51.Why is the eye called the "window of the soul?" Why, the subject of enthusiastic study?51. The Organ of Sight.—The Eye.—The proximity of the eye to the brain, and the important part it performs in giving expression to the emotions, have given it the name of "the window of the soul." The exceeding beauty of its external parts, and the high value of its function, have long made this organ the subject of enthusiastic study. It is chiefly within the last twenty years, however, that this study has been successful and fruitful of practical results. Several ingenious instruments have been invented for the examination of the eye in health and disease, and new operations have been devised for the relief of blindness and of impaired vision. As a result, it is now a well-marked fact that, in civilized lands, the number of those who suffer from loss of sight is proportionally much less than in countries where science is less known and cultivated.52.The most obvious fact? The consequence? The next thing noticed? Its range of view? Of what does the organ of vision consist?52.The most obvious fact in respect to the apparatus ofsight is that there are two eyes, which may either act together as one, and be fixed upon one object, or one eye may be used independently of the other. In consequence of this arrangement the loss of one eye does not necessitate blindness, and, in fact, it not infrequently happens that the sight of one eye may be long impaired or lost before the fact is discovered. We next notice that it is placed at the most elevated part of the body, in front, and near the brain. It also commands a wide range of view, being itself moved with great rapidity, and being further aided by the free motion of the head and neck. The organ of vision consists essentially of two parts: the optical instrument itself—the eyeball—and its enveloping parts, or the case in which the instrument is kept free from harm. The latter, which are external, and which we shall first consider, are chiefly theOrbits, theEyelids, and the apparatus for theTears.53.The protection of the eyeball against injury? The overhanging brow? The opening for the optic nerve?53. The Orbits.—The eyeball, which is a delicate organ, is well defended against external injury within the orbits or bony sockets of the head. These are deep conical hollows, bounded in part by the bones of the skull, and in part by those of the nose and cheek. The orbit juts out beyond the most exposed portion of the eyeball, as may be seen by laying a book over the eye, when it will be found that no part of the eyeball, unless it be very prominent, will be touched by the book; so that the only direction in which an injury is liable to be received is immediately in front of the eye. The overhanging brow is itself covered by a layer of thick skin, studded with short, stout hairs, which are so bent as to prevent the perspiration from running into the eye and obscuring vision. Through a hole in the bottom of the orbit, the nerve of sight passes outward from the brain. The orbit also contains aconsiderableamount of a fatty tissue, upon which, as upon an elastic cushion, the eye rests.Fig. 47.--Front View of Right Eye.Fig. 47.—Front View of Right Eye.(Natural Size.)1. The Lachrymal, or tear gland, lying beneath the upper eyelid.2. The Nasal Duct is shown by the dotted line. The * marks the orifice in the lower lid.The central black spot is thepupil; surrounding it is theiris; and the triangular white spaces are the visible portion of thesclerotic.54.What are the eyelids? The upper lid? The lower one? The mucous membrane of the eye?54. The Eyelids.—The eyelids are two movable curtains, or folds, which, when shut, cover the front part of the orbit, and hide the eye from view. The upper lid is the larger, has a curved margin, and moves freely, while the lower lid is comparatively short and straight, and has but a slight degree of motion (Fig. 47). Skin covers the exterior of the lids, while a fine mucous membrane lines their inner surface, and is likewise spread out over the entire front of the eyeball. This membrane, which is called theConjunctiva, is highly sensitive, and thus plays an important part in protecting the eye against the lodgment of sand, ashes, chaff, and other foreign particles that are blown about in the air. This sensitive membrane will not endure the presence of these particles. If any find access, it causes a constant winking, a flow of tears, and other signs of irritation, until it is removed.55.The eyelashes? The little points within the line of the lashes? Of what use are these glands?55.The long, silky eyelashes, which garnish the edges of the lids, act like a sieve to prevent the entry of dust and the like; and together with the lids, they regulate the amount of light which is permitted to enter the eye, so that it is shielded from a sudden flood or glare of light. The little points seen in the figure just within the line of the lashes, especially on the lower lid, represent the mouths of numerous little sebaceous glands (Fig. 48,D,D), such as are alwaysfound in the neighborhood of hairs. These glands supply a thick, oily material which greases the edges of the lids and prevents their adhering together, and likewise prevents the overflow of the tears upon the cheek.56.The location of the lachrymal gland? The use of the gland?56. The Lachrymal Fluid, or the Tears.—Just within the outer part of the bony arch of the brow, where the bone may be felt to be sharper than in other positions, is lodged a little organ called the lachrymal gland, the situation of which is indicated in Fig. 47, 1. This is the gland whence flows the watery secretion, commonly called thetears, which is designed to perform an exceedingly important duty in lubricating the lids, and in keeping the exposed surface of the eyeball moist and transparent. For, without this or some similar liquid, the front of the eye would speedily become dry and lustreless, like that of a fish which has been removed from the water: the simple exposure of the eye to the air would then suffice to destroy vision.57.When does the secretion of the tears occur? The secretion not used for the eye? Location of the nasal duct? Its use? The overflow of tears in old people?57.This secretion of the tears takes place at all times, during the night as well as the day; but it is seldom noticed, except when under the influence of some strong mental emotion, whether of sorrow or happiness, it is poured forth in excess, so as to overflow the lids. Strong light or a rapid breeze will, among many other causes, excite the flow of the tears. That portion of this secretion which is not used in moistening the eye is carried off into the nose by a canal situated near the inner angle of the eye, called thenasal duct. This duct is shown in Fig. 47, 2, and is connected with each lid by delicate tubes, which are indicated by dotted lines in the figure; the asterisk marks the little opening in the lower lid, by which the tears enter the nasal duct. By gently turning the inner part of that lid downward, and looking in a mirror,this small "lachrymal point" may be seen in your own eye. In old people, these points become everted, and do not conduct the tears to the nasal cavity, so that they are inconvenienced by an overflow of tears upon the face.Fig. 48.--Vertical Section of the Eye.Fig. 48.—Vertical Section of the Eye.(Enlarged.)C, The Cornea. A, The Aqueous Humor. I, The Iris. P, The Pupil. L, The Crystalline Lens. H, The Ligament of the Lens. B, The Ciliary Process. V, The Cavity containing the Vitreous Humor. S, The Sclerotic. Ch, The Choroid. R, The Retina. N, The Optic Nerve. DD, The Eyelids. X, The Levator Muscle of the Upper Lid. Y, The Upper Straight Muscle of the Eye. Z, The Lower Straight Muscle.58.The watery fluid passing over the eyeball? Design of the arrangement? Winking?58.Thus we observe that the gland which forms the tears is placed at the outer part of the eye, while their means of exit is at the inner angle of the eye; which fact renders it necessary that this watery fluid shall pass over the surface of the eyeball before it can escape. This arrangement cannot be accidental, but evinces design, for it thus secures the perfect lubrication of the surface of the eye, and cleanses it from the smaller particles of dustwhich may enter it, in spite of the vigilance of the lids and lashes. The act of winking, which is generally unconsciously performed, and which takes place six or more times in a minute, assists this passage of the tears across the eye, and is especially frequent when the secretion is most abundant.59.Describe the shape of the eyeball. Its structure.59. The Eyeball.