This page contains affiliate links. As Amazon Associates we earn from qualifying purchases.
Language:
Published:
  • 1897
Collection:
Tags:
Buy it on Amazon FREE Audible 30 days

yielding victim.

Opium thus used lays its benumbing hand upon the brain, the mind is befogged, thought and reasoning are impossible. The secretions of the stomach are suspended, digestion is notably impaired, and the gastric nerves are so deadened that the body is rendered unconscious of its needs.

The moral sense is extinguished, persons once honest resort to fraud and theft, if need be, to obtain the drug, till at last health, character, and life itself all become a pitiful wreck.

301. The Use of Opium in Patent Medicines. Some forms of this drug are found in nearly all the various patent medicines so freely sold as a cure-all for every mortal disease. Opiates are an ingredient in different forms and proportions in almost all the soothing-syrups, cough medicines, cholera mixtures, pain cures, and consumption remedies, so widely and unwisely used. Many deaths occur from the use of these opiates, which at first seem indeed to bring relief, but really only smother the prominent symptoms, while the disease goes on unchecked, and at last proves fatal.

These patent medicines may appear to help one person and be fraught with danger to the next, so widely different are the effects of opiates upon different ages and temperaments. But it is upon children that these fatal results oftenest fall. Beyond doubt, thousands of children have been soothed and soothed out of existence.[43]

302. The Victim of the Opium Habit. Occasionally persons convalescing from serious sickness where anodynes were taken, unwisely cling to them long after recovery. Other persons, jaded with business or with worry, and unable to sleep, unwisely resort to some narcotic mixture to procure rest. In these and other similar cases, the use of opiates is always most pernicious. The amount must be steadily increased to obtain the elusive repose, and at best the phantom too often escapes.

Even if the desired sleep is procured, it is hardly the coveted rest, but a troubled and dreamy slumber, leaving in the morning the body quite unrefreshed, the head aching, the mouth dry, and the stomach utterly devoid of appetite. But far worse than even this condition is the slavish yielding to the habit, which soon becomes a bondage in which life is shorn of its wholesome pleasures, and existence becomes a burden.

303. Chloral. There are other preparations which have become instruments of direful and often fatal injury. Chloral is a powerful drug that has been much resorted to by unthinking persons to produce sleep. Others, yielding to a morbid reluctance to face the problems of life, have timidly sought shelter in artificial forgetfulness. To all such it is a false friend. Its promises are treason. It degrades the mind, tramples upon the morals, overpowers the will, and destroys life itself.

304. Cocaine, Ether, Chloroform, and Other Powerful Drugs. Another dangerous drug is Cocaine. Ether and chloroform, those priceless blessings to the human race if properly controlled, become instruments of death when carelessly trifled with. Persons who have been accustomed to inhale the vapor in slight whiffs for neuralgia or similar troubles do so at imminent hazard, especially if lying down. They are liable to become slowly unconscious, and so to continue the inhalation till life is ended.

There is still another class of drugs often carelessly used, whose effect, while less directly serious than those mentioned, is yet far from harmless. These drugs, which have sprung into popular use since the disease _la grippe_ began its dreaded career, include _phenacetine_, _antipyrine_, _antifebrine_, and other similar preparations. These drugs have been seized by the public and taken freely and carelessly for all sorts and conditions of trouble. The random arrow may yet do serious harm. These drugs, products of coal-oil distillation, are powerful depressants. They lower the action of the heart and the tone of the nervous centers. Thus the effect of their continued use is to so diminish the vigor of the system as to aggravate the very disorder they are taken to relieve.

305. Effect of Tobacco on the Nervous System. That the use of tobacco produces a pernicious effect upon the nervous system is obvious from the indignant protest of the entire body against it when it is first used. Its poisonous character is amply shown by the distressing prostration and pallor, the dizziness and faintness, with extreme nausea and vomiting, which follow its employment by a novice.

The morbid effects of tobacco upon the nervous system of those who habitually use it are shown in the irregular and enfeebled action of the heart, with dizziness and muscular tremor. The character of the pulse shows plainly the unsteady heart action, caused by partial paralysis of the nerves controlling this organ. Old, habitual smokers often show an irritable and nervous condition, with sleeplessness, due doubtless to lack of proper brain nutrition.

All these results tend to prove that tobacco is really a nerve poison, and there is reason to suspect that the nervous breakdown of many men in mature life is often due to the continued use of this depressing agent. This is shown more especially in men of sedentary life and habits, as men of active habits and out-door life, experience less of the ill effects of tobacco.

Few, if any, habitual users of tobacco ever themselves approve of it. They all regret the habit, and many lament they are so enslaved to it that they cannot throw it off. They very rarely advise any one to follow their example.

306. Effects of Tobacco on the Mind. With this continuously depressing effect of tobacco upon the brain, it is little wonder that the mind may become enfeebled and lose its capacity for study or successful effort. This is especially true of the young. The growth and development of the brain having been once retarded, the youthful user of tobacco (especially the foolish cigarette-smoker) has established a permanent drawback which may hamper him all his life.

