Chapter XI: The Special Senses (1)

308. The Special Senses. In man certain special organs are set apart the particular duty of which is to give information of the nature of the relations which he sustains to the great world of things, and of which he is but a mere speck. The special senses are the avenues by which we obtain this information as to our bodily condition, the world around us, and the manner in which it affects us.

Animals high in the scale are affected in so many different ways, and by so many agencies, that a subdivision of labor becomes necessary that the sense avenues may be rigidly guarded. One person alone may be a sufficient watch on the deck of a sloop, but an ocean steamer needs a score or more on guard, each with his special duty and at his own post. Or the senses are like a series of disciplined picket-guards, along the outposts of the mind, to take note of events, and to report to headquarters any information which may be within the range of their duty.

Thus it is that we are provided with a number of special senses, by means of which information is supplied regarding outward forces and objects. These are touch, taste, smell, seeing, and hearing, to which may be added the muscular sense and a sense of temperature.

309. General Sensations. The body, as we have learned, is made up of a great number of complicated organs, each doing its own part of the general work required for the life and vigor of the human organism. These organs should all work in harmony for the good of the whole. We must have some means of knowing whether this harmony is maintained, and of receiving timely warning if any organ fails to do its particular duty.

Such information is supplied by the common or general sensations. Thus we have a feeling of hunger or thirst indicating the need of food, and a feeling of discomfort when imperfectly clad, informing us of the need of more clothing.

To these may be added the sensation of pain, tickling, itching, and so on, the needs of which arise from the complicated structure of the human body. The great majority of sensations result from some stimulus or outward agency; and yet some sensations, such as those of faintness, restlessness, and fatigue seem to spring up within us in some mysterious way, without any obvious cause.

310. Essentials of a Sense Organ. Certain essentials are necessary for a sensation. First, there is a special structure adapted to a particular kind of influence. Thus the ear is formed specially for being stimulated by the waves of sound, while the eye is not influenced by sound, but responds to the action of light. These special structures are called terminal organs.

Again, a nerve proceeds from the special structure, which is in direct communication with nerve cells in the brain at the region of consciousness. This last point is important to remember, for if on some account the impression is arrested in the connecting nerve, no sensation will result. Thus a man whose spine has been injured may not feel a severe pinch on either leg. The impression may be quite sufficient to stimulate a nerve center in a healthy cord, so as to produce a marked reflex act, but he has no sensation, because the injury has prevented the impression from being carried up the cord to the higher centers in the brain.

311. The Condition of Sensation. It is thus evident that while an impression may be made upon a terminal organ, it cannot strictly be called a sensation until the person becomes conscious of it. The consciousness of an impression is, therefore, the essential element of a sensation.

It follows that sensation may be prevented in various ways. In the sense of sight, for example, one person may be blind because the terminal organ, or eye, is defective or diseased. Another may have perfect eyes and yet have no sight, because a tumor presses on the nerve between the eye and the brain. In this case, the impression fails because of the break in the communication. Once more, the eye may be perfect and the nerve connection unbroken, and yet the person cannot see, because the center in the brain itself is injured from disease or accident, and cannot receive the impression.

312. The Functions of the Brain Center in the Perception of an Impression. Sensation is really the result of a change which occurs in a nerve center in the brain, and yet we refer impressions to the various terminal organs. Thus, when the skin is pinched, the sensation is referred to the skin, although the perception is in the brain. We may think it is the eyes that see objects; in reality, it is only the brain that takes note of them.

This is largely the result of education and habit. From a blow on the head one sees flashes of light as vividly as if torches actually dance before the eyes. Impressions have reached the seeing-center in the brain from irritation of the optic nerve, producing the same effect as real lights would cause. In this case, however, knowing the cause of the colors, the person is able to correct the erroneous conclusion.

As a result of a depraved condition of blood, the seeing-center itself may be unduly stimulated, and a person may see objects which appear real. Thus in an attack of delirium tremens, the victim of alcoholic poisoning sees horrible and fantastic creatures. The diseased brain refers them as usual to the external world; hence they appear real. As the sufferer’s judgment is warped by the alcoholic liquor, he cannot correct the impressions, and is therefore deceived by them.

313. Organs of Special Sense. The organs of special sense, the means by which we are brought into relation with surrounding objects, are usually classed as five in number. They are sometimes fancifully called “the five gateways of knowledge”—the skin, the organ of touch; the tongue, of taste; the nose, of smell; the eye, of sight; and the ear, of hearing.

Illustration: Fig. 124.—Magnified View of a Papilla of the Skin, with a Touch Corpuscle.

314. The Organ of Touch. The organ of touch, or tactile sensibility, is the most widely extended of all the special senses, and perhaps the simplest. It is certainly the most precise and certain in its results. It is this sense to which we instinctively appeal to escape from the illusions into which the other senses may mislead us. It has its seat in the skin all over the body, and in the mucous membrane of the nostrils. All parts of the body, however, do not have this sense in an equal degree.

