CHAPTER VIII

SENSATION--INTERNAL AND CONTACT SENSES

We have talked a good deal about muscles and the different sorts of

activities they can perform. We have also mentioned the fact that the

skeletal muscles are under accurate nervous control. Our next task is to

investigate the control of this nervous control; in other words to find

out just what it is that causes the nerves to stimulate the muscles so

that they shall perform as skillfully and usefully as they do. In

Chapter II we saw that our bodily movements are adjusted to our needs

through the sense organs. These bring information of the situation and

we act accordingly. We may group the kinds of information which the

sense organs furnish under three heads; first, what is going on inside

our bodies; second, what is happening at the surface of the body, and

third, what is happening at a distance from us. The senses which bring

the first kind of information are called the _internal_ senses; the

second group are the _contact_ senses; and the third are the _distance_

senses. We need to remember that the primary purpose of the senses is to

guide our muscles, and that our muscles are to find food for us, to keep

us from bodily harm, and to assist in the perpetuation of life by

propagating and caring for the young. By keeping these facts in mind we

shall have no difficulty in understanding the way in which the various

senses do their work.

Pain, hunger, and thirst are the internal senses with which we are most

familiar. Pain is evidently a protective sense. It is never aroused

unless something is amiss; for that reason pain should never be

neglected. Of course, in the majority of cases the pain is due to some

simple disturbance which can be located, and if no permanent harm is to

follow, or if no relief is possible, the heroic bearing of the pain is

meritorious; but thousands of women, thinking mistakenly that to

complain of suffering is a sign of weakness, or hoping to spare loved

ones distress, bear in secret or make light of pains that are the signs

of insidious disease, curable if taken in hand early enough, but sure to

cause acutest suffering and untimely death if allowed to go on

unchecked. Unfortunately our most dangerous internal enemies, the

organisms of infectious disease, do not give warning of their attack by

causing pain until the disease itself is so far advanced that there is

no escaping it. In this respect pain falls short of being efficient as a

means of warning us against impending injury.

Hunger and thirst are the stimuli which drive us to the taking of food

and water. It is interesting to think that of all the living things that

roam the earth only men have discovered the connection between the

taking of food and the avoidance of starvation; all other animals are

impelled to nourish themselves wholly through the operation of these

senses. There are two distinct phases to hunger. The first is appetite,

and this by itself seems not to be a sense in the strict meaning of the

word, but rather a memory of agreeable experiences associated with the

taking of food. In man appetite is often sufficient by itself to lead to

eating, as is proved by the frequency with which food is eaten between

meals when there cannot possibly be any genuine hunger, but probably in

animals it acts to arouse genuine hunger, rather than to cause eating by

itself. Genuine hunger is a sense as definite as any other. It is

aroused by spasmodic contractions of the stomach. These contractions

cannot occur except when the wall of the stomach is in a certain state

of tension. Various things can influence the coming on of this degree of

tension in the stomach, and so the possibility of hunger. Appetite

itself probably does this very effectively. Habit seems also to have

something to do with it. Hunger is usually felt just as mealtime draws

near, and it is often much keener at noon or night than before

breakfast, although the stomach has been longer empty at breakfast than

at any other meal. A curious fact about hunger is that it may disappear

completely after a few days of complete starvation. Contrary to the

popular idea that hunger becomes more and more acute as starvation

continues, the testimony of practically all persons who have starved for

more than a few days is that all sensations of hunger, as well as all

strong longings for food, subside and do not return. This is especially

true if the body is kept quiet and if the mind is diverted, so that

recollection of meals particularly enjoyed shall not come up.

Thirst is due to actual drying of the throat. When the cells lining that

region become deficient in moisture the sense is aroused. The drying may

occur from without or from within. When it occurs from without, as in

sleeping with the mouth open, relief can usually be obtained by merely

swallowing saliva copiously. The same treatment helps for the moment

when the lack of moisture is due to deficiency in the amount in the

body, but in this latter case no permanent relief can be had except by

the taking of water. When the amount in the body falls below the proper

level no comfort can be had until the loss has been made good. An

interesting thing about thirst is that it is the only sense which is

said never to be lost or seriously impaired by disease.

In addition to these familiar internal senses we have some that are less

well known. They are for the purpose of what may be described as the

routine guidance of the muscles. The act of walking, as we well know, is

made up of a series of muscular movements which are both accurately

timed and accurately graded. We obtain startling realization of this

when we come to the bottom step on our way down stairs without noticing

that we have arrived there. This timing and grading are done for us by

our bodies without our having to attend to it. The amount of labor that

is saved is shown by walking upon railroad ties. These are irregularly

spaced, and on that account it is necessary for us to pay attention to

every step. There is no comparison between the fatigue of this kind of

walking and ordinary progress along a smooth path. The senses that keep

track of the position of the body and of individual muscles are known

respectively as the equilibrium sense and the muscle-and-joint sense.

The equilibrium sense has as its organ a part of the internal ear.

Deeply imbedded in the bone is a series of chambers and canals lined

with a delicate membrane and filled with liquid. The canals, which are

three in number in each ear, are semicircular in shape, and accordingly

have been named the _semicircular canals_. One of them is horizontal;

the other two are vertical, and the two vertical canals lie at right

angles to one another. This arrangement makes it inevitable that any

movement of the head, in any direction whatsoever, will register

differently on the canal system than any other movement, which is

exactly what is required to make the apparatus efficient as an organ by

which motions of the body are kept track of and guided. Along with the

semicircular canals is a structure known as the _vestibule_ which

registers the position of the head, and so indirectly of the body, when

no movements are being made. We are not ordinarily conscious of the

working of these senses; they carry on their guidance of muscular

movement without our attention. We can, however, pay attention to what

they show if we wish. For example, one who is swimming under water is

never in doubt as to whether his head is turned up or down, even though

his eyes may be shut. His knowledge of position in such a place is

obtained from his equilibrium organ, even though he may not be aware of

the fact. Sometimes the organ becomes diseased. The results, so far as

the victim is concerned, are highly distressing. He usually has to stay

in bed because he cannot balance himself well enough to get about.

