CHAPTER X.

THE SPECIAL SENSES.

_The Production of Sensations--Variety of Sensations--General

Sensibility--Pain and its Function--Special Sensation, Touch, Taste,

Smell, Sight, and Hearing--The Hand, the Organ of Touch--The Sense of

Touch--Delicacy of Touch--Sensation of Temperature and Weight--The

Tongue the Organ of Taste--The Nerves of Taste--The Sense of Taste and

its Relations with the other Senses--The Influence of Education on the

Taste--The Nasal Cavities, or the organs of Smell--The Olfactory

Nerve--The Uses of the Sense of Smell--The Sense of Sight--Light--The

Optic Nerve--The Eyeball and its Coverings--The Function of the

Iris--The Sclerotic, Choroid, and Retina--The Tears and their

Function--The Movements of the Eyeball--The Function of

Accommodation--The Sense of Hearing and Sound--The Ear, or the organ of

Hearing--The External, Middle, and Internal Ear._

[Sidenote: 1. True centre of sensation? Place of the mind's impressions?

What is it convenient to say? What further is stated?]

1. PRODUCTION OF SENSATIONS.--We have already seen that the true centre of

sensation is some organ within the skull, probably among the gray masses at

the base of the brain; but the mind never perceives impressions at that

point; but, on the contrary, always refers them to the external organs of

sensation. Hence, it is convenient to say, that those outer parts possess

the property of sensibility. For instance, we say that we hear with the

ear, taste with the tongue, and feel with the fingers. That this is not the

exact truth is proven by the fact, that whenever the nerve connecting one

of these organs with the brain is severed, it at once loses its capacity

for sensation.

[Sidenote: 2. Consciousness? During sleep? In profound insensibility?]

2. Consciousness, another faculty of the brain, is necessary to complete a

sensation. During sleep, and in other unconscious states, the usual

impressions are presented to {178} the ear, the nose, and the skin, but

they fail to excite sensations, because the nerve-centres are inactive. In

profound insensibility, from chloroform or ether, a limb may be removed

without occasioning the least feeling.

[Sidenote: 3. Sensibility in animals? In the earth-worm? In man?]

3. VARIETY OF SENSATIONS.--All animals have some degree of sensibility. It

is of course feeble and indistinct in the lower forms of life, but

increases in power and variety as we ascend the scale. In the earth-worm,

the nervous system is very simple, the sensibility being moderate and alike

in all parts: hence, if its body be cut into two pieces, each piece will

have the same degree of feeling as before. As we approach man, however, the

sensations multiply and become more acute; the organs are more complex, and

special parts are endowed with special gifts. These special organs cannot

be separated from the rest of the body without the loss of the functions

they are designed to exercise.

[Sidenote: 4. The lowest form of sensation? The highest? Sensations, how

modified? What further can you state as to habitual impressions?]

4. The lowest form of sensation, that of simple contact, is possessed by

the lowest of the animal creation. The highest forms are those by which we

are enabled to know the properties of external objects, such as shape,

size, sound, and color. A variety of means of communicating with the outer

world is the necessary possession of a high intelligence. Sensations are

modified by use. They become more acute and powerful by moderate exercise;

or, they are dulled by undue excitement. The former is shown by the acute

hearing of the Indian, by the sharp sight of the sailor, and by the

delicate touch of the blind. The latter is exemplified by the impaired

hearing of the boiler-maker, and the depraved taste of him who uses pungent

condiments with his food. Again, impressions habitually presented may not

be consciously felt; as is the case with the rumbling of carriages in a

neighboring {179} street, or the regular ticking of a clock. All sensations

become less vivid with the advance of age, especially hearing and vision.

[Sidenote: 5. General sensibility? What have we seen as regards the brain?

Of what other structures is the same true?]

5. GENERAL SENSIBILITY.--There is a property possessed by nearly all parts

of the human body which we call general sensibility. We have recently seen

that the brain is wholly insensitive, and may be cut or pinched without

pain. The same is true of the nails, hair, the scarf-skin or external

covering of the body, and a few other structures. In these parts no nerves

are found. On the other hand, the sensibility of the true skin, and of

mucous membranes, as of the eye and nose, is exquisite, these organs having

a large supply of sensory nerve-fibres. The bones and tendons have less of

these fibres, and are only moderately sensitive.

[Sidenote: 6. The cause of sensibility? Painful part in a surgical

operation? Benumbing the surface? How done by ether?]

6. The sensibility of any part of the body, then, depends upon the number

of nerves present; and, as a rule, the nervous supply is proportional to

the importance of the part, and to its liability to injury. When,

therefore, a surgical operation is performed, the most painful part of it

is the incision through the skin; the muscles, cartilage, and bone being

comparatively without sensation. Hence, if we could benumb the surface,

certain of the lesser operations might be undergone without great

inconvenience. This is, in fact, very successfully accomplished by means of

the cold produced by throwing a spray of ether, or of some other rapidly

evaporating liquid, upon the part to be cut.

[Sidenote: 7. Tickling? Internal sensations? The nerves of general

sensibility?]

7. Tickling is a modification of general sensibility. At first, it excites

a pleasurable sensation, but this soon passes into pain. It is only present

in those parts where the sense of touch is feeble. But all impressions are

not received from without: there are, also, certain internal {180}

sensations, as they are called, which depend upon the condition of the

internal organs, such as appetite, hunger, thirst, the sense of

satisfaction after taking food, dizziness when looking down from some lofty

position, lassitude, drowsiness, fatigue, and other feelings of comfort or

discomfort. General sensibility, whether of the internal or external organs

of the body, chiefly depends upon the sensory fibres of the spinal nerve.

The face, however, is supplied by the sensory cranial nerves. The

sympathetic system has a low grade of feeling in health; but disease in the

parts served by it arouses an intense degree of pain.

[Sidenote: 8. Connection between pain and sensibility?]

8. THE SENSATION OF PAIN.--What then is _pain_? Is it identical with

ordinary sensibility? There seems to be some necessary connection between

the two feelings, for they take place through the same channels, and they

are alike intense in the same situations. But sensibility habitually

contributes to our sources of pleasure, the very opposite of pain; hence,

these feelings cannot be identical.

[Sidenote: 9. Explain the difference between pain and sensibility.]

9. Pain must, therefore, be a modification of the general sensibility,

which follows an excessive degree of excitement of the nerves; there being

a natural limit to the amount of stimulation which they will sustain. So

long as this limit is observed, the part excited may be said to be simply

sensitive; but when it is exceeded, the impression becomes painful. This

difference between sensibility and pain is well shown by the effects of

sunlight upon the eye. The indirect illumination of the sun arouses only

the former feeling, and is indispensable to our comfort and existence;

while the direct ray received into the eye occasions great pain.

[Sidenote: 10. Dread of pain? How may its value be appreciated? Example.]

10. THE USES OF PAIN.--The dread of pain is a valuable monitor to the body.

It puts us on our guard in the presence of danger; teaches moderation in

the use of our {181} powers; indicates the approach of disease; and calls

attention to it when present. The word disease, in fact, according to its

original use, had reference simply to the pain, or want of ease, which

commonly attends disordered health. When we observe the serious mishaps

which occur when sensibility and pain are absent, we cannot fail to

appreciate its value. For example, a paralytic in taking a foot-bath,

forgets to test its temperature, and putting his limbs into water while it

is too hot, is severely scalded without knowing it.

[Sidenote: 11. The case of the traveller? Grain of sand? The sun and

child?]

11. A traveller, overcome by cold and fatigue, lies down and falls asleep

near a large fire, and when he is aroused in the morning, it is discovered

that one of his feet has been insensibly destroyed. A grain of sand,

lodging in an insensitive eye, may cause inflammation and even the loss of

sight. If intense light were not painful to the eye, many a child would

innocently gaze upon the glories of the sun to the ruin of his sight.

[Sidenote: 12. Mission of pain? Painful impressions compared with those of

pleasure?]

12. Pain is, indeed, a present evil, but its relations with the future

prove its mission merciful. Painful impressions cannot be recollected from

past experience; and they cannot be called into existence by the fancy.

Considered in the light of results, pain has a use above that of pleasure;

for while the immoderate pursuit of the latter leads to harm, the tendency

of pain is to restrict the hurtful courses of life, and in this manner to

protect the body.

[Sidenote: 13. What does Magendie say of the relation of pain to pleasure?]

13. The relations of pain to pleasure are thus described by the eminent

physiologist, Magendie:--"By these sensations Nature induces us to concur

in the order which she has established among organized beings. Though it

may appear like sophistry to say that pain is the shadow of pleasure, yet

it is certain that those who have exhausted the ordinary sources of

pleasure have recourse to the {182} causes of pain, and gratify themselves

by their effects. Do we not see in all large cities, that men who are

debauched and depraved find agreeable sensations, where others experience

only intolerable pain?"

[Sidenote: 14. The law of Nature as regards painful sensations among

animals?]

14. As to painful sensation among the inferior animals, the plan of Nature

seems to be, that the higher the intelligence of the creature, and the more

complete its power of defence, the more acute is its sensibility. We infer,

therefore, that animals low in the scale of existence, and helpless, are

not very liable to suffer pain.

[Sidenote: 15. The sensation of contact and pain? Special sensations of

man? How regarded?]

15. SPECIAL SENSATION.--The sensations of simple contact and pain are felt

by nearly all parts of the system, whether external or internal, and are

the necessary consequence of the general sensibility; but, so far as the

objects which surround us are concerned, these impressions are vague and

passive in character, and inform the mind of none of the properties or

powers of these objects. Besides these feelings, therefore, man is endowed

with certain special sensations, which are positive and distinct in

character, and which he can call into exercise at will, and employ in the

pursuit of knowledge. For reasons relating to the original constitution of

the body, these sensations are to be regarded as modifications of the

general sensibility already alluded to, constructed with special reference

to the different forces of Nature, of which we have any knowledge, such as

heat, motion, gravity, sunlight, and the like.

[Sidenote: 16. What are the special senses? Special organs for them?]

