The choroid is the layer of the eyeball
located between the retina
and the sclera
(see sketch of the back of the eyeball below).
It is a thin, highly vascular (i.e. it contains blood
vessels) membrane that is dark brown in colour and contains
a pigment that absorbs excess light and so prevents blurred vision
(due to too much light on the retina).
The choroid is loosely attached to the inner surface of the sclera
by the lamina fusa. The side of the choroid closest to the centre
of the eyeball is attached to the retina. This transparent innermost
layer of the choroid is called Bruch's
The structure of the choroid itself consists mainly
of a dense capillary plexus and of many arterioles and venules transporting
blood to and from this plexus.
The subject of visual (also known as "physiological") optics
is a key component of many courses within the fields of both biology and
physics. It is also an essential consideration in the design of displays
and control units used in many applications from televisions and mobile
telephones to advanced aircraft. In the context of engineering "visual
optics" is one of several medical and psychological topics in the
important area of "Human Factors".