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In ordinary conditions of the eye, this ligament is kept tense so that the front part of the lens is flattened somewhat by the pressure on it. All around the edge, where the cornea, sclerotic, and choroid meet, is a ring of involuntary muscular fibers, forming the ciliary muscle.
Between the ciliary processes and the sclerotic coat is a small muscle, containing both circular and longitudinal fibers, called the ciliary muscle. The iris is a continuation of the choroid coat across the front of the eyeball. It forms a dividing curtain between the two spherical segments and gives the color to the eye.
It also serves to hold the choroid and the retina in position, and to maintain the proper relations of the inner structures of the eye. The iris consists of a framework of connective tissue, the surface of which is lined by cells containing pigment, which gives color to the eye. Bundles of involuntary muscular fibers are found in the substance of the iris.
The eye is bathed with a watery fluid, and protected by the eyelids and the eyebrows; it is moved in various directions, by muscles, all of which will soon be described. The Coats of the Eyeball. The eyeball proper is elastic but firm, and is composed of three coats, or layers, each of which performs important functions. These coats are the sclerotic, the choroid, and the retina.
It lines nearly the whole of the inner surface of the posterior chamber, resting on the inner surface of the choroid. It is with the retina, therefore, that the vitreous humor is in contact. The retina is a very thin, delicate membrane. Although very thin, it is made up of ten distinct layers, and is so complicated in structure that not even a general description will be attempted in this book.
A description of the ordinary type of albino would be as follows: The skin and hair are deprived of pigment; the eyebrows and eyelashes are of a brilliant white or are yellowish; the iris and the choroid are nearly or entirely deprived of coloring material, and in looking at the eye we see a roseate zone and the ordinary pink pupil; from absence of pigment they necessarily keep their eyes three-quarters closed, being photophobic to a high degree.
To prevent any reflected light from striking the plate and interfering with the sharpness of the picture, the interior of the photographic camera box is darkened. The pigmented layer of the choroid coat represents this blackened lining. In the camera, the artist uses a thumb-screw to bring to a focus on the sensitive plate the rays of light coming from objects at different distances.
It serves the purpose of admitting light into the eyeball. *The Middle Coat* consists of three connected portions—the choroid coat, the ciliary processes, and the iris. These surround the larger spherical segment. All three parts are rich in blood vessels, containing the blood supply to the greater portion of the eyeball.
The spinal fluid itself is a secretion of another gland at the base of the brain, the choroid. Nerves and internal secretion are associated here with a closeness symbolic of their general relations. Robertson, an American chemist, separated from the anterior lobe a substance soluble in the fat solvents, like ether and gasoline, which he christened tethelin.
It may even be several minutes before the field of vision really appears black, that is, before the activity of the inner light in the choroid has so far died away that equilibrium prevails between the non-stimulated nerves and the non-stimulated blood. With this insight into the twofold nature of the process of vision we are now able to describe more fully the negative after-image.
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