Describe the structure and function of the parts of the eye, including the parts that surround or fill the eye, that are associated with the light-focusing capability of the eye, and that are associated with the light-receptive capability of the eye.
The adult human eye is surrounded by a layer of tough, elastic connective tissue called the sclera. The portion of the sclera over the front of the eye, called the cornea, is transparent and is the first light-focusing element of the eye. The next layer beneath the sclera is the choroid which serves several important functions. Containing many blood vessels, the choroid supplies blood to the rest of the eye. Darkly pigmented, the choroid absorbs internally reflected light and prevents it from entering the eye. At the junction between the cornea and the sclera, the choroid is thickened and embedded with smooth muscles. Called the ciliary body, this part of the choroid form the 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