The organs for muscle-and-joint sense consist of tiny spindles

distributed around the joints and embedded within the mass of the

muscles. They are arranged so as to be affected by every motion of a

joint or every contraction of a muscle. They register not only the fact

of motion but also the extent. There is a disease, commonly known as

locomotor ataxia, in which the muscle-and-joint sense is impaired or

lost, particularly in the legs. The result is that walking becomes

difficult and unsteady, and usually impossible when the eyes are shut or

the room is dark. This is because the victim learns to make his sight

serve instead of his muscle-and-joint sense for guiding his muscular

movements, and when this also is withdrawn all knowledge of where his

legs are or what they are doing fails, and the only course is to fall

down or lie down as quickly as possible.

We have some additional bodily sensations, such as nausea, repletion,

fatigue, ill feeling or _malaise_, which guide our conduct more or less,

and are not very different in consciousness from hunger or thirst. So

far as is known there are no sense organs by which these sensations are

aroused. They are not strictly senses, therefore. We do not know enough

about how they originate to say anything more about them.

The _contact senses_ are touch, warmth, cold, and taste. Pain that comes

as the result of bodily injury might also be classified as a contact

sense, since its cause is something that comes in direct contact with

the body from outside, but it differs from internal pain only in its

source and not at all in the sensations it arouses, so there is no need

of describing it over again. The sense of touch is the fundamental

sense; the very lowest animals, even those that have no specially

developed sense organs, and few organs of any kind, react to the contact

of objects with their bodies just as the highest animals react to the

sense of touch. When no other information is available, that of simple

contact guides the animal in its securing of food and its avoidance of

harm. In accordance with this primitive character of the touch sense,

the psychologists tell us that we interpret the information from our

more highly developed sense organs, sight particularly, in terms of the

feel of objects. When we look at anything our judgment of it actually

consists in an idea of how it would feel if we were to take hold of it.

Our touch organs consist of tiny spots scattered all over the surface of

the body. They are much closer together on some parts than on others.

The total number is estimated at a half million or more. A good way to

test their sensitiveness is by pressing down on different parts of the

skin with fine hairs. When this is done it is found that the most

sensitive regions--the tip of the tongue, for instance--are fifty or

sixty times as sensitive as the dullest regions, like the small of the

back. To obtain sensations of touch it is necessary that there be

unaffected points alongside those that are affected. If all are acted on

alike, there will be no more sensation than if none is acted upon. This

can be shown by dipping the hand into quicksilver. The very heavy liquid

presses on all the touch points hard enough to affect them, but since it

presses on all alike nothing at all can be felt except along the line

where the hand enters the quicksilver where the pressure is strongly

marked. It is this feature of the touch sense that makes the wearing of

clothing bearable. If we had to feel the contact of the clothes

constantly we should presently find them so trying that we could no

longer endure them. We do feel rough places and are often seriously

annoyed by them, so we can judge what would be the effect if the whole

surface were felt as plainly.

Closely related to touch is the sensation of tickling or itching.

Curious facts about this sensation are the violence of the feeling that

may be aroused by very delicate irritation, drawing a thread along the

corner of the nose, for example; the persistence of the feeling beyond

the actual irritation; and the effectiveness of scratching as a means of

alleviating the condition. Almost nothing is known in explanation of any

of these peculiarities.

In addition to organs of touch the skin contains two kinds of organs for

perceiving differences of temperature. The first of these detects

warmth; the second cold. It is by means of these senses that we judge

whether the place where we are is of a suitable temperature in which to

remain; whether we should be quiet or active; whether special

provisions, like changes in the clothing, are necessary. In the case of

both senses the temperature of the skin is the comparison point. We

judge that an object is warm or cold according as its temperature is

above or below that of the skin which touches it. The ears are usually a

few degrees cooler than the hands; thus it is possible for one and the

same object to feel cold to the hands and warm to the ears. The two

kinds of temperature organs are side by side in the skin, although there

are many more “cold” spots than “warmth” spots. Very warm objects affect

both kinds, and then we get the sensation that we call “hot,” as

distinguished from merely warm. The cold spots are a little nearer the

surface of the skin than are the warmth spots; for this reason a hot

bath may feel cold at the very instant of stepping into it, although the

sensation changes to hot almost at once. We need to remember that our

sensations of warmth or cold depend altogether on the state of the skin,

and tell us nothing at all about whether our bodies as a whole are warm

or cold. Because the blood is always warm a flushed skin always feels

warm, and to produce flushing by means of alcohol has long been used as

a means of making the body feel warm and comfortable. This may be a

serious mistake in cold weather, for to drive the blood to the surface

then may mean that the body as a whole will cool off to the point of

actual injury. It is better to feel cold and conserve the body’s heat

than to feel warm and waste it.

[Illustration: TASTE BUDS

(From Martin’s “Human Body”)]

The last of the contact senses is that of taste. This is found only on

the tongue. Scattered about on that organ are many tiny sense organs

known as taste buds. These are usually in little hollows, so they cannot

be affected unless liquids which can enter the hollows are on the

surface of the tongue. If the tongue is wiped dry and then dry sugar is

sprinkled on it, no sweet taste will develop so long as the dryness

continues. The purpose of taste is evidently to give final information

about the food after it has passed the inspection of the other senses

and has been inserted into the mouth, but before it is swallowed. In the

higher animals there has been a subdivision of this sense into two. The

other is the sense of smell. In large part smell is a distance sense,

and will be treated when we are talking about the distance senses. Smell

has monopolized most of the properties of food-judging, so there is left

for taste proper only four kinds of perception. These are sweet, sour,

salty and bitter. We have, apparently, four kinds of taste buds, one for

each of these kinds of taste. All the other sensations that we call

taste are flavors, and are really smells. Of the four tastes sour and

bitter would probably be called warning and sweet and salty

recommending. Only by practice do we come to care for bitter foods, and

children usually object just as strongly to those that are sour.

Tropical savages, for whom salt is a rarity, esteem it much more highly

than sugar, which they can usually get in abundance.

In concluding this chapter we need to remember that the contact senses

make up the court of last resort; by the time anything comes close

enough to the body to act upon any of them it is so close that the

effect in guiding the muscles must be immediate; there is no time for

deliberation; whatever the muscles are going to do in response to

information thus obtained must be done at once. Later we shall see how

this affects our whole bodily make-up.