16. These distinct and active faculties are termed the special senses, and

are five in number, viz., Touch, Taste, Smell, Sight, and Hearing. For the

exercise of these senses, special organs are furnished, such as the hand,

the tongue, the nose, the eye, and the ear. The manner in which the nerves

of special sense terminate, varies in the {183} case of each organ, so that

each is adapted to one set of sensations alone, and is incapable of

perceiving any other. Thus the nerve of hearing is excited by the

undulations of sound, and not by those of light, while the reverse is true

of the nerve of sight; and the nerve of smell can appreciate neither of

them, being capable only of taking cognizance of the odorous properties of

bodies.

[Sidenote: 17. What is said in relation to one more than the five senses?]

17. By some writers six senses are accorded to man; the additional one

being either the sense of temperature, for as we shall presently see this

is not the same as touch; or according to others, the muscular sense by

which we are enabled to estimate the weights of bodies. The latter also

differs in some respects from the sense of touch.

[Sidenote: 18. The sense of touch, how prevalent? What is said of the

hand?]

18. ORGANS OF TOUCH.--The sense of touch is possessed by nearly all

portions of the general surface of the body, but it finds its highest

development in the hands. The human hand is properly regarded as the model

organ of touch. The minute structure of the skin fits it admirably for this

form of sensation: the cuticle, or scarf-skin, is fine and flexible, while

the cutis, or true skin, contains multitudes of nerve-filaments, arranged

in rows of _papillæ_, or cone-like projections, about one-hundredth of an

inch in length. It is estimated that there are 20,000 of these papillæ in a

square inch of the palmar surface of the hand. Now, although the nerves of

the cutis are the instruments by which impressions are received and

transmitted to the brain, yet the cuticle is essential to the sensation of

touch. This is shown by the fact that whenever the true skin is laid bare,

as by a burn or blister, the only feeling that it experiences from contact

is one of pain, not that of touch.

[Sidenote: 19. Office of the cuticle? Tips of the fingers? The fingers with

thumb?]

19. The office of the cuticle is thus made evident: it is to shield the

nerve filaments from direct contact with {184} external objects. At the

tips of the fingers, where touch is most delicate, the skin rests upon a

cushion of elastic material, and receives firmness and permanence of shape

by means of the nail placed upon the less sensitive side. Besides these

favorable conditions, the form of the arm is such, and its motions are so

easy and varied, that we are able to apply the test of touch in a great

number of directions. The slender, tapering fingers, with their pliant

joints, together with the strong opposable thumb, enable the hand to mold

itself upon and grasp a great variety of objects; so that great as are the

delicacy and grace of the hand, it is not wanting in the elements of power.

[Sidenote: 20. What special importance is attributed to the hand?]

20. Its beauty and adaptation to the wants of man have made the hand an

attractive theme for philosophers. They do not, however, always agree in

their conclusions. One has the opinion that man has acquired his

intelligence and achieved his place as "lord of creation," because he has

this organ. Buffon, in effect, declares that with fingers twice as numerous

and twice as long, we would become proportionally wiser; but Galen long ago

took a more reasonable view, when he taught that "man is the wisest of

animals, not because he possesses the hand; but because he is the wisest

and understands its use, the hand has been given to him; for his mind, not

his hand has taught him the arts." Another has well said, that "no one can

study carefully the human hand and fail to be convinced of the existence of

the Deity."

[Sidenote: 21. The simplicity of touch? What does it teach us?]

21. THE SENSE OF TOUCH.--Touch is the simplest of the senses. It is that

which the child first calls into exercise in solving the early problems of

existence; and it is that which is in the most constant use throughout

life. We are brought by the touch into the most intimate relations with

external objects, and by it we learn the greater number, if not the most

important, of the properties of {185} these objects; such as size, figure,

solidity, motion, and smoothness or roughness of surface.

[Sidenote: 22. Importance of the sense of touch to the development of the

other senses?]

22. The sense of touch assists the other senses, especially that of sight,

giving foundation and reality to their perceptions. Without it, the

impressions received by the eye would be as vague and unreal as the figures

that float through our dreams. A boy who had been blind from birth, at the

age of twelve years received sight by means of a surgical operation: at

first, he was unable to distinguish between a globe and a circular card, of

the same color, before he had touched them. After that, he at once

recognized the difference in their form. He knew the peculiarities of a dog

and a cat by feeling, but not by sight, until one day, happening to take up

the cat, he recognized the connection of the two sorts of impressions,

those of touch and sight; and then, putting the cat down, he said: "So,

puss, I shall know you next time."

[Illustration: FIG. 45.]

[Sidenote: 23. Liability of touch to err? Describe the illustration.]

23. Touch is considered the least liable to error of all the senses; yet,

if that part of the skin by which the sense is exercised is removed from

its customary position, a false impression may be created in the mind. This

is well illustrated by an experiment, which dates from the time of

Aristotle. If we cross the middle finger behind the {186} forefinger, and

then roll a marble, or some small object, upon the tips of the fingers (see

Fig. 45), the impression will be that two marbles are felt. If the fingers,

thus transposed, be applied to the end of the tongue, two tongues will be

felt. When the nose is accidentally destroyed, the surgeon sometimes

performs an operation for the purpose of forming a new one, by

transplanting a partially removed piece of the skin of the forehead upon

the injured part: then, if the new nose be touched or pinched, the feeling

is referred to the forehead. This fact illustrates one important truth,

that the nerves will re-unite after they have been cut, and feeling will be

restored: if it were otherwise, a succession of slight cuts upon the

fingers would seriously impair their tactile sensibility.

[Sidenote: 24. The delicacy of touch? Experiments with a pair of

compasses?]

24. THE DELICACY OF TOUCH.--Although the hand is the proper organ of this

sense, yet it is exercised by various parts of the body, their degree of

sensibility being proportional to the number of papillæ they contain. The

varying degrees of tactile delicacy of the different parts of the surface

have been measured, in an ingenious manner, by means of a pair of

compasses, tipped with small pieces of cork. The two points of the

compasses are touched at the same moment to the skin, the eyes being

closed, and it is found that, in sensitive parts, the distance between the

points may be quite slight, and yet each be plainly felt; while, in less

sensitive parts, the points of the compasses are felt as a single point,

although they are separated one or two inches.

[Sidenote: 25. Further experiments and results?]

25. At the tips of the fingers, the distance between the points being

one-twentieth of an inch, a double impression is felt. The distance must be

twice as great, for the palm; four times as great, for the lips; and, on

the forehead, it must be twenty times greater. At the middle of the back,

where the touch is least acute, the points must be {187} separated more

than two inches before they can be separately felt. Therefore, the sense of

touch in the fingers is said to be fifty times more delicate than upon the

posterior surface of the body.

[Sidenote: 26. Exquisite delicacy of touch? The same among the blind?]

26. Exquisite delicacy of touch is attained by practice. This is shown in

many of the lighter and more graceful employments of daily life. Without

it, the skill of the painter, sculptor, and musician would be rude indeed.

By training, also, the physician acquires the _tactus eruditus_, or

discriminating touch; but among the blind, delicacy of touch is most

remarkable, and it here finds its highest value; for its possession, in a

measure, compensates the loss of sight by enabling them to read, by means

of raised letters, to work with certain tools, and even to play upon

musical instruments. A person born without sight, and without hearing or

voice, may, by the education of the touch, be rescued from apparent

imbecility, and be taught not only to read and write, but even to perform

household and other useful labors.

[Sidenote: 27. Rival candidates for the sixth sense? Give the two reasons

on the subject.]

27. SENSATIONS OF TEMPERATURE AND WEIGHT.--Each of these sensations has

been described by the physiologists as a special sense, and they are rival

candidates, so to speak, for the position and title of the sixth sense. In

the sensation of temperature, or the thermal sense, touch bears a part, but

the two feelings appear to be distinct. In proof of this, we observe,

firstly, that they are not alike intense in the same situations; as, for

example, the skin of the face and elbow, where the sense of touch is

feeble, is very sensitive to impressions of heat and cold. Secondly, the

ability to recognize temperature may be lost by paralysis, while the

sensibility of touch remains unaffected. When the skin comes in contact

with a very hot substance, the sensation felt is that of pain, not of

touch. In like manner, a {188} very cold substance causes pain, not the

feeling of cold. So that a red-hot iron, and solid carbonic acid (the

temperature of which is 108° below zero), feel alike; and each, if pressed

slightly, will produce a blister.

[Sidenote: 28. The muscular sense? State what is said to illustrate the

subject.]

28. The _muscular sense_, by some considered distinct from touch, gives

rise to the sensations of weight, and other forms of external resistance.

That this feeling exists, is shown by the following simple experiment. If

the hand be placed flat upon a table, and a somewhat heavy weight be put

into it, touch alone is exercised and a feeling of pressure results; but if

the hand be raised, a certain amount of muscular effort must be put forth,

and thus the sensation of weight is recognized. Through the muscular sense,

precision of effort is rendered possible; for by it we learn to adjust the

force exerted to the weight of the object to be lifted, moved, or carried.

Without it, all our movements would necessarily become ill-regulated and

spasmodic. In cases of disease, where the sensibility of the lower limbs is

lost, while power of motion remains, the patient is able to stand erect so

long as he can see his limbs; but just as soon as his eyes are closed, he

begins to waver, and will fall unless supported.

[Sidenote: 29. The organ of taste? The tongue? Its powers of motion?]

29. THE ORGAN OF TASTE.--The _tongue_ is the special organ of the sense of

taste; but the back part of the mouth also possesses this faculty. The

tongue is a muscular organ, the muscles composing it being so numerous and

interwoven as to give it the freedom and variety of motion which it

possesses. It can curve itself upward or downward; it can extend or

contract itself; and, with its point, can sweep the cavity of the mouth, in

all directions, in the search for scattered particles of food.

[Sidenote: 30. Peculiarities of the tongue? Uses of the papillæ?]

30. The upper surface of the tongue is peculiar, being marked by the

presence of innumerable _papillæ_, some of {189} which are of microscopic

size, resembling those that abound in the fingers, and in other parts of

the body that have the sense of touch. Others are much larger, and give to

the tongue its roughness of feeling and appearance. Through the medium of

these papillæ, the tongue receives impressions of touch and temperature, as

well as taste: indeed, its extremity is fully as delicate, in respect to

tactile sensations, as the tips of the fingers themselves. It can recognize

the two points of the compasses when separated not more than

one-twenty-fourth of an inch; the back of it is much less sensitive to

touch, while at the same time it is more highly sensitive to impressions of

taste.

[Sidenote: 31. Resemblance to the parts of the tongue? Powers and functions

of the parts?]