—The remarkable optical instrument called the eyeball, or the globe of the eye, upon which sight depends, is, as the name indicates, spherical in shape. It is not a perfect sphere, since the front part projects somewhat beyond the rest, and at the posterior part the optic nerve (Fig. 48,N) is united to it, resembling the junction of the stem with a fruit. In its long diameter, that is, the horizontal or from side to side, it measures a little more than an inch; in other directions it is rather less than an inch. In structure, the ball of the eye is firm, and its tense round contour may in part be felt by pressing the fingers over the closed lids.60.Of what is the eyeball composed? State how.60.The eyeball is composed chiefly of three internal, transparent media, calledhumors; and three investing coats, ortunics. The former are theaqueous humor, Fig. 48,A, thecrystalline lensL, and thevitreous humorV. Of these the lens alone is solid. The three coats of the eyeball are called thescleroticS, thechoroidCH, and theretinaR. This arrangement exists in respect to five-sixths of the globe of the eye, but in the anterior one-sixth, these coats are replaced by thecorneaC, which is thin and transparent, so that the rays of light pass freely through it, as through a clear window-pane.61.The shape of the cornea? Its structure? The "white of the eye?"61.In shape, the cornea is circular and prominent, resembling a miniature watch-glass, about 1/25 of an inch thick. In structure, it resembles horn (as the name signifies), or the nail of the finger, and is destitute ofblood-vessels. TheSclerotic(fromscleros, hard) is composed of dense, white fibrous tissue, and gives to the eyeball its firmness of figure and its white color; in front, it constitutes the part commonly called "the white of the eye." It is one of the strongest tissues in the body; it possesses very few vessels, and is not very sensitive. It affords protection to the extremely delicate interior parts of the eye; and the little muscles which effect its movements are inserted into the sclerotic a short distance behind the cornea (see Fig. 48,Y,Z). It is perforated posteriorly to admit the optic nerve.62.The second or middle coat of the eyeball? Its dark color?62.TheChoroidis the second or middle coat of the eyeball, and lies closely attached to the inner surface of the sclerotic. Unlike the latter tunic, its structure is soft and tender, it is dark in color, and possesses a great abundance of blood-vessels. Its dark color is due to a layer of dark brown or chocolate-colored cells spread out over its inner surface. This dark layer serves to absorb the rays of light after they have traversed the transparent structures in front of it; if the rays were reflected from side to side within the eye, instead of being thus absorbed, confused vision would result from the multitude of images which would be impressed upon the optic nerve.63.Similar mechanism in microscopes? The albinos? White rabbits?63.This mechanism has been unconsciously imitated by the opticians, who, when they make a microscope or telescope, take care that the interior of its tube shall be coated with a thick layer of black paint or lamp-black; for without it, a clear delineation of the object to be viewed is impossible. The albinos, in whom these dark cells of the choroid are wanting, have imperfect vision, especially in the daytime and in strong lights. The dark cells are also wanting in white rabbits, and other animals that have red or pink eyes; their vision appears to be imperfect in the presence of a bright light.64.What is the iris? Its construction? How is the size of the pupil regulated?64. The Iris.—Continuous with the choroid, in the front part of the globe of the eye, is a thin, circular curtain, which occasions the brown, blue, or gray color of the eye in different individuals. On account of the varieties of its color, this membrane has received the nameIris, which is the Greek word for "rainbow" (see Fig. 48,I). A front view of it is shown in Fig. 47. The iris is pierced in its centre by a round opening, called thepupil(P), which is constantly varying in size. In olden times it was spoken of as the "apple of the eye." The hinder surface of the iris, except in albinos, has a layer of dark coloring matter resembling that of the choroid. The iris is a muscular organ, and contains two distinct sets of fibres; one of which is circular, while the other radiates outward from the pupil. The action of these sets of fibres regulates the size of the pupil; for when the circular set acts, the pupil contracts, and when the other set acts, the opening expands. Their action is involuntary, and depends on the reflex system of nerves, which causes the contraction of the pupil when a strong light falls upon the eye, and its expansion when the illumination is feeble.65.The admission of light to the eye? The action of the iris under different circumstances? The lustre of the eye, how affected in youth and old age?65.The iris, accordingly, serves a very useful purpose in regulating the admission of light to the eye. It, however, does not act instantaneously; and hence, when we pass quickly from a dark room into the bright sunlight, the vision is at first confused by the glare of light, but as soon as the pupil contracts, the ability to see becomes perfect. On the other hand, when we enter a dark apartment, such as a cellar, for a short time we can see nothing clearly; but as soon as the pupil expands and admits more light, we are enabled to distinguish the surrounding objects. Animals of the cat species, and others which prowl around after nightfall, are enabled to see in the dark byhaving the iris very dilatable. The size of the pupil affects the lustre of the eye. When it is large, as it usually is during youth, the eye appears clear and brilliant; while in old age the pupil is small and the eye is dull. The brilliancy of the eye is in part, at least, dependent upon the reflection of light from the front surface of the crystalline lens.66.Means used to increase the beauty of the eye? The injurious consequences?66.Certain poisonous vegetables have the property of causing the pupil to dilate, and have been used in small doses to increase the beauty of the eye. One of these drugs has been so largely used by the ladies for this purpose, that it has received the namebelladonna, from the Italian words meaning "beautiful lady." This hazardous practice has resulted more than once in the death of the person desiring thus to increase her personal attraction. The common English name for belladonna is "deadly nightshade." (In the diagram on page214the shape and relations of the iris are more accurately shown than in the figure referred to above.)67.What part does the retina constitute? How formed? Its texture? Color? Sensitiveness?67. The Retinaconstitutes the third and inner coat of the globe of the eye. This, the important part of the eye that is sensitive to light, is a kind of nervous membrane, formed by the expansion of the ultimate filaments of the optic nerve. Its texture is soft, smooth, and very thin; it is translucent and of an opaline, or grayish-white color. It is sensitive to light alone; and if any form of mechanical irritation be applied to it, the sensations of touch and pain are not experienced, but flashes of fire, sparks, and other luminous appearances are perceived. Thus an electric shock given to the eye-ball occasions a flash of light; and a sudden fall, or a blow upon the eye, is often apparently accompanied by the vision of "stars."68.Specific energy of the optic nerve? Trial in Germany?68.These phenomena are due to what is termed the"specific energy" of the optic nerve, which nerve, in common with the other nerves of special sense, obeys a general law of nature, which requires that, whenever one of these nerves is stimulated, it shall respond with the sensation peculiar to itself. These flashes of retinal light have no power to illuminate external objects, although the opposite of this statement has been maintained. On the occasion of a remarkable trial in Germany, it was claimed by a person who had been severely assaulted on a very dark night, that the flashes of light caused by repeated blows upon the head enabled him to see with sufficient distinctness to recognize his assailant. But the evidence of scientific men entirely refuted this claim, by pronouncing that the eye, under the circumstances named, was incapacitated for vision. Too intense light occasions a feeling of pain, but it is of a peculiar kind, and is termed "dazzling."Fig. 49.--Diagram for blind point of eye.Fig. 49.69.Sensitiveness of all parts of the retina? Experiment to prove the existence of the "blind spot."69.All parts of the retina are not equally sensitive, and singularly enough, the point of entry of the nerve of sight, in the back part of the eyeball, is entirely insensible to light, and is called the "blind spot." The existence of this point may be proven by a simple experiment. Hold the accompanying figure, on page 207, directly in front of and parallel with the eyes. Close the left eye, and fix the sight steadily on the left-hand circle; then, by gradually varying the distance of the figure from the eye, at a certain distance (about six inches), the right-hand circle will disappear,but nearer or further than that, it will be plainly seen. The other eye may be also tried, with a similar result: if the gaze be directed to the right-hand circle, the left one will seem to disappear. The experiment may be repeated by using two black buttons on the marble top of a bureau, or on some other white surface. The blind spot does not practically interfere with vision, since the eye is seldom fixed immovably on an object, and the insensitive parts of the two eyes can never be directed upon the same object at the same time.70.Duration of impressions upon the retina? How illustrated?70.Impressions made upon the retina are not at once lost, but persist a measurable length of time, and then gradually fade away. Thus, a bright light or color, gazed at intently, cannot be immediately dismissed from sight by closing or turning away the eyes. A stick lighted at one end, if whirled around rapidly in the dark, presents the appearance of an unbroken luminous ring; and the spokes of a rapidly revolving carriage-wheel seem to be merged into a plane surface. If an object move too rapidly to produce this sort of lasting impression, it is invisible, as in the case of a cannon-ball passing through the air in front of us.71.What further illustration? Winking, why it is not noticed. Ease with which the retina is fatigued or deprived of sensibility? How shown?71.If a card, painted with two primary colors—as red and yellow—be made to rotate swiftly, the eye perceives neither of them distinctly; but the card appears painted with their secondary color—orange. The average duration of retinal images is estimated at one-eighth of a second; and it is because they thus endure, that the act of winking, which takes place so frequently, but so quickly, is not noticed and does not interrupt the vision. The retina is easily fatigued or deprived of its sensibility. After looking steadfastly at a bright light, or at a white object on a black ground, a dark spot, corresponding in shape to the bright object,presents itself in whatever direction we look. This spot passes away as the retina resumes its activity.72.How further shown? How is the result accounted for? "Color-blindness?"72.If a bright color be gazed at intently, and the eyes then be turned to a white surface, a spot will appear; but its color will be the complement of that of the object. Fix the eye upon a red wafer upon a white ground, and on removing the wafer a greenish spot of the same shape takes its place. This result happens because a certain portion of the retina has exhausted its power to perceive the red ray, and perceives only its complementary ray, which is green. The color thus substituted by the exhausted retina is called a physiological or accidental color. In some persons the retina is incapable of distinguishing different colors, when they are said to be affected with "color-blindness." Thus, red and green may appear alike, and then a cherry-tree, full of ripe fruit, will seem of the same color in every part. Railroad accidents have occurred because the engineer of the train, who was color-blind, has mistaken the color of a signal.73.The location of the crystalline lens? How supported? Its color and texture? Shape? Size?73. The Crystalline Lens.