Iris (Anatomy)

The iris is the coloured membrane that sits behind the cornea on the front of the eye. The name iris means rainbow and hence it fits the variety of different shades the iris can be. The pupil resides in the centre of the iris and is responsible for the transmission of light. In front of the iris is a lens.

Structure
The iris is composed of a number of different structures which include membranes, endothelial cells, stroma, muscle fibres, arteries and nerves. All play a different role in the make up of the iris.

At the front of the iris is a layer of endothelial cells which sit on a hyaline membrane. These cells are flattened and sometimes contain pigment granules. The iris is also made up of stroma which is both fibres and cells. The majority of the fibres in the iris radiate towards the pupil to form a mesh like network in which the vessels and nerves are retained. Many branched cells are dispersed between these fibres and these also contain pigment granules in individuals with dark coloured eyes.

The muscle fibre is made up of both circulating and radiating fibres. The circulating fibres form a narrow ring around the pupil and the radiating fibres converge on this margin. The circulating fibres are important in terms of function for they are responsible for forcing the pupil to constrict in the presence of bright light, thereby preventing large amounts of light from entering the eye. This will cause the iris to enlarge. The radiating fibres on the other hand have the opposite effect as they can widen the pupil. The pupil can change in diameter between two and eight centimetres thus the range of light that can be taken in is variable. There are also a numerous arteries supplying the iris. Two arteries called the long ciliary arteries surround the iris and branch to surround the pupil. They converge in a region called the vascular zone.

The nerves of the iris enter from the optic nerve towards the perichoroidal space and form a plexus around the margin of the pupil.

Pigment
The pigment in the iris gives the eye its definitive colour. There are a variety of different shades that can occur depending on genetics. The name iris is associated with the Greek goddess of the rainbow and hence is named due to the range of eye colours found. The colour depends on how much of the pigment shows through the iris. In individuals with grey or brown eyes, the pigment can be found in the stromal cells and in the epithelial layer that covers the front of the iris. Those individuals with blue eyes only have pigment granules within polyhedral cells which are situated on the surface of the iris. This is classed as low pigment and is a recessive trait. Albino eye colour occurs when the pigment granules are absent.