Numerous other reflex centers are described by physiologists, but

enough has been said to emphasize the great importance of the spinal

cord as an independent nerve center, besides its function as a

conductor of nervous impulses to and from the brain.

280. The Brain as a Reflex Center. The brain, as we have just stated,

is the seat of consciousness and intelligence. It is also the seat of

many reflex, automatic, and coordinating centers. These give rise to

certain reflex actions which are as entirely independent of

consciousness as are those of the spinal cord. These acts take place

independently of the will, and often without the consciousness of the

individual. Thus, a sudden flash of light causes the eyes to blink, as

the result of reflex action. The optic nerves serve as the sensory, and

the facial nerves as the motor, conductors. The sudden start of the

whole body at some loud noise, the instinctive dodging a threatened

blow, and the springing back from sudden danger, are the results of

reflex action. The result ensues in these and in many other instances,

without the consciousness of the individual, and indeed beyond his

power of control.

281. The Importance of Reflex Action. Reflex action is thus a marvelous

provision of nature for our comfort, health, and safety. Its vast

influence is not realized, as its numberless acts are so continually

going on without our knowledge. In fact, the greater part of nerve

power is expended to produce reflex action. The brain is thus relieved

of a vast amount of work. It would be impossible for the brain to serve

as a “thinking center” to control every act of our daily life. If we

had to plan and to will every heart-beat or every respiration, the

struggle for life would soon be given up.

The fact that the gray cells of the spinal cord can originate a

countless number of reflex and automatic activities is not only of

great importance in protecting the body from injury, but increases

vastly the range of the activities of our daily life.

Even walking, riding the bicycle, playing on a piano, and numberless

other such acts may be reflex movements. To learn how, requires, of

course, the action of the brain, but with frequent repetition the

muscles become so accustomed to certain successive movements, that they

are continued by the cord without the control of the brain. Thus we may

acquire a sort of artificial reflex action, which in time becomes in a

way a part of our organization, and is carried on without will power or

even consciousness.

So, while the hands are busily doing one thing, the brain can be

intently thinking of another. In fact, any attempt to control reflex

action is more apt to hinder than to help. In coming rapidly down

stairs, the descent will be made with ease and safety if the spinal

cord is allowed entire charge of the act, but the chances of stumbling

or of tripping are very much increased if each step be taken as the

result of the will power. The reflex action of the cord may be

diminished, or inhibited as it is called, but this power is limited.

Thus, we can by an effort of the will stop breathing for a certain

time, but beyond that the reflex mechanism overcomes our will and we

could not, if we would, commit suicide by holding our breath. When we

are asleep, if the palm of the hand be tickled, it closes; when we are

awake we can prevent it.

Illustration: Fig. 120.—Dr. Waller’s Diagrammatic Illustration of the

Reflex Process.

From the sentient surface (1) an afferent impulse passes along (2) to

the posterior root of the spinal cord, the nerve fibers of the

posterior root ending in minute filaments among the small cells of this

part of the cord (3). In some unknown way this impulse passes across

the gray part of the cord to the large cells of the anterior root (5),

the cells of this part being connected by their axis-cylinder with the

efferent fibers (6). These convey the stimulus to the fibers of the

muscle (7), which accordingly contract. Where the brain is concerned in

the action the circuit is longer through S and M.

Experiment 131. _To illustrate reflex action by what is called

knee-jerk._ Sit on a chair, and cross the right leg over the left one.

With the tips of the fingers or the back of a book, strike the right

ligamentum patellæ. The right leg will be raised and thrown forward

with a jerk, owing to the contraction of the quadriceps muscles. An

appreciable time elapses between the striking of the tendon and the

jerk. The presence or absence of the knee-jerk may be a most

significant symptom to the physician.

282. The Sympathetic System. Running along each side of the spine, from

the base of the skull to the coccyx, is a chain of nerve knots, or

ganglia. These ganglia, twenty-four on each side, and their branches

form the sympathetic system, as distinguished from the cerebro-spinal

system consisting of the brain and spinal cord and the nerves springing

from them. The ganglia of the sympathetic system are connected with

each other and with the sensory roots of the spinal nerves by a network

of gray nerve fibers.

At the upper end the chain of each side passes up into the cranium and

is closely connected with the cranial nerves. In the neck, branches

pass to the lungs and the heart. From the ganglia in the chest three

nerves form a complicated network of fibers, from which branches pass

to the stomach, the liver, the intestines, the kidneys, and other

abdominal organs. A similar network of fibers is situated lower down in

the pelvis, from which branches are distributed to the pelvic organs.

At the coccyx the two chains unite into a single ganglion.

Thus, in general, the sympathetic system, while intimately connected

with the cerebro-spinal, forms a close network of nerves which

specially accompany the minute blood-vessels, and are distributed to

the muscles of the heart, the lungs, the stomach, the liver, the

intestines, and the kidneys—that is, the hollow organs of the body.

283. The Functions of the Sympathetic System. This system exercises a

superintending influence over the greater part of the internal organs

of the body, controlling to a certain extent the functions of

digestion, nutrition, circulation, and respiration. The influence thus

especially connected with the processes of organic life is generally

different from, or even opposed to, that conveyed to the same organs by

fibers running in the spinal or cranial nerves. These impulses are

beyond the control of the will.