31. Each lateral half of the tongue resembles the other in structure, and

each receives the same number of nerves--three. One of these regulates

motion, the other two are nerves of special sense. One of the latter

supplies the front half of the tongue, and is called the _gustatory_ nerve.

This is a branch of the great cranial nerve, called the "fifth pair," which

ramifies in all parts of the face. The back of the tongue is endowed with

the power of taste through a nerve known as the _glosso-pharyngeal_,

because it is distributed both to the tongue and throat. This difference in

the nervous supply of the tongue becomes significant, when we learn, as we

shall presently, that each part of it perceives a different class of

flavors.

[Sidenote: 32. Taste? What are the requisites to taste?]

32. THE SENSE OF TASTE.--Taste is the special sense by means of which we

discover the savors, or flavoring properties of the substances, which come

in contact with the tongue. Mere contact with the surface of the tongue,

however, is not sufficient, but contact with the extremities of the nerves

of taste within the papillæ is required. In order that the substance to be

tasted may penetrate the {190} cells covering the nerves, it must either be

liquid in form, or readily soluble in the watery secretion of the mouth,

the saliva. The tongue must be moist also. If the substance be insoluble,

as glass or sand, or the tongue dry, the sense of taste is not awakened. In

sickness, when the tongue is heavily coated, the taste is very defective,

or, as is frequently expressed, "nothing tastes aright."

[Sidenote: 33. Portions of the tongue endowed with taste? Where else does

the sense lodge? What is stated in respect to sweet and bitter flavors?

Reflex effects mentioned?]

33. All portions of the tongue are not alike endowed with the sense of

taste, that function being limited to the posterior third, and to the

margin and tip of this organ. The soft palate, also, possesses the sense of

taste; hence, an article that has an agreeable flavor may very properly be

spoken of as palatable, as is often done. All parts of the tongue do not

perceive equally well the same flavors. Thus, the front extremity and

margin, which is the portion supplied by the "fifth pair" of nerves,

perceives more acutely sweet and sour tastes; but the base of the tongue,

supplied by the _glosso-pharyngeal_ nerve, is especially sensitive to salt

and bitter substances. The nerve of the front part of the tongue, as before

stated, is in active sympathy with those of the face, while the relations

of the other nerve are chiefly with the throat and stomach; so that when an

intensely sour taste is perceived, the countenance is involuntarily

distorted, and is said to wear an acid expression. On the other hand, a

very bitter taste affects certain internal organs, and occasions a

sensation of nausea, or sickness of the stomach.

[Sidenote: 34. What is stated of the relations of taste with other senses?]

34. RELATIONS OF TASTE WITH OTHER SENSES.--Taste is not a simple sense.

Certain other sensations, as those of touch, temperature, smell, and pain,

are blended and confused with it; and certain so-called tastes are really

sensations of another kind. Thus an astringent taste, like that of alum, is

more properly an astringent feeling, and {191} results from an impression

made upon the nerves of touch, that ramify in the tongue. In like manner,

the qualities known as smooth, oily, watery, and mealy tastes, are

dependent upon these same nerves of touch. A burning or pungent taste is a

sensation of pain, having its seat in the tongue and throat. A cooling

taste, like that of mint, pertains to that modification of touch called the

sense of temperature.

[Sidenote: 35. Its dependence on smell? on sight?]

35. Taste is largely dependent upon the sense of smell. A considerable

number of substances, like vanilla, coffee, and garlic, which appear to

possess a strong and distinct flavor, have in reality a powerful odor, but

only a feeble taste. When the sense of smell is interfered with by holding

the nose, it becomes difficult to distinguish between substances of this

class. The same effect is frequently observed when smell is blunted during

an ordinary cold in the head. Sight also contributes to taste. With the

eyes closed, food appears comparatively insipid; and a person smoking

tobacco in the dark is unable to determine by the taste whether his cigar

is lighted or not. Accordingly, it is not a bad plan to close the nose and

shut the eyes when about to swallow some disagreeable medicine.

[Sidenote: 36. The chief use of the sense of taste? The position of its

organs? The rule as regards wholesome and unwholesome food? Remarks

respecting the rule?]

36. INFLUENCE OF EDUCATION ON THE TASTE.--The chief use of the sense of

taste appears to be to act as a guide in the selection of proper food.

Hence its organs are properly placed at the entrance of the digestive

canal. As a general rule, those articles which gratify the taste are

wholesome; while the opposite is true of those which impress it

disagreeably. This statement is more exact in reference to the early years

of life than to later years, when, by reason of mischievous habits, the

sense of taste has become dulled or perverted. The desires of a child are

simple; he is fully satisfied with plain and wholesome articles of diet,

and must usually "learn to like" those {192} which have a strongly marked

flavor. Accordingly, it is far easier at this age to encourage the

preference for plain food, and thus establish healthful habits, than later

in life to uproot habits of indulgence in stimulating substances, after

their ill effects begin to manifest themselves.

[Sidenote: 37. Diversity in tastes of men? How shown? The education of the

sense of taste?]

37. The tastes of men present the most singular diversities, partly the

result of necessity and partly of habit or education. The Esquimaux like

the rank smell of whale oil, which is a kind of food admirably suited to

the requirements of their icy climate; and travellers who go from our

climate to theirs are not slow to develop a liking for the same articles

that the natives themselves enjoy. The sense of taste is rendered very

acute by education, as is shown in an especial manner by those who become

professional "tasters" of tea and wine.

[Sidenote: 38. Location of the sense of smell? The nose? "Roof of the

mouth?"]

38. THE SENSE OF SMELL--THE NASAL CAVITIES.--The sense of smell is located

in the delicate mucous membrane which lines the interior of the nose. That

prominent feature of the face, the nose, which is merely the front boundary

of the true nasal organ, is composed partly of bone and partly of

cartilage. The upper part of it is united with the skull by means of a few

small bones; to which circumstance is due its permanence of shape. The

lower portion, or tip of the nose, contains several thin pieces of

cartilage, which render it flexible and better able to resist the effects

of blows and pressure. Behind the nose we find quite a spacious chamber,

separated from the mouth by the hard palate, forming the "roof of the

mouth," and by the soft palate (see Fig. 46); and divided into two cavities

by a central partition running from before backward.

[Sidenote: 39. Cavities of the nose? Obstruction of the passage of air

through them?]

39. These nasal cavities, constituting the true beginning of the

air-passages, extend from the nose backward to the {193} upper opening of

the throat, and rise as high as the junction of the nose with the forehead.

The inner wall of each cavity is straight and smooth; but from the outer

wall there jut into each cavity three small scroll-like bones. The

structure of these bones is very light, and hence they have been called the

"spongy" bones of the nose. In this manner, while the extent of surface is

greatly increased by the formation of these winding passages, the cavities

are rendered extremely narrow; so much so, in fact, that a moderate

swelling of the mucous membrane which lines them, as from a cold, is

sufficient to obstruct the passage of air through them.

[Illustration: FIG. 46.--SECTION OF THE RIGHT NASAL CAVITY.]

[Sidenote: 40. The special nerve of smell? Its location?]

40. THE NERVE OF SMELL.--The internal surface of the nasal passages is

covered by a delicate and sensitive mucous membrane. Its surface is quite

extensive, following as it does, all the inequalities produced by the

curved spongy bones of the nose. The upper portion of it alone is the seat

of smell, since that part alone receives branches from the "first pair" of

cranial nerves, or the olfactory nerve, which is the special nerve of smell

(see Fig. 43). In Fig. 46 is shown the distribution of this nerve, in the

form of an intricate network upon the two upper spongy bones. The nerve

itself (1) does not issue from the skull, but rests upon a thin bone which

separates it from the cavity of the nose; and the branches which proceed

from it pass through this bone by means of numerous small openings. The

{194} engraving represents the outer surface of the right nasal cavity; the

three wave-like inequalities, upon which the nervous network is spread out,

are due to the spongy bones. The left cavity is supplied in the same

manner.

[Sidenote: 41. Branches of the "fifth pair" of nerves? Nasal mucus? Birds?]

41. The nerves which ramify over the lower part of the membrane, and which

endow it with sensibility to touch and pain, are branches of the "fifth

pair" of nerves. An irritation applied to the parts where this nerve is

distributed occasions sneezing, that is, a spasmodic contraction of the

diaphragm; the object of which is the expulsion of the irritating cause.

The manner in which the olfactory nerve-fibres terminate is peculiar.

Unlike the extremities of other nerves, which are covered in by a greater

or less thickness of tissue, these come directly to the surface of the

mucous membrane, and thus come into very close contact with the odorous

particles that are carried along by the respired air. The surface is at all

times kept in a moist condition by an abundant flow of nasal mucus;

otherwise it would become dry, hard, and insensitive from the continual

passage of air to and fro in breathing. Birds, which respire more actively

than men, have a special gland, for secreting a lubricating fluid, located

in the air-passages of the head.

[Sidenote: 42. Smell? Touch? Taste? Design of smell? Invisible and gaseous

particles? The extreme fineness of the particles? Musk? In other cases?]

42. THE USES OF THE SENSE OF SMELL.--Smell is the special sense which

enables us to appreciate odors. Touch, as we have seen, is largely

concerned with solid bodies; and taste, with fluids, or with solids in

solution. Smell, on the other hand, is designed to afford us information in

reference to substances in a volatile or gaseous form. Invisible and

subtile particles emanate from odorous bodies, and are brought by the

respired air in contact with the terminal filaments of the olfactory nerve,

upon which an agreeable or disagreeable impression is produced. {195} The

fineness of the particles that constitute odors is often so extreme, that

they elude all attempts to measure or weigh them. A piece of musk, for

instance, may be kept for several years, constantly emitting perfume,

without any appreciable loss of weight. In other cases, a loss of substance

is perceptible, such as the essential oils, which enter into the

composition of the ordinary perfumes.

[Sidenote: 43. Aid given by smell? The highest use of the sense? Explain

the manner.]

43. Smell, like taste, aids us in the choice of proper food, leading us to

reject such articles as have a rank or putrid odor, and which are, as a

rule, unfit to be eaten. The highest usefulness of this sense, however,

consists in the protection it affords to the organs of respiration.

