pigmented colorants: dependence on media and time - Cornell ...

The vascular tunic lies on the wall of the eyeball for the rear two thirds, consist-
ing of a heavily <strong>pigmented</strong>, spongy structure called the choroid. It contains a vast
network of blood vessels and capillaries that provide oxygen and nourish a very
important class of cells in the retina, the photoreceptors, that turn light into neu-
ral signals. The choroid’s dark pigmentation also helps absorb light not captured
by the photoreceptors, save stray light from diminishing the quality of the image
that reaches the eye. Incidentally, this is the same reason the inside of a camera
is painted flat black–the paint absorbs scattered light and protects the sharpness
of images on the film. Toward the front of the eye, this middle, choroidal layer
breaks away from the outer layer and runs roughly parallel to the front of the eye.
This section of the middle layer forms a slender spongy structure called the ciliary
body which produces aqueous humor, the watery fluid that fills the cavities in the
eye. This fluid performs important maintenance to the eye, including supplying
oxygen and nutrients to the cornea and lens and carrying waste away. The fluid
also maintains the constant pressure in the eye. If there is too little fluid, the eye
becomes deformed and light does not focus correctly on the back of the eye. If
there is too much pressure for too long, vision can be impaired permanently–the
condition called glaucoma.
As it curls inward, the ciliary body gives way to the iris, that circular tissue
that gives your eye its characteristic color. The iris actually contains two layers, an
outer layer containing pigment and an inner layer containing blood vessels. If the
outer layer is heavily <strong>pigmented</strong>, the iris appears brown. If it is lightly <strong>pigmented</strong>,
the inner layer is partially visible through the outer one and the iris takes on a
lighter color. The pupil, or black region in the eye is not an object, but actually
a gap within two sets of muscles in the iris. The inner set runs in a circle around
58