Illustration: Fig. 121.—The Cervical and Thoracic Portion of the

Sympathetic Nerve and its Main Branches.

A, right pneumogastric;

B, spinal accessory;

C, glosso-pharyngeal;

D, right bronchus;

E, right branch of pulmonary artery;

F, one of the intercostal nerves;

H, great splanchnic nerve;

K, solar plexus;

L, left pneumogastric;

M, stomach branches of right pneumogastric;

N, right ventricle;

O, right auricle;

P, trunk of pulmonary artery;

R, aorta; S, cardiac nerves;

T, recurrent laryngeal nerve;

U, superior laryngeal nerve;

V, submaxillary ganglion;

W, lingual branch of the 5th nerve;

X, ophthalmic ganglion;

Y, motor oculi externus.

Hence, all these actions of the internal organs just mentioned that are

necessary to the maintenance of the animal life, and of the harmony

which must exist between them, are controlled by the sympathetic

system. But for this control, the heart would stop beating during

sleep, digestion would cease, and breathing would be suspended. Gentle

irritation of these nerves, induced by contact of food in the stomach,

causes that organ to begin the churning motion needed for digestion.

Various mental emotions also have a reflex action upon the sympathetic

system. Thus, terror dilates the pupils, fear acts upon the nerves of

the small blood-vessels of the face to produce pallor, and the sight of

an accident, or even the emotions produced by hearing of one, may

excite nausea and vomiting.

The control of the blood-vessels, as has been stated (sec. 195), is one

of the special functions of the sympathetic system. Through the nerves

distributed to the muscular coats of the arteries, the caliber of these

vessels can be varied, so that at one moment they permit a large

quantity of blood to pass, and at another will contract so as to

diminish the supply. This, too, is beyond the control of the will, and

is brought about by the vaso-motor nerves of the sympathetic system

through a reflex arrangement, the center for which is the medulla

oblongata.

284. Need of Rest. The life of the body, as has been emphasized in the

preceding chapters, is subject to constant waste going on every moment,

from the first breath of infancy to the last hour of old age. We should

speedily exhaust our life from this continual loss, but for its

constant renewal with fresh material. This exhaustion of life is

increased by exertion, and the process of repair is vastly promoted by

rest. Thus, while exercise is a duty, rest is equally imperative.

The eye, when exactingly used in fine work, should have frequent

intervals of rest in a few moments of darkness by closing the lids. The

brain, when urged by strenuous study, should have occasional seasons of

rest by a dash of cold water upon the forehead, and a brief walk with

slow and deep inspirations of fresh air. The muscles, long cramped in a

painful attitude, should be rested as often as may be, by change of

posture or by a few steps around the room.

It is not entirely the amount of work done, but the continuity of

strain that wears upon the body. Even a brief rest interrupts this

strain; it unclogs the wheels of action. Our bodies are not designed

for continuous toil. An alternation of labor and rest diminishes the

waste of life. The benign process of repair cannot go on, to any

extent, during strenuous labor, but by interposing frequent though

brief periods of rest, we lessen the amount of exhaustion, refresh the

jaded nerves, and the remaining labor is more easily endured.

285. Benefits of Rest. There is too little repose in our American

nature and in our modes of life. A sense of fatigue is the mute appeal

of the body for a brief respite from labor, and the appeal should, if

possible, be heeded. If this appeal be not met, the future exertion

exhausts far more than if the body had been even slightly refreshed. If

the appeal be met, the brief mid-labor rest eases the friction of toil,

and the remaining labor is more easily borne. The feeling that a

five-minute rest is so much time lost is quite an error. It is a gain

of physical strength, of mental vigor, and of the total amount of work

done.

The merchant burdened with the cares of business life, the soldier on

the long march, the ambitious student over-anxious to win success in

his studies, the housewife wearied with her many hours of exacting

toil, each would make the task lighter, and would get through it with

less loss of vital force, by occasionally devoting a few minutes to

absolute rest in entire relaxation of the strained muscles and

overtaxed nerves.

286. The Sabbath as a Day of Physiological Rest. The divine institution

of a Sabbath of rest, one day in seven, is based upon the highest needs

of our nature. Rest, to be most effective, should alternate in brief

periods with labor.

It is sound physiology, as well as good morals and manners, to cease

from the usual routine of six days of mental or physical work, and rest

both the mind and the body on the seventh. Those who have succeeded

best in what they have undertaken, and who have enjoyed sound health

during a long and useful life, have studiously lived up to the mandates

of this great physiological law. It is by no means certain that the

tendency nowadays to devote the Sabbath to long trips on the bicycle,

tiresome excursions by land and sea, and sight-seeing generally,

affords that real rest from a physiological point of view which nature

demands after six days of well-directed manual or mental labor.

287. The Significance of Sleep as a Periodical Rest. Of the chief

characteristics of all living beings none is so significant as their

periodicity. Plants as well as animals exhibit this periodic character.

Thus plants have their annual as well as daily periods of activity and

inactivity. Hibernating animals pass the winter in a condition of

unconsciousness only to have their functions of activity restored in

early spring. Human beings also present many instances of a periodic

character, many of which have been mentioned in the preceding pages.

Thus we have learned that the heart has its regular alternating periods

of work and rest. After every expiration from the lungs there is a

pause before the next inspiration begins.