Stationed at the gateways of the air-passages, it examines the current of

air as it enters, and warns us of the presence of noxious gases, and of

other and generally invisible enemies to health. Not all dangerous vapors

are offensive, but almost all offensive vapors are unfit to be breathed. A

number of small stiff hairs grow from the margin of the nostrils to prevent

the entrance of dust and other atmospheric impurities, which would be alike

injurious to the olfactory mucous membrane and to the lungs. The benevolent

design of the Maker of our bodies may be observed in all parts of their

mechanism; but, probably, in none is it more clearly displayed than in

connection with the sense of smell.

[Sidenote: 44. Sense of smell in the inferior animals? How, and in what

cases, illustrated?]

44. The sense of smell is developed in a remarkable degree in certain of

the inferior animals, and is especially acute in reference to the peculiar

emanations that appear to characterize the different animals. The lion and

other carnivorous beasts scent their prey from a great distance; and the

fox-hound is able to track the fox through thickets and over open country

for many miles; while the timid, helpless herbivora, such as the deer and

sheep, find in the {196} sense of smell a means of protection against their

natural enemies, of whose approach they are in this manner warned. By

training this sense in the dog, and making it subservient to his use, man

is able to hunt with success certain shy and very fleet animals, which

otherwise he could but seldom approach. Among men, individuals differ

greatly in respect to the development of this sense; and especially in

certain savage tribes it is found to be extremely delicate. Humboldt states

that the natives of Peru can, by this sense, distinguish in the dark

between persons of different races.

[Sidenote: 45. What is sight? What information does it furnish? Composite

visual sensations?]

45. THE SENSE OF SIGHT.--Sight, or Vision, is the special sense by means of

which we appreciate the color, form, size, distance, and other physical

properties of the objects of external nature. Primarily, this sense

furnishes us with information concerning the different shades of color and

the different degrees of brightness: these are the simple sensations of

sight, such as the yellowness and glitter of a gold coin. In addition to

these, there are composite visual sensations, produced by the joint action

of the other senses and by the use of the memory and judgment; such as, in

the case of the coin, its roundness, solidity, size, its distance and

direction from us. So that many of our sensations, commonly considered as

due to sight, are in reality the results of intellectual processes which

take place instantaneously and unconsciously.

[Sidenote: 46. Comparison between sight and hearing? Relative capacity of

deaf and blind?]

46. This faculty not only has value in the practical every-day affairs of

life, but it contributes so largely to the culture of the intellect and to

our higher forms of pleasure, that some writers are disposed to rate it as

the first and most valuable of the senses. Others, however, maintain that

the sense of hearing does not yield in importance to that of sight; and

they cite in support of their position {197} the fact that the blind are

commonly cheerful and gay, while the deaf are inclined to be morose and

melancholy. In respect to the relative capacity for receiving education in

the deaf and blind, it is found that the former learn more quickly, but

their attainments are not profound; while the blind acquire more slowly,

but are able to study more thoroughly.

[Sidenote: 47. Sight, unlike the other senses? In the case of the stars?]

47. LIGHT.--THE OPTIC NERVE.--Unlike the senses previously

considered--touch, taste, and smell--sight does not bring us into immediate

contact with the bodies that are examined; but, by it, we perceive the

existence and qualities of objects that are at a greater or less distance

from us. In the case of the stars, the distance is incalculable, while the

book we read is removed but a few inches. Light is the agent which gives to

this sense its wide range. The nature of this mysterious force is not

known, and it is not here to be discussed; since its study belongs more

properly to the province of natural philosophy.

[Sidenote: 48. The undulatory theory of light? What does the theory

suppose?]

48. It is sufficient, in this connection, to state that the theory of light

now generally accepted, and which best explains the facts of optics, is

that known as the undulatory theory. This theory supposes that there exists

an intangible, elastic medium, which fills all space, and penetrates all

transparent substances, and which is thrown into exceedingly rapid

undulations or waves, by the sun and every other luminous body; the

undulations being propagated with extreme rapidity, and moving not less

than 186,000 miles in a second.

[Sidenote: 49. The sensation of light? Optic nerve?]

49. These waves are thought to produce in the eye the sensation of light,

in the same manner as the sonorous vibrations of the air produce in the ear

the sensation of sound. That part of the eye which is sensitive to these

waves is the expansion of the _optic nerve_. It is sensitive {198} to no

other impression than that of light, and it is the only nerve which is

acted upon by this agent. The optic nerve, also called the "second pair" of

cranial nerves, is the means of communication between the eye and the

brain.

[Sidenote: 50. The two nerves constituting the pair of nerves?]

50. The two nerves constituting the pair, arise from ganglia lying at the

base of the cerebrum, one of them on each side; from which points they

advance to the eyes, being united together in the middle of their course in

the form of the letter X (Fig. 43--2). By this union the two eyes are

enabled to act harmoniously, and in some respects to serve as a double

organ. And by reason of this same intimate nervous communication, when

serious disease affects one eye, the fellow-eye is extremely liable to

become the seat of _sympathetic_ inflammation; and this, if neglected,

almost certainly results in hopeless blindness.

[Sidenote: 51. Why is the eye called the "window of the soul?" Why, the

subject of enthusiastic study?]

51. THE ORGAN OF SIGHT.--THE EYE.--The proximity of the eye to the brain,

and the important part it performs in giving expression to the emotions,

have given it the name of "the window of the soul." The exceeding beauty of

its external parts, and the high value of its function, have long made this

organ the subject of enthusiastic study. It is chiefly within the last

twenty years, however, that this study has been successful and fruitful of

practical results. Several ingenious instruments have been invented for the

examination of the eye in health and disease, and new operations have been

devised for the relief of blindness and of impaired vision. As a result, it

is now a well-marked fact that, in civilized lands, the number of those who

suffer from loss of sight is proportionally much less than in countries

where science is less known and cultivated.

[Sidenote: 52. The most obvious fact? The consequence? The next thing

noticed? Its range of view? Of what does the organ of vision consist?]

52. The most obvious fact in respect to the apparatus of {199} sight is

that there are two eyes, which may either act together as one, and be fixed

upon one object, or one eye may be used independently of the other. In

consequence of this arrangement the loss of one eye does not necessitate

blindness, and, in fact, it not infrequently happens that the sight of one

eye may be long impaired or lost before the fact is discovered. We next

notice that it is placed at the most elevated part of the body, in front,

and near the brain. It also commands a wide range of view, being itself

moved with great rapidity, and being further aided by the free motion of

the head and neck. The organ of vision consists essentially of two parts:

the optical instrument itself--the eyeball--and its enveloping parts, or

the case in which the instrument is kept free from harm. The latter, which

are external, and which we shall first consider, are chiefly the _Orbits_,

the _Eyelids_, and the apparatus for the _Tears_.

[Sidenote: 53. The protection of the eyeball against injury? The

overhanging brow? The opening for the optic nerve?]

53. THE ORBITS.--The eyeball, which is a delicate organ, is well defended

against external injury within the orbits or bony sockets of the head.

These are deep conical hollows, bounded in part by the bones of the skull,

and in part by those of the nose and cheek. The orbit juts out beyond the

most exposed portion of the eyeball, as may be seen by laying a book over

the eye, when it will be found that no part of the eyeball, unless it be

very prominent, will be touched by the book; so that the only direction in

which an injury is liable to be received is immediately in front of the

eye. The overhanging brow is itself covered by a layer of thick skin,

studded with short, stout hairs, which are so bent as to prevent the

perspiration from running into the eye and obscuring vision. Through a hole

in the bottom of the orbit, the nerve of sight passes outward from the

brain. The orbit also contains a considerable amount of a fatty tissue,

upon which, as upon an elastic cushion, the eye rests. {200}

[Illustration: FIG. 47.--FRONT VIEW OF RIGHT EYE. (Natural Size.)

1. The Lachrymal, or tear gland, lying beneath the upper eyelid.

2. The Nasal Duct is shown by the dotted line. The * marks the orifice in

the lower lid.

The central black spot is the _pupil_; surrounding it is the _iris_; and

the triangular white spaces are the visible portion of the _sclerotic_.]

[Sidenote: 54. What are the eyelids? The upper lid? The lower one? The

mucous membrane of the eye?]

54. THE EYELIDS.--The eyelids are two movable curtains, or folds, which,

when shut, cover the front part of the orbit, and hide the eye from view.

The upper lid is the larger, has a curved margin, and moves freely, while

the lower lid is comparatively short and straight, and has but a slight

degree of motion (Fig. 47). Skin covers the exterior of the lids, while a

fine mucous membrane lines their inner surface, and is likewise spread out

over the entire front of the eyeball. This membrane, which is called the

_Conjunctiva_, is highly sensitive, and thus plays an important part in

protecting the eye against the lodgment of sand, ashes, chaff, and other

foreign particles that are blown about in the air. This sensitive membrane

will not endure the presence of these particles. If any find access, it

causes a constant winking, a flow of tears, and other signs of irritation,

until it is removed.

[Sidenote: 55. The eyelashes? The little points within the line of the

lashes? Of what use are these glands?]

55. The long, silky eyelashes, which garnish the edges of the lids, act

like a sieve to prevent the entry of dust and the like; and together with

the lids, they regulate the amount of light which is permitted to enter the

eye, so that it is shielded from a sudden flood or glare of light. The

little points seen in the figure just within the line of the lashes,

especially on the lower lid, represent the mouths of numerous little

sebaceous glands (Fig. 48, D,D), such as are always {201} found in the

neighborhood of hairs. These glands supply a thick, oily material which

greases the edges of the lids and prevents their adhering together, and

likewise prevents the overflow of the tears upon the cheek.

[Sidenote: 56. The location of the lachrymal gland? The use of the gland?]

56. THE LACHRYMAL FLUID, OR THE TEARS.--Just within the outer part of the

bony arch of the brow, where the bone may be felt to be sharper than in

other positions, is lodged a little organ called the lachrymal gland, the

situation of which is indicated in Fig. 47, 1. This is the gland whence

flows the watery secretion, commonly called the _tears_, which is designed

to perform an exceedingly important duty in lubricating the lids, and in

keeping the exposed surface of the eyeball moist and transparent. For,

without this or some similar liquid, the front of the eye would speedily

become dry and lustreless, like that of a fish which has been removed from

the water: the simple exposure of the eye to the air would then suffice to

destroy vision.

