Description and Function of Parts of the Human Eye

Choroid

A thin, highly vascular membrane on which the retina rests; between the retina and sclera

Retina

a ten-layered, delicate, membrane of light-sensitive nervous tissue at the back of the eye; contains photoreceptors (rods and cones) and the neurons which transmit visual impulses from sensory cells through the optic nerve to the brain; is in contact with the choroid, the inner surface with the vitreous body; is soft, semi transparent, and contains rhodopsin, which gives it a purple tint; becomes clouded and opaque if exposed to direct sunlight

Lens (Crystalline Lens)

a transparent, colorless, firm structure of the eye, enclosed in a capsule, located between the iris and the vitreous humor; refracts light to focus images onto the retina at the back of the eye; in old age the lens becomes flattened, more dense, slightly opaque, and amber-tinted.

Fovea (Fovea Centralis)

area at the center of the retina where cone cells are concentrated and there are no rod cells; "blind spot."

Iris

circular; opaque, contractile diaphragm controlling the diameter of the pupil; divides the space between the lens and the cornea into an anterior and a posterior chamber; the colored part of the eye surface containing dark pigment cells variously arranged in different people to produce different colored irises; pigment absent in albinos.

Aqueous Humor

clear, watery fluid circulating in both chambers of eye, associated with the chamber outside the lens.

Ciliary Muscle

controls the diameter of the pupil (how much light passes through the lens);ciliary muscle adjusts the shape of the lens (and hence focal length) by varying the tension of the muscle

Cornea

convex, transparent coating of the eye (made up of callogen-rich epithelial cells) which covers the pupil and iris, consisting of five layers allowing light to pass through to the lens; it is dens, nonvascular, uniform in thickness, and projects like a dome beyond the sclera; the degree of corneal curvature varies in different individuals and in the same person at different ages; curvature is more pronounced in youth than in advanced age.

Optic Nerves

either a pair of second cranial nerves; transmits visual data from retina to brain

Eyelid

movable fold of skin over the eye with lashes and glands along its margin

Rods

one of the photoreceptor cells in the retina; contain photopigments which are very sensitive to low light levels; located mainly around the periphery of the retina; do not code for color

Ciliary Body

part of the eye that joins the iris with the anterior portion of the choroid

Cones

photoreceptor cells in the retina used under conditions of bright illumination; are color coded (red, green, and blue); mediate fine detail vision; contain photopigments with low sensitivity to light (i.e., cannot see colors in dim illumination).

Pupil

the "hole" or circular opening in the iris through which light passes to the lens and the retina; located slightly to the nasal side of the center of the iris; lies behind the anterior chamber of the eye and the cornea and in front of the lens; diameter changes with contraction and relaxation of the muscular fibers of the iris as the eye responds to changes in light, emotional states and other kinds of stimulation.

Conjunctivia

thin layer of mucous membrane lining the inner surface of each eyelid, moves over the eyeball as a protective layer.

Sclera

tough, opaque membrane of the eyebulb which maintains the size and form of the bulb and attaches to muscles that move the bulb; the whites of the eyes.

Vitreous Humor

transparent, semigelatinous fluid in the chamber filling the cavity behind the crystalline lens of the eye.