Now sleep is just another manifestation of this periodic and

physiological rest by which Nature refreshes us. It is during the

periods of sleep that the energy expended in the activities of the

waking hours is mainly renewed. In our waking moments the mind is kept

incessantly active by the demands made on it through the senses. There

is a never-ceasing expenditure of energy and a consequent waste which

must be repaired. A time soon comes when the brain cells fail to

respond to the demand, and sleep must supervene. However resolutely we

may resist this demand, Nature, in her relentless way, puts us to

sleep, no matter what objects are brought before the mind with a view

to retain its attention.[41]

288. Effect of Sleep upon the Bodily Functions. In all the higher

animals, the central nervous system enters once at least in the

twenty-four hours into the condition of rest which we call sleep.

Inasmuch as the most important modifications of this function are

observed in connection with the cerebro-spinal system, a brief

consideration of the subject is properly studied in this chapter. In

Chapter IV. we learned that repose was as necessary as exercise to

maintain muscular vigor. So after prolonged mental exertion, or in fact

any effort which involves an expenditure of what is often called

nerve-force, sleep becomes a necessity. The need of such a rest is

self-evident, and the loss of it is a common cause of the impairment of

health. While we are awake and active, the waste of the body exceeds

the repair; but when asleep, the waste is diminished, and the cells are

more actively rebuilding the structure for to-morrow’s labor. The

organic functions, such as are under the direct control of the

sympathetic nervous system,—circulation, respiration, and

digestion,—are diminished in activity during sleep. The pulsations of

the heart and the respiratory movements are less frequent, and the

circulation is slower. The bodily temperature is reduced, and the

cerebral circulation is diminished. The eyes are turned upward and

inward, and the pupils are contracted.

The senses do not all fall to sleep at once, but drop off successively:

first the sight, then the smell, the taste, the hearing and lastly the

touch. The sleep ended, they awake in an inverse order, touch, hearing,

taste, smell, and sight.

289. The Amount of Sleep Required. No precise rule can be laid down

concerning the amount of sleep required. It varies with age,

occupation, temperament, and climate to a certain extent. An infant

whose main business it is to grow spends the greater part of its time

in sound sleep. Adults of average age who work hard with their hands or

brain, under perfectly normal physiological conditions, usually require

at least eight hours of sleep. Some need less, but few require more.

Personal peculiarities, and perhaps habit to a great extent, exert a

marked influence. Some of the greatest men, as Napoleon I., have been

very sparing sleepers. Throughout his long and active life, Frederick

the Great never slept more than five or six hours in the twenty-four.

On the other hand, some of the busiest brain-workers who lived to old

age, as William Cullen Bryant and Henry Ward Beecher, required and took

care to secure at least eight or nine hours of sound sleep every night.

In old age, less sleep is usually required than in adult life, while

the aged may pass much of their time in sleep. In fact, each person

learns by experience how much sleep is necessary. There is no one thing

which more unfits one for prolonged mental or physical effort than the

loss of natural rest.

290. Practical Rules about Sleep. Children should not be played with

boisterously just before the bedtime hour, nor their minds excited with

weird goblin stories, or a long time may pass before the wide-open eyes

and agitated nerves become composed to slumber. Disturbed or

insufficient sleep is a potent factor towards producing a fretful,

irritable child.

At all ages the last hour before sleep should, if possible, be spent

quietly, to smooth the way towards sound and refreshing rest. The sleep

induced by medicine is very often troubled and unsatisfactory.

Medicines of this sort should not be taken except on the advice of a

physician.

While a hearty meal should not usually be taken just before bedtime, it

is not well to go to bed with a sense of positive faintness and hunger.

Rather, one should take a very light lunch of quite simple food as a

support for the next eight hours.

Illustration: Fig. 122.—Trunk of the Left Pneumogastric.

(Showing its distribution by its branches and ganglia to the larynx,

pharynx, heart, lungs, and other parts.)

It is better, as a rule, not to engage in severe study during the hours

just before bedtime. Neither body nor mind being at its best after the

fatigues of the day, study at that time wears upon the system more, and

the progress is less than at earlier hours. One hour of morning or day

study is worth a much longer time late at night. It is, therefore, an

economy both of time and of nerve force to use the day hours and the

early evening for study.

The so-called “cat naps” should never be made to serve as a substitute

for a full night’s sleep. They are largely a matter of habit, and are

detrimental to some as well as beneficial to others. Late hours are

usually associated with exposure, excitement, and various other drains

upon the nerve force, and hence are injurious.

It is better to sleep on one or other side than on the back. The head

should be somewhat raised, and a mattress is better than a feather bed.

The bedclothes should be sufficient, but not too heavy. Light tends to

prevent sleep, as do loud or abrupt sounds, but monotonous sounds aid

it.

291. Alcohol and the Brain. The unfortunate effects which alcoholic

drinks produce upon the brain and nervous system differ from the

destructive results upon other parts of the body in this respect, that

elsewhere the consequences are usually both less speedy and less

obvious. The stomach, the liver, and even the heart may endure for a

while the trespass of the narcotic poison, and not betray the invasion.

But the nervous system cannot, like them, suffer in silence.

In the other parts of the body the victim may (to a certain extent)

conceal from others the suffering of which he himself is painfully

conscious. But the tortured brain instantly reveals the calamity and

the shame, while the only one who may not fully realize it is the

victim himself. Besides this, the injuries inflicted upon other organs

affect only the body, but here they drag down the mind, ruin the

morals, and destroy the character.

