It is drawn perfectly tight so that the sides of the eyeball exert a continuous tension, or pull, on the membranous capsule, which, in its turn, exerts pressure on the sides of the lens, tending to flatten it.

This arrangement brings the elastic force of the eyeball into opposition to the elastic force of the lens. The ciliary muscle plays between these opposing forces in the following manner:

_To thicken the lens_, the ciliary muscle contracts, pulling forward the suspensory ligament and releasing its tension on the membranous capsule.

This enables the lens to thicken on account of its own elastic force. _To flatten the lens_, the ciliary muscle relaxes, the elastic force of the eyeball resumes its tension on the suspensory ligament, and the membranous capsule resumes its pressure on the sides of the lens. This pressure, overcoming the elastic force of the lens, flattens it.

*Movements of the Eyeb.a.l.l.s.*-In order that the light may enter the eyeb.a.l.l.s to the best advantage, they must be moved in various directions.

These movements are brought about through the action of six small muscles attached to each eyeball. Four of these, named, from their positions, the superior, inferior, internal, and external recti muscles, are attached at one end to the sides of the eyeball and at the other end to the back of the orbit (Fig. 162). These, in the order named, turn the eyes upward, downward, inward, and outward. The other two, the superior and inferior oblique muscles, aid in certain movements of the recti muscles and, in addition, serve to rotate the eyes slightly. The movements of the eyeb.a.l.l.s are similar to those of ball and socket joints.

[Fig. 162]

Fig. 162-*Exterior muscles of eyeball.*

*Binocular Vision.*-In addition to directing the eyeb.a.l.l.s so that light may enter them to the best advantage from different objects, the muscles also enable two eyes to be used as one. Whenever the eyes are directed toward the same object, an image of this object is formed on the retina of each. Double vision is prevented only by having the images fall on corresponding places in the two eyes. This is accomplished by the muscles.

In each act of seeing, it becomes the task of the superior and inferior recti muscles to keep the eyes in the same plane, and of the external and internal recti muscles to give just the right amount of convergence. If slight pressure is exerted against one of the eyes, the action of the muscles is interfered with and, as a consequence, one sees double. The advantages of two eyes over one in seeing lie in the greater distinctness and broader range of vision and in the greater correctness of judgments of distance.

*Visual Sensations.*-The visual sensations include those of _color_ and those of a _general sensibility to light_. Proof of 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.

[Fig. 163]

Fig. 163-*Diagram of irrigating system of the eye.* After wetting the eyeball the tears may also moisten the air entering the lungs.

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 eyeb.a.l.l.s and find their way in each eye to two small ca.n.a.ls whose openings may be seen on the edges of the lids near the inner corner (Fig. 163). These ca.n.a.ls 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 pa.s.sed 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 (_C_, Fig. 164).

[Fig. 164]

Fig. 164-*Diagrams ill.u.s.trating long-sightedness and short-sightedness*, and method of remedying these defects by lenses. _A._ Normal eye. _B._ Long-sighted eye. _C._ Short-sighted eye.

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 (_B_, Fig. 164). These defects in focusing are remedied by wearing gla.s.ses 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 particular defects present in a given eye.

Whenever defects in focusing are present, particularly in astigmatism, extra work is thrown on the ciliary muscle as well as the muscles that move the eyeb.a.l.l.s. 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 gla.s.ses should be fitted by a competent oculist(126) 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 (page 326).

*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 gla.s.ses, this is relieved.

*The Eyes of School Children.*-School children often suffer from defects of vision which render close work burdensome, and cause headache, general nervousness, and disease. Furthermore, the visual defects may be unknown both to themselves and to their parents. Pupils showing indications of eye-strain should be examined by an oculist, and fitted with gla.s.ses should defects be discovered.(127) The precaution, adopted by many schools, of having the eyes of all children examined by a competent physician employed for the purpose, is most excellent and worthy of imitation.

*Reading Gla.s.ses.*-Many people whose eyes are weak, because slightly defective, find great relief in the use of special gla.s.ses for reading and other close work. By using such gla.s.ses they may postpone the time when they are compelled to wear gla.s.ses constantly. It is in the close work that the extra strain comes upon the eyes, and if this is relieved, one can much better withstand the work of distant vision. The reading gla.s.ses should be fitted by a competent oculist, and used only for the purpose for which they are intended.

*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 pa.s.ses 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 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]

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 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 eyeb.a.l.l.s 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?

2. Why does the body not need a light-producing apparatus, corresponding to the larynx in the production of sound?

3. How is the light from a candle made to form an image?

4. What different things must happen in order that one may see an object?

5. Make a sectional drawing of the eyeball, locating and naming all the parts.

6. Of what parts are the outer, middle, and inner coats of the eyeball made up?

7. What portions of the eyeball reflect light? What absorb light? What transmit light? What refract light?

8. Show how the iris, the crystalline lens, the retina, the ciliary muscle, and the cornea aid in seeing.

9. Trace a wave of light from a visible object to the retina.

10. Why does not the inverted image on the retina cause us to see objects upside down?