Part 35

EYELIDS AND TEARS.--The eyelids are close-fitting shutters to screen the eye. The inner side is lined with a mucous membrane that is exceedingly sensitive, and thus aids in protecting the eye from any irritating substance. The looseness of the skin favors swelling from inflammation or the effusion of blood, as in a "black eye." The eyelashes serve as a kind of sieve to exclude the dust, and, with the lids, to shield against a blinding light. Just within the lashes are oil glands, which lubricate the edges of the lids, and prevent them from adhering to each other. The tear or lachrymal gland, G, is an oblong body lodged in the bony wall of the orbit. It empties by several ducts upon the inner surface, at the outer edge of the upper eyelid. Thence the tears, washing the eye, run into the lachrymal lake, D, a little basin with a rounded border fitted for their reception. On each side of this lake two canals, C, C, drain off the overplus through the duct, B, into the nose. In old age and in disease, these canals fail to conduct the tears away, and hence the lachrymal lake overflows upon the face.

FIG. 63.

STRUCTURE OF THE RETINA.--In Fig. 63 is shown a section of the retina, greatly magnified, since this membrane never exceeds 1/80 an inch in thickness. On the inner surface next to the vitreous humor, is a lining membrane not shown in the cut. Next to the choroid and comprising about 1/4 the entire thickness of the retina, is a multitude of transparent, colorless, microscopic rods, a, evenly arranged and packed side by side, like the seeds on the disk of a sunflower. Among them, at regular intervals, are interspersed the cones, b. Delicate nerve fibers pass from the ends of the rods and cones, each expanding into a granular body, c, thence weaving a mesh, d, and again expanding into the granules, f. Last is a layer of fine nerve fibers, g, and gray, ganglionic cells, h, like the gray matter of the brain, whence filaments extend into i, the fibers of the optic nerve. (See p. 354.)

The layer of rods and cones is to the eye what the bristles, otoliths, and Cortian fibers are to the ear. Indeed, the nerve itself is insensible to light. At the point where it enters the eye, there are no rods and cones, and this is called the blind spot. A simple experiment will illustrate the fact. Hold this book directly before the face, and, closing the left eye, look steadily with the right at the left-hand circle in Fig. 64. Move the book back and forth, and a point will be found where the right-hand circle vanishes from sight. At that moment its light falls upon the spot where the rods and cones are lacking.

FIG. 64.

HOW WE SEE.--There is believed to be a kind of universal atmosphere, termed ether, filling all space. This substance is infinitely more subtle than the air, and occupies its pores, as well as those of all other substances. As sound is caused by waves in the atmosphere, so light is produced by waves in the ether. A lamplight, for example, sets in motion waves of ether, which pass in through the pupil of the eye, to the retina, where the rods and cones transmit the vibration through the optic nerve to the brain, and then the mind perceives the light. (Note, p. 236.)

THE USE OF THE CRYSTALLINE LENS. [Footnote: The uses of the eye and ear are dependent upon the principles of Optics and Acoustics. They are therefore best treated in Physics.]--A convex lens, as a common burning glass, bends the rays of light which pass through it, so that they meet at a point called the focus. The crystalline lens converges the rays of light which enter the eye, and brings them to a focus on the retina. [Footnote: The cornea and the humors of the eye act in the same manner as the crystalline lens, but not so powerfully.] The healthy lens has a power of changing its convexity so as to adapt [Footnote: The simplest way of experimenting on the "adjustment of the eye" is to stick two stout needles upright into a straight piece of wood,--not exactly, but nearly in the same straight line, so that, on applying the eye to one end of the piece of wood, one needle (A) shall be seen about six inches off, and the other (B) just on one side of it, at twelve inches distance. If the observer looks at the needle B he will find that he sees it very distinctly, and without the least sense of effort; but the image of A is blurred, and more or less double. Now, let him try to make this blurred image of the needle A distinct. He will find he can do so readily enough, but that the act is accompanied by a sense of fatigue. And in proportion as A becomes distinct, B will become blurred. Nor will any effort enable him to see A and B distinctly at the same time.--HUXLEY.] itself to near and to distant objects. (See Fig. 66.)

FIG. 65.