The brain is indeed the most important organ of the body, as it

presides over all the others. It is the lofty seat of power and

authority. Here the king is on his throne. But if, by this malignant

adversary, the king himself be dethroned, his whole empire falls to

ruins.

292. How Alcohol Injures the Brain. The brain, the nerve centers, and

the nerves are all made up of nerve pulp, the softest and most delicate

tissue in the whole bodily structure. Wherever this fragile material

occurs in our bodies,—in the skull, the spine, the trunk, or the

limbs,—the all-wise Architect has carefully protected it from violence,

for a rough touch would injure it, or even tender pressure would

disturb its function.

It is a further indication of the supreme importance of the brain, that

about one-fifth of the entire blood of the body is furnished to it.

Manifestly, then, this vital organ must be tenderly cared for. It must

indeed be well nourished, and therefore the blood sent to it must be

highly nutrient, capable of supplying oxygen freely. This condition is

essential to successful brain action. But intoxicants bring to it blood

surcharged with a poisonous liquid, and bearing only a limited supply

of oxygen.

Another condition of a healthy brain is that the supply of blood to it

shall be equable and uniform. But under the influence of strong drink,

the blood pours into the paralyzed arteries a surging tide that floods

the head, and hinders and may destroy the use of the brain and the

senses. Still another requirement is that whatever is introduced into

the cerebral tissues, having first passed through the stomach walls and

thence into the blood, shall be bland, not irritating. But in the brain

of the inebriate are found not only the distinct odor but the actual

presence of alcohol. Thus we plainly see how all these three vital

conditions of a healthy brain are grossly violated by the use of

intoxicants.

“I think there is a great deal of injury being done by the use of

alcohol in what is supposed by the consumer to be a most moderate

quantity, to persons who are not in the least intemperate, and to

people supposed to be fairly well. It leads to degeneration of the

tissues; it damages the health; it injures the intellect. Short of

drunkenness, that is, in those effects of it which stop short of

drunkenness, I should say from my experience that alcohol is the most

destructive agent we are aware of in this country.”—Sir William Gull,

the most eminent English physician of our time.

293. Why the Brain Suffers from the Alcoholic Habit. We do not find

that the alcoholic habit has produced in the brain the same coarse

injuries that we see in other organs, as in the stomach, the liver, or

the heart. Nor should we expect to find them; for so delicate and so

sensitive is the structure of this organ, that a very slight injury

here goes a great way,—a disturbance may be overwhelming to the brain

that would be only a trifle to some of the less delicate organs.

Alcohol has different degrees of affinity for different organs of the

body, but much the strongest for the cerebral tissues. Therefore the

brain feels more keenly the presence of alcohol than does any other

organ. Almost the moment that the poison is brought into the stomach,

the nerves send up the alarm that an invading foe has come. At once

there follows a shock to the brain, and very soon its paralyzed

blood-vessels are distended with the rush of blood. This first effect

is, in a certain sense, exhilarating, and from this arousing influence

alcohol has been erroneously considered a stimulant; but the falsity of

this view is pointed out elsewhere in this book.

294. Alcohol, the Enemy of Brain Work. The healthy brain contains a

larger proportion of water than does any other organ. Now alcohol, with

its intense affinity for water, absorbs it from the brain, and thus

condenses and hardens its structure. One of the important elements of

the brain is its albumen; this also is contracted by alcohol. The nerve

cells and fibers gradually become shriveled and their activity is

lowered, the elasticity of the arteries is diminished, the membranes

enveloping the brain are thickened, and thus all proper brain nutrition

is impaired. The entire organ is slowly hardened, and becomes unfitted

for the proper performance of its delicate duties. In brief, alcohol in

any and every form is the enemy of successful and long-continued brain

work.

Illustration: Fig. 123.—Nerve Trunks of the Right Arm.

295. Other Physical Results of Intoxicants. What are some of the

physical results observed? First, we note the failure of the vaso-motor

nerves to maintain the proper tone of the blood-vessels, as in the

turgid face and the congested cornea of the eye. Again, we observe the

loss of muscular control, as is shown by the drop of the lower lip, the

thickened speech, and the wandering eye. The spinal cord, too, is often

affected and becomes unable to respond to the demand for reflex action,

as appears from the trembling hands, the staggering legs, the swaying

body, and the general muscular uncertainty. All these are varied

results of the temporary paralysis of the great nerve centers.

Besides, the sensibility of the nerves is deadened. The inebriate may

seize a hot iron and hardly know it, or wound his hand painfully and

never feel the injury. The numbness is not of the skin, but of the

brain, for the drunken man may be frozen or burned to death without

pain. The senses, too, are invaded and dulled. Double vision is

produced, the eyes not being so controlled as to bring the image upon

corresponding points of the retina.

296. Diseases Produced by Alcohol. The diseases that follow in the

train of the alcoholic habit are numerous and fatal. It lays its

paralyzing hand upon the brain itself, and soon permanently destroys

the integrity of its functions. In some the paralysis is local only,

perhaps in one of the limbs, or on one side of the body; in others

there is a general muscular failure. The vitality of the nerve centers

is so thoroughly impaired that general paralysis often ensues. A

condition of insomnia, or sleeplessness, often follows, or when sleep

does come, it is in fragments, and is far from refreshing to the jaded

body.

