The iris is a circular structure which has a thin layer. It controls the diameter and the size of the pupil as well as the amount of light that reaches the retina.
The color of the eye depends on the iris, which is blue, brown or green. In some people the iris can also be grey or hazel. The muscles that are attached to the iris expand and contract depending on the amount of light that enters the eye. The aperture that is attached with the iris in the middle of the eye is called the pupil. The amount of light that enters the eye depends on the size of the pupil so if the pupil is large more light enters.
There are two layers in the iris- the front pigmented fibrovascular tissue is called the stroma and below the stroma is a pigmented epithelial cell.
The stroma is connected to muscle known as sphincter and this contracts the pupil in a round motion. Another set of muscles known as dilator papillae pulls the iris to enlarge the pupil in folds. The other side of the surface is covered epithelial layer which is a heavily pigmented layer. This layer is as thick as two cells but the surface in front is not covered with epithelium. This surface projects the dilator muscles and this high pigment blocks the light reaching the retina through the iris which restricts it only to the pupil. The root which is the outer boarder of the iris is attached to the sclera as well as the anterior ciliary body. Both these together (ciliary body and iris) are called the anterior uvea.
The front of the root of the iris is an area which is known as trabecular meshwork. This area is through which the aqueous humour continuously drains from the eye. As a result, any disease of the iris affects the intraocular pressure and the body provides a second path for aqueous humour so that it drains from the eye.
Two major regions of the iris
It is the region inside the iris and its boarder forms the outline of the pupil.
The other parts of the iris that stretches to its origin at the ciliary body.
The thickest region of the iris is known as the collarette. It separates the papillary portion from the ciliary portion. The collarette is a coating of the embryonic pupil. This is that part where the dilator muscle and sphincter overlap each other. The radial ridges stretch from the periphery till the papillary zone so that it can supply blood vessels to the iris. The most thinnest and peripheral part of the iris is the root. The iris has smooth muscle cells especially in amphibians and mammals but they are striated in reptiles and birds. Many fish do not have either of these and their irises are not able to contract and dilate, therefore the pupil always remains fixed.