The Eye : Part 4

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 25 of 28)

Visual Sensations. - The visual sensations include those of color and those of a general sensibility to light. Proof the existence of these types of sensation is found in color blindness, a defect which renders the individual unable to distinguish certain colors when he is still able to see objects. Color sensations are the results of light waves of different lengths acting on the retina. While the method by which waves of one length produce one kind of sensation and those of another length a different sensation is not understood, the cones appear to be the portions of the retina acted on to produce the color. On the other hand, the rods are sensitive to all wave lengths and give general sensibility to light.

Visual Perceptions. - "Seeing" is very largely the mental interpretation of the primary sensations and the conditions under which they occur. For example, our ability to see objects in their natural positions when their images are inverted on the retina is explained by the fact that we are not conscious of the retinal image, but of the mind's interpretation of it through experience. Experience has also taught us to locate objects in the direction toward which it is necessary to turn the eyes in order to see them. In other words, we see objects in the direction from which the light enters the eyes. That the object is not always in that direction is shown by the image in the mirror. The apparent size and form of objects are inferences, and they are based in part upon the size and form of the area of the retina stimulated. We judge of distance by the effort required to converge the eyes upon the objects, by the amount of divergence of the waves entering the pupil, and also by the apparent size of the object.

The Lachrymal Apparatus. - Seeing requires that the light penetrate to the retina. For this reason all the structures in front of the retina are transparent. One of these structures, the cornea, on account of its exposure to the air, is liable to become dry, like the skin, and to lose its transparency. To preserve the transparency of the cornea, and also to lubricate the eyelids and aid in the removal of foreign bodies, a secretion, called tears, is constantly supplied.

The lachrymal, or tear, glands are situated at the upper and outer margins of the orbits. They have the general structure of the salivary glands and discharge their liquid by small ducts beneath the upper lids. From here the tears spread over the surfaces of the eyeballs and find their way in each eye to two small canals whose openings may be seen on the edges of the lids near the inner corner. These canals unite to form the nasal duct, which conveys the tears to the nasal cavity on the same side of the nose. When by evaporation the eyeball becomes too dry, the lids close reflexively and spread a fresh layer of tears over the surface. Any excess is passed into the nostrils, where it aids in moistening the air entering the lungs.

Hygiene of the Eye

Defects in Focusing. - The delicacy and complexity of the sense organs of sight render them liable to a number of imperfections, or defects, the most frequent and important being those of focusing. Such defects not only result in the imperfect vision of objects, but they throw an extra strain upon the nervous system and may render the process of seeing exceedingly painful.

A normal eye is able, when relaxed, to focus light accurately from objects which are twenty feet or more away and to accommodate itself to objects as near as five inches. An eye is said to be myopic, or short-sighted, when it is unable to focus light waves from distant objects, but can only distinguish the objects which are near at hand. In such an eye the ball is too long for the converging power of the lenses, and the image is formed in front of the retina.

A long-sighted, or hypermetropic, eye is one which can focus light from distant objects, but not from near objects. In such an eye the ball is too short for the converging power of the lenses and the image tends to form back of the retina. These defects in focusing are remedied by wearing glasses with lenses so shaped as to counteract them. Short-sightedness is corrected by concave lenses and long-sightedness by convex lenses, as shown in diagrams above.

Astigmatism is another defect in the focusing power of the eye. In astigmatism the parts of the eye fail to form the image in the same plane, so that all portions of the object do not appear equally distinct. Certain parts of it are indistinct, or blurred. The cause is found in some difference in curvature of the surfaces of the cornea or crystalline lens. It is corrected by lenses so ground as to correct the particulary defects present in a given eye.

Whenever defects in focusing are present, Particularly in astigmatism, extra work is thrown on the ciliate muscle as well as the muscles that move the eyeballs. The result is frequently to induce a condition, known as muscle weakness, which renders it difficult to use the eyes. Even after the defect in focusing has been remedied, the muscles recover slowly and must be used with care. For this reason glasses should be fitted by a competent oculist as soon as a defect is known to exist. When one is unduly nervous, or suffers from headache, the eyes should be examined for defects in focusing.

Eye Strain and Disease. - The extra work thrown upon the nervous system through seeing with defective eyes, especially in reading and other close work, is now recognized as an important cause of disease. Through the tax made upon the nervous system by the eyes, there may be left an insufficient amount of nervous energy for the proper running of the vital processes. As a result there is a decline of the health. Ample proof that eye strain interferes with the vital processes and causes ill health, is found in the improvements that result when, by means of glasses, this is relieved.

D.C. Heath and Co. - Publishers
Original copyright 1909