The young man addicted to the use of tobacco is often through its use retarded in his career by mental languor or weakening will power, and by mental incapacity. The keenness of mental perception is dulled, and the ability to seize and hold an abstract thought is impaired. True, these effects are not sharply obvious, as it would be impossible to contrast the present condition of any one person with what it might have been. But the comparison of large numbers conveys an instructive lesson. Scholars who start well and give promise of a good future fail by the way. The honors of the great schools, academies, and colleges are very largely taken by the tobacco abstainers. This is proved by the result of repeated and extensive comparisons of the advanced classes in a great number of institutions in this country and in Europe. So true is this that any young man who aspires to a noble career should bid farewell either to his honorable ambition or to his tobacco, for the two very rarely travel together. Consequently our military and naval academies and very many seminaries and colleges prohibit the use of tobacco by their students. For the same reasons the laws of many states very properly forbid the sale to boys of tobacco, and especially of cigarettes.

307. Effect of Tobacco upon Character. Nor does tobacco spare the morals. The tobacco-user is apt to manifest a selfish disregard of the courtesies due to others. He brings to the presence of others a repulsive breath, and clothing tainted with offensive odors. He poisons the atmosphere that others must inhale, and disputes their rights to breathe a pure, untainted air. The free use of tobacco by young people dulls the acuteness of the moral senses, often leads to prevarication and deceit in the indulgence, and is apt to draw one downward to bad associates. It is not the speed but the direction that tells on the future character and destiny of young men.

Additional Experiments.

Experiment 132. _To illustrate the cooperation of certain parts of the body._ Tickle the inside of the nose with a feather. This does not interfere with the muscles of breathing, but they come to the help of the irritated part, and provoke sneezing to clear and protect the nose.

Experiment 133. Pretend to aim a blow at a person’s eye. Even if he is warned beforehand, the lids will close in spite of his effort to prevent them.

Experiment 134. _To illustrate how sensations are referred to the ends of the nerves_. Strike the elbow end of the ulna against anything hard (commonly called “hitting the crazy bone”) where the ulna nerve is exposed, and the little finger and the ring finger will tingle and become numb.

Experiment 135. _To show that every nerve is independent of any other._ Press two fingers closely together. Let the point of the finest needle be carried ever so lightly across from one finger to another, and we can easily tell just when the needle leaves one finger and touches the other.

Experiment 136. _To paralyze a nerve temporarily_. Throw one arm over the sharp edge of a chair-back, bringing the inner edge of the biceps directly over the edge of the chair. Press deep and hard for a few minutes. The deep pressure on the nerve of the arm will put the arm “asleep,” causing numbness and tingling. The leg and foot often “get asleep” by deep pressure on the nerves of the thigh.

Experiment 137. Press the ulnar nerve at the elbow, the prickling sensation is referred to the skin on the ulnar side of the hand.

Experiment 138. Dip the elbow in ice-cold water; at first one feels the sensation of cold, owing to the effect on the cutaneous nerve-endings. Afterwards, when the trunk of the ulnar nerve is affected, pain is felt in the skin of the ulnar side of the hand, where the nerve terminates.

Chapter XI.

The Special Senses.

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: a oAx]

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 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_.

[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.]

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.
]

Various mental emotions, as joy and grief, may produce similar results. In these cases the glands secrete the fluid in such quantities that it cannot escape by the lacrymal canals, and the excess rolls over the cheeks as tears. Excessive grief sometimes acts on the nerve centers in exactly the opposite manner, so that the activity of the glands is arrested and less fluid is secreted. This explains why some people do not shed tears in times of deep grief.

Experiment 155. Gently turn the inner part of your lower eyelid down. Look in a mirror, and the small lacrymal point, or opening into the nasal duct, may be observed.

337. Color-blindness. There is an abnormal condition of vision called color-blindness, in which the power of discrimination between different colors is impaired. Experiment shows that ninety-six out of every one hundred men agree as to the identity or the difference of color, while the remaining four show a defective perception of color.

The first may be said to have _normal vision_; the second are called _color-blind_. It is a curious fact that ten times more men than women are color-blind.

In its true sense, color-blindness is always congenital, often hereditary. This condition of abnormal vision is totally incurable. A person may be color-blind and not know it until the defect is accidentally revealed. The common form of defective color-vision is the inability to distinguish between _red_ and _green_. As green lights mean safety, and red lights danger, on railroads, on shipboard, and elsewhere, it becomes of paramount importance that no one who is color-blind should be employed in such service. Various tests are now required by statute law in many states to be used for the detection of such defects of vision among employees in certain occupations.

338. School Life and the Eyesight. The eyes of children need more care than those of adults, because their eyes are still in the course of development. The eyes, like any other organ which is yet to attain its full growth, require more care in their use than one which has already reached its full size. They are peculiarly liable to be affected by improper or defective light. Hence the care of the eyes during school life is a matter of the most practical importance.

In no matter of health can the teacher do a more distinct service than in looking after the eyesight of the pupils. Children suffering from defective vision are sometimes punished by teachers for supposed stupidity. Such pupils, as well as the deaf, are peculiarly sensitive to their defects. Every schoolroom should have plenty of light; it should come from either side or the rear, and should be regulated with suitable shades and curtains.

Pupils should not be allowed to form the bad habit of reading with the book held close to the eyes. The long search on maps for obscure names printed in letters of bad and trying type should be discouraged. Straining the eyes in trying to read from slates and blackboards, in the last hour of the afternoon session, or in cloudy weather, may do a lifelong injury to the eyesight. Avoid the use, so far as possible, especially in a defective light, of text-books which are printed on battered type and worn plates.

The seat and desk of each scholar should be carefully arranged to suit the eyesight, as well as the bones and muscles. Special pains should be taken with the near-sighted pupils, and those who return to school after an attack of scarlet fever, measles, or diphtheria.