CHAPTER IX

SENSATION--DISTANCE SENSES

The three senses that give us information of what is happening beyond

the surface of our bodies are smell, hearing, and sight. Since smell is

closely related to taste, which was talked about in the last chapter, we

shall take it up first. Smell is like taste in that it is aroused by

chemical substances, but to be smelled these must be in gaseous form,

not dissolved in water, as for taste. The organ for smell is in the

upper part of the nasal chamber. There are really two of them, one in

each nostril. They are made up simply of little patches of mucous

membrane, as the membrane that lines the nose is called, in which are

many of the particular kind of cells that are affected by odors. An

interesting thing about these patches is that they are not in the part

of the nostrils through which the main current of air sweeps in

breathing, but in a little pocket off this main channel. If air

containing an odorous substance is breathed in or out, a little of it

works its way into the side pocket and is smelled. If we wish to get

more of the odor we do it by sniffing, which is changing the shape of

the nostrils to throw the air current more directly against the smell

organs.

These organs are amazingly sensitive. It is hard to appreciate the

minuteness of the amounts of material that can be smelled. Especially is

this true of those animals that have a really keen sense of

[Illustration: Photo, Fifth Avenue Hospital

A NEW HOSPITAL DESIGNED TO SUPPLY THE MAXIMUM OF LIGHT AND AIR--CHIEF

REQUISITES FOR HEALTH--TO EACH ROOM]

[Illustration: Copyright, Fifth Avenue Hospital

THE ARCHITECT’S PLAN WAS MADE WITH THE NEED FOR LIGHT AND AIR IN MIND]

smell. The amount of substance that rubs off from a rabbit’s feet onto

the ground at each step cannot be much to begin with; yet this continues

for hours to give off gas into the air, and a dog coming along at any

time meanwhile will get enough of the gas into his nostrils to smell it.

Fortunately for our comfort the sense of smell fatigues very rapidly. An

odor that is excessively disagreeable at first presently no longer

troubles us. If it were not for this, it would be almost, if not quite,

impossible to obtain laborers in those industries where the odor is

necessarily bad. There is, however, a source of danger in this quick

subsidence of smell perception. About our only method of judging offhand

as to the ventilation of a room is by the smell, and this fails as a

guide when we have been in the room for a time. Persons coming in from

outside are often struck by the bad state of the air in rooms whose

occupants are not conscious that there is anything amiss. Because of

this the ventilation of schoolrooms usually is not, and never should be,

left merely to the judgment of the teacher, but definite rules are laid

down as to opening of ventilators or windows.

When the gas that is smelled is part of an inward air current we

recognize it as coming from the outside and call it an odor; when it is

part of an outward current we call it a flavor. On account of the rapid

fatigue of the sense of smell we are unconscious of the smell of our own

breath, but can get fresh smells from within, and these come,

practically always, from materials that have just been taken into the

mouth. In comparison with taste flavor furnishes great variety of

perception. As persons become connoisseurs in food their enjoyment

depends more and more on flavor and less and less on taste. The

sensation from spices is a combination of flavor with irritation of the

tongue that is partly pain and partly touch. The sense of smell has

evidently a two-fold use; it makes us aware that there is food in the

vicinity, or sometimes that disagreeable things are near at hand; and it

shares with taste the duty of enabling us to judge of the food as it is

being eaten. Agreeable tastes, flavors, and odors add much to the

enjoyment of life. Within reasonable limits it is well to cultivate this

kind of enjoyment, for while there is no doubt that it can be overdone,

as in the excessive lengths to which the decadent Romans went to gratify

their taste and smell, neither is there any doubt that bodily health in

general, and the bodily function of digestion in particular, benefit

definitely from the kind of enjoyment that savory food and delightful

odors bring. It is the duty of those charged with the responsibility of

preparing and serving food to take pains that full advantage is taken of

the possibilities present in what food is to be prepared. This does not

mean expensive food; what it does call for is skillful handling of all

food, whether cheap or costly.

In introducing the subject of hearing we shall have to say a few words

about that which is heard, namely sound. Any object that has any degree

of elasticity at all is apt, if struck or rubbed or otherwise set

suddenly in motion, to start vibrating back and forth; the vibrations

will nearly always be regular, and will occur at a rate that is the same

for that particular object whenever it vibrates. The rate depends on the

size, the character, and the degree of stretch of the object. Air is to

all intents and purposes perfectly elastic; it is set vibrating by any

object that is vibrating in it, but since it has no particular size nor

degree of stretch it takes the vibration rate of the object that started

it going in the first place. The vibrations once started in air spread

in all directions, just as waves spread from a stone thrown into a pond,

and when these air waves strike upon another object that is free to

vibrate they will set it going at the same rate. The human hearing

apparatus is a device which is set in vibration by air waves, and the

result is called sound. The ear is limited in its ability to respond to

vibrations; they must be neither too fast nor too slow; if slower than

16 a second, most people will fail to hear them, and the same is true if

they are more rapid than about 40,000 a second. Between these limits

vibrations that strike upon the ear are heard as sounds.

Differences in vibration rate between one sound and another can be

recognized by the ear; the difference is a matter of pitch. By the pitch

of a tone we mean the vibration rate which it has. More rapid vibrations

give tones of higher pitch; a slow rate means a low pitch. Middle C on

the piano has a rate of either 256 or 261 a second according to the

system used by the tuner. The human voice has an extreme range starting

with the lowest note that the bass voice can compass with a rate of

about 80 vibrations a second, to the highest note that famous sopranos

can attain at about 1,400 a second. There is a record of a singer who

could achieve a tone with a rate of 2,100 a second, but this has not

been duplicated so far as is known. Of course no single voice can cover

more than a fraction of this range. Most men produce all their tones at

rates of between 90 and 500 a second, and women between 200 and 800 a

second. Not every different vibration rate is heard as a tone of

different pitch; the ear is not sensitive enough for that. The interval

between one note and the next includes several vibrations, more the

higher one goes in the scale. A perfectly true tone has exactly the rate

called for; a departure of one or two vibrations a second may not be

noticed, but if the error is greater the singer is sharping or flatting

his tone, according as he is above or below the true rate. A note that

has just double the rate of another one is said to be its octave. For

convenience the interval of the octave has been split up into twelve

tones, and all our music is constructed on that basis.

It is evident that the ear must be a very complicated organ; not only

must it perceive differences in pitch, as just indicated, but

differences in loudness must also register differently. More than that,

the ear has to be able to deal with sounds made up of a great many tones

coming into it all at once. When we listen to an orchestra or band, the

waves that strike our ears represent the commotion set up in the air by

all the instruments together. It is a remarkable fact that in this case,

instead of getting a meaningless jumble, we actually get a blend of

tones from which, if we are sufficiently musical, we can pick out the

individual elements.