In Chapter IX. we learned that the superficial layers of the skin covers and dips in between the papillæ. We also learned that these papillæ are richly provided with blood-vessels and sensory nerve fibers (sec. 234). Now these nerve fibers terminate in a peculiar way in those parts of the body which are endowed with a very delicate sense of touch. In every papilla are oval-shaped bodies about 1/300 of an inch long, around which the nerve fibers wind, and which they finally enter. These are called touch-bodies, or tactile corpuscles, and are found in great numbers on the feet and toes, and more scantily in other places, as on the edges of the eyelids.

Again, many of the nerve fibers terminate in corpuscles, the largest about 1/20 of an inch long, called Pacinian corpuscles. These are most numerous in the palm of the hand and the sole of the foot. In the papillæ of the red border of the lips the nerves end in capsules which enclose one or more fibers, and are called end-bulbs.

The great majority of the nerve fibers which supply the skin do not end in such well-defined organs. They oftener divide into exceedingly delicate filaments, the terminations of which are traced with the greatest difficulty.

315. The Sense of Touch. Touch is a sensation of contact referred to the surface of the body. It includes three things,—the sense of contact, the sense of pressure, and the sense of heat and cold.

The sense of contact is the most important element in touch. By it we learn of the form, size, and other properties of objects, as their smoothness and hardness. As we all know, the sense of touch varies in different parts of the skin. It is most acute where the outer skin is thinnest. The tips of the fingers, the edges of the lips, and the tip of the tongue are the most sensitive parts.

Even the nails, the teeth, and the hair have the sense of touch in a slight degree. When the scarf skin is removed, the part is not more sensitive to sense of contact. In fact, direct contact with the unprotected true skin occasions pain, which effectually masks the feeling of touch. The sense of touch is capable of education, and is generally developed to an extraordinary degree in persons who are deprived of some other special sense, as sight or hearing. We read of the famous blind sculptor who was said to model excellent likenesses, guided entirely by the sense of touch. An eminent authority on botany was a blind man, able to distinguish rare plants by the fingers, and by the tip of the tongue. The blind learn to read with facility by passing their fingers over raised letters of a coarse type. It is impossible to contemplate, even for a moment, the prominence assigned to the sense of touch in the physical organism, without being impressed with the manifestations of design—the work of an all-wise Creator.

316. Muscular Sense; Sense of Temperature; Pain. When a heavy object is laid upon certain parts of the body, it produces a sensation of pressure. By it we are enabled to estimate differences of weight. If an attempt be made to raise this object, it offers resistance which the muscles must overcome. This is known as the muscular sense. It depends on sensory nerves originating in the muscles and carrying impressions from them to the nerve centers.

The skin also judges, to a certain extent, of heat and cold. These sensations can be felt only by the skin. Direct irritation of a nerve does not give rise to them. Thus, the exposed pulp of a diseased tooth, when irritated by cold fluids, gives rise to pain, and not to a sensation of temperature. Various portions of the body have different degrees of sensibility in this respect. The hand will bear a degree of heat which would cause pain to some other parts of the body. Then, again, the sensibility of the outer skin seems to affect the sensibility to heat, for parts with a thin skin can bear less heat than portions with a thick cuticle.

Experiment 139. _To illustrate how the sense of touch is a matter of habit or education_. Shut both eyes, and let a friend run the tips of your fingers first lightly over a hard plane surface; then press hard, then lightly again, and the surface will seem to be concave.

Experiment 140. Cross the middle over the index finger, roll a small marble between the fingers; one has a distinct impression of two marbles. Cross the fingers in the same way, and rub them against the point of the nose. A similar illusion is experienced.

Experiment 141. _To test the sense of locality_. Ask a person to shut
his eyes, touch some part of his body lightly with the point of a pin,
and ask him to indicate the part touched.
As to the general temperature, this sense is relative and is much
modified by habit, for what is cold to an inhabitant of the torrid
zone would be warm to one accustomed to a very cold climate.
Pain is an excessive stimulation of the sensory nerves, and in it
all finer sensations are lost. Thus, when a piece of hot iron burns
the hand, the sensation is the same as when the iron is very cold,
and extreme cold feels like intense heat.

317. The Organ of Taste. The sense of taste is located chiefly in the tongue, but may also be referred even to the regions of the fauces. Taste, like touch, consists in a particular mode of nerve termination.

The tongue is a muscular organ covered with mucous membrane, and is richly supplied with blood-vessels and nerves. By its complicated movements it is an important factor in chewing, in swallowing, and in articulate speech. The surface of the tongue is covered with irregular projections, called papillæ,—fine hair-like processes, about 1/12 of an inch high. Interspersed with these are the fungiform papillæ. These are shaped something like a mushroom, and may often be detected by their bright red points when the rest of the tongue is coated.

Towards the root of the tongue is another kind of papillæ, the circumvallate, eight to fifteen in number, arranged in the form of the letter V, with the apex directed backwards. These are so called because they consist of a fungiform papilla surrounded by a fold of mucous membrane, presenting the appearance of being walled around.

In many of the fungiform and most of the circumvallate papillæ are peculiar structures called taste buds or taste goblets. These exist in great numbers, and are believed to be connected with nerve fibers. These taste buds are readily excited by savory substances, and transmit the impression along the connected nerve.

The tongue is supplied with sensory fibers by branches from the fifth and eighth pairs of cranial nerves. The former confers taste on the front part of the tongue, and the latter on the back part. Branches of the latter also pass to the soft palate and neighboring parts and confer taste on them. The motor nerve of the tongue is the ninth pair, the hypoglossal.