Experiment 156. _To test color-blindness._ On no account is the person being tested to be asked to name a color. In a large class of students one is pretty sure to find some who are more or less color-blind. The common defects are for red and green.

Place worsteds on a white background in a good light. Select, as a test color, a skein of light green color, such as would be obtained by mixing a pure green with white. Ask the examinee to select and pick out from the heap all those skeins which appear to him to be of the same color, whether of lighter or darker shades. A color-blind person will select amongst others some of the confusion-colors, _e.g._, pink, yellow. A colored plate showing these should be hung up in the room. Any one who selects all the greens and no confusion-colors has normal color vision. If, however, one or more confusion-colors be selected, proceed as follows: select as a test color a skein of pale rose. If the person be red-blind, he will choose blue and violet; if green-blind, gray and green.

Select a bright red skein. The red-blind will select green and brown; the green-blind picks out reds or lighter brown.

339. Practical Hints on the Care of the Eyes. The eye is an exceedingly delicate and sensitive organ. While it is long-suffering, its endurance has a limit. Like all the other organs of the body, the eyes are better for moderate and rational use. More than any other organ they require attention to the general health, as the condition of the skin, exercise in the open air, good food, and proper habits of daily living.

The tissues of the eyes are peculiarly sensitive to any general influence. Certain constitutional diseases, like rheumatism, lead-poisoning, diphtheria, and measles often affect the eyes. Special care should be taken with children’s eyes during and after an attack of measles and scarlet fever. The eyes of young infants should not be exposed to glaring lights or to the direct rays of the sun, as when taken out in baby carriages.

[Illustration: Fig. 136.–Showing the Relative Position of the Lacrymal Apparatus, the Eyeball, and the Eyelids.

A, lacrymal canals, with the minute orifices represented as two black dots (puncta lacrymalia) to the right; B, tendon of the orbicularis palpebrarum muscle; apparently under B is seen the lacrymal sac. The minute openings of the Meibomian glands are seen on the free margins of the eyelids.

Below A is seen a small conical elevation, with black dots (the lacrymal papilla or caruncle).]

Glasses should be worn when they are needed. A failure to do this ususally causes much unnecessary suffering. It is far from wise to postpone as long as possible the first use of glasses. The selection and proper fitting of glasses call for the combined skill of both the physician and the optician. Obstinate headaches are often caused by defective vision, and may disappear after discontinuing improper glasses.

The habit of reading, in the cars or elsewhere, the daily paper and poorly printed books, with their blurred and indistinct type, is a severe strain on the accommodation apparatus of the eyes. It is a dangerous practice to read in bed at night, or while lying down in a darkened or shaded room. This is especially true during recovery from illness. The muscles of the eyes undergo excessive strain in accommodating themselves to the unnatural position. The battered type, wood-pulp paper, and poor presswork, now so commonly used in the cheap editions of books and periodicals, are often injurious to the eyesight.

Reading-matter should not be held nearer to the eyes than is necessary to make the print appear perfectly sharp and distinct. No print should be read continuously that cannot be seen clearly at about eighteen inches. Those who read music are especially liable to strain the eyes, because exact vision is required to follow the notes. Persons who wear glasses for reading should be careful to use them while reading music, and good light is necessary to avoid any undue strain.

After reading steadily for some time, the eyes should be rested by closing them a short period or by looking at some distant object, even if only for a few moments. The book, the sewing, and work generally, should be held as far from the eyes as is compatible with good vision. The natural tendency is to reverse this rule. We should never read, write, sew, stitch, or otherwise use the eyes when they smart or tingle, or when the sight is dim or blurred. The eyes are then tired and need a rest. Much injury may be done by reading in twilight, or by artificial light in the early morning, and by reading and working in badly lighted and ill-ventilated rooms.

Good artificial light is much to be preferred to insufficient sunlight. The artificial light should be sufficiently bright and steady; a fickering light is always bad. Riding against a strong wind, especially on a bicycle, may prove hurtful, at least for eyes that are inclined to any kind of inflammation. The light reflected from snow is a common source of injury to the eyes. It is a wise caution in passing from a dark room to avoid looking immediately at the sun, an incandescent light, the glistening snow, or other bright objects.

The eyes should never be rubbed, or the fingers thrust into them,[46] and much less when they are irritated by any foreign substance. The sooner the offending substance is removed the better.

[Illustration: Fig. 137.–Lacrymal Canals, Lacrymal Sac, and Nasal ducts, opened by their Anterior Portion.]

340. Effect of Alcohol upon the Eye. The earlier and slighter forms of injury done to the eye by the use of intoxicants are quite familiar: the watery condition of the eye and of the lids, and the red and bleared aspect of the organ. Both are the result of chronic inflammation, which crowds the blood into the vessels of the cornea, making them bloodshot and visible. The nerves controlling the circulation of the eye are partially paralyzed, and thus the relaxed vessels become distended.

But more serious results ensue. Long use of intoxicants produces diseases of the retina, involving in many cases marked diminution of acuteness as well as quickness of vision, and at times distorted images upon the surface of the retina. In other instances, the congestion of the optic nerve is so serious as to involve a progressive wasting of that organ, producing at first a hazy dimness of vision which gradually becomes worse and worse, till total blindness may ensue.

It is beyond question that a wide comparison of cases by careful observers proves that a large fraction of those who indulge in strong drink suffer from some form of disease of the eye.

