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A Practical Physiology by Albert F. Blaisdell

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yielding victim.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Additional Experiments.

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

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

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

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

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

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

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

Chapter XI.

The Special Senses.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Experiment 141. _To test the sense of locality_. Ask a person to
shut his eyes, touch some part of his body lightly with the point of a
pin, and ask him to indicate the part touched.

As to the general temperature, this sense is relative and is much
modified by habit, for what is cold to an inhabitant of the torrid zone
would be warm to one accustomed to a very cold climate.

Pain is an excessive stimulation of the sensory nerves, and in it all
finer sensations are lost. Thus, when a piece of hot iron burns the hand,
the sensation is the same as when the iron is very cold, and extreme cold
feels like intense heat.

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

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

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

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

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

[Illustration: Fig. 125.--The Tongue.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[Illustration: Fig. 127.--Section of the Human Eye.]

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

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

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

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

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

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

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

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

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

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

[Illustration: a oAx]

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

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

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

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

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

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

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

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

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

329. The Crystalline Lens. Just behind the pupil and close to the iris is
a semi-solid, double-convex body, called the crystalline lens. It is
shaped like a magnifying glass, convex on each side, but with the
posterior surface more convex than the anterior. In health it is perfectly
clear and transparent, and highly elastic. When the lens becomes opaque,
from change in old age, or from ulcers or wounds, we have the disease
known as _cataract_.

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

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

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

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

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

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

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

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

[Illustration: Fig. 130.--Illustrating the manner in which the Image of an
Object is brought to a Focus in a Photographer's Camera.]

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

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

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

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

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

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

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

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

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

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

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

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

Experiment 151. _To show that impressions made upon the retina do
not disappear at once_. Look steadily at a bright light for a moment or
two, and then turn away suddenly, or shut the eyes. A gleam of light
will be seen for a second or two.

Look steadily at a well-lighted window for a few seconds, and then turn
the eyes suddenly to a darkened wall. The window frame may be plainly
seen for a moment.

Glance at the sun for a moment, close the eyes and the image of the sun
may be seen for a few seconds.

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

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

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

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

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

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

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

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

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

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

[Illustration: Fig. 134.--Muscles of the Eyeball.

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

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

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

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

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

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

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

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

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

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

[Illustration: Fig. 135.--Lacrymal Gland and Ducts.

A, lachrymal gland, the size of a small almond lodged in a shallow
depression in the bones of the orbit;
B, lachrymal ducts (usually seven), which form a row of openings into
the conjunctival fold.
]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Additional Experiments.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[Illustration: Fig. 145.]

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

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

[Illustration: Fig. 146.--Optic Disks.

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

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

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

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

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

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

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

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

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

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

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

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

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

Chapter XII.

The Throat and the Voice.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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