Monday, September 4, 2017

The Retina

Retina is a delicate servos membrane, upon the surface of which the images of external objects are received. Its outer surface is in contact which the choroid; its inner with the vitreous body. Behind it is continuous with the optic nerve; it is gradually diminishes in thickness from behind forward; and, in front, extends nearly as far as the ciliary body, where it appears to terminate is a jagged margin, the ora scrrata. Here the nervous tissues of the retina end, a thin prolongation of the membrane extends forward over the back of the ciliary processes and iris, forming the parts ciliaris retinae and parts iridiae retina, already referred to. This forward prolongation consists of the pigmentary layer of the retina together with a stratum of columnar epithelium.



The retina is soft, semi transparent, and of a purple tint in the fresh state, owing to the presence of a coloring material named rhodopsin or visual purple; but it soon becomes eluded, opaque and bleached when exposed to sunlight. Exactly in the center of the posterior park of the retina, corresponding to the axis of the eye, and at a point in the fresh state, owing to the presence of a coloring material named rhodopsin or visual purple; but it soon becomes clouded, opaque and bleached when exposed to sunlight. Exactly in the center of the posterior part of the retina, corresponding to the exist of the eye, and at a point in which the sense of vision is most perfect is an oval yellowish spot, called, after its discover, the yellow spot or macula lutea of Sommering , having a central depression, the fovea centralis.

The retina in the situation of the fovea centralis is exceedingly thin, and the dark color of the choroid is distinetly seen through it; so that it presents more the appearance of a foramen, and hence the name “foramen of Sommering” at first given to it. It exist only in man, the quadrumana and some suarian reptiles. About one-eight of an inch to the inner side of the yellow spot is the point of entrance of the optic nerve (porus opticus); here the nervous substance is slightly raised so as to form an eminence (colliculus nervi optivi); the arteria centralis retinae pierce its centre. That is the only part of the surface of the retina from which the power of vision is absent, and is termed the “blind spot.”

If the membrane damage than our eyes need use an aid tools like glass or contact lense, right now people don't need use this glass if use current technology by laser surgery LASIK.

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