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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
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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