Illustration: Fig. 125.—The Tongue.

A, epiglottis;
B, glands at the base of tongue;
C, tonsil;
D, median circumvallate papilla,
E, circumvallate papillæ;
F, filiform papillæ;
H, furrows on border of the tongue;
K, fungiform papillæ.

318. The Sense of Taste. The sense of taste is excited by stimulation of the mucous membrane of the tongue and of the palate, affecting the ends of the nerve fibers. Taste is most acute in or near the circumvallate papillæ. The middle of the tongue is scarcely sensitive to taste, while the edges and the tip are, as a rule, highly sensitive.

Certain conditions are necessary that the sense of taste may be exercised. First, the substance to be tasted must be in _solution_, or be soluble in the fluids of the mouth. Insoluble substances are tasteless. If we touch our tongue to a piece of rock crystal, there is a sensation of contact or cold, but no sense of taste. On the other hand, when we bring the tongue in contact with a piece of rock salt, we experience the sensations of contact, coolness, and saline taste.

Again, the mucous membrane of the mouth must be _moist_. When the mouth is dry, and receives substances not already in solution, there is no saliva ready to dissolve them; hence, they are tasteless. This absence of taste is common with the parched mouth during a fever.

The tongue assists in bringing the food in contact with the nerves, by pressing it against the roof of the mouth and the soft palate, and thus is produced the fullest sense of taste.

319. Physiological Conditions of Taste. The tongue is the seat of sensations which are quite unlike each other. Thus, besides the sense of taste, there is the sensation of touch, pressure, heat and cold, burning or acrid feelings, and those produced by the application of the tongue to an interrupted electric current. These are distinct sensations, due to some chemical action excited probably in the touch cells, although the true tastes may be excited by causes not strictly chemical. Thus a smart tap on the tongue may excite the sensation of taste.

In the majority of persons the back of the tongue is most sensitive to bitters, and the tip to sweets. Saline matters are perceived most distinctly at the tip, and acid substances at the sides. The nerves of taste are sensitive in an extraordinary degree to some articles of food and certain drugs. For example, the taste of the various preparations of quinine, peppermint, and wild cherry is got rid of with difficulty.

Like the other special senses, that of taste may become fatigued. The repeated tasting of one substance rapidly deadens the sensibility, probably by over-stimulation. Some savors so impress the nerves of taste that others fail to make any impression. This principle is used to make disagreeable medicine somewhat tasteless. Thus a few cloves, or grains of coffee, or a bit of pepper, eaten before a dose of castor oil, renders it less nauseous.

Flavor is something more than taste. It is in reality a mixed sensation, in which smell and taste are both concerned, as is shown by the common observation that one suffering from a cold in the head, which blunts his sense of smell, loses the proper flavor of his food. So if a person be blindfolded, and the nose pinched, he will be unable to distinguish between an apple and an onion, if one be rubbed on the tongue after the other. As soon as the nostrils are opened the difference is at once perceived.

Experiment 142. Put a drop of vinegar on a friend’s tongue, or on your own. Notice how the papillæ of the tongue start up.

Experiment 143. Rub different parts of the tongue with the pointed end of a piece of salt or gum-aloes, to show that the _back_ of the tongue is most sensitive to salt and bitter substances.

Experiment 144. Repeat the same with some sweet or sour substances, to show that the _edges_ of the tongue are the most sensitive to these substances.

Experiment 145. We often fail to distinguish between the sense of taste and that of smell. Chew some pure, roasted coffee, and it seems to have a distinct taste. Pinch the nose hard, and there is little taste. Coffee has a powerful odor, but only a feeble taste. The same is true of garlic, onions, and various spices.

Experiment 146. Light helps the sense of taste. Shut the eyes, and palatable foods taste insipid. Pinch the nose, close the eyes, and see how palatable one half of a teaspoonful of cod-liver oil becomes.

Experiment 147. Close the nostrils, shut the eyes, and attempt to distinguish by taste alone between a slice of an apple and one of a potato.

320. Modifications of the Sense of Taste. Taste is modified to a great extent by habit, education, and other circumstances. Articles of food that are unpleasant in early life often become agreeable in later years. There is occasionally a craving, especially with people of a peculiar nervous organization, for certain unnatural articles (as chalk and laundry starch) which are eaten without the least repugnance. Again, the most savory dishes may excite disgust, while the simplest articles may have a delicious flavor to one long deprived of them. The taste for certain articles is certainly acquired. This is often true of raw tomatoes, olives, and especially of tobacco.

The organs of taste and smell may be regarded as necessary accessories of the general apparatus of nutrition, and are, therefore, more or less essential to the maintenance of animal life. While taste and smell are generally maintained until the close of life, sight and hearing are often impaired by time, and may be altogether destroyed, the other vital functions remaining unimpaired.

321. Effect of Tobacco and Alcohol upon Taste. It would be remarkable if tobacco should fail to injure the sense of taste. The effect produced upon the tender papillæ of the tongue by the nicotine-loaded juices and the acrid smoke tends to impair the delicate sensibility of the entire surface. The keen appreciation of fine flavors is destroyed. The once clear and enjoyable tastes of simple objects become dull and vapid; thus highly spiced and seasoned articles of food are in demand, and then follows continued indigestion, with all its suffering.

