Eye

The human eye consists of the eyeball and protective accessory parts (eyelids, lacrimal apparatus, etc.). It is suspended within the orbital, and is composed of connective tissue.

The outermost layer of the eyeball is the sclera which is composed of fibrous tissue and commonly known as the white of the eye. The anterior part of the sclera is the cornea. The cornea is transparent and lies over the colored part of the eye, or the iris. The next layer of the eyeball is the choroid coat. It contains many blood vessels and a colored ring of tissue made of muscles and pigment called the iris. In the center of the iris is the pupil, a circular opening through which light can pass. The amount of light entering the eye is controlled by adjustment in the size of the pupil.

The innermost layer of the eyeball is the retina. This is the light sensitive part of the eye and is composed of two layers. The outer layer of the retina consists of light receptors, the cones and rods. Cones function in bright light, and detect color and fine detail. Rods function in dim light, and perceive images in black and white. The innermost layer consists of nerve cells which transmit signals from the receptors to the brain. The fovea centralis is an area on the retina with a high concentration of cones, but no rods; it is an area where discrimination is acute and where images are usually focused. The optic disc is also known as the blind spot: it contains no receptors, thus images cannot be detected in this area. It is where nerves leave the retina to go to the brain.

The refractive structures are the cornea, lens, aqueous humor, and vitreous humor. The lens is a transparent biconvex disc which lies just in back of the iris. It is connected to the choroid coat by small muscles whose function is to change the shape of the lens, for focusing on distant or near images. The anterior cavity, between the cornea and iris, is filled with aqueous humor. The posterior cavity is filled with vitreous humor.

Light passes through the cornea and the pupil, is focused through the lens, and projected upon the retina. The image projected is upside down when perceived by the receptors. Light stimulates the nerve cells which transmit an impulse to the brain; the brain corrects the image.


References
Antony, C.P. & G.A. Thibodeaw. 1979. Textbook of Anatomy and Physiology. The C.V. Mosby Company, St. Louis. 731 pp.

Gray, H. 1977. Gray's Anatomy. Crown Publishers, Inc, New York. 1257 pp.

Lockhart, R.D., G.F. Hamilton, et. al. 1974. Anatomy of The Human Body. Faber and Faber Limited. London. 697 pp.

Van Amerongen, C. The Way Things Work; Book Of The Body. New York: Simon and Schuster, 1979.