Can You Describe The Physiology Of The Human Eye?


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Steve Theunissen Profile
The eyeball is a complex, three-dimensional, full-color motion-picture camera. It is shaped like a sphere and, for the most part, is filled with a jellylike transparent substance called vitreous humor. Encasing this semiliquid substance is a three-layered "skin." The outer layer is known as the sclera, the middle one is the choroid and the inner layer is the retina. But none of these three layers encircle the entire eyeball. They all leave space at the front.

In the forepart of the eyeball the sclera merges into a convex transparent tissue called the cornea. It is shaped somewhat like a tiny bowl placed at the front of the eye with the bottom facing out. Behind the cornea is the iris, a thin circular curtain of muscular tissue that constitutes the colored part of the eye. The iris has a hole at its center, known as the pupil. Two sets of muscles in the iris can change the size of the pupil, controlling the amount of light that enters. Filling the space between the cornea and the iris is a clear fluid called aqueous humor.

Your eyes do not 'see' anything? Instead, they bend and focus rays of light, which the optic nerve (attached to the rear of the eyeball) transmits to the visual center of the brain. To make that possible, the eye has another major part.

Just behind the iris is a transparent crystalline lens, which is about the size of an aspirin tablet. The lens is biconvex, that is, it is convex at both its front and back. The cornea and the lens work together to bend light rays and focus them at the back of the eyeball on the inner of its three surrounding layers, the retina. A group of muscles attached to the lens can make it change shape so as to bring things into focus. A cross-section view of the lens would reveal that, when focusing on a distant object, it is oval-shaped like a football. The lens would appear rounder when the eye views something up close.

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