Again, the burning, almost caustic effect of the stronger alcoholic drinks, and the acrid pungency of tobacco smoke, are disastrous to the finer perceptions of both taste and odors.

322. Smell. The sense of smell is lodged in the delicate membrane which lines the nasal cavities. The floor, sides, and roof of these cavities are formed by certain bones of the cranium and the face. Man, in common with all air-breathing animals, has two nasal cavities. They communicate with the outer air by two nostrils opening in front, while two other passages open into the pharynx behind.

To increase the area of the air passages, the two light, spongy turbinated bones, one on each side, form narrow, winding channels. The mucous membrane, with the branches of the olfactory nerve, lines the dividing wall and the inner surfaces of these winding passages. Below all these bones the lower turbinated bones may be said to divide the olfactory chamber above from the ordinary air passages.

Illustration: Fig. 126.—Distribution of Nerves over the Interior of the Nostrils. (Outer wall.)

A, branches of the nerves of smell—olfactory nerve, or ganglion;
B, nerves of common sensation to the nostril;
E, F, G, nerves to the, palate springing from a ganglion at C;
H, vidian nerve, from which branches
D, I, and J spring to be distributed to the nostrils.

The nerves which supply the nasal mucous membrane are derived from the branches of the fifth and the first pair of cranial nerves,—the olfactory. The latter, however, are the nerves of smell proper, and are spread out in a kind of thick brush of minute nerve filaments. It is in the mucous membrane of the uppermost part of the cavity of the nostril that the nerve endings of smell proper reside. The other nerves which supply the nostrils are those of common sensation (sec. 271).

323. The Sense of Smell. The sense of smell is excited by the contact of odorous particles contained in the air, with the fibers of the olfactory nerves, which are distributed over the delicate surface of the upper parts of the nasal cavities. In the lower parts are the endings of nerves of ordinary sensation. These latter nerves may be irritated by some substance like ammonia, resulting in a powerfully pungent sensation. This is not a true sensation of smell, but merely an irritation of a nerve of general sensation.

In ordinary quiet breathing, the air simply flows along the lower nasal passages into the pharynx, scarcely entering the olfactory chamber at all. This is the reason why, when we wish to perceive a faint odor, we sniff up the air sharply. By so doing, the air which is forcibly drawn into the nostrils passes up even into the higher olfactory chamber, where some of the floating particles of the odorous material come into contact with the nerves of smell.

One of the most essential conditions of the sense of smell is that the nasal passages be kept well bathed in the fluid secreted by the lining membrane. At the beginning of a cold in the head, this membrane becomes dry and swollen, thus preventing the entrance of air into the upper chamber, deadening the sensibility of the nerves, and thus the sense of smell is greatly diminished.

The delicacy of the sense of smell varies greatly in different individuals and in different animals. It is generally more acute in savage races. It is highly developed in both the carnivora and the herbivora. Many animals are more highly endowed with this sense than is man. The dog, for example, appears to depend on the sense of smell almost as much as on sight. It is well known, also, that fishes have a sense of smell. Fragments of bait thrown into the water soon attract them to a fishing ground, and at depths which little or no light can penetrate. Deer, wild horses, and antelopes probably surpass all other animals in having a vivid sense of smell.

Smell has been defined as “taste at a distance,” and it is obvious that these two senses not only form a natural group, but are clearly associated in their physical action, especially in connection with the perception of the flavor of food. The sense of odor gives us information as to the quality of food and drink, and more especially as to the quality of the air we breathe. Taste is at the gateway of the alimentary canal, while smell acts as the sentinel of the respiratory tract. Just as taste and flavor influence nutrition by affecting the digestive process, so the agreeable odors about us, even those of the perfumes, play an important part in the economy of life.

324. The Sense of Sight. The sight is well regarded as the highest and the most perfect of all our senses. It plays so common and so beneficent a part in the animal economy that we scarcely appreciate this marvelous gift. Sight is essential not only to the simplest matters of daily comfort and necessity, but is also of prime importance in the culture of the mind and in the higher forms of pleasure. It opens to us the widest and the most varied range of observation and enjoyment. The pleasures and advantages it affords, directly and indirectly, have neither cessation nor bounds.

Apart from its uses, the eye itself is an interesting and instructive object of study. It presents beyond comparison the most beautiful example of design and artistic workmanship to be found in the bodily structure. It is the watchful sentinel and investigator of the external world. Unlike the senses of taste and smell we seem, by the sense of vision, to become aware of the existence of objects which are entirely apart from us, and which have no direct or material link connecting them with our bodies. And yet we are told that in vision the eye is affected by something which is as material as any substance we taste or smell.

Note. “The higher intelligence of man is intimately associated with the perfection of the eye. Crystalline in its transparency, sensitive in receptivity, delicate in its adjustments, quick in its motions, the eye is a fitting servant for the eager soul, and, at times, the truest interpreter between man and man of the spirit’s inmost workings. The rainbow’s vivid hues and the pallor of the lily, the fair creations of art and the glance of mutual affection, all are pictured in its translucent depths, and transformed and glorified by the mind within. Banish vision, and the material universe shrinks for us to that which we may touch; sight alone sets us free to pierce the limitless abyss of space.”—M’Kendrick and Snodgrass’s _Physiology of the Senses_.