—Across the front of the eye, just behind the iris, is situated theCrystalline lens, enclosed within its own capsule. It is supported in its place partly by a delicate circular ligament, and partly by the pressure of adjacent structures. It is colorless and perfectly transparent, and has a firm but elastic texture. In shape it is doubly convex, and may be rudely compared to a small lemon-drop. The front face of the lens is flatter than the other, and is in contact with the iris near its pupillary margin, as is represented in the diagram on page214. It is only one-fourth of an inch thick.74.Cataract? Aqueous humor? Vitreous humor?74.When this little body becomes opaque, and no longer affords free passage to the rays of light, as often happenswith the advance of age, an affection termed "cataract" is produced. Between the crystalline lens and the cornea is a small space which contains theaqueous humor(see Fig. 48,A). This humor consists of five or six drops of a clear, colorless liquid very much like water, as its name implies. That part of the globe of the eye lying behind the lens is occupied by thevitreous humor, so called from its fancied resemblance to melted glass (Fig. 48,V). This humor is a transparent, jelly-like mass, enclosed within an exceedingly thin membrane. It lies very closely applied to the retina, or nervous membrane of the eye, and constitutes fully two-thirds of the bulk of the eyeball.Fig. 50.--The Retinal Image.Fig. 50.—The Retinal Image.75.What is a lens and its focus? The miniature image, how produced?75. The Uses of the Crystalline Lens.—A convex lens has the property of converging the rays of light which pass through it; and the point at which it causes them to meet is termed its focus. If a lens of this description, such as a magnifying or burning-glass, be held in front of an open window, in such a position as to allow its focus to fall upon a piece of paper, it will be found to depict upon the paper a miniature image of the scene outside of the window. It will be further noticed that the image is inverted, or upside down, and that the paperat the place upon which the image is thrown is much brighter than any other part.76.How are figures painted upon the retina? How proved?76.Now all the transparent structures of the eye, but especially the crystalline lens, operate upon its posterior part, or retina, as the convex lens acts upon the paper; that is, they paint upon the retina a bright inverted miniature of the objects that appear in front of the eye (Fig. 50). That this actually takes place may be proved by experiment. If the eyeball of a white rabbit, the walls of which are transparent, be examined while a lighted candle is held before the cornea, an image of the candle-flame may be seen upon the retina.77.What can be said in respect to the form and structure of the crystalline lens?77.The form and structure of the crystalline lens endow it with a remarkable degree of refractive power, and enable it to converge all the rays of light that enter it through the pupil, to a focus exactly at the surface of the retina. When this lens is removed from the eye, as is frequently done for the cure of cataract, it is found that the rays of light then have their focus three-eighths of an inch behind the retina; that the image is four times larger than in the healthy eye, that it is less brilliant, and that its outline is very indistinct. From this we learn that one of the uses of the crystalline lens is to make the retinal image bright and sharply-defined, at the same time that it reduces its size. Indeed, the small size of the image is a great advantage, as it enables the limited surface of the retina to receive, at a glance, impressions from a considerable field of vision.78.How is the inverted image upon the retina presented in its true position to the mind?78.As the image upon the retina is inverted, how does the mind perceive the object in its true, erect position? Many explanations have been advanced, but the simplest and most satisfactory appears to be found in the fact thatthe retina observes no difference, so to speak, between the right and left or the upper and lower positions of objects. In fact, the mind is never conscious of the formation of a retinal image, and until instructed, has no knowledge that it exists. Consequently, our knowledge of the relative location of external objects must be obtained from some other source than the retina. The probable source of this knowledge is the habitual comparison of those objects with the position of our own bodies: thus, to see an elevated object, we know we must raise the head and eyes; and to see one at our right hand, we must turn the head and eyes to the right.Fig. 51.--The Different Shapes of the Globe of the Eye.Fig. 51.—The Different Shapes of the Globe of the Eye.N, The Natural Eye. M, The Short-sighted Eye. H, The Long-sighted Eye. S, Parallel Rays from the Sun.79.The uniform perfection of the eye? Examples? The most common imperfection?79. Long-sight or Hyperopia, and Short-sight or Myopia.—The eye is not in all cases perfectly formed. For example, persons may from birth have the cornea too prominent or too flat, or the lens may be too thick or too thin. In either of these conditions sight will be more or less defective from the first, and the defect will not tend to disappear as life advances. The most common imperfection, however, is in the shape of the globe; which may be short (Fig. 51,H), as compared with the natural eye,N, or it may be too long,M.80.How is "long-sight" explained? "Short-sight?"80.When the globe is short, objects can only be clearlyseen that are at a distance, and the condition of the vision is known as "long-sight," or hyperopia. It will be observed, by reference to Fig. 51, that the focus of the rays of light would fall behind the retina of this eye. When the globe is too long, objects can only be clearly seen that are very near to the eye; and the condition resulting from this defect is termed "short-sight," or myopia. The focus of the rays of light is, in this case, formed in the interior of the eye in front of the retina.81.Long-sight, how common? With what must it not be confounded? Kind of glasses for short-sight? Why? Squint?81.Long-sight, or hyperopia, is common among schoolchildren, nearly as much so as short-sight, and must not be confounded with the defect known as the "far sight" of old people; although in both affections the sight is improved by the use of convex glasses. Children not infrequently discover that they see much better when they chance to put on the spectacles of old persons. For the relief of short-sight, concave glasses should be employed; as they so scatter the rays of light as to bring the focus to the retina, and thus cause the vision of remote objects to become at once distinct. That form of "squint," in which the eyes are turned inward, is generally dependent upon long-sight, while that rarer form, when they turn outward, is due to short-sight.82.What is stated in connection with the opera-glass? Experiment with pencil and distant object?82. The Function of Accommodation.—If, after looking through an opera-glass at a very distant object, it is desired to view another nearer at hand, it will be found impossible to obtain a clear vision of the second object unless the adjustment of the instrument is altered; which is effected by means of the screw. If an object, like the end of a pencil, be held near the eye, in a line with another object at the other side of the room, or out of the window, and the eye be fixed first upon one and then upon the other, it will be found that when the pencil is clearly seen, thefurther object is indistinct; and when the latter is seen clearly, the pencil appears indistinct; and that it is impossible to see both clearly at the same time. Accordingly, the eye must have the capacity of adjusting itself to distances, which is in some manner comparable to the action of the screw of the opera-glass.Fig. 52.--The Function of Accommodation.Fig. 52.—The Function of Accommodation.The right half of the diagram shows the eye at rest. The left half shows the lens accommodated for near vision.83.Function of accommodation? In what does it consist? How is the function explained?83.This, which has been called the function of accommodation, is one of the most admirable of all the powers of the eye, and is exercised by the crystalline lens. It consists essentially in a change in the curvature of the front surface of the lens, partly through its own elasticity, and partly through the action of the ciliary muscle. When the eye is at rest, that is, when accommodated for a distant object, the lens is flatter and its curvature diminished (see Fig. 52); but when strongly accommodated for near vision the lens becomes thicker, its curvature increases, and the image on the retina is made more sharp and distinct. Since a strong light is not required in viewing near objects, the pupil contracts, as is shown in the left-hand half of the diagram.84.Change of sight with the approach of old age? Explain the change?84. Old-sight, or Presbyopia.