[Sidenote: 57. When does the secretion of the tears occur? The secretion

not used for the eye? Location of the nasal duct? Its use? The overflow of

tears in old people?]

57. This secretion of the tears takes place at all times, during the night

as well as the day; but it is seldom noticed, except when under the

influence of some strong mental emotion, whether of sorrow or happiness, it

is poured forth in excess, so as to overflow the lids. Strong light or a

rapid breeze will, among many other causes, excite the flow of the tears.

That portion of this secretion which is not used in moistening the eye is

carried off into the nose by a canal situated near the inner angle of the

eye, called the _nasal duct_. This duct is shown in Fig. 47, 2, and is

connected with each lid by delicate tubes, which are indicated by dotted

lines in the figure; the asterisk marks the little opening in the lower

lid, by which the tears enter the nasal duct. By gently turning the inner

part of that lid downward, and looking in a mirror, {202} this small

"lachrymal point" may be seen in your own eye. In old people, these points

become everted, and do not conduct the tears to the nasal cavity, so that

they are inconvenienced by an overflow of tears upon the face.

[Illustration: FIG. 48.--VERTICAL SECTION OF THE EYE. (Enlarged.)

C, The Cornea. A, The Aqueous Humor. I, The Iris. P, The Pupil. L, The

Crystalline Lens. H, The Ligament of the Lens. B, The Ciliary Process.

V, The Cavity containing the Vitreous Humor. S, The Sclerotic. Ch, The

Choroid. R, The Retina. N, The Optic Nerve. DD, The Eyelids. X, The Levator

Muscle of the Upper Lid. Y, The Upper Straight Muscle of the Eye. Z, The

Lower Straight Muscle.]

[Sidenote: 58. The watery fluid passing over the eyeball? Design of the

arrangement? Winking?]

58. Thus we observe that the gland which forms the tears is placed at the

outer part of the eye, while their means of exit is at the inner angle of

the eye; which fact renders it necessary that this watery fluid shall pass

over the surface of the eyeball before it can escape. This arrangement

cannot be accidental, but evinces design, for it thus secures the perfect

lubrication of the surface of the eye, and cleanses it from the smaller

particles of dust {203} which may enter it, in spite of the vigilance of

the lids and lashes. The act of winking, which is generally unconsciously

performed, and which takes place six or more times in a minute, assists

this passage of the tears across the eye, and is especially frequent when

the secretion is most abundant.

[Sidenote: 59. Describe the shape of the eyeball. Its structure.]

59. THE EYEBALL.--The remarkable optical instrument called the eyeball, or

the globe of the eye, upon which sight depends, is, as the name indicates,

spherical in shape. It is not a perfect sphere, since the front part

projects somewhat beyond the rest, and at the posterior part the optic

nerve (Fig. 48, N) is united to it, resembling the junction of the stem

with a fruit. In its long diameter, that is, the horizontal or from side to

side, it measures a little more than an inch; in other directions it is

rather less than an inch. In structure, the ball of the eye is firm, and

its tense round contour may in part be felt by pressing the fingers over

the closed lids.

[Sidenote: 60. Of what is the eyeball composed? State how.]

60. The eyeball is composed chiefly of three internal, transparent media,

called _humors_; and three investing coats, or _tunics_. The former are the

_aqueous humor_, Fig. 48, A, the _crystalline lens_ L, and the _vitreous

humor_ V. Of these the lens alone is solid. The three coats of the eyeball

are called the _sclerotic_ S, the _choroid_ CH, and the _retina_ R. This

arrangement exists in respect to five-sixths of the globe of the eye, but

in the anterior one-sixth, these coats are replaced by the _cornea_ C,

which is thin and transparent, so that the rays of light pass freely

through it, as through a clear window-pane.

[Sidenote: 61. The shape of the cornea? Its structure? The "white of the

eye?"]

61. In shape, the cornea is circular and prominent, resembling a miniature

watch-glass, about 1/25 of an inch thick. In structure, it resembles horn

(as the name signifies), or the nail of the finger, and is destitute of

{204} blood-vessels. The _Sclerotic_ (from _scleros_, hard) is composed of

dense, white fibrous tissue, and gives to the eyeball its firmness of

figure and its white color; in front, it constitutes the part commonly

called "the white of the eye." It is one of the strongest tissues in the

body; it possesses very few vessels, and is not very sensitive. It affords

protection to the extremely delicate interior parts of the eye; and the

little muscles which effect its movements are inserted into the sclerotic a

short distance behind the cornea (see Fig. 48, Y, Z). It is perforated

posteriorly to admit the optic nerve.

[Sidenote: 62. The second or middle coat of the eyeball? Its dark color?]

62. The _Choroid_ is the second or middle coat of the eyeball, and lies

closely attached to the inner surface of the sclerotic. Unlike the latter

tunic, its structure is soft and tender, it is dark in color, and possesses

a great abundance of blood-vessels. Its dark color is due to a layer of

dark brown or chocolate-colored cells spread out over its inner surface.

This dark layer serves to absorb the rays of light after they have

traversed the transparent structures in front of it; if the rays were

reflected from side to side within the eye, instead of being thus absorbed,

confused vision would result from the multitude of images which would be

impressed upon the optic nerve.

[Sidenote: 63. Similar mechanism in microscopes? The albinos? White

rabbits?]

63. This mechanism has been unconsciously imitated by the opticians, who,

when they make a microscope or telescope, take care that the interior of

its tube shall be coated with a thick layer of black paint or lamp-black;

for without it, a clear delineation of the object to be viewed is

impossible. The albinos, in whom these dark cells of the choroid are

wanting, have imperfect vision, especially in the daytime and in strong

lights. The dark cells are also wanting in white rabbits, and other animals

that have red or pink eyes; their vision appears to be imperfect in the

presence of a bright light. {205}

[Sidenote: 64. What is the iris? Its construction? How is the size of the

pupil regulated?]

64. THE IRIS.--Continuous with the choroid, in the front part of the globe

of the eye, is a thin, circular curtain, which occasions the brown, blue,

or gray color of the eye in different individuals. On account of the

varieties of its color, this membrane has received the name _Iris_, which

is the Greek word for "rainbow" (see Fig. 48, I). A front view of it is

shown in Fig. 47. The iris is pierced in its centre by a round opening,

called the _pupil_ (P), which is constantly varying in size. In olden times

it was spoken of as the "apple of the eye." The hinder surface of the iris,

except in albinos, has a layer of dark coloring matter resembling that of

the choroid. The iris is a muscular organ, and contains two distinct sets

of fibres; one of which is circular, while the other radiates outward from

the pupil. The action of these sets of fibres regulates the size of the

pupil; for when the circular set acts, the pupil contracts, and when the

other set acts, the opening expands. Their action is involuntary, and

depends on the reflex system of nerves, which causes the contraction of the

pupil when a strong light falls upon the eye, and its expansion when the

illumination is feeble.

[Sidenote: 65. The admission of light to the eye? The action of the iris

under different circumstances? The lustre of the eye, how affected in youth

and old age?]

65. The iris, accordingly, serves a very useful purpose in regulating the

admission of light to the eye. It, however, does not act instantaneously;

and hence, when we pass quickly from a dark room into the bright sunlight,

the vision is at first confused by the glare of light, but as soon as the

pupil contracts, the ability to see becomes perfect. On the other hand,

when we enter a dark apartment, such as a cellar, for a short time we can

see nothing clearly; but as soon as the pupil expands and admits more

light, we are enabled to distinguish the surrounding objects. Animals of

the cat species, and others which prowl around after nightfall, are enabled

to see in the dark by {206} having the iris very dilatable. The size of the

pupil affects the lustre of the eye. When it is large, as it usually is

during youth, the eye appears clear and brilliant; while in old age the

pupil is small and the eye is dull. The brilliancy of the eye is in part,

at least, dependent upon the reflection of light from the front surface of

the crystalline lens.

[Sidenote: 66. Means used to increase the beauty of the eye? The injurious

consequences?]

66. Certain poisonous vegetables have the property of causing the pupil to

dilate, and have been used in small doses to increase the beauty of the

eye. One of these drugs has been so largely used by the ladies for this

purpose, that it has received the name _belladonna_, from the Italian words

meaning "beautiful lady." This hazardous practice has resulted more than

once in the death of the person desiring thus to increase her personal

attraction. The common English name for belladonna is "deadly nightshade."

(In the diagram on page 214 the shape and relations of the iris are more

accurately shown than in the figure referred to above.)

[Sidenote: 67. What part does the retina constitute? How formed? Its

texture? Color? Sensitiveness?]

67. THE RETINA constitutes the third and inner coat of the globe of the

eye. This, the important part of the eye that is sensitive to light, is a

kind of nervous membrane, formed by the expansion of the ultimate filaments

of the optic nerve. Its texture is soft, smooth, and very thin; it is

translucent and of an opaline, or grayish-white color. It is sensitive to

light alone; and if any form of mechanical irritation be applied to it, the

sensations of touch and pain are not experienced, but flashes of fire,

sparks, and other luminous appearances are perceived. Thus an electric

shock given to the eye-ball occasions a flash of light; and a sudden fall,

or a blow upon the eye, is often apparently accompanied by the vision of

"stars."

[Sidenote: 68. Specific energy of the optic nerve? Trial in Germany?]

68. These phenomena are due to what is termed the {207} "specific energy"

of the optic nerve, which nerve, in common with the other nerves of special

sense, obeys a general law of nature, which requires that, whenever one of

these nerves is stimulated, it shall respond with the sensation peculiar to

itself. These flashes of retinal light have no power to illuminate external

objects, although the opposite of this statement has been maintained. On

the occasion of a remarkable trial in Germany, it was claimed by a person

who had been severely assaulted on a very dark night, that the flashes of

light caused by repeated blows upon the head enabled him to see with

sufficient distinctness to recognize his assailant. But the evidence of

scientific men entirely refuted this claim, by pronouncing that the eye,

under the circumstances named, was incapacitated for vision. Too intense

light occasions a feeling of pain, but it is of a peculiar kind, and is

termed "dazzling."

[Illustration: FIG. 49.]

[Sidenote: 69. Sensitiveness of all parts of the retina? Experiment to

prove the existence of the "blind spot."]

69. All parts of the retina are not equally sensitive, and singularly

enough, the point of entry of the nerve of sight, in the back part of the

eyeball, is entirely insensible to light, and is called the "blind spot."