Physicists tell us that this material, known as the _luminiferous ether_, permeates the universe, and by its vibrations transmits movements which affect the eye, giving rise to the sensation of light, and the perception of even the most distant objects. Our eyes are so constructed as to respond to the vibrations of this medium for the transmission of light.

325. The Eye. The eye, the outer instrument of vision, is a most beautiful and ingenious machine. All its parts are arranged with such a delicate adjustment to one another, and such an exquisite adaptation of every part to the great object of the whole, that the eye is properly regarded as one of the wonders of nature.

The eyeball is nearly spherical in shape, but is slightly elongated from before backwards. The front part is clear and transparent, and bulges somewhat prominently to allow the entrance of the rays of light. The eye rests in a bowl-shaped socket, called the orbit, formed by parts of various bones of the head and face. The margins of this cavity are formed of strong bone which can withstand heavy blows. The socket is padded with loose, fatty tissue, and certain membranes, which serve as a soft and yielding bed in which the eyeball can rest and move without injury. In a severe sickness this fatty tissue is absorbed, and this fact explains the sunken appearance of the eyes.

The orbit is pierced through its posterior surface by an opening through which the nerve of sight, the optic, passes to the eyeball. We may think of the optic nerve holding the eyeball much as the stem holds the apple. It is the function of this most important nerve to transmit retinal impressions to the seat of consciousness in the brain, where they are interpreted.

The eye is bathed with a watery fluid, and protected by the eyelids and the eyebrows; it is moved in various directions, by muscles, all of which will soon be described.

Illustration: Fig. 127.—Section of the Human Eye.

326. The Coats of the Eyeball. The eyeball proper is elastic but firm, and is composed of three coats, or layers, each of which performs important functions. These coats are the sclerotic, the choroid, and the retina.

The sclerotic coat is the outside layer and enclosing membrane of the eyeball. It is a tough, fibrous coat for the protection and maintenance of the shape of the eye. It is white and glistening in appearance, and is in part visible, to which the phrase, “the white of the eye,” is applied. To this coat, which serves as a kind of framework for the eye, are attached the muscles which move the eyeball. In front of the globe, the sclerotic passes into a transparent circular portion forming a window through which one can see into the interior. This is the cornea.

The cornea, a clear, transparent, circular disk, fits into the sclerotic, somewhat as the crystal fits into the metallic case of a watch, forming a covering for its dial. It projects from the general contour of the eyeball, not unlike a rounded bay-window, and is often spoken of as the “window of the eye.”

Lining the inner surface of the sclerotic is the second coat, the choroid. It is dark in color and fragile in structure, and is made up almost entirely of blood-vessels and nerves. As the choroid approaches the front part of the eyeball, its parts become folded upon themselves into a series of ridges, called ciliary processes. These folds gradually become larger, and at last merge into the ciliary or accommodation muscle of the eye. The circular space thus left in front by the termination of the choroid is occupied by the iris, a thin, circular curtain, suspended in the aqueous humor behind the cornea and in front of the crystalline lens. In its center is a round opening for the admission of light.

This is the pupil, which appears as if it were a black spot. The back of the iris is lined with dark pigment, and as the coloring matter is more or less abundant, we may have a variety of colors. This pigment layer and that of the choroid and retina absorb the light entering the eye, so that little is reflected.

The pupil appears black, just as the open doorway to a dark closet seems black. The margin of the iris is firmly connected with the eyeball all round, at the junction of the sclerotic and the cornea.

327. The Retina. The third and innermost coat of the eyeball is the retina. This is the perceptive coat, without which it would be impossible to see, and upon which the images of external objects are received. It lines nearly the whole of the inner surface of the posterior chamber, resting on the inner surface of the choroid. It is with the retina, therefore, that the vitreous humor is in contact.

The retina is a very thin, delicate membrane. Although very thin, it is made up of ten distinct layers, and is so complicated in structure that not even a general description will be attempted in this book. It does not extend quite to the front limits of the posterior chamber, but stops short in a scalloped border, a little behind the ciliary processes. This is the nerve coat of the eye, and forms the terminal organ of vision. It is really an expansion of the ultimate fibers of the optic nerve, by means of which impressions are sent to the brain.

The retina contains curious structures which can be seen only with the aid of the microscope. For instance, a layer near the choroid is made up of nerve cells arranged in innumerable cylinders called “rods and cones,” and packed together not unlike the seeds of a sunflower. These rods and cones are to be regarded as the peculiar modes of termination of the nerve filaments of the eye, just as the taste buds are the modes of termination of the nerve of taste in the tongue, and just as the touch corpuscles are the terminations of the nerves in the skin.

Experiment 148. Close one eye and look steadily at the small a in the figure below. The other letters will also be visible at the same time. If now the page be brought slowly nearer to the eye while the eye is kept steadily looking at the small a, the large A will disappear at a certain point, reappearing when the book is brought still nearer.