—But this marvellously beautiful mechanism becomes worn with use; or, more strictly speaking, the lens, like other structures of the body, becomes harder with the approach of old age. The material composing the lens becomes less elastic, the power to increase its curvature is gradually lost, and as a consequence, the person is obliged to hold the book further away when reading, and to seek a stronger light. In a word, the function of accommodation begins to fail, and is about the first evidence that marks the decline of life. By looking at the last preceding diagram, and remembering that the increased curvature of the lens cannot take place, it will be at once understood why old-sight is benefitted in near vision by the convex lens, such as the spectacles of old people contain. It acts as a substitute for the deficiency of the crystalline lens.85.Hearing or audition? What is sound? How propagated commonly? Stone thrown in water?85. The Sense of Hearing.—Sound.—Hearing, or audition, is the special sense by means of which we are made acquainted withsound. What is sound? It is an impression made upon the organs of hearing, by the vibrations of elastic bodies. This impression is commonly propagated by means of the air, which is thrown into delicate undulations, in all directions from the vibrating substance. When a stone is thrown into smooth water, a wave of circular form is set in motion, from the point where the stone struck, which constantly increases in size and diminishes in force, as it advances.86.Sound-wave in the atmosphere? Its shape? Rate of motion? Sound, in water, air, and solid bodies?86.Somewhat resembling this, is the undulation, or sound-wave, which is imparted by a sonorous vibration to the surrounding atmosphere. Its shape, however, is spherical, rather than circular, since it radiates upward, downward, and obliquely as well as horizontally, like the wavein water. The rate of motion of this spherical wave of air is about 1050 feet per second, or one mile in five seconds. In water, sound travels four times as fast as in air, and still more rapidly through solid bodies; along an iron rod, its velocity is equal to two miles per second.87.The earth as a conductor of sound? To what has the western Indian been taught? Solid substances as conductors? As regards sound, in what respect is air necessary? Sound in a vacuum?87.The earth, likewise, is a good conductor of sound. It is said that the Indian of our western prairies can, by listening at the surface of the ground, hear the advance of a troop of cavalry, while they are still out of sight, and can even discriminate between their tread and that of a herd of buffaloes. Solid substances also convey sounds with greater power than air. If the ear be pressed against one end of a long beam, the scratching of a pin at the other extremity may be distinctly heard, which will not be at all audible when the ear is removed from the beam. Although air is not the best medium for conveying sound, it is necessary for its production. Sound cannot be produced in a vacuum, as is shown by ringing a bell in the exhausted receiver of an air-pump, for it is then entirely inaudible. But let the air be readmitted gradually, then the tones become more and more distinct, and when the receiver is again full of air, they will be as clear as usual.88.All sonorous bodies do not vibrate with the same degree of rapidity, and upon this fact depends thepitchof the sounds that they respectively produce. The more frequent the number of vibrations within a given time, the higher will be the pitch; and the fewer their number, the lower or graver will it be. Now, the rate of the successive vibrations of different notes has been measured, and it has thus been found that if they are less than sixteen in a second, no sound is audible; while if they exceed 60,000 per second the sound is very faint, and is painful to theear. The extreme limit of the capacity of the human ear may be considered as included between these points; but the sounds which we ordinarily hear are embraced between 100 and 3,000 vibrations per second.89.Theear, which is the proper organ of hearing, is the most complicated of all the structures that are employed in the reception of external impressions. The parts of which it is composed are numerous, and some of them are extremely small and delicate. Nearly all these parts are located in an irregularly shaped cavity hollowed out in the temporal, or "temple," bone of each side of the head. That part of the bone in which the auditory cavity is placed has the densest structure of all bones of the body, and has therefore been called the "petrous," or rocky part of the temporal bone. In studying the ear, it is necessary to consider it as divided into three portions, which are called, from their relative positions, theexternalear, themiddleear, and theinternalear. (In the diagram, Fig. 53,A, the first is not shaded, the second is lightly shaded, and the last has a dark background.)90.Of what does the external portion of the organ of hearing consist? Describe the portal of that organ known as the ear. Its use?90. The External Ear.—The external portion of the organ of hearing, designated in Fig. 53,A, includes, first, that outer part (a), which is commonly spoken of as "the ear," but which in fact is only the portal of that organ; and, secondly, theauditory canal(b). The former consists of a flat flexible piece of cartilage, projecting slightly from the side of the head, attached to it by ligaments, and supplied with a few weak muscles. Its surface is uneven, and curiously curved, and from its resemblance to a shell, it has been called theconcha. It probably serves to collect sounds, and to give them an inward direction; although its removal is said not to impair the acuteness of hearing more than a few days.91.The ear in the animals of delicate hearing? Rabbit? Fishes?91.In those animals whose hearing is more delicate than that of man, the corresponding organ is of greater importance, it being larger and supplied with muscles of greater power, so that it serves as a natural kind of ear-trumpet, which is easily movable in the direction of any sound that attracts the attention of the animal. Bold, predaceous animals generally have the concavity of this organ directed forward, while in timorous animals, like the rabbit, it is directed backward. Fishes have no outer ear, but sounds are transmitted directly through the solid bones of the head, to the internal organ of hearing.Fig. 53.--The Ear and its Different Parts.Fig. 53.—The Ear and its Different Parts.A, Diagram of the Ear.a,b, External Ear.c, Membrane tympani.d, Middle Ear.e, Internal Ear.B to B''', Bones of the Middle Ear (magnified).C, The Labyrinths, or Internal Ear (highly magnified).92.What is the auditory canal? Describe it.92.Theauditory canal(Fig. 53,A,b), which is continuous with the outer opening of the ear, is a blind passage, an inch and a quarter in length, its inner extremity being bounded by a closely-fitting, circular membrane. This canal is of oval form, is directed forward and inward,and is slightly curved; so that the inner end is ordinarily concealed from view. The pouch of the skin which lines this passage is smooth and thin, especially at the lower end, where it covers the membrane just mentioned.93.How is it guarded and protected? Ear-wax?93.As in the case of the nostrils, a number of small, stiff hairs garnish the margin of the auditory canal, and guard it, to some extent, against the entrance of insects and other foreign objects. The skin, too, covering its outer half, is furnished with a belt of little glands which secrete a yellow, viscid, and bitter substance, called "ear-wax," which is especially obnoxious to small insects. As the outer layer of this wax-like material loses its useful properties, it becomes dry, and falls out of the ear in the form of minute, thin scales, a fresh supply being furnished from the little glands beneath. In its form, the auditory canal resembles the tube of an ear-trumpet, and serves to convey the waves of sound to the middle portion of the ear.94.What is the middle ear? Why called tympanum?94. The Middle Ear, or Tympanum.—The middle ear is a small cavity, or chamber, of irregular shape, about one-fourth of an inch across from side to side, and half an inch long (see Fig. 53,A,d). From the peculiar arrangement of its various parts it has very properly been called thetympanum, or the "drum of the ear." The middle ear, like the external canal, contains air.
51.Why is the eye called the "window of the soul?" Why, the subject of enthusiastic study?
51. The Organ of Sight.—The Eye.—The proximity of the eye to the brain, and the important part it performs in giving expression to the emotions, have given it the name of "the window of the soul." The exceeding beauty of its external parts, and the high value of its function, have long made this organ the subject of enthusiastic study. It is chiefly within the last twenty years, however, that this study has been successful and fruitful of practical results. Several ingenious instruments have been invented for the examination of the eye in health and disease, and new operations have been devised for the relief of blindness and of impaired vision. As a result, it is now a well-marked fact that, in civilized lands, the number of those who suffer from loss of sight is proportionally much less than in countries where science is less known and cultivated.