In time follows another and a terrible disease known as _delirium

tremens_; and this may occur in those who claim to be only moderate

drinkers, rarely if ever intoxicated. It accompanies an utter breakdown

of the nervous system. Here reason is for the time dethroned, while at

some times wild and frantic, or at others a low, mumbling delirium

occurs, with a marked trembling from terror and exhaustion.

There is still another depth of ruin in this downward course, and that

is _insanity_. In fact, every instance of complete intoxication is a

case of temporary insanity, that is, of mental unsoundness with loss of

self-control. Permanent insanity may be one of the last results of

intemperance. Alcoholism sends to our insane asylums a large proportion

of their inmates, as ample records testify.

297. Mental and Moral Ruin Caused by Alcoholism. Alcoholism, the evil

prince of destroyers, also hastens to lay waste man’s mental and moral

nature. Just as the inebriate’s senses, sight, hearing, and touch, fail

to report correctly of the outer world, so the mind fails to preside

properly over the inner realm. Mental perceptions are dulled. The

stupefied faculties can hardly be aroused by any appeal. Memory fails.

Thus the man is disqualified for any responsible labor. No railroad

company, no mercantile house, will employ any one addicted to drinking.

The mind of the drunkard is unable to retain a single chain of thought,

but gropes about with idle questionings. The intellect is debased.

Judgment is impossible, for the unstable mind cannot think, compare, or

decide.

The once active power of the will is prostrate, and the victim can no

longer resist the feeblest impulse of temptation. The grand faculty of

self-control is lost; and as a result, the baser instincts of our lower

nature are now uppermost; greed and appetite rule unrestrained.

But the moral power is also dragged down to the lowest depths. All the

finer sensibilities of character are deadened; all pride of personal

appearance, all nice self-respect and proper regard for the good

opinion of others, every sense of decorum, and at last every pretence

of decency. Dignity of behavior yields to clownish silliness, and the

person lately respected is now an object of pity and loathing. The

great central convictions of right and wrong now find no place in his

nature; conscience is quenched, dishonesty prevails. This is true both

as to the solemn promises, which prove mere idle tales, and also as to

property, for he resorts to any form of fraud or theft to feed the

consuming craving for more drink.

298. Evil Results of Alcoholism Inherited. But the calamity does not

end with the offender. It may follow down the family line, and fasten

itself upon the unoffending children. These often inherit the craving

for drink, with the enfeebled nature that cannot resist the craving,

and so are almost inevitably doomed to follow the appalling career of

their parents before them.

Nor does this cruel taint stop with the children. Even their

descendants are often prone to become perverse. As one example, careful

statistics of a large number of families, more than two hundred

descended from drunkards, show that a very large portion of them gave

undoubted proof of well-marked degeneration. This was plain in the

unusual prevalence of infant mortality, convulsions, epilepsy,

hysteria, fatal brain diseases, and actual imbecility.[42]

It is found that the long-continued habitual user of alcoholic drinks,

the man who is never intoxicated, but who will tell you that he has

drunk whiskey all his life without being harmed by it, is more likely

to transmit the evil effects to his children than the man who has

occasional drunken outbreaks with intervals of perfect sobriety

between. By his frequently repeated small drams he keeps his tissues

constantly “alcoholized” to such an extent that they are seldom free

from some of the more or less serious consequences. His children are

born with organisms which have received a certain bias from which they

cannot escape; they are freighted with some heredity, or predisposition

to particular forms of degeneration, to some morbid tendency, to an

enfeebled constitution, to various defective conditions of mind and

body. Let the children of such a man attempt to imitate the drinking

habits of the father and they quickly show the effects. Moderate

drinking brings them down.

Among other consequences of an alcoholic inheritance which have been

traced by careful observers are: Morbid changes in the nerve centers,

consisting of inflammatory lesions, which vary according to the age in

which they occur; alcoholic insanity; congenital malformations; and a

much higher infant death rate, owing to lack of vitality, than among

the children of normal parents.

Where the alcoholic inheritance does not manifest itself in some

definite disease or disorder, it can still be traced in the limitations

to be found in the drinking man’s descendants. They seem to reach a

level from which they cannot ascend, and where from slight causes they

deteriorate. The parents, by alcoholic poisoning, have lowered the race

stock of vitality beyond the power of ascent or possibility to rise

above or overcome the downward tendency.

Of course these effects of alcoholics differ widely according to the

degree of intoxication. Yet, we must not forget that the real nature of

inebriety is always the same. The end differs from the beginning only

in degree. He who would avoid a life of sorrow, disgrace, and shame

must carefully shun the very first glass of intoxicants.

299. Opium. Opium is a gum-like substance, the dried juice of the

unripe capsule of the poppy. The head of the plant is slit with fine

incisions, and the exuding white juice is collected. When it thickens

and is moulded in mass, it becomes dark with exposure. _Morphine_, a

white powder, is a very condensed form of opiate; _laudanum_, an

alcoholic solution of marked strength; and _paregoric_, a diluted and

flavored form of alcoholic tincture.

300. Poisonous Effects of Opium. Some persons are drawn into the use of

opium, solely for its narcotic and intoxicating influence. Every early

consent to its use involves a lurking pledge to repeat the poison, till

soon strong cords of the intoxicant appetite bind the now 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:

On the reappearance of the A it will be noted that it comes into view

from the inner side, the x being seen before it. If now we move the

book towards its original place, the A will again disappear, coming

again into view from the outer side when the o is seen before it.

