  Sight is considered the most important of the sensations. It is the chief means of bringing the body into proper relations with its surroundings and, even more than the sensation of hearing, is an avenue for the reception of ideas. The sense organs for the production of sight are the eyes; the external stimulus is Light.—Light, like sound, consists of certain vibrating movements, or waves. They differ from sound waves, however, in form, velocity, and in method of origin and transmission. Light waves are able to pass through a vacuum, thus showing that they are not dependent upon air for their transmission. They are supposed to be transmitted by what the physicist calls ether—a highly elastic and exceedingly thin substance which fills all space and penetrates all matter. As a rule, light waves originate in bodies that are highly heated, being started by the vibrations of the minute particles of matter. Light is influenced in its movements by various conditions. In a substance of uniform density it moves with an unchanging velocity and in a straight line. If it enters a less dense, or rarer, substance, its velocity increases; if one more dense, its velocity diminishes; and if it enters either the rarer or denser substance in any direction other than perpendicularly, it is bent out of its course, or refracted. If it strikes against a body lying in its course, it may be thrown off (reflected), or it may enter the body and either be passed on through (transmitted) or absorbed (Fig. 157). Light which is absorbed is transformed into heat. [pg 371]
Fig. 157 Fig. 157—Diagram illustrating passage of light waves.On the right the light is transmitted by the glass, reflected by the mirror, refracted by the prism, and absorbed by the black cloth. On the left the light from the candle forms an image by passing through a small hole in a cardboard and falling upon a screen. Formation of Images.—Another principle necessary to seeing is that of refraction. Refraction means the bending, or turning, of light from a straight course. One of the most interesting effects of refraction is the formation of images of objects, such as may be accomplished by light from them passing in a certain manner through convex lenses. If, for example, a convex lens be moved back and forth[pg 372]
Fig. 158 Fig. 158—Diagram illustrating formation of images. On the right the image is formed by a double convex lens; on the left by the lenses of the eye. The candle flame represents a luminous, or light-giving, body; but light passes from the large arrow by reflection. (See text.) In order to form an image, the light waves spreading out from the object must be brought together, or focused. Focusing means literally the bringing of light to a point, but it is evident in the formation of an image that all the waves are not brought to a single point. If they were, there would be no image. In the example of the candle given above, the explanation is as follows: The light from the candle comes from a great number of separate and distinct points in the candle flame. The lens, by its peculiar shape, bends the waves coming from any single point so that they are brought to a corresponding point on the screen. Furthermore, the points of focused light are made to occupy the same relative positions on the screen as the points from which they emanate in the candle flame (Fig. 158). This is why the area of light on the screen has the same form as the candle, or makes an image of it. The same explanation applies if, instead of the luminous candle, a body that simply reflects light, as a book, is used. The Problem of Seeing.—What we call seeing is vastly more than the stimulation of the brain through the action of light upon afferent neurons. It is the perceiving of all the different things that make up our surroundings. If[pg 373] Sense Organs of Sight.—The sense organs of sight consist mainly of the two eyeballs. Each of these is located in a cavity of the skull bones, called the orbit, where it is held in position by suitable tissues and turned in different directions by a special set of muscles. A cup-shaped receptacle is provided within the orbit, by layers of fat, and a smooth surface is supplied by a double membrane that lies between the fat and the eyeball. In front the eyeballs are provided with movable coverings, called the eyelids. These are composed of dense layers of connective tissue, covered on the outside by the skin and lined within by a sensitive membrane, called the conjunctiva. At the base of the lids the conjunctiva passes to the eyeball and forms a firmly attached covering over its front surface. This membrane prevents the passage of foreign materials back of the eyeball, and by its sensitiveness stimulates effort for the removal of irritating substances from beneath the lids. The eyelashes and the eyebrows are also a means of protecting the eyeballs. The Eyeball, or globe of the eye, is a device for focusing light upon a sensitized nervous surface which it incloses and protects. In shape it is nearly spherical, being about [pg 374]
Fig. 159 Fig. 159—Diagram of the eyeball in position. 1. Yellow spot. 2. Blind spot. 3. Retina. 4. Choroid coat. 5. Sclerotic coat. 6. Crystalline lens. 7. Suspensory ligament. 8. Ciliary processes and ciliary muscle. 9. Iris containing the pupil. 10. Cornea. 11. Lymph duct. 12. Conjunctiva. 13. Inferior and superior recti muscles. 14. Optic nerve. 15. Elevator muscle of eyelid. 16. Bone. A. Posterior chamber containing the vitreous humor. B. Anterior chamber containing the aqueous humor. The Outer Coat surrounds the entire globe of the eye and consists of two parts—the sclerotic coat and the cornea. The sclerotic coat covers the greater portion of the larger spherical segment and is recognized in front as "the white of the eye." It is composed mainly of fibrous connective tissue and is dense, opaque, and tough. It preserves the form of the eyeball and protects the portions within. It is pierced at the back by a small opening which admits the[pg 375] The cornea forms the transparent covering over the lesser spherical segment of the eyeball, shading into the sclerotic coat at its edges. It has a complex structure, consisting in the main of a transparent form of connective tissue. It serves the purpose of admitting light into the eyeball. The Middle Coat consists of three connected portions—the choroid coat, the ciliary processes, and the iris. These surround the larger spherical segment. All three parts are rich in blood vessels, containing the blood supply to the greater portion