Thursday, December 31, 2009

Eye to Detect of Light

There are many type of eyes, the chemical processes that transforms light from outside source into nerve impulses in the eye is bassically similar in all land vertebrates and marine fish, and in some insect. George Wald from Havard get a Nobel price for physiology or medicine for discovering the details of the first step, which occurs in the retina or ommatidium.

The eyes of different animals vary greatly in structure and complexity. Most lower animals, such as the planaria, a flatworm, do not posses image forming eyes but simple photorecetors, or light sensing organs. In the eye spots of a planaria, retinal clubs transmit information to the cerebral ganlia, or brain, from the light absorbing pigmen cells that form the outer layer of the spots. This information enables the animal to detect different degrees of light intensity and to locate the source of the light stimulus.

More advanced animals have image forming eyes, in addition to or instead of such simple light-sensitive organ. Insects, such as the honeybee typically have compound eyes composed of many individual facets, each of which is a photoreceptor unit linked by a nerve to the brain. Light entering the crystalline come behind the cornea is focused on the rhabdom, or light sensitive zone, the result is a mosaic and, in some case, a combined image.

Eyes of the most other higher animals are cameralike in function and, in similar in structure, sometimes providing stricking instances of convergent evolution. Thus the eyes of an invertebrate such as the squid, a mollusk and of a vertebrate such as the lizard arose in separate ways over the course of geological time, but many of the structures of the eyes, and their functions are practically the same.

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