The existence of this point may be proven by a simple experiment. Hold the

accompanying figure, on page 207, directly in front of and parallel with

the eyes. Close the left eye, and fix the sight steadily on the left-hand

circle; then, by gradually varying the distance of the figure from the eye,

at a certain distance (about six inches), the right-hand circle will

disappear, {208} but nearer or further than that, it will be plainly seen.

The other eye may be also tried, with a similar result: if the gaze be

directed to the right-hand circle, the left one will seem to disappear. The

experiment may be repeated by using two black buttons on the marble top of

a bureau, or on some other white surface. The blind spot does not

practically interfere with vision, since the eye is seldom fixed immovably

on an object, and the insensitive parts of the two eyes can never be

directed upon the same object at the same time.

[Sidenote: 70. Duration of impressions upon the retina? How illustrated?]

70. Impressions made upon the retina are not at once lost, but persist a

measurable length of time, and then gradually fade away. Thus, a bright

light or color, gazed at intently, cannot be immediately dismissed from

sight by closing or turning away the eyes. A stick lighted at one end, if

whirled around rapidly in the dark, presents the appearance of an unbroken

luminous ring; and the spokes of a rapidly revolving carriage-wheel seem to

be merged into a plane surface. If an object move too rapidly to produce

this sort of lasting impression, it is invisible, as in the case of a

cannon-ball passing through the air in front of us.

[Sidenote: 71. What further illustration? Winking, why it is not noticed.

Ease with which the retina is fatigued or deprived of sensibility? How

shown?]

71. If a card, painted with two primary colors--as red and yellow--be made

to rotate swiftly, the eye perceives neither of them distinctly; but the

card appears painted with their secondary color--orange. The average

duration of retinal images is estimated at one-eighth of a second; and it

is because they thus endure, that the act of winking, which takes place so

frequently, but so quickly, is not noticed and does not interrupt the

vision. The retina is easily fatigued or deprived of its sensibility. After

looking steadfastly at a bright light, or at a white object on a black

ground, a dark spot, corresponding in shape to the bright object, {209}

presents itself in whatever direction we look. This spot passes away as the

retina resumes its activity.

[Sidenote: 72. How further shown? How is the result accounted for?

"Color-blindness?"]

72. If a bright color be gazed at intently, and the eyes then be turned to

a white surface, a spot will appear; but its color will be the complement

of that of the object. Fix the eye upon a red wafer upon a white ground,

and on removing the wafer a greenish spot of the same shape takes its

place. This result happens because a certain portion of the retina has

exhausted its power to perceive the red ray, and perceives only its

complementary ray, which is green. The color thus substituted by the

exhausted retina is called a physiological or accidental color. In some

persons the retina is incapable of distinguishing different colors, when

they are said to be affected with "color-blindness." Thus, red and green

may appear alike, and then a cherry-tree, full of ripe fruit, will seem of

the same color in every part. Railroad accidents have occurred because the

engineer of the train, who was color-blind, has mistaken the color of a

signal.

[Sidenote: 73. The location of the crystalline lens? How supported? Its

color and texture? Shape? Size?]

73. THE CRYSTALLINE LENS.--Across the front of the eye, just behind the

iris, is situated the _Crystalline lens_, enclosed within its own capsule.

It is supported in its place partly by a delicate circular ligament, and

partly by the pressure of adjacent structures. It is colorless and

perfectly transparent, and has a firm but elastic texture. In shape it is

doubly convex, and may be rudely compared to a small lemon-drop. The front

face of the lens is flatter than the other, and is in contact with the iris

near its pupillary margin, as is represented in the diagram on page 214. It

is only one-fourth of an inch thick.

[Sidenote: 74. Cataract? Aqueous humor? Vitreous humor?]

74. When this little body becomes opaque, and no longer affords free

passage to the rays of light, as often happens {210} with the advance of

age, an affection termed "cataract" is produced. Between the crystalline

lens and the cornea is a small space which contains the _aqueous humor_

(see Fig. 48, A). This humor consists of five or six drops of a clear,

colorless liquid very much like water, as its name implies. That part of

the globe of the eye lying behind the lens is occupied by the _vitreous

humor_, so called from its fancied resemblance to melted glass (Fig. 48,

V). This humor is a transparent, jelly-like mass, enclosed within an

exceedingly thin membrane. It lies very closely applied to the retina, or

nervous membrane of the eye, and constitutes fully two-thirds of the bulk

of the eyeball.

[Illustration: FIG. 50.--THE RETINAL IMAGE.]

[Sidenote: 75. What is a lens and its focus? The miniature image, how

produced?]

75. THE USES OF THE CRYSTALLINE LENS.--A convex lens has the property of

converging the rays of light which pass through it; and the point at which

it causes them to meet is termed its focus. If a lens of this description,

such as a magnifying or burning-glass, be held in front of an open window,

in such a position as to allow its focus to fall upon a piece of paper, it

will be found to depict upon the paper a miniature image of the scene

outside of the window. It will be further noticed that the image is

inverted, or upside down, and that the paper {211} at the place upon which

the image is thrown is much brighter than any other part.

[Sidenote: 76. How are figures painted upon the retina? How proved?]

76. Now all the transparent structures of the eye, but especially the

crystalline lens, operate upon its posterior part, or retina, as the convex

lens acts upon the paper; that is, they paint upon the retina a bright

inverted miniature of the objects that appear in front of the eye (Fig.

50). That this actually takes place may be proved by experiment. If the

eyeball of a white rabbit, the walls of which are transparent, be examined

while a lighted candle is held before the cornea, an image of the

candle-flame may be seen upon the retina.

[Sidenote: 77. What can be said in respect to the form and structure of the

crystalline lens?]

77. The form and structure of the crystalline lens endow it with a

remarkable degree of refractive power, and enable it to converge all the

rays of light that enter it through the pupil, to a focus exactly at the

surface of the retina. When this lens is removed from the eye, as is

frequently done for the cure of cataract, it is found that the rays of

light then have their focus three-eighths of an inch behind the retina;

that the image is four times larger than in the healthy eye, that it is

less brilliant, and that its outline is very indistinct. From this we learn

that one of the uses of the crystalline lens is to make the retinal image

bright and sharply-defined, at the same time that it reduces its size.

Indeed, the small size of the image is a great advantage, as it enables the

limited surface of the retina to receive, at a glance, impressions from a

considerable field of vision.

[Sidenote: 78. How is the inverted image upon the retina presented in its

true position to the mind?]

78. As the image upon the retina is inverted, how does the mind perceive

the object in its true, erect position? Many explanations have been

advanced, but the simplest and most satisfactory appears to be found in the

fact that {212} the retina observes no difference, so to speak, between the

right and left or the upper and lower positions of objects. In fact, the

mind is never conscious of the formation of a retinal image, and until

instructed, has no knowledge that it exists. Consequently, our knowledge of

the relative location of external objects must be obtained from some other

source than the retina. The probable source of this knowledge is the

habitual comparison of those objects with the position of our own bodies:

thus, to see an elevated object, we know we must raise the head and eyes;

and to see one at our right hand, we must turn the head and eyes to the

right.

[Illustration: FIG. 51.--THE DIFFERENT SHAPES OF THE GLOBE OF THE EYE.

N, The Natural Eye. M, The Short-sighted Eye. H, The Long-sighted Eye.

S, Parallel Rays from the Sun.]

[Sidenote: 79. The uniform perfection of the eye? Examples? The most common

imperfection?]

79. LONG-SIGHT OR HYPEROPIA, AND SHORT-SIGHT OR MYOPIA.--The eye is not in

all cases perfectly formed. For example, persons may from birth have the

cornea too prominent or too flat, or the lens may be too thick or too thin.

In either of these conditions sight will be more or less defective from the

first, and the defect will not tend to disappear as life advances. The most

common imperfection, however, is in the shape of the globe; which may be

short (Fig. 51, H), as compared with the natural eye, N, or it may be too

long, M.

[Sidenote: 80. How is "long-sight" explained? "Short-sight?"]

80. When the globe is short, objects can only be clearly {213} seen that

are at a distance, and the condition of the vision is known as

"long-sight," or hyperopia. It will be observed, by reference to Fig. 51,

that the focus of the rays of light would fall behind the retina of this

eye. When the globe is too long, objects can only be clearly seen that are

very near to the eye; and the condition resulting from this defect is

termed "short-sight," or myopia. The focus of the rays of light is, in this

case, formed in the interior of the eye in front of the retina.

[Sidenote: 81. Long-sight, how common? With what must it not be confounded?

Kind of glasses for short-sight? Why? Squint?]

81. Long-sight, or hyperopia, is common among schoolchildren, nearly as

much so as short-sight, and must not be confounded with the defect known as

the "far sight" of old people; although in both affections the sight is

improved by the use of convex glasses. Children not infrequently discover

that they see much better when they chance to put on the spectacles of old

persons. For the relief of short-sight, concave glasses should be employed;

as they so scatter the rays of light as to bring the focus to the retina,

and thus cause the vision of remote objects to become at once distinct.

That form of "squint," in which the eyes are turned inward, is generally

dependent upon long-sight, while that rarer form, when they turn outward,

is due to short-sight.

[Sidenote: 82. What is stated in connection with the opera-glass?

Experiment with pencil and distant object?]

82. THE FUNCTION OF ACCOMMODATION.--If, after looking through an

opera-glass at a very distant object, it is desired to view another nearer

at hand, it will be found impossible to obtain a clear vision of the second

object unless the adjustment of the instrument is altered; which is

effected by means of the screw. If an object, like the end of a pencil, be

held near the eye, in a line with another object at the other side of the

room, or out of the window, and the eye be fixed first upon one and then

upon the other, it will be found that when the pencil is clearly seen, the

{214} further object is indistinct; and when the latter is seen clearly,

the pencil appears indistinct; and that it is impossible to see both

clearly at the same time. Accordingly, the eye must have the capacity of

adjusting itself to distances, which is in some manner comparable to the

action of the screw of the opera-glass.

[Illustration: FIG. 52.--THE FUNCTION OF ACCOMMODATION.

The right half of the diagram shows the eye at rest. The left half shows

the lens accommodated for near vision.]

[Sidenote: 83. Function of accommodation? In what does it consist? How is

the function explained?]