341. Effect of Tobacco upon Vision. Tobacco, in its distribution of evil effects, does not neglect the senses and especially the eye. A variety of vicious results is produced. The pungent smoke inflames the lids. The narcotic dilates the pupil, causing dimness and confusion of vision. A diseased condition occurs with severe pain in the eye followed by impaired vision.

Oculists speak impressively of the ill effects of tobacco, and especially of cigarettes, upon the eyes of the young. They mention a well-known disease, tobacco blindness, usually beginning with color-blindness, and progressing occasionally with increasing dimness of vision to entire loss of sight.[47]

342. The Sense of Hearing. The structure of the human ear is much more complicated than is generally supposed. It is an apparatus constructed to respond to the waves of sound. As a whole, it may be considered a peculiar form of nerve-ending.

The external ear forms only a part of a most elaborate apparatus whereby sound waves may be transmitted inwards to the real organ of hearing. The really sensitive part of the ear, in which the auditory nerve ends, is buried for protection deep out of sight in the bones of the head; so deep that sounds cannot directly affect it. Some arrangement, therefore, is required for conducting the sounds inwards to this true organ.

[Illustration: Fig. 138.–The Pinna, or Auricle.]

In studying the structure of the ear, and how it is fitted to respond to sonorous vibrations, we may divide it into three parts: the sound-conducting part, known as the external ear, the middle ear, and the deeply placed nerve portion, the inner ear.

343. The External Ear. The external ear consists of an expanded portion known as the pinna or _auricle_, and of a passage, the auditory canal or _meatus_, leading inwards from it. The surface of the auricle is convoluted to collect and transmit the vibrations of air by which sound is produced the auditory canal conducts these vibrations to the tympanic membrane. Many animals move the auricle in the direction of the sound. Thus the horse pricks up its ears when it hears a noise, the better to judge of the direction of sounds.[48]

The external auditory meatus, the passage to the middle ear, is curved and is about an inch and a quarter long. Near its outer portion are a number of fine hairs slanting outwards to prevent the entrance of insects. Embedded in the deeper parts of the canal are glands which secrete the _cerumen_, or ear-wax, which keeps the canal moist, and helps to protect it against foreign bodies and insects. As the result of a cold, this wax may collect in sufficient quantities to block the passage, and to diminish to a considerable extent the power of hearing.

344. The Middle Ear. At the inner end of the outer ear passage is the tympanum, known as “the drum of the ear.” It is a thin, oval membrane, stretched at an angle across the deep end of the passage, which it completely closes. The tympanum is thus a partition between the passage of the outer ear and the cavity of the middle ear. On its inner side is a small air chamber in the petrous portion of the temporal bone, called the cavity of the tympanum. Its bony walls are lined with mucous membrane similar to that lining the nose, mouth, and throat. On the inner wall of the tympanum are two openings, the round window, or _foramen rotundum_, and the oval window, or _foramen ovale_.

The tympanic cavity communicates with the back part of the throat, by the Eustachian tube. This tube is about one and a half inches long and lined with mucous membrane similar to that of the tympanic chamber and the throat. This passage is usually closed, but is opened in the act of swallowing. In health there is no communication between the chamber of the middle ear and the outside, except by the Eustachian tube. Thus a throat cold, with redness and swelling of the mucous membrane, is usually accompanied with some degree of deafness, because the swelling may block the lumen of the tube, and thus prevent the free passage of air to and fro.

[Illustration: Fig. 139.–General View of the Organ of Hearing.

A, pinna;
B, cavity of the concha, showing the orifices of a great number of sebaceous glands;
C, external auditory meatus;
D, membrana tympani;
F, incus;
H, malleus;
K, handle of malleus applied to the internal surface of the membrana tympani;
L, tensor tympani muscle;
between M and K is the tympanic cavity; N, Eustachian tube;
O, P, semicircular canals;
R, internal auditory canal;
S, large nerve given off from the facial ganglion; T, facial and auditory nerves.
]

A most curious feature of the ear is the chain of tiny movable bones which stretch across the cavity of the middle ear. They connect the tympanic membrane with the labyrinth, and serve to convey the vibrations communicated to the membrane across the cavity of the tympanum to the internal ear. These bones are three in number, and from their shape are called the malleus, or _hammer_, incus, or _anvil_; and
stapes, or _stirrup_.

The hammer is attached by its long handle to the inner surface of the drum of the ear. The round head is connected with the anvil by a movable joint, while the long projection of the anvil is similarly connected with the stirrup bone. The plate of the stirrup is fixed by a membrane into the oval window of the inner wall of the tympanic chamber.

These little bones are connected with each other and the tympanum by ligaments and moved by three tiny muscles. Two are attached to the hammer, and tighten and relax the drum; the other is attached to the stirrup, and prevents it from being pushed too deeply into the oval window.

[Illustration: Fig. 140.–Ear-Bones. (Anterior View.)

1, malleus, or hammer;
2, incus, or anvil;
3, stapes, or stirrup.
]

345. The Internal Ear. This forms one of the most delicate and complex pieces of mechanism in the whole body. It is that portion of the organ which receives the impression of sound, and carries it directly to the seat of consciousness in the brain. We are then able to say that we hear.

The internal ear, or bony labyrinth, consists of three distinct parts, or variously shaped chambers, hollowed out in the temporal bone,–the vestibule, the semicircular canals, and the cochlea, or snail’s shell.

[Illustration: Fig. 141.–A Cast of the External Auditory Canal. (Posterior view)]

The vestibule is the common cavity with which all the other portions of the labyrinth connect. It is an oval-shaped chamber, about ⅓ of an inch in diameter, occupying the middle part of the internal ear. It is on the inner side of the oval window, which was closed, as we have seen, by the stirrup bone. From one side of this vestibule, or central hall, the three semicircular canals pass off, and from the other side, the cochlea.