328. Inner Structure of the Eye. Let us imagine an eyeball divided

through the middle from above downwards. Let us now start in front and

observe its parts (Fig. 127). We come first to the cornea, which has

just been described. The iris forms a sort of vertical partition,

dividing the cavity of the eyeball into two chambers.

Illustration: Fig. 128.—Diagram illustrating the Manner in which the

Image of an Object is brought to a Focus on the Retina.

The anterior chamber occupies the space between the cornea and the

iris, and is filled with a thin, watery fluid called the aqueous humor.

The portion behind the iris forms the posterior chamber, and contains

the crystalline lens and a transparent, jelly-like fluid, the vitreous

humor. This fluid is never renewed, and its loss is popularly described

by the phrase, “when the eye runs out.”

Experiment 149. The retina is not sensitive where the optic nerve

enters the eyeball. This is called the “blind spot.” Put two

ink-bottles about two feet apart, on a table covered with white paper.

Close the left eye, and fix the right steadily on the left-hand

inkstand, gradually varying the distance from the eye to the

ink-bottle. At a certain distance the right-hand bottle will disappear;

but nearer or farther than that, it will be plainly seen.

The vitreous humor fills about four-fifths of the eyeball and prevents

it from falling into a shapeless mass. It also serves to hold the

choroid and the retina in position, and to maintain the proper

relations of the inner structures of the eye.

The iris consists of a framework of connective tissue, the surface of

which is lined by cells containing pigment, which gives color to the

eye.

Bundles of involuntary muscular fibers are found in the substance of

the iris. Some are arranged in a ring round the margin of the pupil;

others radiate from it like the spokes of a wheel. When the circular

fibers contract, the pupil is made smaller, but if these fibers relax,

the radiating fibers cause the pupil to dilate more or less widely.

329. The Crystalline Lens. Just behind the pupil and close to the iris

is a semi-solid, double-convex body, called the crystalline lens. It is

shaped like a magnifying glass, convex

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

Accommodation.

On the right the lens is arranged for distant vision, the ciliary

muscle is relaxed and the ligament D is tense, so flattening by its

compression the front of the lens C; on the left the muscle A is

acting, and this relaxes the ligament and allows the lens B to become

more convex, and so fitted for the vision of near objects]

on each side, but with the posterior surface more convex than the

anterior. In health it is perfectly clear and transparent, and highly

elastic. When the lens becomes opaque, from change in old age, or from

ulcers or wounds, we have the disease known as _cataract_.

The lens is not placed loosely in the eyeball, but is enclosed in a

transparent and elastic capsule suspended throughout its circumference

by a ligament called the suspensory ligament. This ligament not only

retains the lens in place, but is capable of altering its shape. In

ordinary conditions of the eye, this ligament is kept tense so that the

front part of the lens is flattened somewhat by the pressure on it.

All around the edge, where the cornea, sclerotic, and choroid meet, is

a ring of involuntary muscular fibers, forming the ciliary muscle. When

these fibers contract, they draw forwards the attachment of the

suspensory ligament of the lens, the pressure of which on the lens is

consequently diminished. The elasticity of the lens causes it at once

to bulge forwards, and it becomes more convex.

The ciliary muscle is thus known as the muscle of accommodation,

because it has the power to accommodate the eye to near and distant

objects. In this respect it corresponds in its use to the adjusting

screw in the opera-glass and the microscope.

330. The Eye Compared to the Photographic Camera. As an optical

instrument, the eye may be aptly compared, in many particulars, to the

photographic camera. The latter, of course, is much simpler in

structure. The eyelid forms the cap, which being removed, the light

from the object streams through the eye and passes across the dark

chamber to the retina behind, which corresponds to the sensitive plate

of the camera. The transparent structures through which the rays of

light pass represent the lenses. To prevent any reflected light from

striking the plate and interfering with the sharpness of the picture,

the interior of the photographic camera box is darkened. The pigmented

layer of the choroid coat represents this blackened lining.

In the camera, the artist uses a thumb-screw to bring to a focus on the

sensitive plate the rays of light coming from objects at different

distances. Thus the lens of the camera may be moved nearer to or

farther from the object. In order to obtain clear images, the same

result must be accomplished by the eye. When the eye is focused for

near objects, those at a distance are blurred, and when focused for

distant objects, those near at hand are indistinct. Now, in the eye

there is no arrangement to alter the position of the lenses, as in the

camera, but the same result is obtained by what is called

“accommodation.”

Again, every camera has an arrangement of diaphragms regulating the

amount of light. This is a rude contrivance compared with the iris,

which by means of its muscular fibers can in a moment alter the size of

the pupil, thus serving a similar purpose.

Illustration: Fig. 130.—Illustrating the manner in which the Image of

an Object is brought to a Focus in a Photographer’s Camera.

331. The Refractive Media of the Eye. The eye is a closed chamber into

which no light can pass but through the cornea. All the rays that enter

the eye must also pass through the crystalline lens, which brings them

to a focus, as any ordinary lens would do.

