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Updated: May 5, 2025
The retina lines about two-thirds of the posterior portion of the shell of the eye. It is made up of seven layers. The essential layer is named from its appearance, rods and cones. The middle coat is the choroid. This is a dark, pigmented, vascular and muscular membrane. The posterior portion is in contact with the retina. Anteriorly it forms the ciliary processes and the iris.
It is dark in color and fragile in structure, and is made up almost entirely of blood-vessels and nerves. As the choroid approaches the front part of the eyeball, its parts become folded upon themselves into a series of ridges, called ciliary processes. These folds gradually become larger, and at last merge into the ciliary or accommodation muscle of the eye.
The choroid coat lies immediately beneath the sclerotic coat at all places except a small margin toward the front of the eyeball. It is composed chiefly of blood vessels and a delicate form of connective tissue that holds them in place. It contains numerous pigment cells which give it a dark appearance and serve to absorb surplus light.
What we call 'seeing' is far more the result of an interplay between the retina carrying the nerves, and the choroid carrying the blood-vessels. In this interplay the nerves are the passive, receptive organ for the inworking of external light, while the blood-activity comes to meet the nerve-process with a precisely correlated action. In this action we find what Goethe called the 'inner light'.
Sydney Jones states that in the case of an old epileptic who had been accustomed to take nitrate of silver as a remedy, the choroid plexuses were remarkably dark, and from their surface could be scraped a brownish black, soot-like material, and a similar substance was found lying quite free in the cavity of the fourth ventricle, apparently detached from the choroid plexus. Path.
What we have described as the polar interplay of blood and nerve in the act of sight is not confined to the narrow field of the eye. Just as the nerve processes arising in the retina are continued to the optic centre in the cerebrum, so must we look for the origin of the corresponding blood process not in the choroid itself, but in the lower regions of the organism.
Near where the sclerotic coat joins the cornea, the choroid coat separates from the outer wall and, by folding, forms many slight projections into the interior space. These are known as the ciliary processes. The effect of these folds is to collect a large number of capillaries into a small space and to give this part of the eyeball an extra supply of blood.
The circular space thus left in front by the termination of the choroid is occupied by the iris, a thin, circular curtain, suspended in the aqueous humor behind the cornea and in front of the crystalline lens. In its center is a round opening for the admission of light. This is the pupil, which appears as if it were a black spot.
A patient, then aged 21, suffered three years ago from a scotoma almost central; and was first seen six months after that with a macular choroidal atrophy and abnormal pigmentation. She suffered, we afterwards concluded, from choroidal tuberculosis. A recurrence involving adjoining choroid occurred fourteen months ago.
It is surrounded by coats, which contain refracting mediums, called humors. There are three coats, called the sclerotic, the choroid, and the retina; and three humors, called the aqueous, the crystalline, and the vitreous. The sclerotic or outer coat, called also the white of the eye, is an opaque, fibrous membrane.
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