The three semicircular canals, so called from their shape, are simply bony tubes about 1/20 of an inch in width, making a curve of about 1/4 of an inch in diameter. They pass out from the vestibule, and after bending around somewhat like a hoop, they return again to the vestibule. Each bony canal contains within it a membranous canal, at the end of which it is dilated to form an _ampulla_.

Experiment 157. _To vibrate the tympanic membrane and the little ear-bones._ Shut the mouth, and pinch the nose tightly. Try to force air through the nose. The air dilates the Eustachian tube, and is forced into the ear-drum. The distinct crackle, or clicking sound, is due to the movement of the ear-bones and the tympanic membrane.

The cochlea, or snail’s shell, is another chamber hollowed out in the solid bone. It is coiled on itself somewhat like a snail’s shell. There is a central pillar, around which winds a long spiral canal. One passage from the cochlea opens directly into the vestibule; the other leads to the chamber of the middle ear, and is separated from it by the little round window already described.

The cochlea contains thousands of the most minute cords, known as the fibers or _organ of Corti_.[49] Under the microscope they present the appearance of the keyboard of a piano. These fibers appear to vibrate in sympathy with the countless shades of sounds which daily penetrate the ear. From the hair-like processes on these tightly stretched fibers, auditory impulses appear to be transmitted to the brain.

The tubes and chambers of the inner ear enclose and protect a delicate membranous sac of exactly the same shape as themselves. Between the bony walls of the passages and the membranous bag inside is a thin, clear fluid, the _perilymph_. The membranous bag itself contains a similar fluid, the _endolymph_. In this fluid are found some minute crystals of lime like tiny particles of sand, called _otoliths_, or ear-stones. Every movement of the fluid itself throws these grains from side to side.

[Illustration: Fig. 142.–Bony internal Ear of Right Side. (Magnified; the upper figure of the natural size.)

A, oval window (foramen ovale);
B, C, D, semicircular canals;
* represents the bulging part (ampulla) of each canal; E, F, G cochlea, H, round window (foramen rotundum). ]

The auditory nerve, or nerve of hearing, passes to the inner ear, through a passage in the solid bone of the skull. Its minute filaments spread at last over the inner walls of the membranous labyrinth in two branches,–one going to the vestibule and the ampullæ at the ends of the semicircular canals, the other leading to the cochlea.

346. Mechanism of Hearing. Waves of sound reach the ear, and are directed by the concha to the external passage, at the end of which they reach the tympanic membrane. When the sound-waves beat upon this thin membrane, it is thrown into vibration, reproducing in its movements the character of the air-vibrations that have fallen upon it.

Now the vibrations of the tympanic membrane are passed along the chain of bones attached to its inner surface and reach the stirrup bone. The stirrup now performs a to-and-fro movement at the oval window, passing the auditory impulse inwards to the internal ear.

Every time the stirrup bone is pushed in and drawn out of the oval window, the watery fluid (the perilymph) in the vestibule and inner ear is set in motion more or less violently, according to the intensity of the sound. The membranous labyrinth occupies the central portion of the vestibule and the passages leading from it. When, therefore, the perilymph is shaken it communicates the impulse to the fluid (endolymph) contained in the inner membranous bag. The endolymph and the tiny grains of ear-sand now perform their part in this marvelous and complex mechanism. They are driven against the sides of the membranous bag, and so strike the ends of the nerves of hearing, which transmit the auditory impulses to the seat of sensation in the brain.

It is in the seat of sensation in the brain called the _sensorium_ that the various auditory impulses received from different parts of the inner ear are fused into one, and interpreted as sounds. It is the extent of the vibrations that determines the loudness of the sound; the number of them that determines the pitch.

Experiment 158. Hold a ticking watch between the teeth, or touch the upper incisors with a vibrating tuning-fork; close both ears, and observe that the ticking or vibration is heard louder. Unstop one ear, and observe that the ticking or vibration is heard loudest in the stopped ear.

Experiment 159. Hold a vibrating tuning-fork on the incisor teeth until you cannot hear it sounding. Close one or both ears, and you will hear it.

Experiment 160. Listen to a ticking watch or a tuning-fork kept vibrating electrically. Close the mouth and nostrils, and take either a deep inspiration or deep expiration, so as to alter the tension of the air in the tympanum; in both cases the sound is diminished.

Experiment 161. With a blindfolded person test his sense of the direction of sound, _e.g._, by clicking two coins together. It is very imperfect. Let a person press both auricles against the side of the head, and hold both hands vertically in front of each meatus. On a person making a sound in front, the observed person will refer it to a position behind him.

347. Practical Hints on the Care of the Ear. This very delicate and complicated organ is often neglected when skilled treatment is urgently needed, and it is often ignorantly and carelessly tampered with when it should be let alone.

Never insert into the ear canal the corners of towels, ear spoons, the ends of toothpicks, hairpins, or any other pointed instruments. It is a needless and dangerous practice, usually causing, in time, some form of inflammation. The abrasion of the skin in the canal thus produced affords a favorable soil for the growth of vegetable parasites.

[Illustration: Fig. 143.–Diagram of the Middle and Internal Ear.]