83. This, which has been called the function of accommodation, is one of

the most admirable of all the powers of the eye, and is exercised by the

crystalline lens. It consists essentially in a change in the curvature of

the front surface of the lens, partly through its own elasticity, and

partly through the action of the ciliary muscle. When the eye is at rest,

that is, when accommodated for a distant object, the lens is flatter and

its curvature diminished (see Fig. 52); but when strongly accommodated for

near vision the lens becomes thicker, its curvature increases, and the

image on the retina is made more sharp and distinct. Since a strong light

is not required in viewing near objects, the pupil contracts, as is shown

in the left-hand half of the diagram. {215}

[Sidenote: 84. Change of sight with the approach of old age? Explain the

change?]

84. OLD-SIGHT, OR PRESBYOPIA.--But this marvellously beautiful mechanism

becomes worn with use; or, more strictly speaking, the lens, like other

structures of the body, becomes harder with the approach of old age. The

material composing the lens becomes less elastic, the power to increase its

curvature is gradually lost, and as a consequence, the person is obliged to

hold the book further away when reading, and to seek a stronger light. In a

word, the function of accommodation begins to fail, and is about the first

evidence that marks the decline of life. By looking at the last preceding

diagram, and remembering that the increased curvature of the lens cannot

take place, it will be at once understood why old-sight is benefitted in

near vision by the convex lens, such as the spectacles of old people

contain. It acts as a substitute for the deficiency of the crystalline

lens.

[Sidenote: 85. Hearing or audition? What is sound? How propagated commonly?

Stone thrown in water?]

85. THE SENSE OF HEARING.--SOUND.--Hearing, or audition, is the special

sense by means of which we are made acquainted with _sound_. What is sound?

It is an impression made upon the organs of hearing, by the vibrations of

elastic bodies. This impression is commonly propagated by means of the air,

which is thrown into delicate undulations, in all directions from the

vibrating substance. When a stone is thrown into smooth water, a wave of

circular form is set in motion, from the point where the stone struck,

which constantly increases in size and diminishes in force, as it advances.

[Sidenote: 86. Sound-wave in the atmosphere? Its shape? Rate of motion?

Sound, in water, air, and solid bodies?]

86. Somewhat resembling this, is the undulation, or sound-wave, which is

imparted by a sonorous vibration to the surrounding atmosphere. Its shape,

however, is spherical, rather than circular, since it radiates upward,

downward, and obliquely as well as horizontally, like the wave {216} in

water. The rate of motion of this spherical wave of air is about 1050 feet

per second, or one mile in five seconds. In water, sound travels four times

as fast as in air, and still more rapidly through solid bodies; along an

iron rod, its velocity is equal to two miles per second.

[Sidenote: 87. The earth as a conductor of sound? To what has the western

Indian been taught? Solid substances as conductors? As regards sound, in

what respect is air necessary? Sound in a vacuum?]

87. The earth, likewise, is a good conductor of sound. It is said that the

Indian of our western prairies can, by listening at the surface of the

ground, hear the advance of a troop of cavalry, while they are still out of

sight, and can even discriminate between their tread and that of a herd of

buffaloes. Solid substances also convey sounds with greater power than air.

If the ear be pressed against one end of a long beam, the scratching of a

pin at the other extremity may be distinctly heard, which will not be at

all audible when the ear is removed from the beam. Although air is not the

best medium for conveying sound, it is necessary for its production. Sound

cannot be produced in a vacuum, as is shown by ringing a bell in the

exhausted receiver of an air-pump, for it is then entirely inaudible. But

let the air be readmitted gradually, then the tones become more and more

distinct, and when the receiver is again full of air, they will be as clear

as usual.

88. All sonorous bodies do not vibrate with the same degree of rapidity,

and upon this fact depends the _pitch_ of the sounds that they respectively

produce. The more frequent the number of vibrations within a given time,

the higher will be the pitch; and the fewer their number, the lower or

graver will it be. Now, the rate of the successive vibrations of different

notes has been measured, and it has thus been found that if they are less

than sixteen in a second, no sound is audible; while if they exceed 60,000

per second the sound is very faint, and is painful to the {217} ear. The

extreme limit of the capacity of the human ear may be considered as

included between these points; but the sounds which we ordinarily hear are

embraced between 100 and 3,000 vibrations per second.

89. The _ear_, which is the proper organ of hearing, is the most

complicated of all the structures that are employed in the reception of

external impressions. The parts of which it is composed are numerous, and

some of them are extremely small and delicate. Nearly all these parts are

located in an irregularly shaped cavity hollowed out in the temporal, or

"temple," bone of each side of the head. That part of the bone in which the

auditory cavity is placed has the densest structure of all bones of the

body, and has therefore been called the "petrous," or rocky part of the

temporal bone. In studying the ear, it is necessary to consider it as

divided into three portions, which are called, from their relative

positions, the _external_ ear, the _middle_ ear, and the _internal_ ear.

(In the diagram, Fig. 53, A, the first is not shaded, the second is lightly

shaded, and the last has a dark background.)

[Sidenote: 90. Of what does the external portion of the organ of hearing

consist? Describe the portal of that organ known as the ear. Its use?]

90. THE EXTERNAL EAR.--The external portion of the organ of hearing,

designated in Fig. 53, A, includes, first, that outer part (_a_), which is

commonly spoken of as "the ear," but which in fact is only the portal of

that organ; and, secondly, the _auditory canal_ (_b_). The former consists

of a flat flexible piece of cartilage, projecting slightly from the side of

the head, attached to it by ligaments, and supplied with a few weak

muscles. Its surface is uneven, and curiously curved, and from its

resemblance to a shell, it has been called the _concha_. It probably serves

to collect sounds, and to give them an inward direction; although its

removal is said not to impair the acuteness of hearing more than a few

days. {218}

[Sidenote: 91. The ear in the animals of delicate hearing? Rabbit? Fishes?]

91. In those animals whose hearing is more delicate than that of man, the

corresponding organ is of greater importance, it being larger and supplied

with muscles of greater power, so that it serves as a natural kind of

ear-trumpet, which is easily movable in the direction of any sound that

attracts the attention of the animal. Bold, predaceous animals generally

have the concavity of this organ directed forward, while in timorous

animals, like the rabbit, it is directed backward. Fishes have no outer

ear, but sounds are transmitted directly through the solid bones of the

head, to the internal organ of hearing.

[Illustration: FIG. 53.--THE EAR AND ITS DIFFERENT PARTS.

A, Diagram of the Ear.

_a_, _b_, External Ear. _c_, Membrane tympani. _d_, Middle Ear.

_e_, Internal Ear.

B to B''', Bones of the Middle Ear (magnified).

C, The Labyrinths, or Internal Ear (highly magnified).]

[Sidenote: 92. What is the auditory canal? Describe it.]

92. The _auditory canal_ (Fig. 53, A, _b_), which is continuous with the

outer opening of the ear, is a blind passage, an inch and a quarter in

length, its inner extremity being bounded by a closely-fitting, circular

membrane. This canal is of oval form, is directed forward and inward, {219}

and is slightly curved; so that the inner end is ordinarily concealed from

view. The pouch of the skin which lines this passage is smooth and thin,

especially at the lower end, where it covers the membrane just mentioned.

[Sidenote: 93. How is it guarded and protected? Ear-wax?]

93. As in the case of the nostrils, a number of small, stiff hairs garnish

the margin of the auditory canal, and guard it, to some extent, against the

entrance of insects and other foreign objects. The skin, too, covering its

outer half, is furnished with a belt of little glands which secrete a

yellow, viscid, and bitter substance, called "ear-wax," which is especially

obnoxious to small insects. As the outer layer of this wax-like material

loses its useful properties, it becomes dry, and falls out of the ear in

the form of minute, thin scales, a fresh supply being furnished from the

little glands beneath. In its form, the auditory canal resembles the tube

of an ear-trumpet, and serves to convey the waves of sound to the middle

portion of the ear.

[Sidenote: 94. What is the middle ear? Why called tympanum?]

94. THE MIDDLE EAR, OR TYMPANUM.--The middle ear is a small cavity, or

chamber, of irregular shape, about one-fourth of an inch across from side

to side, and half an inch long (see Fig. 53, A, _d_). From the peculiar

arrangement of its various parts it has very properly been called the

_tympanum_, or the "drum of the ear." The middle ear, like the external

canal, contains air.

[Sidenote: 95. What is the membrana tympani? Describe it.]

95. The circular membrane, already mentioned as closing the auditory canal,

is the partition which separates the middle from the external ear, and is

called the _membrana tympani_ (_c_), and may be considered as the outer

head of the drum of the ear. It is sometimes itself spoken of as the

"drum," but this is incorrect; since a drum is not a membrane, but is the

hollow space across which the membrane is stretched. This membranous

drum-head is very tense and elastic, and so thin as to be almost

transparent; {220} its margin is fastened into a circular groove in the

adjacent bone. Each wave of sound that impinges against this delicate

membrane causes it to vibrate, and it, in turn, excites movements in the

parts beyond.

[Sidenote: 96. What are the ossicles? Their number and names? Their

arrangement?]

96. Within the tympanum is arranged a chain of remarkable "little bones,"

or _ossicles_. They are chiefly three in number, and from their peculiar

shapes bear the following names: _malleus_, or the mallet; _incus_, or the

anvil; and _stapes_, or the stirrup. A fourth, the smallest bone in the

body, in early life intervenes between the incus and stapes, but at a later

period it becomes a part of the incus. It is called the _orbicular_ bone.

Small as are these ossicles--and they, together, weigh only a few

grains--they have their little muscles, cartilages, and blood-vessels, as

perfectly arranged as the larger bones of the body. One end of the chain of

ossicles, the mallet, is attached to the membrane of the tympanum, or outer

drum-head, while the other end, the stirrup, is firmly joined by its

foot-piece to a membrane in the opposite side of the cavity. The chain,

accordingly, hangs suspended across the drum between the two membranes; and

when the outer one vibrates under the influence of the sound-wave, the

chain swings inward and transmits the vibration to the entrance of the

inner ear.

[Sidenote: 97. The Eustachian tube? Describe it, and state its use.]