52.The most obvious fact? The consequence? The next thing noticed? Its range of view? Of what does the organ of vision consist?
52.The most obvious fact in respect to the apparatus ofsight is that there are two eyes, which may either act together as one, and be fixed upon one object, or one eye may be used independently of the other. In consequence of this arrangement the loss of one eye does not necessitate blindness, and, in fact, it not infrequently happens that the sight of one eye may be long impaired or lost before the fact is discovered. We next notice that it is placed at the most elevated part of the body, in front, and near the brain. It also commands a wide range of view, being itself moved with great rapidity, and being further aided by the free motion of the head and neck. The organ of vision consists essentially of two parts: the optical instrument itself—the eyeball—and its enveloping parts, or the case in which the instrument is kept free from harm. The latter, which are external, and which we shall first consider, are chiefly theOrbits, theEyelids, and the apparatus for theTears.
53.The protection of the eyeball against injury? The overhanging brow? The opening for the optic nerve?
53. The Orbits.—The eyeball, which is a delicate organ, is well defended against external injury within the orbits or bony sockets of the head. These are deep conical hollows, bounded in part by the bones of the skull, and in part by those of the nose and cheek. The orbit juts out beyond the most exposed portion of the eyeball, as may be seen by laying a book over the eye, when it will be found that no part of the eyeball, unless it be very prominent, will be touched by the book; so that the only direction in which an injury is liable to be received is immediately in front of the eye. The overhanging brow is itself covered by a layer of thick skin, studded with short, stout hairs, which are so bent as to prevent the perspiration from running into the eye and obscuring vision. Through a hole in the bottom of the orbit, the nerve of sight passes outward from the brain. The orbit also contains aconsiderableamount of a fatty tissue, upon which, as upon an elastic cushion, the eye rests.
Fig. 47.--Front View of Right Eye.Fig. 47.—Front View of Right Eye.(Natural Size.)1. The Lachrymal, or tear gland, lying beneath the upper eyelid.2. The Nasal Duct is shown by the dotted line. The * marks the orifice in the lower lid.The central black spot is thepupil; surrounding it is theiris; and the triangular white spaces are the visible portion of thesclerotic.
1. The Lachrymal, or tear gland, lying beneath the upper eyelid.
2. The Nasal Duct is shown by the dotted line. The * marks the orifice in the lower lid.
The central black spot is thepupil; surrounding it is theiris; and the triangular white spaces are the visible portion of thesclerotic.
54.What are the eyelids? The upper lid? The lower one? The mucous membrane of the eye?
54. The Eyelids.—The eyelids are two movable curtains, or folds, which, when shut, cover the front part of the orbit, and hide the eye from view. The upper lid is the larger, has a curved margin, and moves freely, while the lower lid is comparatively short and straight, and has but a slight degree of motion (Fig. 47). Skin covers the exterior of the lids, while a fine mucous membrane lines their inner surface, and is likewise spread out over the entire front of the eyeball. This membrane, which is called theConjunctiva, is highly sensitive, and thus plays an important part in protecting the eye against the lodgment of sand, ashes, chaff, and other foreign particles that are blown about in the air. This sensitive membrane will not endure the presence of these particles. If any find access, it causes a constant winking, a flow of tears, and other signs of irritation, until it is removed.
55.The eyelashes? The little points within the line of the lashes? Of what use are these glands?
55.The long, silky eyelashes, which garnish the edges of the lids, act like a sieve to prevent the entry of dust and the like; and together with the lids, they regulate the amount of light which is permitted to enter the eye, so that it is shielded from a sudden flood or glare of light. The little points seen in the figure just within the line of the lashes, especially on the lower lid, represent the mouths of numerous little sebaceous glands (Fig. 48,D,D), such as are alwaysfound in the neighborhood of hairs. These glands supply a thick, oily material which greases the edges of the lids and prevents their adhering together, and likewise prevents the overflow of the tears upon the cheek.
56.The location of the lachrymal gland? The use of the gland?
56. The Lachrymal Fluid, or the Tears.—Just within the outer part of the bony arch of the brow, where the bone may be felt to be sharper than in other positions, is lodged a little organ called the lachrymal gland, the situation of which is indicated in Fig. 47, 1. This is the gland whence flows the watery secretion, commonly called thetears, which is designed to perform an exceedingly important duty in lubricating the lids, and in keeping the exposed surface of the eyeball moist and transparent. For, without this or some similar liquid, the front of the eye would speedily become dry and lustreless, like that of a fish which has been removed from the water: the simple exposure of the eye to the air would then suffice to destroy vision.
57.When does the secretion of the tears occur? The secretion not used for the eye? Location of the nasal duct? Its use? The overflow of tears in old people?
57.This secretion of the tears takes place at all times, during the night as well as the day; but it is seldom noticed, except when under the influence of some strong mental emotion, whether of sorrow or happiness, it is poured forth in excess, so as to overflow the lids. Strong light or a rapid breeze will, among many other causes, excite the flow of the tears. That portion of this secretion which is not used in moistening the eye is carried off into the nose by a canal situated near the inner angle of the eye, called thenasal duct. This duct is shown in Fig. 47, 2, and is connected with each lid by delicate tubes, which are indicated by dotted lines in the figure; the asterisk marks the little opening in the lower lid, by which the tears enter the nasal duct. By gently turning the inner part of that lid downward, and looking in a mirror,this small "lachrymal point" may be seen in your own eye. In old people, these points become everted, and do not conduct the tears to the nasal cavity, so that they are inconvenienced by an overflow of tears upon the face.
Fig. 48.--Vertical Section of the Eye.Fig. 48.—Vertical Section of the Eye.(Enlarged.)C, The Cornea. A, The Aqueous Humor. I, The Iris. P, The Pupil. L, The Crystalline Lens. H, The Ligament of the Lens. B, The Ciliary Process. V, The Cavity containing the Vitreous Humor. S, The Sclerotic. Ch, The Choroid. R, The Retina. N, The Optic Nerve. DD, The Eyelids. X, The Levator Muscle of the Upper Lid. Y, The Upper Straight Muscle of the Eye. Z, The Lower Straight Muscle.
C, The Cornea. A, The Aqueous Humor. I, The Iris. P, The Pupil. L, The Crystalline Lens. H, The Ligament of the Lens. B, The Ciliary Process. V, The Cavity containing the Vitreous Humor. S, The Sclerotic. Ch, The Choroid. R, The Retina. N, The Optic Nerve. DD, The Eyelids. X, The Levator Muscle of the Upper Lid. Y, The Upper Straight Muscle of the Eye. Z, The Lower Straight Muscle.
58.The watery fluid passing over the eyeball? Design of the arrangement? Winking?
58.Thus we observe that the gland which forms the tears is placed at the outer part of the eye, while their means of exit is at the inner angle of the eye; which fact renders it necessary that this watery fluid shall pass over the surface of the eyeball before it can escape. This arrangement cannot be accidental, but evinces design, for it thus secures the perfect lubrication of the surface of the eye, and cleanses it from the smaller particles of dustwhich may enter it, in spite of the vigilance of the lids and lashes. The act of winking, which is generally unconsciously performed, and which takes place six or more times in a minute, assists this passage of the tears across the eye, and is especially frequent when the secretion is most abundant.
59.Describe the shape of the eyeball. Its structure.
59. The Eyeball.—The remarkable optical instrument called the eyeball, or the globe of the eye, upon which sight depends, is, as the name indicates, spherical in shape. It is not a perfect sphere, since the front part projects somewhat beyond the rest, and at the posterior part the optic nerve (Fig. 48,N) is united to it, resembling the junction of the stem with a fruit. In its long diameter, that is, the horizontal or from side to side, it measures a little more than an inch; in other directions it is rather less than an inch. In structure, the ball of the eye is firm, and its tense round contour may in part be felt by pressing the fingers over the closed lids.
60.Of what is the eyeball composed? State how.
