The Iris is the coloured part
of the human eye.
That is, the anterior surface of the iris has different colours in different
individuals and is also marked by lines that converge toward the pupil
(as studied in iridology). However, the posterior (back) surface of this
iris has a deep purple tint due to two layers of pigmented columnar epithelium.
This pigmented epithelium is usually referred to as the "pars iridica
retinae" but is sometimes called simply "uvea" due to the
similarity of its colour to that of a ripe purple grape.
The Iris is a thin circular contractile curtain located
in the aqueous
humour - in front of the lens
but behind the cornea.
It contains a circular aperture (or "hole" - in non-medical
terms !) called the pupil
and located just to the nasal side of the centre of the iris.
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"). That is, 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, 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.
The iris is composed of a series of layers, including: (1.) Flattened
endothelial cells on a hyaline basement-membrane; (2.) Stroma - consisting
of fibres and cells; (3.) Muscular Fibre - consisting of circular and
radiating fibres; (4.) Pigment - the location of pigment cells differing
in different irides; (5.) Arteries of the iris, and (6.) Nerves of the
Choroid and Iris.