of the eyeball. The choroid coat lies immediately beneath the sclerotic coat at all places except a small margin toward the front of the eyeball. It is composed chiefly of blood vessels and a delicate form of connective tissue that holds them in place. It contains numerous pigment cells which give it a dark appearance and serve to absorb surplus light. Near where the sclerotic coat joins the cornea, the choroid coat separates from the outer wall and, by folding, forms many slight projections into the interior space. These are known as the ciliary processes. The effect of these folds is to collect a large number of capillaries into a small space and to give this part of the eyeball an extra supply of blood. Between the ciliary processes and the sclerotic coat is a small muscle, containing both circular and longitudinal fibers, called the ciliary muscle. The iris is a continuation of the choroid coat across the front of the eyeball. It forms a dividing curtain between the two spherical segments and gives the color to the eye. At its center is a circular opening, called the pupil, which admits light to the back of the eyeball. By varying the[pg 376]
Fig. 160 Fig. 160—Diagram showing main nervous elements in the retina. Light waves stimulate the rods and cones at back surface of the retina, starting impulses which excite the ganglion cells at the front surface. Fibers from the ganglion cells pass into the optic nerve. The Inner Coat, or Retina.—This is a delicate membrane containing the expanded termination of the optic nerve. It rests upon the choroid coat and spreads over about two thirds of the back surface of the eyeball. Although not more than one fiftieth of an inch in thickness, it presents a very complex structure, essentially nervous, and is[pg 377] In the center of the retina is a slight oval depression having a faint yellowish color, and called, on that account, the yellow spot. This is the part of the retina which is most sensitive to light. Directly over the place of entrance of the optic nerve is a small area from which the rods and cones are absent and which, therefore, is not sensitive to light. This is called the blind spot. (See Practical Work.) The Crystalline Lens.—Immediately back of the iris and touching it is a transparent, rounded body, called the crystalline lens. This is about one fourth of an inch thick and one third of an inch through its long diameter, and is more curved on the back than on the front surface. It is inclosed in a thin sheath, called the membranous capsule, which connects with a divided sheath from the sides of the eyeball, called the suspensory ligament (Fig. 159). Both the lens and the capsule are highly elastic. Chambers and "Humors" of the Eyeball.—The crystalline lens together with the suspensory ligament and the ciliary processes form a partition across the eyeball. This divides the eye space into two separate compartments, which are filled with the so-called "humors" of the eye. The front cavity of the eyeball, which is again divided in part by the iris, is filled with the aqueous humor. This is a clear, lymph-like liquid which contains an occasional[pg 378] The back portion of the eyeball is filled with a soft, transparent, jelly-like substance, called the vitreous humor. It is in contact with the surface of the retina at the back and with the attachments of the lens in front, being surrounded by a thin covering of its own, called the hyaloid membrane. The aqueous and vitreous humors aid in keeping the eyeball in shape and also in focusing. How we see Objects.—To see an object at least four things must happen: 1. Light must pass from the object into the eye. Objects cannot be seen where there is no light or where, for some reason, it is kept from entering the eye. 2. The light from the object must be focused (made to form an image) on the retina. In forming the image, an area of the retina is stimulated which corresponds to the form of the object. 3. Impulses must pass from the retina to the brain, stimulating it to produce the sensations. 4. The sensations must be so interpreted by the mind as to give an impression of the object. Focusing Power of the Eyeball.—The eyeball is essentially a device for focusing light. All of its transparent portions are directly concerned in this work, and the portions that are not transparent serve to protect and operate these parts and hold them in place. Of chief importance are the crystalline lens and the cornea. Both of these are lenses. The cornea with its inclosed liquid is a plano-convex lens, while the crystalline lens is[pg 379]
Fig. 161 Fig. 161—Diagram showing changes in shape of crystalline lens to adapt it to near and distant vision. Accommodation.—A difficulty in focusing arises from the fact that the degree of divergence of the light waves entering the eye from different objects, varies according to their distance. Since the waves from any given point on an object pass out in straight lines in all directions, the waves that enter the eye from distant objects are at a different angle from those that enter from near objects. In reality waves from distant objects are practically parallel, while those from very near objects diverge to a considerable degree. To adjust the eye to different distances requires some change in the focusing parts that corresponds to the differences in the divergence of the light.[pg 380] 1. In looking from a distant to a near object, the lens becomes more convex, i.e., rounder and thicker (Fig. 161). This change is necessary because the greater divergence of the light from the near objects requires a greater converging power on the part of the lens.125 2. In looking from near to distant objects, the lens becomes flatter and thinner (Fig. 161). This change is necessary because the less divergent waves from the distant objects require less converging power on the part of the lens. The method employed in changing the shape of the lens is difficult to determine and different theories have been advanced to account for it. The following, proposed by Helmholtz, is the theory most generally accepted: The lens is held in place back of the pupil by the suspensory ligament. This is attached at its inner margin to the membranous capsule, and at its outer margin to the sides of the eyeball, and entirely surrounds the lens. 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[pg 381] Movements of the Eyeballs.—In order that the light may enter the eyeballs 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 eyeballs are similar to those of ball and socket joints.