60.The eyeball is composed chiefly of three internal, transparent media, calledhumors; and three investing coats, ortunics. The former are theaqueous humor, Fig. 48,A, thecrystalline lensL, and thevitreous humorV. Of these the lens alone is solid. The three coats of the eyeball are called thescleroticS, thechoroidCH, and theretinaR. This arrangement exists in respect to five-sixths of the globe of the eye, but in the anterior one-sixth, these coats are replaced by thecorneaC, which is thin and transparent, so that the rays of light pass freely through it, as through a clear window-pane.
61.The shape of the cornea? Its structure? The "white of the eye?"
61.In shape, the cornea is circular and prominent, resembling a miniature watch-glass, about 1/25 of an inch thick. In structure, it resembles horn (as the name signifies), or the nail of the finger, and is destitute ofblood-vessels. TheSclerotic(fromscleros, hard) is composed of dense, white fibrous tissue, and gives to the eyeball its firmness of figure and its white color; in front, it constitutes the part commonly called "the white of the eye." It is one of the strongest tissues in the body; it possesses very few vessels, and is not very sensitive. It affords protection to the extremely delicate interior parts of the eye; and the little muscles which effect its movements are inserted into the sclerotic a short distance behind the cornea (see Fig. 48,Y,Z). It is perforated posteriorly to admit the optic nerve.
62.The second or middle coat of the eyeball? Its dark color?
62.TheChoroidis the second or middle coat of the eyeball, and lies closely attached to the inner surface of the sclerotic. Unlike the latter tunic, its structure is soft and tender, it is dark in color, and possesses a great abundance of blood-vessels. Its dark color is due to a layer of dark brown or chocolate-colored cells spread out over its inner surface. This dark layer serves to absorb the rays of light after they have traversed the transparent structures in front of it; if the rays were reflected from side to side within the eye, instead of being thus absorbed, confused vision would result from the multitude of images which would be impressed upon the optic nerve.
63.Similar mechanism in microscopes? The albinos? White rabbits?
63.This mechanism has been unconsciously imitated by the opticians, who, when they make a microscope or telescope, take care that the interior of its tube shall be coated with a thick layer of black paint or lamp-black; for without it, a clear delineation of the object to be viewed is impossible. The albinos, in whom these dark cells of the choroid are wanting, have imperfect vision, especially in the daytime and in strong lights. The dark cells are also wanting in white rabbits, and other animals that have red or pink eyes; their vision appears to be imperfect in the presence of a bright light.
64.What is the iris? Its construction? How is the size of the pupil regulated?
64. The Iris.—Continuous with the choroid, in the front part of the globe of the eye, is a thin, circular curtain, which occasions the brown, blue, or gray color of the eye in different individuals. On account of the varieties of its color, this membrane has received the nameIris, which is the Greek word for "rainbow" (see Fig. 48,I). A front view of it is shown in Fig. 47. The iris is pierced in its centre by a round opening, called thepupil(P), which is constantly varying in size. In olden times it was spoken of as the "apple of the eye." The hinder surface of the iris, except in albinos, has a layer of dark coloring matter resembling that of the choroid. The iris is a muscular organ, and contains two distinct sets of fibres; one of which is circular, while the other radiates outward from the pupil. The action of these sets of fibres regulates the size of the pupil; for when the circular set acts, the pupil contracts, and when the other set acts, the opening expands. Their action is involuntary, and depends on the reflex system of nerves, which causes the contraction of the pupil when a strong light falls upon the eye, and its expansion when the illumination is feeble.
65.The admission of light to the eye? The action of the iris under different circumstances? The lustre of the eye, how affected in youth and old age?
65.The iris, accordingly, serves a very useful purpose in regulating the admission of light to the eye. It, however, does not act instantaneously; and hence, when we pass quickly from a dark room into the bright sunlight, the vision is at first confused by the glare of light, but as soon as the pupil contracts, the ability to see becomes perfect. On the other hand, when we enter a dark apartment, such as a cellar, for a short time we can see nothing clearly; but as soon as the pupil expands and admits more light, we are enabled to distinguish the surrounding objects. Animals of the cat species, and others which prowl around after nightfall, are enabled to see in the dark byhaving the iris very dilatable. The size of the pupil affects the lustre of the eye. When it is large, as it usually is during youth, the eye appears clear and brilliant; while in old age the pupil is small and the eye is dull. The brilliancy of the eye is in part, at least, dependent upon the reflection of light from the front surface of the crystalline lens.
66.Means used to increase the beauty of the eye? The injurious consequences?
66.Certain poisonous vegetables have the property of causing the pupil to dilate, and have been used in small doses to increase the beauty of the eye. One of these drugs has been so largely used by the ladies for this purpose, that it has received the namebelladonna, from the Italian words meaning "beautiful lady." This hazardous practice has resulted more than once in the death of the person desiring thus to increase her personal attraction. The common English name for belladonna is "deadly nightshade." (In the diagram on page214the shape and relations of the iris are more accurately shown than in the figure referred to above.)
67.What part does the retina constitute? How formed? Its texture? Color? Sensitiveness?
67. The Retinaconstitutes the third and inner coat of the globe of the eye. This, the important part of the eye that is sensitive to light, is a kind of nervous membrane, formed by the expansion of the ultimate filaments of the optic nerve. Its texture is soft, smooth, and very thin; it is translucent and of an opaline, or grayish-white color. It is sensitive to light alone; and if any form of mechanical irritation be applied to it, the sensations of touch and pain are not experienced, but flashes of fire, sparks, and other luminous appearances are perceived. Thus an electric shock given to the eye-ball occasions a flash of light; and a sudden fall, or a blow upon the eye, is often apparently accompanied by the vision of "stars."
68.Specific energy of the optic nerve? Trial in Germany?
68.These phenomena are due to what is termed the"specific energy" of the optic nerve, which nerve, in common with the other nerves of special sense, obeys a general law of nature, which requires that, whenever one of these nerves is stimulated, it shall respond with the sensation peculiar to itself. These flashes of retinal light have no power to illuminate external objects, although the opposite of this statement has been maintained. On the occasion of a remarkable trial in Germany, it was claimed by a person who had been severely assaulted on a very dark night, that the flashes of light caused by repeated blows upon the head enabled him to see with sufficient distinctness to recognize his assailant. But the evidence of scientific men entirely refuted this claim, by pronouncing that the eye, under the circumstances named, was incapacitated for vision. Too intense light occasions a feeling of pain, but it is of a peculiar kind, and is termed "dazzling."
Fig. 49.--Diagram for blind point of eye.Fig. 49.
69.Sensitiveness of all parts of the retina? Experiment to prove the existence of the "blind spot."
69.All parts of the retina are not equally sensitive, and singularly enough, the point of entry of the nerve of sight, in the back part of the eyeball, is entirely insensible to light, and is called the "blind spot." The existence of this point may be proven by a simple experiment. Hold the accompanying figure, on page 207, directly in front of and parallel with the eyes. Close the left eye, and fix the sight steadily on the left-hand circle; then, by gradually varying the distance of the figure from the eye, at a certain distance (about six inches), the right-hand circle will disappear,but nearer or further than that, it will be plainly seen. The other eye may be also tried, with a similar result: if the gaze be directed to the right-hand circle, the left one will seem to disappear. The experiment may be repeated by using two black buttons on the marble top of a bureau, or on some other white surface. The blind spot does not practically interfere with vision, since the eye is seldom fixed immovably on an object, and the insensitive parts of the two eyes can never be directed upon the same object at the same time.
70.Duration of impressions upon the retina? How illustrated?
70.Impressions made upon the retina are not at once lost, but persist a measurable length of time, and then gradually fade away. Thus, a bright light or color, gazed at intently, cannot be immediately dismissed from sight by closing or turning away the eyes. A stick lighted at one end, if whirled around rapidly in the dark, presents the appearance of an unbroken luminous ring; and the spokes of a rapidly revolving carriage-wheel seem to be merged into a plane surface. If an object move too rapidly to produce this sort of lasting impression, it is invisible, as in the case of a cannon-ball passing through the air in front of us.
71.What further illustration? Winking, why it is not noticed. Ease with which the retina is fatigued or deprived of sensibility? How shown?