This, in turn, may lead to a chronic inflammation of the canal and of the tympanic membrane. Again, there is always risk that the elbow may be jogged and the instrument pushed through the drum-head. There is, of course, a natural impulse to relieve the itching of the ear. This should be done with the tips of the fingers or not at all.

The popular notion that something should be put into the ear to cure toothache is erroneous. This treatment does not cure a toothache, and may lead to an injury to the delicate parts of the ear. A piece of absorbent cotton, carefully inserted into the ear, may be worn out of doors, when the cold air causes pain, but should be removed on coming into the house.

Frequent bathing in the cold water of ponds and rivers is liable to injure both the ears and the general health. In salt-water bathing, the force of the waves striking against the ears often leads to earache, long-continued inflammation, or defective hearing; to diminish this risk, insert into the ears a small plug of absorbent cotton.

The ears are often carelessly exposed to cold water and inclement weather. Very cold water should never be used to bathe the ears and nostrils. Bathe moderately and gently in lukewarm water, using a wash-rag in preference to a sponge; dry gently and thoroughly. Children’s ears are often rudely washed, especially in the auditory canal. This is not at all necessary to cleanliness, and may result in a local inflammation.

Never shout suddenly in a person’s ear. The ear is not prepared for the shock, and deafness has occasionally resulted. A sudden explosion, the noise of a cannon, may burst the drum-head, especially if the Eustachian tube be closed at the time. During heavy cannonading, soldiers are taught to keep the mouth open to allow an equal tension of air.

[Illustration: Fig. 144.–Section of Cochlea.

From A straight downwards is the direction of the central column, to which E points. B points to the projecting ridge, almost dividing the canal of the tube into an upper compartment (D), and a lower (C).]

Insects may gain entrance to the ears and occasion annoyance, pain, and fright, perhaps leading to vomiting, even to convulsions, with nervous children. A lighted lamp held at the entrance of the ear will often induce the offending insect to crawl out towards the light. A few drops of warm water, sweet oil, or molasses, dropped into the ear, will help remove the intruder.

When a discharge occurs from the ears, it is not best to plug them with cotton wads. It only keeps in what should be got rid of. Do not go to sleep with the head on a window sill or in any position, with the ears exposed to draughts of cold or damp air.

No effort should be made to remove the ear wax unless it accumulates unduly. The skin of the canal grows outward, and the extra wax and dust will be naturally carried out, if let alone. Never employ any of the many articles or “drops,” advertised to cure deafness. Neuralgic pain in the canal, usually classed as earache, may be due to decayed or improperly filled teeth.

Quinine, so generally used in its many preparations for malaria, causes a peculiar ringing or buzzing in the ears. This is a warning that it should be taken in smaller doses, or perhaps stopped for a time. In some cases quinine may produce temporary deafness.

The practice of snuffing up cold water into the nostrils is occasionally followed by an acute inflammation of the middle ear, some of the water finding its way through the Eustachian tube into this part of the organ of hearing. The nasal douche, so often advised as a home remedy for nasal catarrh, should be used only with great caution, and always in accordance with detailed directions from a physician.

348. Effect of Tobacco upon the Hearing. The sense of hearing is often injured by the use of tobacco. The irritating smoke filling all the inner cavity of the mouth and throat, readily finds its way up the Eustachian tube, dries the membrane, and irritates or inflames the delicate mechanism of the inner ear. Thus may be produced a variety of serious aural disturbances, such as unnatural noises, whistling, and roaring, followed oftentimes by a partial loss of hearing.

Hearing may be impaired by the use of alcoholic beverages. Alcohol inflames the mucous membrane of the throat, then by its nearness the lining of the Eustachian tube, and finally may injure the delicate apparatus of the internal ear.

Additional Experiments.

Experiment 162. Use a small pair of wooden compasses, or an ordinary pair of dividers with their points guarded by a small piece of cork. Apply the points of the compasses lightly and simultaneously to different parts of the body, and ascertain at what distance apart the points are felt as two. The following is the order of sensibility: tip of tongue, tip of the middle finger, palm, forehead, and back of hand.

Experiment 163. Test as in preceding experiment the skin of the arm, beginning at the shoulder and passing downwards. Observe that the sensibility is greater as one tests towards the fingers, and also in the transverse than in the long axis of the limb. In all cases compare the results obtained on both sides of the body.

Experiment 164. By means of a spray-producer, spray the back of the hand with ether, and observe how the sensibility is abolished.

Experiment 165. Touch your forehead with your forefinger; the finger appears to feel the contact, but on rubbing the forefinger rapidly over the forehead, it is the latter which is interpreted as “feeling” the finger.

Experiment 166. Generally speaking, the sensation of touch is referred to the cutaneous surfaces. In certain cases, however, it is referred even beyond this. Holding firmly in one hand a cane or a pencil, touch an object therewith; the sensation is referred to the extremity of the cane or pencil.

If, however, the cane or pencil be held loosely in one’s hand, one experiences two sensations: one corresponding to the object touched, and the other due to the contact of the rod with the skin. The process of mastication affords a good example of the reference of sensations to and beyond the periphery of the body.

Experiment 167. Prepare a strong solution of sulphate of quinine with the aid of a little sulphuric acid to dissolve it (_bitter_), a five-per-cent solution of sugar (_sweet_), a ten-per-cent solution of common salt (_saline_), and a one-per-cent solution of acetic acid (_acid_). Wipe the tongue dry, and lay on its tip a crystal of sugar. It is not tasted until it is dissolved.

Experiment 168. Apply a crystal of sugar to the tip, and another to the back of the tongue. The sweet taste is more pronounced at the tip.