The ear consists, in the first place, of a vibrator that will respond

accurately to any vibration rate or combination of vibration rates

within its range, and secondly of a sensitive apparatus that is acted

upon by the vibrator. The vibrator must respond freely to feeble

impulses, and, what is of prime importance, to any vibration rate as

readily as to any other. Almost all elastic bodies have a preferred

vibration rate; that is, they will respond better to some rates than to

others. About the only exception to this rule

[Illustration: DIAGRAM OF THE EAR

_A_, auditory canal, leading to the eardrum _B_; _C_, cavity of the

middle ear, communicating by the Eustachian tube with the throat _D_ and

containing the ear bones; _E_, semicircular canals; _F_, true hearing

organ; _G_, auditory nerve. (“The Human Mechanism,” by Hough and

Sedgwick.)]

is in the case of membranes that are not tightly stretched. A stretched

membrane, like a drumhead, has its own vibration rate, but one that is

not on the stretch is able to vibrate at almost any rate. This fact is

taken advantage of in the telephone and the phonograph, both of which

depend on being able to vibrate at various rates almost equally well. In

the ear, also, an unstretched membrane is the vibrator. We are familiar

with it as the eardrum. It is located at the bottom of the ear canal,

but cannot be seen by looking therein, because of a slight curve at

about the middle. When the ear specialist wants to examine the eardrum

he thrusts a small metal tube into the canal. This straightens it out

enough to bring the drum into view.

The eardrum does not act directly upon the sensitive hearing apparatus,

but its vibrations are transmitted across a space known as the middle

ear. The necessity for this space is found in the fact that atmospheric

pressure is not constant, but changes frequently from day to day,

besides falling off as one ascends higher above sea level. The free

action of the eardrum depends on its not being stretched; if there were

no means of readjustment it might be properly set for one air pressure,

but greater pressures would bulge it inward, putting it on the stretch,

and so cutting down its ability to respond to a wide range of tones. The

middle ear, which is the space behind the drum, connects with the

outside air by a tube leading from it to the back of the throat, which

latter communicates freely with the air through the nose, as well as

through the mouth whenever it is open. The tube is known as the

Eustachian tube. Its walls are ordinarily collapsed, so it is not an

open passage, but every time one swallows the tube is pulled open, thus

allowing differences in air pressure on the two sides of the eardrum to

equalize. Whenever one ascends a high hill quickly, as by train or

automobile, or even in going to the top of a high building by elevator,

the difference in air pressure behind and in front of the eardrum can be

felt. The sensation is disagreeable, and there is definite impairment of

hearing. Repeated swallowing gives relief.

The vibrations of the eardrum are transmitted across the space of the

middle ear by a chain of three tiny bones; these are very irregular in

shape, and are attached to one another in such a way that every

movement of the drum is followed exactly as to time and direction, but

with reduced size and increased power. The hearing apparatus, which is

part of the internal ear, but not the same part as makes up the

equilibrium organ, contains liquid which is set moving by the last of

the chain of bones, and this liquid acts upon the actual sensitive cells

which make up the sense organ proper. These latter are arranged in an

exceedingly complicated fashion. There are various theories as to the

precise manner in which the vibrations of the liquid in the inner ear

arouse the sensitive cells. It is thought that different cells respond

to tones of different pitch, but exactly how this is accomplished is not

known.

Deafness may result from failure of the sensitive inner ear to respond,

or from poor transmission of vibrations across the middle ear by the

chain of bones, or by interference with the freedom of action of the

eardrum, or by preventing the air waves from striking upon the drum.

Injury to the inner ear is rare, because of its secure position within

the bone. A common form of deafness is the result of hardening of the

connections between the ear bones, so that the chain no longer follows

well the vibrations of the drum. This usually begins to come on during

the twenties or thirties, and causes almost complete deafness by the age

of fifty. In most if not all cases it is hereditary. Interference with

the action of the eardrum may be due to the partial destruction of the

drum itself. Scarlet fever and measles are particularly likely to leave

the drum in a delicate condition, and any strain upon it then may

rupture it beyond repair. Continuous closing of the Eustachian tube, by

preventing equalization of air pressure on the two sides of the drum,

causes partial deafness. Inflammation accompanying a cold may cause

this, or the growth of adenoids in the back of the throat. Adenoids will

be described later; here they are mentioned only because they may press

upon and close the Eustachian tubes. There is danger from the closure of

the tubes by inflammation, because the inflammation may creep up to the

middle ear and cause serious trouble, both there and in the mastoid

region adjoining. Earache in children should be carefully watched, since

it usually means that the middle ear has been invaded by the same

inflammatory condition that is present in the throat in colds, and may

do serious damage to the delicate structures there. Often in children,

and sometimes in adults, the hearing is impaired by the accumulation of

wax in the ear canal. This wax is a sticky secretion that serves to

catch particles entering the ear canal and to prevent them from striking

against the drum. Unless the ear canals are washed out frequently with

_hot_ water the wax accumulates and hardens into a plug which closes the

ear canal, shutting off faint sounds. The wax dissolves in hot water,

but not in cold, so its accumulation is to be prevented by taking pains

that hot water actually gets to the bottom of the ear canal once in a

while. Digging the wax out with hard instruments should be done only by

an expert with greatest caution, lest the drum be injured.

The last of the senses to be described is sight; this is the one of

which we make the most use ordinarily, and curiously enough is the only

one that we can turn on and off. Loud noises or penetrating smells must

be endured, but by shutting our eyes tightly we can escape sight

whenever we want to. Altogether there are three kinds of information

which the sense of sight brings to us; the first is the knowledge

simply of light and darkness; the second is the knowledge of the shape

and size of objects; the third is the knowledge of color. Nearly all

animals seem to have some power of distinguishing between light and

shade. Even the one-celled kinds, that have no eyes or anything

corresponding to eyes, behave in a way that proves them to have this

power. A lot of them can be put in a dish of water that is well lighted

on one side and in shade on the other and in a few minutes all of them

will be found to have traveled to one side or the other according as

they happen to be a light-seeking kind or a dark-seeking species. As we

go higher up the animal scale we find that parts of the body show this

same power of distinguishing light from darkness. In the case of the

common angleworm, or earthworm as it is more properly called, the front

end has the power but the rear end has not. In very highly organized

animals only the special organs known as eyes possess the power; all the

rest of the body has lost it. The next feature of sight, the ability to

perceive the shape and size of objects, requires a special apparatus,

the eye, so animals that lack eyes cannot perceive objects, although

they may be able to tell light from darkness.