71.If a card, painted with two primary colors—as red and yellow—be made to rotate swiftly, the eye perceives neither of them distinctly; but the card appears painted with their secondary color—orange. The average duration of retinal images is estimated at one-eighth of a second; and it is because they thus endure, that the act of winking, which takes place so frequently, but so quickly, is not noticed and does not interrupt the vision. The retina is easily fatigued or deprived of its sensibility. After looking steadfastly at a bright light, or at a white object on a black ground, a dark spot, corresponding in shape to the bright object,presents itself in whatever direction we look. This spot passes away as the retina resumes its activity.
72.How further shown? How is the result accounted for? "Color-blindness?"
72.If a bright color be gazed at intently, and the eyes then be turned to a white surface, a spot will appear; but its color will be the complement of that of the object. Fix the eye upon a red wafer upon a white ground, and on removing the wafer a greenish spot of the same shape takes its place. This result happens because a certain portion of the retina has exhausted its power to perceive the red ray, and perceives only its complementary ray, which is green. The color thus substituted by the exhausted retina is called a physiological or accidental color. In some persons the retina is incapable of distinguishing different colors, when they are said to be affected with "color-blindness." Thus, red and green may appear alike, and then a cherry-tree, full of ripe fruit, will seem of the same color in every part. Railroad accidents have occurred because the engineer of the train, who was color-blind, has mistaken the color of a signal.
73.The location of the crystalline lens? How supported? Its color and texture? Shape? Size?
73. The Crystalline Lens.—Across the front of the eye, just behind the iris, is situated theCrystalline lens, enclosed within its own capsule. It is supported in its place partly by a delicate circular ligament, and partly by the pressure of adjacent structures. It is colorless and perfectly transparent, and has a firm but elastic texture. In shape it is doubly convex, and may be rudely compared to a small lemon-drop. The front face of the lens is flatter than the other, and is in contact with the iris near its pupillary margin, as is represented in the diagram on page214. It is only one-fourth of an inch thick.
74.Cataract? Aqueous humor? Vitreous humor?
74.When this little body becomes opaque, and no longer affords free passage to the rays of light, as often happenswith the advance of age, an affection termed "cataract" is produced. Between the crystalline lens and the cornea is a small space which contains theaqueous humor(see Fig. 48,A). This humor consists of five or six drops of a clear, colorless liquid very much like water, as its name implies. That part of the globe of the eye lying behind the lens is occupied by thevitreous humor, so called from its fancied resemblance to melted glass (Fig. 48,V). This humor is a transparent, jelly-like mass, enclosed within an exceedingly thin membrane. It lies very closely applied to the retina, or nervous membrane of the eye, and constitutes fully two-thirds of the bulk of the eyeball.
Fig. 50.--The Retinal Image.Fig. 50.—The Retinal Image.
75.What is a lens and its focus? The miniature image, how produced?
75. The Uses of the Crystalline Lens.—A convex lens has the property of converging the rays of light which pass through it; and the point at which it causes them to meet is termed its focus. If a lens of this description, such as a magnifying or burning-glass, be held in front of an open window, in such a position as to allow its focus to fall upon a piece of paper, it will be found to depict upon the paper a miniature image of the scene outside of the window. It will be further noticed that the image is inverted, or upside down, and that the paperat the place upon which the image is thrown is much brighter than any other part.
76.How are figures painted upon the retina? How proved?
76.Now all the transparent structures of the eye, but especially the crystalline lens, operate upon its posterior part, or retina, as the convex lens acts upon the paper; that is, they paint upon the retina a bright inverted miniature of the objects that appear in front of the eye (Fig. 50). That this actually takes place may be proved by experiment. If the eyeball of a white rabbit, the walls of which are transparent, be examined while a lighted candle is held before the cornea, an image of the candle-flame may be seen upon the retina.
77.What can be said in respect to the form and structure of the crystalline lens?
77.The form and structure of the crystalline lens endow it with a remarkable degree of refractive power, and enable it to converge all the rays of light that enter it through the pupil, to a focus exactly at the surface of the retina. When this lens is removed from the eye, as is frequently done for the cure of cataract, it is found that the rays of light then have their focus three-eighths of an inch behind the retina; that the image is four times larger than in the healthy eye, that it is less brilliant, and that its outline is very indistinct. From this we learn that one of the uses of the crystalline lens is to make the retinal image bright and sharply-defined, at the same time that it reduces its size. Indeed, the small size of the image is a great advantage, as it enables the limited surface of the retina to receive, at a glance, impressions from a considerable field of vision.
78.How is the inverted image upon the retina presented in its true position to the mind?
78.As the image upon the retina is inverted, how does the mind perceive the object in its true, erect position? Many explanations have been advanced, but the simplest and most satisfactory appears to be found in the fact thatthe retina observes no difference, so to speak, between the right and left or the upper and lower positions of objects. In fact, the mind is never conscious of the formation of a retinal image, and until instructed, has no knowledge that it exists. Consequently, our knowledge of the relative location of external objects must be obtained from some other source than the retina. The probable source of this knowledge is the habitual comparison of those objects with the position of our own bodies: thus, to see an elevated object, we know we must raise the head and eyes; and to see one at our right hand, we must turn the head and eyes to the right.
Fig. 51.--The Different Shapes of the Globe of the Eye.Fig. 51.—The Different Shapes of the Globe of the Eye.N, The Natural Eye. M, The Short-sighted Eye. H, The Long-sighted Eye. S, Parallel Rays from the Sun.
N, The Natural Eye. M, The Short-sighted Eye. H, The Long-sighted Eye. S, Parallel Rays from the Sun.
79.The uniform perfection of the eye? Examples? The most common imperfection?
79. Long-sight or Hyperopia, and Short-sight or Myopia.—The eye is not in all cases perfectly formed. For example, persons may from birth have the cornea too prominent or too flat, or the lens may be too thick or too thin. In either of these conditions sight will be more or less defective from the first, and the defect will not tend to disappear as life advances. The most common imperfection, however, is in the shape of the globe; which may be short (Fig. 51,H), as compared with the natural eye,N, or it may be too long,M.
80.How is "long-sight" explained? "Short-sight?"
80.When the globe is short, objects can only be clearlyseen that are at a distance, and the condition of the vision is known as "long-sight," or hyperopia. It will be observed, by reference to Fig. 51, that the focus of the rays of light would fall behind the retina of this eye. When the globe is too long, objects can only be clearly seen that are very near to the eye; and the condition resulting from this defect is termed "short-sight," or myopia. The focus of the rays of light is, in this case, formed in the interior of the eye in front of the retina.
81.Long-sight, how common? With what must it not be confounded? Kind of glasses for short-sight? Why? Squint?
81.Long-sight, or hyperopia, is common among schoolchildren, nearly as much so as short-sight, and must not be confounded with the defect known as the "far sight" of old people; although in both affections the sight is improved by the use of convex glasses. Children not infrequently discover that they see much better when they chance to put on the spectacles of old persons. For the relief of short-sight, concave glasses should be employed; as they so scatter the rays of light as to bring the focus to the retina, and thus cause the vision of remote objects to become at once distinct. That form of "squint," in which the eyes are turned inward, is generally dependent upon long-sight, while that rarer form, when they turn outward, is due to short-sight.
82.What is stated in connection with the opera-glass? Experiment with pencil and distant object?
82. The Function of Accommodation.—If, after looking through an opera-glass at a very distant object, it is desired to view another nearer at hand, it will be found impossible to obtain a clear vision of the second object unless the adjustment of the instrument is altered; which is effected by means of the screw. If an object, like the end of a pencil, be held near the eye, in a line with another object at the other side of the room, or out of the window, and the eye be fixed first upon one and then upon the other, it will be found that when the pencil is clearly seen, thefurther object is indistinct; and when the latter is seen clearly, the pencil appears indistinct; and that it is impossible to see both clearly at the same time. Accordingly, the eye must have the capacity of adjusting itself to distances, which is in some manner comparable to the action of the screw of the opera-glass.
Fig. 52.--The Function of Accommodation.Fig. 52.—The Function of Accommodation.The right half of the diagram shows the eye at rest. The left half shows the lens accommodated for near vision.