Now, if the media through which the light from an object passes to

reach the retina were all of the same density as the air, and were also

plane surfaces, an impression would be produced, but the image would

not be distinct. The action of the lens is aided by several refractive

media in the eye. These media are the cornea, the aqueous humor, and

the vitreous humor. By reason of their shape and density these media

refract the rays of light, and bring them to a focus upon the retina,

thus aiding in producing a sharp and distinct image of the object. Each

point of the image being the focus or meeting-place of a vast number of

rays coming from the corresponding point of the object is sufficiently

bright to stimulate the retina to action.[44]

Thus, the moment rays of light enter the eye they are bent out of their

course. By the action of the crystalline lens, aided by the refractive

media, the rays of light that are parallel when they fall upon the

normal eye are brought to a focus on the retina.

If the entire optical apparatus of the eye were rigid and immovable,

one of three things would be necessary, in order to obtain a clear

image of an object; for only parallel rays (that is, rays coming from

objects distant about thirty feet or more), are brought to a focus in

the average normal eye, unless some change is brought about in the

refractive media. First, the posterior wall of the eye must be moved

further back, or the lens would have to be capable of movement, or

there must be some way of increasing the focusing power of the lens. In

the eye it is the convexity of the lens that is altered so that the eye

is capable of adjusting itself to different distances.[45]

Illustration: Fig. 131.—The Actual Size of the Test-Type, which should

be seen by the Normal Eye at a Distance of Twenty Feet.

332. The More Common Defects of Vision. The eye may be free from

disease and perfectly sound, and yet vision be indistinct, because the

rays of light are not accurately brought to a focus on the retina. “Old

sight,” known as presbyopia, is a common defect of vision in advancing

years. This is a partial loss of the power to accommodate the eye to

different distances. This defect is caused by an increase in the

density of the crystalline lens, and an accompanying diminution in the

ability to change its form. The far point of vision is not changed, but

the near point is removed so far from the eye, that small objects are

no longer visible.

Illustration: Fig. 132.—Diagram illustrating the Hypermetropic

(far-sighted) Eye.

The image P′ of a point P falls behind the retina in the unaccommodated

eye. By means of a convex lens it may be focused on the retina without

accommodation (dotted lines). (To save space P is placed much too near

the eye.)

Hence, when a person about forty-five years of age complains of dim

light, poor print, and tired eyes, the time has come to seek the advice

of an optician. A convex lens may be needed to aid the failing power to

increase the convexity of the lens, and to assist it in bringing the

divergent rays of light to a focus.

In “long sight,” or hypermetropia both the near and far point of vision

are concerned, and there is no distinct vision at any distance without

a strain. It is a defect in the focus, dependent upon the form of the

eyes, and exists in childhood. The axis of the eyeball is too short,

and the focus falls beyond the retina, which is too near the cornea. In

childhood this strain may pass unnoticed, but, sooner or later it

manifests itself by a sense of fatigue, dizziness, and a blurred and

indistinct vision. The remedy is in the use of convex glasses to

converge parallel rays of light before they enter the eye. The muscles

of accommodation are thus relieved of their extra work.

“Short sight,” known as myopia, is one of the commonest defects of

vision. In this defect the axis of the eye, or the distance between the

cornea and the retina, is too long and the rays of light are brought to

a focus in front of the retina. The tendency to short-sightedness

exists in many cases at birth, and is largely hereditary. It is

alarmingly common with those who make a severe demand upon the eyes.

During childhood there is a marked increase of near-sightedness. The

results of imprudence and abuse, in matters of eyesight, are so

disastrous, especially during school life, that the question of short

sight becomes one of paramount importance.

Experiment 150. With a hand-mirror reflect the sunlight on a white

wall. Look steadily at the spot for a full minute, and then let the

mirror suddenly be removed. The “complementary” color—a dark spot—will

appear.

Experiment 151. _To show that impressions made upon the retina do not

disappear at once_. Look steadily at a bright light for a moment or

two, and then turn away suddenly, or shut the eyes. A gleam of light

will be seen for a second or two.

Look steadily at a well-lighted window for a few seconds, and then

turn the eyes suddenly to a darkened wall. The window frame may be

plainly seen for a moment.

Glance at the sun for a moment, close the eyes and the image of the

sun may be seen for a few seconds.

Experiment 152. Take a round piece of white cardboard the size of a

saucer, and paint it in alternate rings of red and yellow,—two primary

colors. Thrust a pin through the center and rotate it rapidly. The eye

perceives neither color, but orange,—the secondary color.

Experiment 153. To note the shadows cast upon the retina by opaque

matters in the vitreous humor (popularly known as floating specks, or

gossamer threads), look through a small pin-hole in a card at a bright

light covered by a ground-glass shade.

Experiment 154. _To illustrate accommodation_. Standing near a source

of light, close one eye, hold up both forefingers not quite in a line,

keeping one finger about six or seven inches from the other eye, and

the other forefinger about sixteen to eighteen inches from the eye.

Look at the _near_ finger; a distinct image is obtained of it, while

the far one is blurred or indistinct. Look at the far image; it becomes

distinct, while the near one becomes blurred. Observe that in

accommodating for the near object, one is conscious of a distinct

effort.

In many cases near-sightedness becomes a serious matter and demands

skillful advice and careful treatment. To remedy this defect, something

must be done to throw farther back the rays proceeding from an object

so that they will come to a focus exactly on the retina. This is done

by means of concave glasses, properly adjusted to meet the conditions

of the eyes. The selection of suitable glasses calls for great care, as

much harm may be done by using glasses not properly fitted to the eye.