In order to see an object it is necessary that a pattern or image of it

be thrown on a sensitive surface; this surface registers the details of

the pattern, and so the object is seen. What we have to do here is to

find out how these patterns or images are formed in our eyes. In the

first place we must realize that every visible object has rays of light

going off from every part of its surface in every direction. So-called

_self-luminous_ objects, like lamps or the sun, produce the light

within themselves; all others merely reflect light that falls on them

from some source. Whenever light falls on any object, unless it is a

perfect mirror, part is absorbed by the object and the rest is

reflected; that which is reflected presently strikes against another

object and is again in part absorbed and in part reflected; as this

process is repeated over and over again the light becomes so broken up

that rays of it are traveling in every direction from every point on

every object. Any spot so protected that no rays can strike it evidently

cannot reflect any out again, and such a spot will be absolutely dark.

For the formation of an image of any object all that is necessary is

that some of the rays of light from every point on the object be caused

to fall in exactly corresponding positions on the image. The simplest

possible means of doing this takes advantage of the fact that rays of

light travel in straight lines. If we inclose an incandescent bulb in a

tight box with a round hole in one side of it, every spot on the

incandescent filament will be giving off light in every direction, but

all the light will be cut off by the box except that which happens to

have the direction which takes it out through the hole. From every

incandescent spot, then, there will be a beam of light in the form of a

cone escaping from the box through the hole. The tip of the cone will be

the incandescent spot; the slope of its sides will depend upon the size

of the hole. If a screen is placed in front of the hole, all these cones

of light will strike on it, and it will be illuminated in a pattern

which is made up of all the cones from all the incandescent spots which

make up the filament, but these will overlap so much that one cannot be

told from another. Now if the hole in

[Illustration: DIAGRAM OF A PINHOLE CAMERA

Showing how clear images can be formed by the use of a hole so small

that only pencils of light can pass through it to strike on the screen

at the back.]

the box is made small enough, a pinhole, in fact, and if the screen is

placed close to the hole, the cones of light from the different

incandescent spots become so narrow that when they strike the screen

they overlap scarcely at all, and what we get is a tiny spot of light on

the screen corresponding to every incandescent spot on the filament and

straight in line with it through the hole. Here we have exactly what we

have been talking about, namely a pattern or image of an object. The

image will be upside down, because those rays from the top of the bulb

that strike the tiny hole will be below on the outside, and those from

the bottom will be above. The same thing can be worked exactly in

reverse; we can place a box with a pinhole in it in front of any object

and get an image inside the box on the back; by placing a photographic

plate or film there an excellent picture can be taken. There is just one

reason why this scheme is not used in all cameras; that is that unless

the object is very brightly illuminated indeed the amount of light that

passes through the pinhole is not enough to affect the plate or film

except on long exposure. Perfect pictures can be taken with a pinhole

camera wherever long exposures are possible, or wherever the object

shines brightly enough. This difficulty is gotten around in the ordinary

camera by gathering up all the light in a wide cone from each spot on

the object and condensing it again on the plate or film. The image is

formed just as before, but now each spot on the image includes, not only

the beam of light that comes in a straight line from the corresponding

spot on the object, but in addition that in a wide cone surrounding the

straight beam. It is naturally brighter the wider the cone; which

explains why in poor light we open the diaphragm of the camera wider

than when the light is good; a brightly illuminated object will pass

enough light through a narrow opening, but a dimmer object must have as

wide an opening as possible in order that enough may get through.

The method of condensing the light in a spreading cone so that it shall

come back to a point again is by means of a lens; not only is this true

of cameras, but also of the eye; in fact everything that has been said

thus far about cameras applies perfectly to the eye. There is one thing

about the way in which light is brought to a point by a lens that makes

the formation of images by this method troublesome in comparison with

their formation in pinhole cameras. That is that the cone of light which

strikes the lens is condensed as an opposite cone on the other side, and

since the formation of an image requires that every point of the object

shall be reproduced as a _point_ in the image, there is only one place

where the image can come, which is where the tips of the cones of light

are. This place is spoken of as the _focus_. Unless the screen or film

is exactly there the image will be made up of overlapping circles of

light instead of points, and so will be blurred. In a pinhole camera the

cones are so small that they cannot overlap, so there is no one place

where the image is better than elsewhere; in other words, there is no

necessity of focusing. The chief reason that focusing gives trouble is

that the farther away from the lens the object is the closer to the lens

will be its image; hence if the field of view consists of several

objects at different distances they will not all focus at the same

level; the distant objects will focus near the lens; the near objects

farther back. In practice this trouble is met by using a thick lens,

which has a very short focus to begin with, so that a considerable range

of distances will be covered without serious blurring, and for finer

work by adjusting the distance between the lens and the back of the

camera, where the film or plate is, so that there shall be a good focus

of the particular object that is desired.

In the eye the clear part that projects between the lids, and is called

the _cornea_, is the important lens. Just behind is the arrangement that

corresponds to the diaphragm of the camera; this is the colored part

with a round hole in the center; it is called the _iris_ and the round

hole is the _pupil_. Behind the iris, and resting right against it, is

the secondary lens of the eye, known as the _crystalline lens_. The eye

as a whole is a globe just under an inch in diameter; at the back of it,

straight behind the lenses and pupil, is the sensitive surface upon

which images are formed. This is the _retina_; it extends pretty well

around to the sides, but the part we use most in seeing is the small

portion straight in line with the pupil. The cornea by itself is a lens

whose focus is

[Illustration: VERTICAL SECTION OF THE RIGHT EYE AND ITS LIDS

_c_, cornea; _l_, crystalline lens, its margin shielded by _i_, iris;

_p_, pupil; _r_, retina; _m_, muscles that move the eyeball; _o. n._,

optic nerve. (From “Human Physiology,” Stiles.)]