Fig. 162 Fig. 162—Exterior muscles of eyeball. Binocular Vision.—In addition to directing the eyeballs so that light may enter them to the best advantage from different objects, the[pg 382] 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. [pg 383]
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 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 (Fig. 163). 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 EYEDefects 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[pg 384] 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 illustrating 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 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[pg 385] 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 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 oculist126 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 glasses, 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,[pg 386] Reading Glasses.—Many people whose eyes are weak, because slightly defective, find great relief in the use of special glasses for reading and other close work. By using such glasses they may postpone the time when they are compelled to wear glasses 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 glasses 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 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]
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[pg 388] 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] 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? 11. What change occurs in the shape of the crystalline lens when we look from distant to near objects? From near to distant objects? Why are these changes necessary? How are they brought about? 12. How does the method of adjustment, or accommodation, of the eyeball differ from that of a telescope or a photographer's camera? 13. With two eyes how are we kept from seeing double? 14. What different purposes are served by the tears. Trace them from the lachrymal glands to the nostrils. 15. Show how the proper lenses remedy short- and long-sightedness. 16. Describe the conjunctiva and give its functions. Why should it be so sensitive? 17. How may eye strain cause disease in parts of the body remote from the eyes? 18. How does "image stimulation" differ from light stimulation in general? PRACTICAL WORKTo illustrate Simple Properties of Light.—1. Heat an iron or platinum wire in a clear gas flame. Observe that when a high temperature is reached it gives out light or becomes luminous. 2. Cover one hand with a white and the other with a black piece of cloth, and hold both for a short time in the direct rays of the sun. Note and account for the difference in temperature which is felt. [pg 390] 4. Place a coin in the center of an empty pan and let the members of the class stand where the coin is barely out of sight over the edges of the pan. Fill the pan with water and account for the coin's coming into view. Show by a drawing how light, in passing from the water into the air, is so bent as to enter the eye. 5. With a convex lens, in a darkened room, focus the light from a candle flame so that it falls on a white screen and forms an image of the candle. Observe that the image is inverted. In a well-lighted room focus the light from a window upon a white screen. Show that, as the distance from the window to the screen is changed, the position of the lens must also be changed. (Accommodation.) 6. Hold a piece of cardboard, about eight inches square and having a smooth, round hole an eighth of an inch in diameter in the center, in front of a lighted candle in a darkened room. Back of the opening place a muslin or paper screen (Fig. 157). Observe that a dim image is formed. Account for the fact that it is inverted. Hold a lens between the cardboard and the screen so that the light passes through it also. The image should now appear smaller and more distinct.
Fig. 166 Fig. 166—Diagram for proving presence of the blind spot. To prove the Presence of the Blind Spot.—Close the left eye and with the right gaze steadily at the spot on the left side of this page (Fig. 166). Then starting with the book a foot or more from the face, move it slowly toward the eye. A place will be found where the spot on the right entirely disappears. On bringing it nearer, however, it is again seen. As the book is moved forward or backward, the position of the[pg 391] Dissection of the Eyeball.—Procure from the butcher two or three eyeballs obtained from cattle. After separating the fat, connective tissue, and muscle, place them in a shallow vessel and cover with water. Insert the blade of a pair of sharp scissors at the junction of the sclerotic rotic coat with the cornea and cut from this point nearly around the entire circumference of the eyeball, passing near the optic nerve. Spread open in the water and identify the different parts from the description in the text. Open the second eyeball in water by cutting away the cornea. Examine the parts in front of the lens.
Fig. 167 Fig. 167—Model for demonstrating the eyeball. To illustrate Accommodation.—Paste together the ends of a strip of stiff writing paper (two by five inches) making a ring a little less than three inches in diameter. This is to represent the crystalline lens. Now paste a piece of thin paper (two by seven inches) upon a second strip of the same size, leaving an open place in the middle for the insertion of the paper lens. A flexible piece of cardboard (three by twelve inches) is now bent into the form of a half circle and to its ends are fastened the strips of paper containing the ring. Make a small hole in each of the four corners of the bent cardboard. Through these holes pass two loops of thread, or fine string, in opposite directions, letting the ends hang loose from the cardboard. When everything is in position, the tension from the cardboard flattens the paper lens, while pulling the strings releases this tension and permits the lens to become more rounded. With this simple device the changes in the curvature of the lens for near and distant vision are easily shown. |
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