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. |
