanocytes in the stroma:

Few stromal melanocytes → light scatter from the posterior pigment epithelium produces a blue appearance
More melanocytes → progressively green, hazel, or brown eyes
The posterior pigment epithelium is uniformly dark in all individuals; it is only the stromal melanocyte density that varies

Clinical Relevance

The iridocorneal angle (where iris meets cornea) contains the trabecular meshwork through which aqueous humor drains into the scleral venous sinus (canal of Schlemm). Obstruction here causes glaucoma.
Laser iridotomy creates a small opening at the base of the iris to widen the iridocorneal angle in angle-closure glaucoma.
Mydriatic drugs (atropine, phenylephrine) dilate the pupil by blocking parasympathetics or stimulating sympathetics.
Miotic drugs (pilocarpine) constrict the pupil via parasympathomimetic action.

Sources: Gray's Anatomy for Students, p. 1086-1090 | Histology: A Text and Atlas (Pawlina), p. 2392-2393

This is a shared conversation. Sign in to Orris to start your own chat.