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Human Sight
Dátum pridania: 30.11.2002 Oznámkuj: 12345
Autor referátu: nirm
 
Jazyk: Angličtina Počet slov: 1 623
Referát vhodný pre: Stredná odborná škola Počet A4: 5.4
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The inner segment also contains a synaptic terminal, which allows the photoreceptor cells to communicate chemically with other retinal cells.
Each eye has got about 125 million rods and 7 million cones. Most of the cones are concentrated in the center of the retina directly behind the lens in area called the macula lutea (lat. “yellow spot”), especially in a small, depressed, rod-free area called the fovea, or fovea centralis. The vision is sharpest and color perception is optimal on this places.
The portion of retina where the optic nerve exists from the eyeball contains neither rods nor cones; it is called the blind spot or optic disc.

Accommodation
Images from an object less than 6m away would normally be focused behind the retina instead of on it. To bring images into perfect focus on the retina is the role of the adjustable lens. This is accomplished by a reflex called accommodation. When you want to focus your eyes on an object close to you, you involuntarily contract the ciliary muscles in your eye, which pull the ciliary body slightly forward and inward, reducing the tension on the suspensory ligaments attached to the lens capsule. When the tension is reduced, the elastic lens becomes thicker. By the focusing the distant objects, the ciliary muscles are released.
As the lens becomes harder and less elastic with age, it becomes more and more difficult to focus on near objects, a condition known as presbyopia.

Convergence
Human beings have binocular vision. Each eye receives an image from a slightly different angle and this creates the impression of distance, depth, and three-dimensionality. For this reason is this vision also called stereoscopic vision. In binocular vision, the two eyeballs turn slightly inward to focus on a close object so that both images fall on the corresponding points of both retinas at the same time. This action is called convergence. In order to produce a single image , the six pairs of extraocular muscles must move together with perfect coordination. As a result of convergence, the simultaneous stimulation of both retinas produces the perception of a single image in the occipital lobe of the cerebral cortex.


Accessory structures of the Eye
The eye is enclosed in an orbital cavity, or orbit, which protects it from external buffeting. The eyelids (palpebrae) are folds of skin that create an almond-shaped opening around the eyeball when the eye is open. The points of the almond, where the upper and lower eyelids meet, are called canthi.
 
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