While it is quite technical to deal in physiological technicalities, which can be found in any textbook on ophthalmology, it may be helpful to summarize briefly the structure of the eye. The eyeball is embedded in fat and fibrous tissue and lodged in bony sockets called orbits. It is held in place and rotated by means of six extrinsic muscles, attached to the posterior bony wall of the socket; at the anterior pole, they are inserted into the eyeball.
The eyeball itself is made up of three distinct layers, or coats. The outer part, which we know as the white of the eye, but which is more properly called the sclera, is a tough, fibrous tissue, which begins where the cornea leaves off, and extends back to where it is pierced by the optic nerve. Four-fifths of the sclera is opaque; the remaining one-fifth is a translucent area directly at the front of the eye, called the cornea.
Inside the sclera is a second coat, or lining, known as the choroid, composed of blood vessels and pigment, and is essentially a nutrient organ, providing nutritious fluid to the retina.
Within the choroid is a third coat, an extension of the optic nerve, a sensitive membrane called the retina. This is an exceedingly thin and highly complicated membrane, in which are the terminations of the nerve tendrils, which are of two kinds: rods and cones. The cones are the most sensitive and are found toward the center of the retina where the keenest seeing is done.
This center, about a sixteenth of an inch in diameter, is called the macula. Here form and color and sharp definition are registered. Beyond this spot, cones and rods mingle, but toward the periphery only rods are found. These are sensitive to dim light and therefore are used for night seeing.
About a tenth of an inch to the nasal side from the center is the optic disc, the point of entrance of the optic nerve, commonly called the blind spot. You can demonstrate this blind spot easily for yourself. Cover your left eye and look at the circle on the left with your right eye, moving the book slowly from side to side. When the right-hand circle vanishes, its image has fallen on the blind spot of the retina.
When light rays reach the surface of the retina, the radiant energy undergoes a process of chemical change, and this new form of energy is carried by the optic nerve into the visual centers of the brain.
It has long been known that the eye adjusts to various distances, a process called accommodation. In myopia, or near-sighted vision, the rays of light are thrown to a point in front of the retina. In hypermetropia, or long-sightedness, the rays of light focus in a point behind the retina. In astigmatism, which causes distorted images, the light rays are spread in a diffused area on the retina, rather than in a point. In normal vision, the image is focused in a point directly upon the retina itself.
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