Experiment 169. Repeat the process with sulphate of quinine in solution. It is scarcely tasted on the tip, but is tasted immediately on the back part of the tongue. Test where salines and acids are tasted most acutely.

Experiment 170. _To illustrate the muscular sense_. Take two equal iron or lead weights; heat one and leave the other cold. The cold weight will feel the heavier.

Experiment 171. Place a thin disk of _cold_ lead, the size of a silver dollar, on the forehead of a person whose eyes are closed; remove the disk, and on the same spot place two warm disks of equal size. The person will judge the latter to be about the same weight, or lighter, than the single cold disk.

Experiment 172. Compare two similar wooden disks, and let the diameter of one be slightly greater than that of the other. Heat the smaller one to over 120° F., and it will be judged heavier than the larger cold one.

Experiment 173. _To illustrate the influence of excitation of one sense organ on the other sense organs_. Small colored patches the shape and color of which are not distinctly visible may become so when a tuning-fork is kept vibrating near the ears. In other individuals the visual impressions are diminished by the same process.

On listening to the ticking of a watch, the ticking sounds feebler or louder on looking at a source of light through glasses of different colors.

If the finger be placed in cold or warm water the temperature appears to rise when a red glass is held in front of the eyes.

Experiment 174. _Formation of an inverted image on the retina_. Take a freshly removed ox-eye; dissect the sclerotic from that part of its posterior segment near the optic nerve. Roll up a piece of blackened paper in the form of a tube, black surface innermost, and place the eye in it with the cornea directed forward. Look at an object–_e.g._, a candle-flame–and observe the inverted image of the flame shining through the retina and choroid, and notice how the image moves when the candle is moved.

Experiment 175. Focus a candle-flame or other object on the ground-glass plate of an ordinary photographic camera, and observe the small inverted image.

Experiment 176. _To illustrate spherical aberration_. Make a pin-hole in a blackened piece of cardboard; look at a light placed at a greater distance than the normal distance of accommodation. One will see a radiate figure with four to eight radii. The figures obtained from opposite eyes will probably differ in shape.

Experiment 177. Hold a thin wooden rod or pencil about a foot from the eyes and look at a distant object. Note that the object appears double. Close the right eye; the left image disappears, and _vice versa_.

Experiment 178. _To show the movements of the iris_. It is an extremely beautiful experiment, and one that can easily be made. Look through a pin-hole in a card at a uniform white surface as the white shade of an ordinary reading-lamp. With the right eye look through the pin-hole, the left eye being closed. Note the size of the (slightly dull) circular visual field. Open the left eye, the field becomes brighter and smaller (contraction of pupil); close the left eye, after an appreciable time, the field (now slightly dull) is seen gradually to expand. One can thus see and observe the rate of movements of his own iris.

[Illustration: Fig. 145.]

Experiment 179. _To show the blind spot_. The left eye being shut, let the right eye be fixed upon the cross as in Fig. 145. When the book is held at arm’s length, both cross and round spot will be visible; but if the book be brought to about 8 inches from the eye, the gaze being kept steadily upon the cross, the round spot will at first disappear, but as the book, is brought still nearer both cross and round spot will again be seen.

Experiment 180. _To illustrate the duration of retinal impressions_. On a circular white disk, about halfway between the center and circumference, fix a small, black, oblong disk, and rapidly rotate it by means of a rotating wheel. There appears a ring of gray on the black, showing that the impression on the retina lasts a certain time.

[Illustration: Fig. 146.–Optic Disks.

The disk A, having black and white sectors, when rotated rapidly gives an even gray tint as in B.]

Experiment 181. Mark off a round piece of cardboard into black and white sectors as in A (Fig. 146). Attach it so as to rotate it rapidly, as on a sewing machine. An even gray tint will be produced as in B.

Experiment 182._To illustrate imperfect visual judgments_. Make three round black dots, A, B, C, of the same size, in the same line, and let A and C be equidistant from B. Between A and B make several more dots of the same size. A and B will then appear to be farther apart than B and C.

[Illustration:
* * * * * * *
A B C
]

For the same reason, of two squares absolutely identical in size, one marked with alternately clear and dark cross-bands, and the other with alternately clear and dark upright markings, the former will appear broader and the latter higher than the other.

Experiment 183. Make on a white card two squares of equal size. Across the one draw _horizontal_ lines at equal distances, and in the other make similar _vertical_ lines. Hold them at some distance. The one with horizontal lines appears higher than it really is, while the one with vertical lines appears broader, i.e., both appear oblong.

Experiment 184. Look at the row of letters (S) and figures (8). To

[Illustration:
S S S S S S S S 8 8 8 8 8 8 8 8 ]

some the upper halves of the letters and figures may appear to be of the same size as the lower halves, to others the lower halves may appear larger. Hold the figure upside down, and observe that there is a considerable difference between the two, the lower halves being considerably larger.

Experiment 185. _To illustrate imperfect visual judgment_. The length of a line appears to vary according to the angle and direction of certain other lines in relation to it (Fig. 147). The length of the two vertical lines is the same, yet B appears much longer than A.

[Illustration: Fig. 147.–To show False Estimate of Size.

\ /
\ /
/|\ |
/ | \ |
| |
A | B |
| |
\ | / |
\|/ |
/ \
/ \
]

Experiment 186. In indirect vision the appreciation of direction is still more imperfect. While leaning on a large table, fix a point on the table, and then try to arrange three small pieces of colored paper in a straight line. Invariably, the papers, being at a distance from the fixation-point, and being seen by indirect vision, are arranged, not in a straight line, but in the arc of a circle with a long radius.

