armature for the lens. Anterior to the ciliary body, the choroid extends into the eyeball cavity. Called the iris, this part of the choroid is a ring of pigmented tissue. The iris has both circularly and radially arranged smooth muscle fibers which control the size of the pupil, the opening in the center of the iris that admits light to the eye. Contraction of the iris's circular fibers causes the pupil to become smaller admitting less light to the eye. Contraction of the iris's radial fibers causes the pupil to become larger admitting more light to the eye. The lens, suspended from the ciliary body by suspensory ligaments, is the second light-focusing element of the eye. The lens and its ligaments divide the eyeball cavity into two chambers. The chamber between the cornea and the lens is filled with a watery fluid called aqueous humor while the chamber between the lens and the retina is filled with a viscous fluid called vitreous humor. The retina, a thin tissue covering the inner surface of the choroid, contains the receptor cells for the sense of vision: the rods and the cones.
Concentrated most heavily at the periphery of the retina, rod cells are the most sensitive to light enabling humans to see in conditions with very low lighting. Yet, the images the rods foster are grainy and poorly defined as well as being only monochromatic since rod cells cannot detect color. Each rod cell has an inner segment which contains the nucleus and synaptic body and a cylinder-shaped outer segment. The outer segment is a specialized, non-motile cilium which contains light-sensitive pigment in the vesicles on its surface. The pigment, rhodopsin, is composed of a