Illustration:

On the reappearance of the A it will be noted that it comes into view from the inner side, the x being seen before it. If now we move the book towards its original place, the A will again disappear, coming again into view from the outer side when the o is seen before it.

328. Inner Structure of the Eye. Let us imagine an eyeball divided through the middle from above downwards. Let us now start in front and observe its parts (Fig. 127). We come first to the cornea, which has just been described. The iris forms a sort of vertical partition, dividing the cavity of the eyeball into two chambers.

Illustration: Fig. 128.—Diagram illustrating the Manner in which the Image of an Object is brought to a Focus on the Retina.

The anterior chamber occupies the space between the cornea and the iris, and is filled with a thin, watery fluid called the aqueous humor.

The portion behind the iris forms the posterior chamber, and contains the crystalline lens and a transparent, jelly-like fluid, the vitreous humor. This fluid is never renewed, and its loss is popularly described by the phrase, “when the eye runs out.”

Experiment 149. The retina is not sensitive where the optic nerve enters the eyeball. This is called the “blind spot.” Put two ink-bottles about two feet apart, on a table covered with white paper. Close the left eye, and fix the right steadily on the left-hand inkstand, gradually varying the distance from the eye to the ink-bottle. At a certain distance the right-hand bottle will disappear; but nearer or farther than that, it will be plainly seen.

The vitreous humor fills about four-fifths of the eyeball and prevents it from falling into a shapeless mass. It also serves to hold the choroid and the retina in position, and to maintain the proper relations of the inner structures of the eye.

The iris consists of a framework of connective tissue, the surface of which is lined by cells containing pigment, which gives color to the eye.

Bundles of involuntary muscular fibers are found in the substance of the iris. Some are arranged in a ring round the margin of the pupil; others radiate from it like the spokes of a wheel. When the circular fibers contract, the pupil is made smaller, but if these fibers relax, the radiating fibers cause the pupil to dilate more or less widely.

329. The Crystalline Lens. Just behind the pupil and close to the iris is a semi-solid, double-convex body, called the crystalline lens. It is shaped like a magnifying glass, convex

Illustration: Fig. 129.—Diagram showing the Change in the Lens during Accommodation.

On the right the lens is arranged for distant vision, the ciliary muscle is relaxed and the ligament D is tense, so flattening by its compression the front of the lens C; on the left the muscle A is acting, and this relaxes the ligament and allows the lens B to become more convex, and so fitted for the vision of near objects]

on each side, but with the posterior surface more convex than the anterior. In health it is perfectly clear and transparent, and highly elastic. When the lens becomes opaque, from change in old age, or from ulcers or wounds, we have the disease known as _cataract_.

The lens is not placed loosely in the eyeball, but is enclosed in a transparent and elastic capsule suspended throughout its circumference by a ligament called the suspensory ligament. This ligament not only retains the lens in place, but is capable of altering its shape. In ordinary conditions of the eye, this ligament is kept tense so that the front part of the lens is flattened somewhat by the pressure on it.

All around the edge, where the cornea, sclerotic, and choroid meet, is a ring of involuntary muscular fibers, forming the ciliary muscle. When these fibers contract, they draw forwards the attachment of the suspensory ligament of the lens, the pressure of which on the lens is consequently diminished. The elasticity of the lens causes it at once to bulge forwards, and it becomes more convex.

The ciliary muscle is thus known as the muscle of accommodation, because it has the power to accommodate the eye to near and distant objects. In this respect it corresponds in its use to the adjusting screw in the opera-glass and the microscope.

330. The Eye Compared to the Photographic Camera. As an optical instrument, the eye may be aptly compared, in many particulars, to the photographic camera. The latter, of course, is much simpler in structure. The eyelid forms the cap, which being removed, the light from the object streams through the eye and passes across the dark chamber to the retina behind, which corresponds to the sensitive plate of the camera. The transparent structures through which the rays of light pass represent the lenses. To prevent any reflected light from striking the plate and interfering with the sharpness of the picture, the interior of the photographic camera box is darkened. The pigmented layer of the choroid coat represents this blackened lining.

In the camera, the artist uses a thumb-screw to bring to a focus on the sensitive plate the rays of light coming from objects at different distances. Thus the lens of the camera may be moved nearer to or farther from the object. In order to obtain clear images, the same result must be accomplished by the eye. When the eye is focused for near objects, those at a distance are blurred, and when focused for distant objects, those near at hand are indistinct. Now, in the eye there is no arrangement to alter the position of the lenses, as in the camera, but the same result is obtained by what is called “accommodation.”

Again, every camera has an arrangement of diaphragms regulating the amount of light. This is a rude contrivance compared with the iris, which by means of its muscular fibers can in a moment alter the size of the pupil, thus serving a similar purpose.

Illustration: Fig. 130.—Illustrating the manner in which the Image of an Object is brought to a Focus in a Photographer’s Camera.

331. The Refractive Media of the Eye. The eye is a closed chamber into which no light can pass but through the cornea. All the rays that enter the eye must also pass through the crystalline lens, which brings them to a focus, as any ordinary lens would do.

