sclera

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eye

eye, organ of vision and light perception. In humans the eye is of the camera type, with an iris diaphragm and variable focusing, or accommodation. Other types of eye are the simple eye, found in many invertebrates, and the compound eye, found in insects and many other arthropods. In an alternate pathway to the one that transmits visual images, the eye perceives sunlight. This information stimulates the hypothalamus, which passes the information on to the pineal gland. The pineal gland then regulates its production of the sleep-inducing chemical, melatonin, essentially setting the body's circadian clock (see rhythm, biological).

The Human Eye

Anatomy and Function

The human eye is a spheroid structure that rests in a bony cavity (socket, or orbit) on the frontal surface of the skull. The thick wall of the eyeball contains three covering layers: the sclera, the choroid, and the retina. The sclera is the outermost layer of eye tissue; part of it is visible as the “white” of the eye. In the center of the visible sclera and projecting slightly, in the manner of a crystal raised above the surface of a watch, is the cornea, a transparent membrane that acts as the window of the eye. A delicate membrane, the conjunctiva, covers the visible portion of the sclera.

Underneath the sclera is the second layer of tissue, the choroid, composed of a dense pigment and blood vessels that nourish the tissues. Near the center of the visible portion of the eye, the choroid layer forms the ciliary body, which contains the muscles used to change the shape of the lens (that is, to focus). The ciliary body in turn merges with the iris, a diaphragm that regulates the size of the pupil. The iris is the area of the eye where the pigmentation of the choroid layer, usually brown or blue, is visible because it is not covered by the sclera. The pupil is the round opening in the center of the iris; it is dilated and contracted by muscular action of the iris, thus regulating the amount of light that enters the eye. Behind the iris is the lens, a transparent, elastic, but solid ellipsoid body that focuses the light on the retina, the third and innermost layer of tissue.

The retina is a network of nerve cells, notably the rods and cones, and nerve fibers that fan out over the choroid from the optic nerve as it enters the rear of the eyeball from the brain. Unlike the two outer layers of the eye, the retina does not extend to the front of the eyeball. Between the cornea and iris and between the iris and lens are small spaces filled with aqueous humor, a thin, watery fluid. The large spheroid space in back of the lens (the center of the eyeball) is filled with vitreous humor, a jellylike substance.

Accessory structures of the eye are the lacrimal gland and its ducts in the upper lid, which bathe the eye with tears, keeping the cornea moist, clean, and brilliant, and drainage ducts that carry the excess moisture to the interior of the nose. The eye is protected from dust and dirt by the eyelashes, eyelid, and eyebrows. Six muscles extend from the eyesocket to the eyeball, enabling it to move in various directions.

Eye Disorders

In addition to errors of refraction (astigmatism, farsightedness, and nearsightedness), the human eye is subject to various types of injury, infection, and changes due to systemic disease. Strabismus is a condition in which the eye turns in or out because of an imbalance in the eye musculature. A cornea damaged by accident or illness can sometimes be corrected by excimer laser or surgically replaced with a healthy one from a deceased person. Experimental retinal implants, consisting of electrode arrays that receive visual data from an external camera, have been used to partially restore sight to persons with damaged retinas, enabling some recognition of shapes, light and dark areas, and motion. Eyes that are used in various ways for surgical repairs are supplied by eye banks. People can arrange to have their eyes donated to such organizations after their death.

Eyes in Other Animals

The camera type of eye, which forms excellent images, is found in all vertebrates, in cephalopods (such as the squid and octopus), and in some spiders. In each of those groups the camera type of eye evolved independently. In some species, e.g., kestrels, the eye can perceive ultraviolet light, an aid to tracking prey.

Simple eyes, or ocelli, are found in a great variety of invertebrate animals, including flatworms, annelid worms (such as the earthworm), mollusks, crustaceans, and insects. An ocellus has a layer of photosensitive cells that can set up impulses in nerve fibers; the more advanced types also have a rigid lens for concentrating light on this layer. Simple eyes can perceive light and dark, enabling the animal to perceive the location and movement of objects. They form no image, or a very poor one.

The compound eye is found in a large number of arthropods, including various species of insects, crustaceans, centipedes, and millipedes. A compound eye consists of from 12 to over 1,000 tubular units, called ommatidia, each with a rigid lens and photosensitive cells; each omnatidium is surrounded by pigment cells and receives only the light from its own lens. The lenses fit together on the surface of the eye, forming the large, many-faceted structure that can be seen, for example, in the fly. Each ommatidium supplies a small piece of the image perceived by the animal. The compound eye creates a poor image and cannot perceive small or distant objects; however, it is superior to the camera eye in its ability to discriminate brief flashes of light and movement, and in some insects (e.g., bees) it can detect the polarization of light. Because arthropods are so numerous, the compound eye is the commonest type of animal eye.

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Sclera

 

the external tough fibrous tunic of the eye, which performs a supportive and protective function. In most vertebrates the sclera contains cartilaginous tissue and bony laminae that form a sclerotic ring. (Chondrichthians and modern amphibians and mammals do not have sclerotic rings.)

In humans the sclera consists of a dense fibrous tissue in which intermixed collagenous and elastic fibers are arranged predominantly in meridional and equatorial directions. Connective tissue cells are located between the fibers, and pigment cells (melanocytes) are found where the optic nerve leaves the eye. The external layer of the sclera contains a highly motile system of slender collagenous fibers and laminae separated from one another by slitlike cavities (Tenon’s capsule); the fibers and laminae facilitate rotation of the eyeball in various directions. The external surface of the sclera is attached to the tendons of the oculomotor muscles.

In humans the thickness of the sclera at the posterior pole of the eyeball is approximately 1 mm, at the equator approximately 0.3–0.4 mm, and at the anterior pole of the eyeball at the site of the transition into the cornea approximately 0.6 mm. Small branching cavities are found at the junction of the sclera and cornea; the cavities join to form Schlemm’s canal, which drains fluid from the anterior chamber of the eye.

O. G. STROEVA

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

sclera

[′skler·ə]
(anatomy)
The hard outer coat of the eye, continuous with the cornea in front and with the sheath of the optic nerve behind.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

sclera

the firm white fibrous membrane that forms the outer covering of the eyeball
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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However, we have shown that the collagen molecular spacing in both cornea and sclera remains fairly constant either when vitamin C is used alone or when it is used in the presence of sugar.
Our experiments do suggest that vitamin C was the least effective of all the inhibitors in terms of minimizing changes in collagen-associated fluorescence in both cornea and sclera even though it seemed to prevent structural changes associated with glycation.
The viable options therefore include epikeratophakia anterior chamber IOL implant iris fixated intraocular lens and sclera fixated posterior chamber IOL implants.