| The most encouraging fact about your eyes is | | | | nerve, a sensitive membrane called the retina. |
| that the less effort you make to see the better | | | | This is an exceedingly thin and highly complicated |
| you can see. It is difficult for people with | | | | membrane, in which are the terminations of the |
| imperfect sight to believe that perfect sight | | | | nerve tendrils, which are of two kinds: rods and |
| requires no effort; indeed, any effort to improve | | | | cones. |
| the sight makes it worse. | | | | The cones are the most sensitive and are found |
| Curiously enough, we have long known that | | | | toward the center of the retina where the |
| before we can swim or dance, play tennis or golf, | | | | keenest seeing is done. This center, about a |
| or master a musical instrument, our muscles must | | | | sixteenth of an inch in diameter, is called the |
| be relaxed. Acquiring that relaxation of the | | | | macula. Here form and color and sharp definition |
| muscles has often been the hardest part of | | | | are registered. Beyond this spot, cones and rods |
| learning any of these things, but without it all the | | | | mingle, but toward the periphery only rods are |
| other lessons are useless. It is a similar case with | | | | found. These are sensitive to dim light and |
| the eyes. | | | | therefore are used for night seeing. |
| It may be helpful here to summarize briefly the | | | | About a tenth of an inch to the nasal side from |
| structure of the eye. The eyeball is embedded in | | | | the center is the optic disc, the point of entrance |
| fat and fibrous tissue and lodged in bony sockets | | | | of the optic nerve, commonly called the blind spot. |
| called orbits. It is held in place and rotated by | | | | When light rays reach the surface of the retina, |
| means of six extrinsic muscles, attached to the | | | | the radiant energy undergoes a process of |
| posterior bony wall of the socket; at the anterior | | | | chemical change, and this new form of energy is |
| pole, they are inserted into the eyeball. | | | | carried by the optic nerve into the visual centers |
| The eyeball itself is made up of three distinct | | | | of the brain. |
| layers, or coats. The outer part, which we know | | | | It has long been known that the eye adjusts to |
| as the white of the eye, but which is more | | | | various distances, a process called |
| properly called the sclera, is a tough, fibrous | | | | accommodation. In myopia, or near-sighted vision, |
| tissue, which begins where the cornea leaves off, | | | | the rays of light are thrown to a point in front of |
| and extends back to where it is pierced by the | | | | the retina. In hypermetropia, or long-sightedness, |
| optic nerve. Four-fifths of the sclera is opaque; | | | | the rays of light focus in a point behind the retina. |
| the remaining one-fifth is a translucent area | | | | While in astigmatism, which causes distorted |
| directly at the front of the eye, called the cornea. | | | | images, the light rays are spread in a diffused |
| Inside the sclera is a second coat, or lining, known | | | | area on the retina, rather than in a point. In |
| as the choroid, composed of blood vessels and | | | | normal vision, the image is focused in a point |
| pigment, and is essentially a nutrient organ, | | | | directly upon the retina itself. |
| providing nutritive fluid to the retina. Within the | | | | Such is the wonder and complexity of our eyes. |
| choroid is a third coat, an extension of the optic | | | | |