Now, if the media through which the light from an object passes to reach the retina were all of the same density as the air, and were also plane surfaces, an impression would be produced, but the image would not be distinct. The action of the lens is aided by several refractive media in the eye. These media are the cornea, the aqueous humor, and the vitreous humor. By reason of their shape and density these media refract the rays of light, and bring them to a focus upon the retina, thus aiding in producing a sharp and distinct image of the object. Each point of the image being the focus or meeting-place of a vast number of rays coming from the corresponding point of the object is sufficiently bright to stimulate the retina to action.[44]

Thus, the moment rays of light enter the eye they are bent out of their course. By the action of the crystalline lens, aided by the refractive media, the rays of light that are parallel when they fall upon the normal eye are brought to a focus on the retina.

If the entire optical apparatus of the eye were rigid and immovable, one of three things would be necessary, in order to obtain a clear image of an object; for only parallel rays (that is, rays coming from objects distant about thirty feet or more), are brought to a focus in the average normal eye, unless some change is brought about in the refractive media. First, the posterior wall of the eye must be moved further back, or the lens would have to be capable of movement, or there must be some way of increasing the focusing power of the lens. In the eye it is the convexity of the lens that is altered so that the eye is capable of adjusting itself to different distances.[45]

Illustration: Fig. 131.—The Actual Size of the Test-Type, which should be seen by the Normal Eye at a Distance of Twenty Feet.

332. The More Common Defects of Vision. The eye may be free from disease and perfectly sound, and yet vision be indistinct, because the rays of light are not accurately brought to a focus on the retina. “Old sight,” known as presbyopia, is a common defect of vision in advancing years. This is a partial loss of the power to accommodate the eye to different distances. This defect is caused by an increase in the density of the crystalline lens, and an accompanying diminution in the ability to change its form. The far point of vision is not changed, but the near point is removed so far from the eye, that small objects are no longer visible.

Illustration: Fig. 132.—Diagram illustrating the Hypermetropic (far-sighted) Eye.

The image P′ of a point P falls behind the retina in the unaccommodated eye. By means of a convex lens it may be focused on the retina without accommodation (dotted lines). (To save space P is placed much too near the eye.)

Hence, when a person about forty-five years of age complains of dim light, poor print, and tired eyes, the time has come to seek the advice of an optician. A convex lens may be needed to aid the failing power to increase the convexity of the lens, and to assist it in bringing the divergent rays of light to a focus.

In “long sight,” or hypermetropia both the near and far point of vision are concerned, and there is no distinct vision at any distance without a strain. It is a defect in the focus, dependent upon the form of the eyes, and exists in childhood. The axis of the eyeball is too short, and the focus falls beyond the retina, which is too near the cornea. In childhood this strain may pass unnoticed, but, sooner or later it manifests itself by a sense of fatigue, dizziness, and a blurred and indistinct vision. The remedy is in the use of convex glasses to converge parallel rays of light before they enter the eye. The muscles of accommodation are thus relieved of their extra work.

“Short sight,” known as myopia, is one of the commonest defects of vision. In this defect the axis of the eye, or the distance between the cornea and the retina, is too long and the rays of light are brought to a focus in front of the retina. The tendency to short-sightedness exists in many cases at birth, and is largely hereditary. It is alarmingly common with those who make a severe demand upon the eyes. During childhood there is a marked increase of near-sightedness. The results of imprudence and abuse, in matters of eyesight, are so disastrous, especially during school life, that the question of short sight becomes one of paramount importance.

Experiment 150. With a hand-mirror reflect the sunlight on a white wall. Look steadily at the spot for a full minute, and then let the mirror suddenly be removed. The “complementary” color—a dark spot—will appear.

Experiment 151. _To show that impressions made upon the retina do not
disappear at once_. Look steadily at a bright light for a moment or
two, and then turn away suddenly, or shut the eyes. A gleam of light
will be seen for a second or two.
Look steadily at a well-lighted window for a few seconds, and then
turn the eyes suddenly to a darkened wall. The window frame may be
plainly seen for a moment.
Glance at the sun for a moment, close the eyes and the image of the
sun may be seen for a few seconds.

Experiment 152. Take a round piece of white cardboard the size of a saucer, and paint it in alternate rings of red and yellow,—two primary colors. Thrust a pin through the center and rotate it rapidly. The eye perceives neither color, but orange,—the secondary color.

Experiment 153. To note the shadows cast upon the retina by opaque matters in the vitreous humor (popularly known as floating specks, or gossamer threads), look through a small pin-hole in a card at a bright light covered by a ground-glass shade.

Experiment 154. _To illustrate accommodation_. Standing near a source of light, close one eye, hold up both forefingers not quite in a line, keeping one finger about six or seven inches from the other eye, and the other forefinger about sixteen to eighteen inches from the eye. Look at the _near_ finger; a distinct image is obtained of it, while the far one is blurred or indistinct. Look at the far image; it becomes distinct, while the near one becomes blurred. Observe that in accommodating for the near object, one is conscious of a distinct effort.

In many cases near-sightedness becomes a serious matter and demands skillful advice and careful treatment. To remedy this defect, something must be done to throw farther back the rays proceeding from an object so that they will come to a focus exactly on the retina. This is done by means of concave glasses, properly adjusted to meet the conditions of the eyes. The selection of suitable glasses calls for great care, as much harm may be done by using glasses not properly fitted to the eye.