longer than the length of the eyeball, and the crystalline lens by

itself also has a long focus, but the two in combination give a focus

that just corresponds with the length of the eyeball, so that the images

of all objects at a distance of eighteen feet or more fall sharply on

the retina. Since near objects focus farther away from the lens than far

objects, the effect of this is to make objects nearer than eighteen feet

out of focus. For them a longer eyeball would be needed, and it would

have to become longer and longer the nearer the object was brought to

the eye. We all know that when we look at near objects we make an

adjustment in the eyes. This is known as accommodation; for a long time

it was supposed that _accommodation_ was actually secured by lengthening

or shortening the eyeball to bring the focus right, but we now know

that the eyeball does not change in shape when we accommodate. The same

result is secured by another means, namely, by letting the crystalline

lens bulge out and become thicker. It was stated incidentally a few

pages back that thick lenses have shorter focuses than thin. So when the

crystalline lens thickens it shortens the focus of the eye, and throws

the image forward. This will locate the image of near objects on the

retina, instead of behind it, as in the unaccommodated eye. The

crystalline lens is not stiff like glass, but rather like a thick jelly;

it is in a transparent bag, or capsule, which is fastened to the inside

of the eyeball all around, and the pull on the capsule stretches it out

pretty flat, making the lens thin. There are tiny muscles inside the

eyeball, known as the _ciliary_ muscles. These are so fastened that when

they contract they pull the eyeball forward and inward, loosening the

tension on the capsule of the lens. This then bulges out, taking up all

the slack. When we learn, as babies, to accommodate for near objects we

find out just how much the capsule must be loosened to give the proper

adjustment for any distance. As people get along in years the

crystalline lens often loses its elasticity so that it does not bulge

when the capsule is loosened. After this happens vision for near objects

is no longer clear. Relief is obtained by wearing reading glasses. These

are merely glass lenses worn in front of the eyes selected so that their

focus fits in with that of the lenses of the eye itself to bring the

image of objects at reading distance sharply on the retina. It is

usually necessary to replace these glasses from time to time as the

crystalline lens becomes stiffer and stiffer and ordinary accommodation

fails more and more.

There is a disease known as _cataract_ in which the crystalline lenses

become cloudy and finally completely opaque. Of course this means

blindness, since the passage of light to the retina is interfered with.

Relief is obtained by the simple expedient of removing the opaque lenses

bodily. This is possible merely because the crystalline lens is not the

chief lens of the eye. To be sure the cornea by itself will not focus on

the retina, but a glass lens can be placed in front of it which will add

itself to the cornea and the combined lenses will. There is no

possibility of accommodation in a case like this, so the patient has to

be furnished with bifocal lenses; the main part gives clear distance

vision; the lower section gives clear vision at the reading distance.

The patient has to get along with blurred vision in the regions between.

Not all eyes are exactly the right size so that distant objects shall

focus sharply on the retina. In fact a large proportion of them are

either too long or too short. It is clear that in an eyeball that is too

long the image of distant objects will fall in front of the retina, but

near objects that happen to be at just the right distance will focus

exactly on the retina. The distance at which this happens depends, of

course, on how much too long the eyeball is. Persons that have unduly

long eyeballs are, therefore, nearsighted. The condition is called

_myopia_. The correction for it consists in the use of lenses in front

of the eyes that instead of shortening the focus shall lengthen it.

Concave lenses, namely, those that are thick at the edge and thin in the

middle, will do this, and these are the kind that are worn by

near-sighted people.

When the eyeball is too short the image of distant objects falls behind

the retina, and of course that of near objects tends to fall farther

back yet. Since by

[Illustration: Photo, Fifth Avenue Hospital

SOFT, RESTFUL COLORS AND ATTRACTIVE SURROUNDINGS ARE CONDUCIVE TO GOOD

SPIRITS AND THEREFORE TO HEALTH]

[Illustration: Copyright, Paul Thompson

THE MAGNET IS HERE PUT TO THE UNUSUAL SERVICE OF REMOVING IRON FILINGS

FROM THE EYE]

accommodation the focus can be thrown forward, most persons with short

eyeballs can see distant objects clearly by accommodating for them, and

near objects that are not too near by extreme accommodation. For this

reason _hyperopia_, as this condition is called, is usually not

discovered until the person begins to feel the strain of the constant

accommodation that is necessary whenever the eyes are open. Eyestrain

usually shows itself in headaches; in fact, so large a proportion of

headaches come from this cause that anyone who suffers from them at all

frequently should have his eyes examined by a competent oculist. Relief

for hyperopia is by means of glasses that shorten the focus and thus

bring the image of distant objects forward to where the retina is in the

short eyeball.

[Illustration: EYE LENS--NORMAL AND DEFECTIVE

_A_, normal eye; _B_, myopic eye; _C_, hyperopic eye (From Martin’s

“Human Body”)]

There is one other defect of vision that is so common as to call for a

word; this is _astigmatism_. It is the condition in which the cornea is

not curved equally in all directions; the vertical curvature may be

greater or less than the horizontal. The effect is that points on

objects do not focus sharply as points in the image, but as little

elliptical spots. If one adjusts the accommodation so that the top and

bottom edges of objects are sharp the sides will be blurred and vice

versa. Usually the blurring is not great enough to be noticeable, but

only enough to make the person unconsciously dissatisfied with the

accommodation. He, therefore, constantly tries to improve it by changing

the tension of his ciliary muscles, and so brings on eyestrain. The

correction for this condition is glasses that are not equally curved in

all directions, but so selected that their less curvatures shall fit in

with the greater curvatures of the cornea. In fact, glasses that are

curved only in one direction are usually used, this curvature being just

enough to bring up the total curvature in that direction to equal the

other curvatures of the cornea. It ought not be necessary to give

warning that only competent persons should be allowed to examine and

prescribe for the eyes. Great skill is needed to determine accurately

just how far from correct the eyeball is, and unless this is known there

is no means except guesswork by which to decide on a prescription.

