The iris is the *coloured part of the human eye. It is a thin circular contractile 'curtain' located in the aqueous humour, which is in front of ('anterior to') the lens but behind ('posterior to') the cornea. It contains a circular aperture (or 'hole', in plain non-medical English) called the pupil that is located just to the nasal side of the centre of the iris.
How does the iris work ?
A simple description of the iris is that it is a coloured diaphragm of variable size whose function is to adjust the size of the pupil to regulate the amount of light admitted into the eye.
It does this via the pupillary reflex (which is also known as the 'light reflex').
It works as follows: When bright light reaches the retina, nerves of the parasympathetic nervous system are stimulated, a ring of muscle around the margin of the iris contracts, so the size of the pupil is reduced, hence less light is able to enter the eye. Conversely, in dim lighting conditions the pupil opens due to stimulation of the sympathetic nervous system that contracts of radiating muscles, hence increases the size of the pupil.
More about the colour of the iris
*Although, in simple terms, the iris may be referred to as the coloured part of the eye, it is really only the front surface of the iris that is 'coloured'.
More accurately the anterior (front) surface of the iris has different colours in different people. It is also marked by lines that converge toward the pupil, as studied in iridology. The posterior (back) surface of the iris has a deep purple tint due to two layers of pigmented columnar epithelium. This pigmented epithelium is called the 'pars iridica retinae' due to the similarity of its colour to that of a ripe purple grape, but is often just referred to as the uvea.
More about the structure of the iris of the eye
The iris is composed of a series of layers, including:
More about Ophthalmology:
This section includes short definitions and descriptions of the parts of the eye.
For other descriptions in this category, choose from the list to the left (but note that this is not a complete / exhaustive list).
Other related sections include:
- A labelled diagram of the eye
- A concise description of the human retina with brief descriptions of the parts of the retina
- Diseases and disorders of the human eye and the human visual system
- Clinical and surgical procedures re. eyes and human visual system
For further information see also our pages of books about ophthalmology.