General Precautions in the Use of the Eyes.—If proper care is exercised in the use of the eyes, many of their common ailments and defects may be avoided. Any one, whether his eyes are weak or strong, will do well to observe the following precautions:
1. Never read in light that is very intense or very dim. 2. When the eyes hurt from reading, stop using them. 3. Never hold a book so that the smooth page reflects light into the eyes. The best way is to sit or stand so that the light passes over the shoulder to the book. 4. Never study by a lamp that is not shaded. 5. Practice cleanliness in the care of the eyes. Avoid rubbing the eyes with the fingers unless sure the fingers are clean.
If the eyes are weak, use them less and avoid, if possible, reading by artificial light. Weak eyes are sometimes[pg 387] benefited by bathing them in warm water, or with water containing enough salt to make them smart slightly. Boracic acid dissolved in water (40 grains to 4 ounces of distilled water) is also highly recommended as a wash for weak eyes.
Fig. 165—Method of procedure in lifting the eyelid (Pyle).
Removal of Foreign Bodies from the Eyes.—Foreign bodies embedded in the eyeball should be removed by the oculist or physician. Small particles of dust or cinder may be removed without the aid of the physician, by exercising proper care. First let the tears, if possible, wash the offending substance to the corner of the eye, or edge of the lid, where it can be removed with a soft cloth. If it sticks to the ball or the under surface of the lid, it will be necessary to find where it is located, and then dislodge it from its position. Begin by examining the lower lid. Pull it down sufficiently to expose the inner surface, and, if the foreign substance be there, wipe it off with the hem of a clean handkerchief. If it is not under the lower lid, it will be necessary to fold back the upper lid. "The patient is told to look down, the edge of the lid and the lashes are seized with the forefinger and thumb of the right hand (Fig. 165), and the lid is drawn at first downward and forward away from the globe; then upward and backward over the point of the thumb or forefinger of the left hand, which is held stationary on the lid, and acts as a fulcrum."[128] The foreign body is now removed in[pg 388] the same manner as from the lower lid. A large lens may be used to good advantage in finding the irritating substance.
Strong Chemicals in the Eyes.—Students in the laboratory frequently, through accident, get strong chemicals, as acids and bases, in the eyes. The first thing to do in such cases is quickly and thoroughly to flood the eyes with water. Any of the chemical which remains may then be counteracted by the proper reagent, care being taken to use a very dilute solution. To counteract an acid, use sodium bicarbonate (cooking soda), and for bases use a very dilute solution of acetic acid (vinegar). To guard against getting the counteractive agent too strong for the inflamed eye, it should first be tried on an eye that has not been injured.
Summary.—The nervous impulses that cause the sensation of sight are started by light waves falling upon a sensitized nervous surface, called the retina. By means of refractive agents, forming a part of the eyeball in front of the retina, light from different objects is focused and made to form images of the objects upon the surface. In this way the light is made to stimulate a portion of the retina corresponding to the form of the object. This, the image method of stimulation, enables the mind to recognize objects and to locate them in their various positions. While the greater portion of the eyeball is concerned in the focusing of light, the crystalline lens, operated by the ciliary muscle, serves as the special instrument of accommodation. Muscles attached to the eyeballs turn them in different directions, and so adjust them with reference to each other that double vision is avoided.
Exercises.—1. Under what conditions are light waves reflected, refracted, and absorbed?
[pg 389]2. Why does the body not need a light-producing apparatus, corresponding to the larynx in the production of sound?