The third feature of vision is the perception of color. Color is to

light what pitch is to sound; that is, it depends on the vibration rate

of the light waves. Light, as already explained, is one of the forms in

which energy reaches the earth from the sun. Heat is another form. Both

are portions of a great energy stream to which we give the name of

_radiant energy_. This, as it comes from the sun, is made up of a

mixture of vibrations having almost every imaginable rate, except that

the slowest are many times faster than the highest pitched sound. At a

certain rate, and one that for these vibrations ranks as slow, the

energy is what we know as heat, and over a considerable range it

continues to be called heat; more rapid vibrations, and they are so

rapid that they have to be expressed in trillions per second, cause the

effect on our retinas that we call light. The slowest that we can see

give the sensation of red; the most rapid the sensation of violet; the

other colors of the rainbow, which in order after red are orange,

yellow, green and blue, are vibration rates between those that give red

and those that give violet. It will be noticed that only six colors are

given for the rainbow instead of seven. This is because there is not

enough real difference between blue and indigo to justify making them

separate colors. The distinction was made at the time when it was

supposed that there was something specially wonderful about the number

seven, which made it necessary that every important feature in nature

should show that number. We now know of no reason why seven should have

virtue over any other number. When all the vibration rates are mixed

together, as they are in the sunlight, the sensation is white. There are

other mixtures of colors that give white also, but they are not exactly

equivalent to the white of sunlight, as is proven when one tries to

match colors under artificial light. A great deal of labor has been

devoted to the attempt to get an artificial light that shall be

practically equivalent to sunlight, and only lately have good results

been obtained. When no light enters the eye the sensation is of black,

and it is worth while to note that so far as our sensations are

concerned black is as much a color as white or any other,

notwithstanding the fact that no light falls on the retina when the

black sensation is being felt. Contrary to ordinary belief, blind

persons who are blind because their eyes have been destroyed do not see

black all the time; they simply have no sensations at all from the eyes.

On the other hand, persons blind because of cataract do see black,

because in them the retinas are still present, but no light falls on

them.

The perception of color is very complicated, and not at all well

understood. Persons who do not have the same color perception as most of

us are called color-blind, and by learning some things about

color-blindness we shall best get an idea of color perception itself.

About four men in one hundred have defective color sense; the proportion

in women is only about a tenth as great. By far the commonest type is

one in which neither red nor green is seen correctly, but both are seen

as neutral tints, and in many cases look so much alike that the person

cannot tell one from the other. The practical importance of knowing

whether or not this defect is present is seen when we think that red and

green lights are used more than any other colors in signaling, so that

railroad men and others who work by signals must have normal color

sense. It has been discovered that even people that have normal color

vision are color-blind in the margins of the retina. This can easily be

demonstrated by bringing a red or green disk slowly around in front of

the eye of a person who, meanwhile, keeps looking straight ahead. He

will see the disk out of the corner of his eye some time before he can

tell what color it is. In fact, if a red or green disk is used, he

usually will not be certain as to the color until it is almost straight

in front. Blue or yellow disks can be told with certainty much farther

out, but even these colors are not perceived clear to the edge of the

field of vision. These experiments show that the retina becomes more and

more highly developed as an organ for perceiving color as we get closer

and closer to the center; at the extreme edges there is no color sense

at all, but only the primitive ability to tell light from darkness;

closer in blue and yellow are distinguished and not red and green; only

in the central part are all colors clearly seen.

In addition to the kinds of information which have been described thus

far, that the distance senses bring in, there is another kind, fully as

important as any in our actual use of our senses; that is information as

to the “direction from us” of the object or objects which are arousing

the sense. We can get this through all the distance senses, but much

more perfectly in the case of sight than in the others. We locate the

direction of objects that we smell by turning the head this way and

that, sniffing meanwhile, and noting the position in which the odor is

caught most clearly. Animals with a keen sense of smell, like dogs, can

locate directions very accurately by this means. In the case of hearing

the method is to turn the head until the sound is equally loud in both

ears. We would expect that a person who was hard of hearing in one ear

would never be able to locate sounds by this method; but, as a matter of

fact, such persons unconsciously allow for the difference in hearing in

the two ears, and so can judge the direction of sounds about as well as

any of us. Animals, like horses or rabbits, that have very movable outer

ears, undoubtedly can locate sounds much more accurately than we can.

Our outer ears are of almost no use in hearing; persons who have had the

misfortune to lose them hear practically as well as anyone.

[Illustration: DIAGRAM SHOWING HOW DISTANCE AFFECTS THE SIZE OF THE

IMAGE]

We locate directions with the sense of sight with perfect accuracy,

because unless the image of the object we are looking at falls on the

center of the retina it is not seen clearly. The only way to make the

image fall just there is to look directly at the object. The muscle

sense in the eye muscles is extremely delicate, so that if the eyes are

rolled at all in looking at anything we know it and can judge, also, how

much they are turned from the straight position. In this way we are able

to tell exactly the direction from us of any object we can see.

We can judge the _distance_ of a near object very accurately by noting

the degree to which the two eyes have to be turned in in order to see it

clearly with both. We are quite unconscious of this means of making the

judgment; all we know is that we can tell. It is easy to prove that it

depends on the two eyes by closing one and trying to make movements that

depend on accurate knowledge of distance. A good example is threading a

needle sideways. With both eyes open this can be done fairly easily, but

with one shut it cannot be done at all, except by chance. Objects so far

away that the eyes are not turned in perceptibly in looking at them are

judged as to distance wholly on the basis of their size. It is clear

that the actual size of any image on the retina will depend in part on

how large the object is, and in part on how far away it is. If an object

that we know to be large casts a small image on the retina, we conclude

that it is far away. It follows that unless there are some familiar

objects in view, judgments of distance are not at all trustworthy. A

good illustration is in looking up a bare hillside, and trying to

estimate the distance to the top. If, while this estimation is being

made, a man or horse suddenly comes into view at the top, the man or

horse will nearly always appear unexpectedly large, showing that the top

of the hill is not actually as far away as it was judged to be.

The possession of two eyes instead of one is an advantage to us in

another way, in addition to helping in the estimation of near distances.

This is in making objects appear solid, or in other words, in helping

the estimation of depth. When we look about us we have no difficulty in

realizing that some objects are near and others far, and that the

objects themselves have some parts that are nearer to us than other

parts. A great many things assist us in this realization. First and

foremost comes that which is known in art as perspective, namely the

tendency of distant objects or distant parts of objects to appear

smaller than those that are near. This can best be illustrated in the

case of parallel lines extending away from the eye, as when one stands

on a straight railroad track and looks along it. Although we know

perfectly well that the rails are the same distance apart all along, if

we were to believe our eyesight implicitly we should think that they

came gradually together. It is on account of this matter of perspective

that drawings of solid objects must show supposedly parallel sides

nearer together at the far end than at the near. Besides perspective

there are the shadows to be taken into account. Only solid objects cast

shadows, so if we see a shadow apparently cast by any object we

naturally conclude that it is solid. Both perspective and shadows can be

and are used by artists in making drawings and paintings look real. In

fact, they have almost no other means of doing this. As we all know,

even the cleverest paintings do not give an impression of depth equal to

that which comes from actually looking at solid objects. This is because

of help we get from the two eyes in the latter case. The reason for the

difference is that when we look at a solid object with both eyes the

view we get with one eye is not exactly the same as with the other; we

see a little farther around on the left side with the left eye, and on

the right side with the right eye. The combined view with the two eyes

gives us an impression of solidity that cannot possibly be had when the

view with the two eyes is exactly the same, as when we look at a

picture. The only way in which the impression of depth can really be

gotten in a picture is by using the familiar method of the stereoscope,

where two pictures are taken simultaneously by two cameras, placed a

little farther apart than the two eyes; and then the two are looked at

together through a special pair of prisms.