NEAR AND FAR SIGHT.--If the lens be too convex, it will bring the rays to a focus before they reach the retina; if too flat, they will reach the retina before coming to a focus. In either case, the sight will be indistinct. A more common defect, however, is in the shape of the globe of the eye, which is either flattened or elongated. In the former case (see G, Fig. 67), objects at a distance can be seen most distinctly-- hence that is called farsightedness. [Footnote: This should not be confounded with the long sight of old people, which is caused by the stiffness of the ciliary muscles, whereby the lens can not adapt itself to the varying distances of objects.] In the latter, objects near by are clearer, and hence this is termed nearsightedness. Farsightedness is remedied by convex glasses; nearsightedness, by concave. When glasses will improve the sight they should be worn; [Footnote: Dr. Henry W. Williams, the celebrated ophthalmologist, says that, in some cases, glasses are more necessary at six or eight years of age than to the majority of healthy eyes at sixty. Sometimes children find accidentally that they can see better through grandmother's spectacles. They should then be supplied with their own.] any delay will be liable to injure the eyes, by straining their already impaired power. Cataract is a disease in which there is an opacity of the crystalline lens or its capsules, which obscures the vision. The lens may be caused to be absorbed, or may be removed by a skillful surgeon and the defect remedied by wearing convex glasses.

FIG. 66.

FIG. 67.

COLOR-BLIND PERSONS receive only two of the three elementary color sensations (green, red, violet). The spectrum appears to them to consist of two decidedly different colors, with a band of neutral tint between. The extreme red end is invisible, and a bright scarlet and a deep green appear alike. They are unable to distinguish between the leaves of a cherry tree and its fruit by the color of the two, and see no difference between blue and yellow cloth. Whittier, the poet, it is said, could not tell red from green unless in direct sunlight. Once he patched some damaged wall paper in his library by matching a green vine in the pattern with one of a bright autumnal crimson. This defect in the eye is often unnoticed, and many railway accidents have doubtless happened through an inability to detect the color of signal lights.

CARE OF THE EYES.--The shape of the eye can not be changed by rubbing and pressing it, as many suppose, but the sight may thus be fatally injured. Children troubled by nearsightedness should not lean forward at their work, as thereby the vessels of the eye become overcharged with blood. They should avoid fine print, and try, in every possible way, to spare their eyes. If middle age be reached without especial difficulty of sight, the person is comparatively safe. Most cases of squinting are caused by longsightedness, the muscles being strained in the effort to obtain distinct vision. In childhood, it may be cured by a competent surgeon, who will generally cut the muscle that draws the eye out of place.

After any severe illness, especially after measles, scarlatina, or typhoid fever, the eyes should be used with extreme caution, since they share in the general debility of the body, and recover their strength slowly. Healthy eyes even should never be used to read fine print or by a dim light. Serious injury may be caused by an imprudence of this kind. Reading upon the cars is also a fruitful source of harm. The lens, striving to adapt itself to the incessantly varying distance of the page, soon becomes wearied. Whenever the eyes begin to ache, it is a warning that they are being overtaxed and need rest.

Objects that get into the eye should be removed before they cause inflammation; rubbing in the meantime only irritates and increases the sensitiveness. If the eye be shut for a few moments, so as to let the tears accumulate, and the upper lid be then lifted by taking hold of it at the center, the cinder or dust is often washed away at once. Trifling objects can be removed by simply drawing the upper lid as far as possible over the lower one; when the lid flies back to its place, the friction will detach any light substance. If it becomes necessary, turn the upper lid over a pencil, and the intruder may then be wiped off with a handkerchief. "Eye-stones" are a popular delusion. When they seem to take out a cinder, it is only because they raise the eyelid, and allow the tears to wash it out. No one should ever use an eyewash, except by medical advice. The eye is too delicate an organ to be trifled with, and when any disease is suspected, a reliable physician should be consulted. This is especially necessary, since, when one eye is injured, the other, by sympathy, is liable to become inflamed, and perhaps be destroyed.

When reading or working, the light should be at the left side, or at the rear; never in front.

The constant increase of defective eyesight among the pupils in our schools is an alarming fact. Dr. Agnew considers that our schoolrooms are fast making us a spectacle-using people. Nearsightedness seems to increase from class to class, until in the upper departments, there are sometimes as high as fifty per cent of the pupils thus afflicted. The causes are (1), desks so placed as to make the light from the windows shine directly into the eyes of the scholars; (2), cross lights from opposite windows; (3), insufficient light; (4), small type that strains the eyes; and (5), the position of the pupil as he bends over his desk or slate, causing the blood to settle in his eyes. All these causes can be remedied; the position of the desks can be changed; windows can be shaded, or new ones inserted; books and newspapers that try the eyes can be rejected; and every pupil can be taught how to sit at study.

PRACTICAL QUESTIONS.

1. Why does a laundress test the temperature of her flatiron by holding it near her cheek?

2. When we are cold, why do we spread the palms of our hands before the fire?

3. What is meant by a "furred tongue"?

4. Why has sand or sulphur no taste?

5. What was the origin of the word palatable?

6. Why does a cold in the head injure the flavor of our coffee?

7. Name some so-called flavors that are really sensations of touch.

8. What is the object of the hairs in the nostrils?

9. What use does the nose subserve in the process of respiration?

10. Why do we sometimes hold the nose when we take unpleasant medicine?