General Structure
The iris consists of two layers: the front pigmented fibrovascular tissue known as a stroma and, beneath the stroma, pigmented epithelial cells.
The stroma connects to a sphincter muscle (sphincter pupillae), which contracts the pupil in a circular motion, and a set of dilator muscles (dilator pupillae) which pull the iris radially to enlarge the pupil, pulling it in folds. The back surface is covered by a heavily pigmented epithelial layer that is two cells thick (the iris pigment epithelium), but the front surface has no epithelium. This anterior surface projects as the dilator muscles. The high pigment content blocks light from passing through the iris to the retina, restricting it to the pupil. The outer edge of the iris, known as the root, is attached to the sclera and the anterior ciliary body. The iris and ciliary body together are known as the anterior uvea. Just in front of the root of the iris is the region referred to as the trabecular meshwork, through which the aqueous humour constantly drains out of the eye, with the result that diseases of the iris often have important effects on intraocular pressure, and body provide a lesser secondary pathway for the aqueous humour to drain from the eye.
The iris is divided into two major regions:
- The pupillary zone is the inner region whose edge forms the boundary of the pupil.
- The ciliary zone is the rest of the iris that extends to its origin at the ciliary body.
The collarette is the thickest region of the iris, separating the pupillary portion from the ciliary portion. The collarette is a rudiment of the coating of the embryonic pupil. It is typically defined as the region where the sphincter muscle and dilator muscle overlap. Radial ridges extend from the periphery to the pupillary zone, to supply the iris with blood vessels. The root of the iris is the thinnest and most peripheral.
The muscle cells of the iris are smooth muscle in mammals and amphibians, but are striated muscle in birds and reptiles. Many fish have neither, and, as a result, their irides are unable to dilate and contract, so that the pupil always remains of a fixed size.
Read more about this topic: Iris (anatomy)
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