Chapter XII.

The Throat and the Voice.

349. The Throat. The throat is a double highway, as it were, through which the air we breathe traverses the larynx on its way to the lungs, and through which the food we swallow reaches the œsophagus on its passage to the stomach. It is, therefore, a very important region of the body, being concerned in the great acts of respiration and digestion.

The throat is enclosed and protected by various muscles and bony structures, along which run the great blood-vessels that supply the head, and the great nerve trunks that pass from the brain to the parts below.

We have already described the food passages (Chapter VI.) and the air passages (Chapter VIII.).

To get a correct idea of the throat we should look into the wide-open mouth of some friend. Depressing the tongue we can readily see the back wall of the pharynx, which is common to the two main avenues leading to the lungs and the stomach. Above, we notice the air passages, which lead to the posterior cavities of the nose. We have already described the hard palate, the soft palate, the uvula, and the tonsils (Fig. 46).

On looking directly beyond these organs, we see the beginning of the downward passage,–the pharynx. If now the tongue be forcibly drawn forward, a curved ridge may be seen behind it. This is the epiglottis, which, as we have already learned shuts down, like the lid of a box, over the top of the larynx (secs. 137 and 203).

The throat is lined with mucous membrane covered with ciliated epithelium, which secretes a lubricating fluid which keeps the parts moist and pliable. An excess of this secretion forms a thick, tenacious mass of mucus, which irritates the passages and gives rise to efforts of hawking and coughing to get rid of it.

350. The Larynx. The larynx, the essential organ of voice, forms the box-like top of the windpipe. It is built of variously shaped cartilages, connected by ligaments. It is clothed on the outside with muscles; on the inside it is lined with mucous membrane, continuous with that of the other air passages.

[Illustration: Fig. 148.–View of the Cartilages in front project and form the lages and Ligaments of the “Adam’s apple,” plainly seen and Larynx. (Anterior view.)

A, hyoid bone;
B, thyro-hyoid membrane;
C, thyroid cartilage;
D, erico-thyroid membrane;
E, cricoid cartilage, lateral ligaments seen on each side; F, upper ring of the trachea.
(“Adam’s apple” is in the V-shaped groove on a line with B and C.) ]

The larynx has for a framework two cartilages, the thyroid and the cricoid, one above the other. The larger of these, called the thyroid, from a supposed resemblance to a shield, consists of two extended wings which join in front, but are separated by a wide interval behind. The united edges in front project and form the “Adam’s apple” plainly seen and easily felt on most people, especially on very lean men.

Above and from the sides rise two horns connected by bands to the hyoid bone from which the larynx is suspended. This bone is attached by muscles and ligaments to the skull. It lies at the base of the tongue, and can be readily felt by the finger behind the chin at the angle of the jaw and the neck (sec. 41 and Fig. 46). From the under side of the thyroid two horns project downwards to become jointed to the cricoid. The thyroid thus rests upon, and is movable on, the cricoid cartilage.

The cricoid cartilage, so called from its fancied resemblance to a signet-ring, is smaller but thicker and stronger than the thyroid, and forms the lower and back part of the cavity of the larynx. This cartilage is quite sensitive to pressure from the fingers, and is the cause of the sharp pain felt when we try to swallow a large and hard piece of food not properly chewed.

[Illustration: Fig. 149.–Diagram of a Sectional of Nasal and Throat Passages.

C, nasal cavities;
T, tongue;
L, lower jaw;
M, mouth;
U, uvula;
E, epiglottis;
G, larynx;
O, œsophagus.
]

On the upper edge of the cricoid cartilage are perched a pair of very singular cartilages, pyramidal in shape, called the arytenoid, which are of great importance in the production of the voice. These cartilages are capped with little horn-like projections, and give attachment at their anterior angles to the true vocal cords, and at their posterior angles to the muscles which open and close the glottis, or upper opening of the windpipe. When in their natural position the arytenoid cartilages resemble somewhat the mouth of a pitcher, hence their name.

351. The Vocal Cords. The mucous membrane which lines the various cartilages of the larynx is thrown into several folds. Thus, one fold, the free edge of which is formed of a band of elastic fibers, passes horizontally outwards from each side towards the middle line, at the level of the base of the arytenoid cartilages. These folds are called the true vocal cords, by the movements of which the voice is produced.

Above them are other folds of mucous membrane called the false vocal cords, which take no part in the production of the voice. The arrangement of the true vocal cords, projecting as they do towards the middle line, reduces to a mere chink the space between the part of the larynx above them and the part below them. This constriction of the larynx is called the glottis.

[Illustration: Fig. 150.–View of the Cartilages and Ligaments of Larynx. (Posterior view.)

A, epiglottis;
B, thyroid cartilage;
C, arytenoid cartilage;
D, ligament connecting lower cornu of the thyroid with the back of the cricoid cartilage;
E, cricoid cartilage;
F, upper ring of the trachea.
]

352. The Mechanism of the Voice. The mechanism of the voice may be more easily understood by a study of Fig. 150. We have here the larynx, viewed from behind, with all the soft parts in connection with it. On looking down, the folds forming the true vocal cords are seen enclosing a V-shaped aperture (the glottis), the narrow part being in front.

The form of this aperture may be changed by the delicately coordinate activities of the muscles of the larynx. For instance, the vocal cords may