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Vision is the most fundamental of human senses. The eye is a very complex and truly amazing organ. It is approximately one inch wide and deep, and
0.9 inches tall.
Human eyes allow humans to appreciate all the beauty of the world they live in, to read and gain knowledge, and to communicate their thoughts and desires to each other through visual expression and visual arts. The human eye is wrapped in three layers of tissue: the external layer, formed by the sclera and cornea; the intermediate layer, divided into two parts: anterior (iris and ciliary body) and posterior (choroid); and the internal layer, or the sensory part of the eye, the retina.
The major structures of the human eye are the sclera, cornea, choroid, ciliary body, iris, pupil, retina, mocula and fovea, optic nerve, optic disc, vitreous humor, aqueous humor, canal of schlemm, lens and conjunctiva. Each of the structures is described below.
The eye functions on much the same principle as a camera. The cornea has the effect of a glass lens and represents the foremost lens of the camera. When the cornea sees the image of an object, it does the focusing work. This image is then sent to the pupil that represents the aperture of the camera. This image is focused further by the lens inside the eye that represents the remaining lenses which are incorporated into camera lens. This focused image at this time is upside down.
Finally, this reverse image is transmitted to the brain for processing by the retina, which resembles the film inside a camera.
Sclera
The sclera is the tough, outermost layer of the eye. The white membrane of the sclera maintains the shape of the eye.
Cornea
The cornea is located at the front of the eye. The moist and dust-free cornea admits light to the interior of the eye and bends the light rays to that they can be brought to a focus.
Choroids
The middle layer of the human eye structure, choroids reduce reflection of stray light within the eye.
Ciliary
Body
The ciliary body consists of the ciliary muscle. When the muscle relaxes, the diameter of the ciliary body increases. By contrast, when the muscle contracts, the diameter of the ciliary body reduces. These changes enable the eye to adjust its focus between far objects and near objects.
Iris
Formed by the choroid coat, the iris is located in the front of the eye. The iris is pigmented and is responsible for eye color.
Pupil
Controlled by the automatic nervous system, the pupil is responsible for adjusting to the amount of light. In dim light the pupil opens wider. In bright light the pupil closes down.
Retina
The inner layer of the eye, the retina contains the light receptors, the rods and cones. The retina also has many interneurons that process the signals arising in the rods and cones before passing them back to the brain.
Macula and Fovea
The macula is located in the back of the eye and in the center of the retina
. In the center of the macula is an area called the fovea centralis. This area contains only cones and is responsible for seeing fine detail clearly.
Optic Nerve
Formed by the retinal nerve fibers, the optic nerve conducts the electrical impulses to the brain. Vision is based on the modulation of these nerve impulses.
Optic Disc
The optic disc is the place where the optic nerve and blood vessels exit the retina. This area is a blind spot on the retina because there are no rods or cones at that location.
Vitreous Humor
Located inside the eyeball and separated by the lens, the vitreous humor is a clear, gel-like material.
Aqueous Humor
A clear, watery material, the aqueous humor is divided into two sections called the anterior chamber (in front of the iris) and the posterior chamber (behind the iris).
Canal of Schlemm
A circular canal between the cornea and the iris, the canal of Schlemm provides an exit for the aqueous humor from the eye into the bloodstream.
Lens
A transparent structure, the lens lies between the iris and the vitreous humor. This lens is flexible and can be made fatter or slimmer to give more or less focusing power to view near and far objects.
Conjunctiva
Covering the inside surface of the eyelids and sclera is a mucous membrane, the conjunctiva helps to keep the eye moist.
From:
(
http://faculty.atu.edu/cbrucker/Engl2053/Samples/Yang13
.htm)
- published: 12 May 2016
- views: 694