Illustration: Fig. 133.—Diagram illustrating the Myopic (near-sighted) Eye.

The image P′ of a distant object P falls in front of the retina even without accommodation. By means of a concave lens (L) the image may be made to fall on the retina (dotted lines). (To save space P is placed much too near the eye).

There is an optical condition of the eye known as astigmatism, in which the cornea is usually at fault. In this defect of vision the curvature of the cornea is greater in one meridian than in another. As a result the rays from an object are not all brought to the same focus. Objects appear distorted or are seen with unequal clearness. Glasses of a peculiar shape are required to counteract this defect.

333. The Movements of the Eyes. In order that our eyes may be efficient instruments of vision, it is necessary that they have the power of moving independently of the head. The mechanical arrangement by which the eyeballs are moved in different directions is quite simple. It is done by six little muscles, arranged in three pairs, which, with one exception, originate in the back of the cavity in which the eye rests. Four of these muscles run a straight course and are called the _recti_. The remaining two muscles bend in their course and are called _oblique_. The coördination of these tiny muscles is marvellous in its delicacy, accuracy, and rapidity of action.

When, for any cause, the coördination is faulty, “cross eye,” technically called strabismus, is produced. Thus, if the internal rectus is shortened, the eye turns in; if the external rectus, the eye turns out, producing what is known as “wall eye.” It is thus evident that the beauty of the internal mechanism of the eye has its fitting complement in the precision, delicacy, and range of movement conferred upon it by its muscles.

334. The Eyelids and Eyebrows. The eye is adorned and protected by the eyelids, eyelashes, and eyebrows.

Illustration: Fig. 134.—Muscles of the Eyeball.

A, attachment of tendon connected with the three recti muscles;
B, external rectus, divided and turned downward, to expose the
internus rectus;
C, inferior rectus;
D, internal rectus;
E, superior rectus;
F, superior oblique;
H, pulley and reflected portion of the superior oblique;
K, inferior oblique; L, levator palpebri superioris;
M, middle portion of the same muscle (L);
N, optic nerve.

The eyelids, two in number, move over the front of the eyeball and protect it from injury. They consist of folds of skin lined with mucous membrane, kept in shape by a layer of fibrous material. Near the inner surface of the lids is a row of twenty or thirty glands, known as the _Meibomian glands_, which open on the free edges of each lid. When one of these glands is blocked by its own secretion, the inflammation which results is called a “sty.”

The inner lining membrane of the eyelids is known as the conjunctiva; it is richly supplied with blood-vessels and nerves. After lining the lids it is reflected on to the eyeballs. It is this membrane which is occasionally inflamed from taking cold.

The free edges of the lids are bordered with two or more rows of hairs called the eyelashes, which serve both for ornament and for use. They help to protect the eyes from dust, and to a certain extent to shade them. Their loss gives a peculiar, unsightly look to the face.

The upper border of the orbit is provided with a fringe of short, stiff hairs, the eyebrows. They help to shade the eyes from excessive light, and to protect the eyelids from perspiration, which would otherwise cause serious discomfort.

335. The Lacrymal Apparatus. Nature provides a special secretion, the tears, to moisten and protect the eye. The apparatus producing this secretion consists of the lacrymal or tear gland and lacrymal canals or tear passages (Fig. 136).

Outside of the eyeball, in the loose, fatty tissue of the orbit, in the upper and outer corner is the lacrymal or tear gland. It is about the size of a small almond and from it lead several little canals which open on the inner surface of the upper lid. The fluid from the gland flows out by these openings over the eyeball, and is collected at the inner or nasal corner. Here in each lid is a little reddish elevation, or _lacrymal caruncle_, in which is an opening, communicating with a small canal in the lid which joins the lacrymal sac, lodged between the orbit and the bridge of the nose (Fig. 137).

From this sac there passes a channel, the nasal duct, about one-half of an inch long, leading into the lower portion of the nostril. The fluid which has flowed over the eye is drained off by these canals into the nose. During sleep this secretion is much diminished. When the eyes are open the quantity is sufficient to moisten the eyeball, the excess being carried into the nose so gradually that the attention is not attracted to it.

The lacrymal canals are at times blocked by inflammation of the nasal duct, and the fluid collects in the corners of the eyelids and overflows down the cheeks, producing much inconvenience. The lining membrane of the eyelids through these canals is continuous with that of the nostrils. Hence, when the lining membrane of the eye is red and swollen, as during a cold, the nasal passages are also irritated, and when the nasal membrane is inflamed, the irritation is apt to pass upwards and affect the eyelids.

336. The Tears. The lacrymal or tear gland is under the control of the nervous system. Thus, if anything irritates the eyelids, the sensory nerves are stimulated and the impression is carried to the brain. Thence the nerve impulses travel to the lacrymal glands, leading to an increased flow of their secretion. The irritation of the sensory nerves in the nasal passages by smelling such substances as onions, or pungent salts, often causes a copious flow of tears.

Illustration: Fig. 135.—Lacrymal Gland and Ducts.

A lachrymal gland, the size of a small almond lodged in a shallow depression in the bones of the orbit;

B, lachrymal ducts (usually seven), which form a row of openings into the conjunctival fold.