The right half of the diagram shows the eye at rest. The left half shows the lens accommodated for near vision.
83.Function of accommodation? In what does it consist? How is the function explained?
83.This, which has been called the function of accommodation, is one of the most admirable of all the powers of the eye, and is exercised by the crystalline lens. It consists essentially in a change in the curvature of the front surface of the lens, partly through its own elasticity, and partly through the action of the ciliary muscle. When the eye is at rest, that is, when accommodated for a distant object, the lens is flatter and its curvature diminished (see Fig. 52); but when strongly accommodated for near vision the lens becomes thicker, its curvature increases, and the image on the retina is made more sharp and distinct. Since a strong light is not required in viewing near objects, the pupil contracts, as is shown in the left-hand half of the diagram.
84.Change of sight with the approach of old age? Explain the change?
84. Old-sight, or Presbyopia.—But this marvellously beautiful mechanism becomes worn with use; or, more strictly speaking, the lens, like other structures of the body, becomes harder with the approach of old age. The material composing the lens becomes less elastic, the power to increase its curvature is gradually lost, and as a consequence, the person is obliged to hold the book further away when reading, and to seek a stronger light. In a word, the function of accommodation begins to fail, and is about the first evidence that marks the decline of life. By looking at the last preceding diagram, and remembering that the increased curvature of the lens cannot take place, it will be at once understood why old-sight is benefitted in near vision by the convex lens, such as the spectacles of old people contain. It acts as a substitute for the deficiency of the crystalline lens.
85.Hearing or audition? What is sound? How propagated commonly? Stone thrown in water?
85. The Sense of Hearing.—Sound.—Hearing, or audition, is the special sense by means of which we are made acquainted withsound. What is sound? It is an impression made upon the organs of hearing, by the vibrations of elastic bodies. This impression is commonly propagated by means of the air, which is thrown into delicate undulations, in all directions from the vibrating substance. When a stone is thrown into smooth water, a wave of circular form is set in motion, from the point where the stone struck, which constantly increases in size and diminishes in force, as it advances.
86.Sound-wave in the atmosphere? Its shape? Rate of motion? Sound, in water, air, and solid bodies?
86.Somewhat resembling this, is the undulation, or sound-wave, which is imparted by a sonorous vibration to the surrounding atmosphere. Its shape, however, is spherical, rather than circular, since it radiates upward, downward, and obliquely as well as horizontally, like the wavein water. The rate of motion of this spherical wave of air is about 1050 feet per second, or one mile in five seconds. In water, sound travels four times as fast as in air, and still more rapidly through solid bodies; along an iron rod, its velocity is equal to two miles per second.
87.The earth as a conductor of sound? To what has the western Indian been taught? Solid substances as conductors? As regards sound, in what respect is air necessary? Sound in a vacuum?
87.The earth, likewise, is a good conductor of sound. It is said that the Indian of our western prairies can, by listening at the surface of the ground, hear the advance of a troop of cavalry, while they are still out of sight, and can even discriminate between their tread and that of a herd of buffaloes. Solid substances also convey sounds with greater power than air. If the ear be pressed against one end of a long beam, the scratching of a pin at the other extremity may be distinctly heard, which will not be at all audible when the ear is removed from the beam. Although air is not the best medium for conveying sound, it is necessary for its production. Sound cannot be produced in a vacuum, as is shown by ringing a bell in the exhausted receiver of an air-pump, for it is then entirely inaudible. But let the air be readmitted gradually, then the tones become more and more distinct, and when the receiver is again full of air, they will be as clear as usual.
88.All sonorous bodies do not vibrate with the same degree of rapidity, and upon this fact depends thepitchof the sounds that they respectively produce. The more frequent the number of vibrations within a given time, the higher will be the pitch; and the fewer their number, the lower or graver will it be. Now, the rate of the successive vibrations of different notes has been measured, and it has thus been found that if they are less than sixteen in a second, no sound is audible; while if they exceed 60,000 per second the sound is very faint, and is painful to theear. The extreme limit of the capacity of the human ear may be considered as included between these points; but the sounds which we ordinarily hear are embraced between 100 and 3,000 vibrations per second.
89.Theear, which is the proper organ of hearing, is the most complicated of all the structures that are employed in the reception of external impressions. The parts of which it is composed are numerous, and some of them are extremely small and delicate. Nearly all these parts are located in an irregularly shaped cavity hollowed out in the temporal, or "temple," bone of each side of the head. That part of the bone in which the auditory cavity is placed has the densest structure of all bones of the body, and has therefore been called the "petrous," or rocky part of the temporal bone. In studying the ear, it is necessary to consider it as divided into three portions, which are called, from their relative positions, theexternalear, themiddleear, and theinternalear. (In the diagram, Fig. 53,A, the first is not shaded, the second is lightly shaded, and the last has a dark background.)
90.Of what does the external portion of the organ of hearing consist? Describe the portal of that organ known as the ear. Its use?
90. The External Ear.—The external portion of the organ of hearing, designated in Fig. 53,A, includes, first, that outer part (a), which is commonly spoken of as "the ear," but which in fact is only the portal of that organ; and, secondly, theauditory canal(b). The former consists of a flat flexible piece of cartilage, projecting slightly from the side of the head, attached to it by ligaments, and supplied with a few weak muscles. Its surface is uneven, and curiously curved, and from its resemblance to a shell, it has been called theconcha. It probably serves to collect sounds, and to give them an inward direction; although its removal is said not to impair the acuteness of hearing more than a few days.
91.The ear in the animals of delicate hearing? Rabbit? Fishes?
91.In those animals whose hearing is more delicate than that of man, the corresponding organ is of greater importance, it being larger and supplied with muscles of greater power, so that it serves as a natural kind of ear-trumpet, which is easily movable in the direction of any sound that attracts the attention of the animal. Bold, predaceous animals generally have the concavity of this organ directed forward, while in timorous animals, like the rabbit, it is directed backward. Fishes have no outer ear, but sounds are transmitted directly through the solid bones of the head, to the internal organ of hearing.
Fig. 53.--The Ear and its Different Parts.Fig. 53.—The Ear and its Different Parts.A, Diagram of the Ear.a,b, External Ear.c, Membrane tympani.d, Middle Ear.e, Internal Ear.B to B''', Bones of the Middle Ear (magnified).C, The Labyrinths, or Internal Ear (highly magnified).
A, Diagram of the Ear.
a,b, External Ear.c, Membrane tympani.d, Middle Ear.e, Internal Ear.
B to B''', Bones of the Middle Ear (magnified).
C, The Labyrinths, or Internal Ear (highly magnified).
92.What is the auditory canal? Describe it.
92.Theauditory canal(Fig. 53,A,b), which is continuous with the outer opening of the ear, is a blind passage, an inch and a quarter in length, its inner extremity being bounded by a closely-fitting, circular membrane. This canal is of oval form, is directed forward and inward,and is slightly curved; so that the inner end is ordinarily concealed from view. The pouch of the skin which lines this passage is smooth and thin, especially at the lower end, where it covers the membrane just mentioned.
93.How is it guarded and protected? Ear-wax?
93.As in the case of the nostrils, a number of small, stiff hairs garnish the margin of the auditory canal, and guard it, to some extent, against the entrance of insects and other foreign objects. The skin, too, covering its outer half, is furnished with a belt of little glands which secrete a yellow, viscid, and bitter substance, called "ear-wax," which is especially obnoxious to small insects. As the outer layer of this wax-like material loses its useful properties, it becomes dry, and falls out of the ear in the form of minute, thin scales, a fresh supply being furnished from the little glands beneath. In its form, the auditory canal resembles the tube of an ear-trumpet, and serves to convey the waves of sound to the middle portion of the ear.
94.What is the middle ear? Why called tympanum?
94. The Middle Ear, or Tympanum.—The middle ear is a small cavity, or chamber, of irregular shape, about one-fourth of an inch across from side to side, and half an inch long (see Fig. 53,A,d). From the peculiar arrangement of its various parts it has very properly been called thetympanum, or the "drum of the ear." The middle ear, like the external canal, contains air.