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

Eye.

The image P′ of a distant object P falls in front of the retina even

without accommodation. By means of a concave lens (L) the image may be

made to fall on the retina (dotted lines). (To save space P is placed

much too near the eye).

There is an optical condition of the eye known as astigmatism, in which

the cornea is usually at fault. In this defect of vision the curvature

of the cornea is greater in one meridian than in another. As a result

the rays from an object are not all brought to the same focus. Objects

appear distorted or are seen with unequal clearness. Glasses of a

peculiar shape are required to counteract this defect.

333. The Movements of the Eyes. In order that our eyes may be efficient

instruments of vision, it is necessary that they have the power of

moving independently of the head. The mechanical arrangement by which

the eyeballs are moved in different directions is quite simple. It is

done by six little muscles, arranged in three pairs, which, with one

exception, originate in the back of the cavity in which the eye rests.

Four of these muscles run a straight course and are called the _recti_.

The remaining two muscles bend in their course and are called

_oblique_. The coördination of these tiny muscles is marvellous in its

delicacy, accuracy, and rapidity of action.

When, for any cause, the coördination is faulty, “cross eye,”

technically called strabismus, is produced. Thus, if the internal

rectus is shortened, the eye turns in; if the external rectus, the eye

turns out, producing what is known as “wall eye.” It is thus evident

that the beauty of the internal mechanism of the eye has its fitting

complement in the precision, delicacy, and range of movement conferred

upon it by its muscles.

334. The Eyelids and Eyebrows. The eye is adorned and protected by the

eyelids, eyelashes, and eyebrows.

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

A, attachment of tendon connected with the three recti muscles;

B, external rectus, divided and turned downward, to expose the

internus rectus;

C, inferior rectus;

D, internal rectus;

E, superior rectus;

F, superior oblique;

H, pulley and reflected portion of the superior oblique;

K, inferior oblique; L, levator palpebri superioris;

M, middle portion of the same muscle (L);

N, optic nerve.

The eyelids, two in number, move over the front of the eyeball and

protect it from injury. They consist of folds of skin lined with mucous

membrane, kept in shape by a layer of fibrous material. Near the inner

surface of the lids is a row of twenty or thirty glands, known as the

_Meibomian glands_, which open on the free edges of each lid. When one

of these glands is blocked by its own secretion, the inflammation which

results is called a “sty.”

The inner lining membrane of the eyelids is known as the conjunctiva;

it is richly supplied with blood-vessels and nerves. After lining the

lids it is reflected on to the eyeballs. It is this membrane which is

occasionally inflamed from taking cold.

The free edges of the lids are bordered with two or more rows of hairs

called the eyelashes, which serve both for ornament and for use. They

help to protect the eyes from dust, and to a certain extent to shade

them. Their loss gives a peculiar, unsightly look to the face.

The upper border of the orbit is provided with a fringe of short, stiff

hairs, the eyebrows. They help to shade the eyes from excessive light,

and to protect the eyelids from perspiration, which would otherwise

cause serious discomfort.

335. The Lacrymal Apparatus. Nature provides a special secretion, the

tears, to moisten and protect the eye. The apparatus producing this

secretion consists of the lacrymal or tear gland and lacrymal canals or

tear passages (Fig. 136).

Outside of the eyeball, in the loose, fatty tissue of the orbit, in the

upper and outer corner is the lacrymal or tear gland. It is about the

size of a small almond and from it lead several little canals which

open on the inner surface of the upper lid. The fluid from the gland

flows out by these openings over the eyeball, and is collected at the

inner or nasal corner. Here in each lid is a little reddish elevation,

or _lacrymal caruncle_, in which is an opening, communicating with a

small canal in the lid which joins the lacrymal sac, lodged between the

orbit and the bridge of the nose (Fig. 137).

From this sac there passes a channel, the nasal duct, about one-half of

an inch long, leading into the lower portion of the nostril. The fluid

which has flowed over the eye is drained off by these canals into the

nose. During sleep this secretion is much diminished. When the eyes are

open the quantity is sufficient to moisten the eyeball, the excess

being carried into the nose so gradually that the attention is not

attracted to it.

The lacrymal canals are at times blocked by inflammation of the nasal

duct, and the fluid collects in the corners of the eyelids and

overflows down the cheeks, producing much inconvenience. The lining

membrane of the eyelids through these canals is continuous with that of

the nostrils. Hence, when the lining membrane of the eye is red and

swollen, as during a cold, the nasal passages are also irritated, and

when the nasal membrane is inflamed, the irritation is apt to pass

upwards and affect the eyelids.

336. The Tears. The lacrymal or tear gland is under the control of the

nervous system. Thus, if anything irritates the eyelids, the sensory

nerves are stimulated and the impression is carried to the brain.

Thence the nerve impulses travel to the lacrymal glands, leading to an

increased flow of their secretion. The irritation of the sensory nerves

in the nasal passages by smelling such substances as onions, or pungent

salts, often causes a copious flow of tears.

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

A lachrymal gland, the size of a small almond lodged in a shallow

depression in the bones of the orbit;

B, lachrymal ducts (usually seven), which form a row of openings into

the conjunctival fold.

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.