97. The musical instrument, the drum, is not complete if the air within be

perfectly confined: we therefore find in all instruments of this kind a

small opening in the side, through which air may pass freely. By this means

the pressure of the air upon the vellum which forms the head of the drum is

made equal upon all sides, and the resonance of the drum remains unaffected

by the varying density of the atmosphere. It will, therefore, emit its

proper sound, whether it be struck in the rarified air of the mountain-top,

or in the condensed air of a mine. The tympanum, {221} or drum of the ear,

in like manner has an opening by means of which it communicates freely with

the external air. This opening is a narrow canal, about an inch and a half

long, called the _Eustachian tube_, after the name of its discoverer,

Eustachius.

[Illustration: FIG. 54.--SECTION OF THE RIGHT EAR.

A, The Concha. B, Auditory Canal. C, Membrane of the Drum, (the lower

half.) D, A small muscle. E, Incus, or Anvil. M, Malleus, or Mallet.

I, Eustachian Tube. G, Semicircular Canals. H, Cochlea, or snail's shell.]

[Sidenote: 98. What can you state of the action of the Eustachian tube?]

98. The course of this passage is indicated in Fig. 54, I, directed

downward and inward: its other extremity opens into the upper part of the

throat. The passage itself is ordinarily closed, but whenever the act of

swallowing or gaping takes place, the orifice in the throat is stretched

open, and the air of the cavity of the tympanum may then be renewed. Air

may at will be made to enter through this tube, by closing the mouth and

nose, and then trying {222} to force air through the latter. When this is

done, a distinct crackle or clicking sound is perceived, due to the

movement of the membranes, and of the little bones of the ear.

[Sidenote: 99. What other purpose does the Eustachian tube serve? How is

this shown? "Throat-deafness?" Primary use of the Eustachian tube?]

99. The Eustachian tube serves, also, as an escape-pipe for the fluids

which form within the middle ear; and hence, when its lining membrane

becomes thickened, in consequence of a cold, or sore throat, and the

passage is thus more or less choked up, the fluids are unable to escape as

usual, and therefore accumulate within the ear. When this takes place, the

vibrations of the membrane are interfered with; the sounds heard appear

muffled and indistinct; and a temporary difficulty of hearing, which is

known as "throat-deafness," is the result. This result resembles the effect

produced by interrupting the vibrations of a sonorous body, such as all are

familiar with; if the finger be placed upon a piano-string or bell when it

is struck, the proper sound is no longer fully and clearly emitted. But the

primary use of this tube is to afford a free communication between the

middle ear and the external atmosphere, and thus secure an equal pressure

upon both sides of the membrane of the drum of the ear, however the density

of the atmosphere may vary. If, from undue tension of the membranes, pain

is experienced in the ears, when ascending into a rare atmosphere, as in a

balloon, or descending into a dense one, as in a diving-bell, it may be

relieved by repeating the act of swallowing, from time to time, in order

that the inner and outer pressure may thus be promptly equalized.

[Sidenote: 100. The essential part of the organ of hearing? Its location?

Formation?]

100. THE INTERNAL EAR, OR LABYRINTH.--The most essential part of the organ

of hearing is the distribution of the _auditory nerve_. This is found

within the cavity of the internal ear, which, from its exceedingly tortuous

shape, {223} has been termed the _labyrinth_ (see Fig. 53, C). This cavity

is hollowed out in dense bone, and consists of three parts; the _vestibule_

(_a_), or ante-chamber, which is connected with the other two; the

_cochlea_ (_b_), or snail's shell; and the three _semicircular canals_

(_c_). The manner in which the nerve of hearing is distributed is

remarkable, and is peculiar to this nerve. In the vestibule and the canals

its fibres are spread out over the inner surface, not of the bony cavity

but of a membranous bag, which conforms to and partially fills that cavity;

and which floats in it, being both filled and surrounded with a clear,

limpid fluid.

[Sidenote: 101. Where is the "ear-sand" found? Give the theory as to its

use.]

101. A singular addition to the mechanism of hearing is observed within

this membranous bag of the labyrinth. This consists of two small oval

ear-stones, and a quantity of fine powder of a calcareous nature, which is

called "ear-sand." When examined under the microscope, these sandy

particles are seen to lie scattered upon and among the delicate filaments

of the auditory nerve; and it is probable, that as the tremulous sound-wave

traverses the fluid of the vestibule, the sand rises and falls upon the

nerve filaments, and thus intensifies the sonorous impression.

[Sidenote: 102. In the cochlea or snail's shell? "Key-board" in the

internal ear? The vestibules? Semicircular canals?]

102. In the cochlea, or snail's shell, which contains the fluid, but no

membrane, the nerve ramifies upon a spiral shelf, which, like the cochlea

itself, takes two and a half turns, growing continuously smaller as it

winds upward. As many as three thousand nerve fibres of different lengths

have been counted therein; which, it has been thought, form the grand, yet

minutely small key-board, upon which strike all the musical tones that are

destined to be conveyed to the brain. The vestibule, it is also supposed,

takes cognizance of noise as distinguished from musical sounds; while the

office of the semicircular canals is, in part at least, to prevent internal

echoes, or reverberations. {224}

[Sidenote: 103. With what does the vestibule communicate? What is the

theory by which sound is conducted to the brain?]

103. The vestibule communicates with the chain of bones of the middle ear

by means of a small opening, called the "oval window," or _fenestra

ovalis_. Across this window is stretched the membrane, which has already

been alluded to as being joined to the stirrup-bone of the middle ear.

Through this window, then, the sound-wave, which traverses the external and

middle ear, arrives at last at the labyrinth. The limpid fluid which the

latter contains, and which bathes the terminal fibres of the nerve of

hearing, is thus agitated, the nerve-fibres are excited, and a sonorous

impression is conducted to the brain, or, as we say, a sound is heard.

[Sidenote: 104. The formation of the organ of hearing with a view to its

protection?]

104. PROTECTION OF THE SENSE OF HEARING.--From what has been seen of the

complicated parts which compose the organ of hearing, it is evident that

while many of them possess an exquisite delicacy of structure, Nature has

well and amply provided for their protection. We have observed the

concealed situation of the most important parts of the mechanism of the

ear, the length of its cavity, its partitions, the hardness of its walls,

and its communication with the atmosphere; all these provisions rendering

unnecessary any supervision or care on our part in reference to the

interior of the ear. But in respect to its external parts, which are under

our control and within the reach of harm, it is otherwise. We may both

observe the dangers which threaten them, and learn the means necessary to

protect them.

[Sidenote: 105. Danger to which the hearing may be subjected? Advice?]

105. One source of danger to the hearing consists in lowering the

temperature of the ear, especially by the introduction of cold water into

the auditory canal. Every one is familiar with the unpleasant sensation of

distension and the confusion of sounds which accompany the filling of the

ear with water when bathing: the weight of the {225} water within it really

distends the membrane, and the cold chills the adjacent sensitive parts. It

is not surprising, therefore, that the frequent introduction of cold water

and its continued presence in the ear enfeeble the sense of hearing. Care

should be taken to remove water from the ear after bathing, by holding the

head on one side, and, at the same time, slightly expanding the outer

orifice, so that the fluid may run out. For a like reason, the hair about

the ears should not be allowed to remain wet, but should be thoroughly

dried as soon as possible.

[Sidenote: 106. The general rule as to the use of water for the ear?]

106. It may be stated as a general rule, to which there are but few

exceptions, that no cold liquid should ever be allowed to enter the ear.

When a wash or injection is rendered necessary, it should always be warmed

before use. The introduction of cold air is likewise hurtful, especially

when it pours through a crevice directly into the ear, as it may often do

through the broken or partially closed window of a car. The avoidance of

this evil gives rise to another almost as great; namely, the introduction

of cotton or other soft substances into the ear to prevent it from

"catching cold." This kind of protection tends to make the part unnaturally

susceptible to changes of temperature, and its security seems to demand the

continued presence of the "warm" covering. As a consequence of its

presence, sounds are not naturally conveyed, and the sensitiveness of the

nerve of hearing is gradually impaired.

[Sidenote: 107. Chief source of injury to the ear? Directions for removing

foreign objects from the ear? Of a live insect?]

107. The chief source of injury, however, to the ear is from the

introduction of solid substances into the auditory canal, with the design

of removing insects or other foreign objects that have found their way into

the ear; or with the design of scraping out the ear-wax. For displacing a

foreign object, it is usually sufficient to syringe the ear gently with

warm water, the head being so held that the {226} fluid easily escapes. If

a live insect has gained entrance to the ear, it may first be suffocated by

pouring a little oil upon it, and afterward removed by syringing the ear as

just mentioned.

108. The removal of ear-wax is generally unnecessary; for, as we have

before seen, Nature provides that the excess of it shall become dry, and

then spontaneously fall out in the form of fine scales. The danger from the

introduction of solid implements into the outer ear is chiefly found in the

fact that the membrane which lies at the bottom of it is very fragile, and

that any injury of it is liable to be permanent, and to permanently impair

the hearing of the injured ear.

QUESTIONS FOR TOPICAL REVIEW.

_Give as full statements as you can on the following subjects_:

1. Production of sensation 177, 178

2. Variety of sensations 178, 179

3. General sensibility 179, 180

4. The sensation of pain 180

5. The uses of pain 180, 181, 182

6. Special sensation 182, 183

7. Organs of touch 183, 184

8. The sense of touch 184, 185, 186

9. The delicacy of touch 186, 187

10. Sensations of temperature and weight 187, 188

11. The organ of taste 188, 189

12. The sense of taste 189, 190

13. Relations of taste, etc. 190, 191

14. Influence of education, etc. 191, 192

15. The sense of smell 192, 193

16. The nerve of smell 193, 194

17. Uses of the sense of smell 194, 195

18. The sense of sight 196, 197

19. Light, and the optic nerve 197, 198

20. The organ of sight 198, 199

21. The orbits 199

22. The eyelids 200, 201

23. The lachrymal fluid 201, 202, 203

24. The eyeball 203, 204

25. The iris 205, 206

26. The retina 206, 207, 208, 209

27. The crystalline lens 209, 210

28. Uses of the lens 210, 211, 212

29. Long and short sight 212, 213

30. Function of accommodation 213, 214

31. Old sight, or presbyopia 215

32. Hearing and sound 215, 216, 217

33. The external ear 217, 218, 219

34. The middle ear 219-222

35. The internal ear 222, 223, 224

36. Protection of the sense of hearing 224, 225, 226

* * * * *

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