By means of the three distance senses, smell, hearing, and sight, we are

informed pretty completely as to what is around us. All three give an

idea as to the direction from us of objects; although sight does this

better than either of the others. Sight, also, lets us know accurately

as to the distance away of objects, provided they are fairly near. Smell

and hearing, as well as sight, may give us some idea as to the distance

of far objects, but only when we are dealing with familiar sensations. A

very faint smell, or faint sound, means a distant object provided we

know enough about the source to know that if the object were near the

sensations would be keener. Our judgment of distant objects by sight is

better than this, but not by any means perfect. When we contrast the

distance senses with the contact senses we see at once that the great

advantage coming from the possession of distance senses is in the time

that is permitted for action. In the chapter on contact senses we

emphasized the fact that the response to them must be immediate; there

is no time to pick and choose. When we can learn of the presence of

objects before they reach us, and something of their direction and

distance as well, we can usually take time to select the most fitting

course of action. We do not have to jump into a mud puddle to escape an

automobile if we see it soon enough. We shall learn in a later chapter

how this opportunity of choice is wrapped up with our development into

highly intelligent animals.

CHAPTER X

THE NERVOUS SYSTEM AND SIMPLE NERVOUS ACTIONS

In the second chapter we saw that to make our muscles act in accordance

with the information brought in by the sense organs some means of

communication between them is necessary; we saw, also, that this means

consists of the nervous system. Now that we have learned something about

both muscles and sense organs we are ready to look into the way in which

communication between them is carried on. First of all, we must realize

that living protoplasm does this. The nerve cells are alive and have

their basic metabolism just as do all other living cells. They also have

their functional metabolism; but in them, instead of taking the form of

forcible motion, as in muscles, or of the manufacture of special

materials, as in gland cells, it takes the form of the transmission of a

disturbance from one part of the cell to another. An interesting and

important fact about this transmission of disturbances is that the

actual amount of functional metabolism required by it is very small.

Only by the most careful measurements has it been shown that nerve cells

that are functioning have a greater metabolism than those that are at

rest. For a long time it was thought that a nerve cell acted very much

like a telephone or a telegraph wire, transmitting some kind of a

disturbance which was set up in it, but not having any active part

itself in the process. We now know that the special activity of nerve

cells is a form of functional metabolism, just as is the special

activity of muscle cells or gland cells.

The nerve cells have to make communication between sense organs and

muscles, and, as we have already seen, these are often quite a distance

apart. It is necessary, therefore, that the nerve cells be long enough

to reach over these distances. As a matter of fact, it is not necessary

for single cells to have this great length, because it is possible for

them to be arranged end to end, making a path of living protoplasm

consisting not of one cell but of a chain of them. If we look at a nerve

cell under the microscope we see that it is made up of a little central

mass of protoplasm to which has been given the name of “cell body.” From

this cell body extends a tiny thread of living protoplasm. This thread

is called the _axon_. It is so very slender that it cannot be seen

except under a powerful microscope, and yet in our own bodies and the

bodies of all large animals many of these are three feet or more in

length without a break. This tiny protoplasmic thread, the axon, was

formed originally by growing out from the cell body. As it grew it

became surrounded by a sheath, which probably gives it strength and

decreases the danger of its being broken. Another thing which helps to

keep the axons from being injured is that they are always in bundles.

Instead of one of these very slender axons lying all by itself, it will

be bound up with several hundred others; the arrangement is similar to

that in a telephone cable, where a great many single wires are bound

together in the large and very strong cable. The living protoplasm of

the nerve cell has a gray color, so that wherever this shows we have

what is commonly called gray matter. We saw a moment ago that every axon

is inclosed in a sheath. Some of these sheaths are transparent, so that

the gray color can be seen underneath, but most of them have a layer of

white material, which makes them look white instead of gray. The bundles

of axons corresponding to the telephone cables make up what we call the

nerves. Nearly all nerves are white in color because of the white

material in the sheaths.

The sense organs, as we have seen in Chapters VIII and IX, are some of

them inside the body, others spread over the surface of the skin, and

the rest in the special sense organs, like the eyes or the ears. A very

complex organ, like the eye or the ear, has thousands of axons leading

from it. In the case of the eye these are grouped into a large nerve

leading away from it at the back, which is called the optic nerve. There

is a similar large nerve leading from the ear. When any sense organ is

acted upon, as when light falls in the eye or sound on the ear, it

starts a disturbance in some or all of the axons leading away from it.

As we have said over and over, the purpose of the nervous system is to

arouse the muscles to activity, and to guide them in that activity. We

must ask next, then, how the nerves are distributed to the muscles. If

we dissect the body of any animal or bird, we can find nerves passing to

various parts. For example, a large nerve goes down each leg; this nerve

subdivides here and there. Since we know that the nerve consists of a

great many axons bundled together, we will realize that this

subdivision is not a real branching, but simply a passing of some of

the axons away from the main trunk along the smaller stem. Some of these

smaller stems can be traced to endings in the skin; these contain the

axons connecting with sense organs. Others lead directly into muscles.

Some of these axons may also connect with sense organs, since, as we

have already seen, every muscle has embedded in it the organs of muscle

sense, but in addition any nerve that leads to a muscle contains a great

many axons which pass directly to the muscle fibers. These are the axons

by which the muscles are aroused to activity. It is a general rule of

the nervous system that no nerve cell extends without a break from any

sense organ to any muscle fiber. The axon which communicates with the

sense organ belongs to one nerve cell; the axon which connects with the

muscle fiber belongs to a different nerve cell. The first is called a

_sensory_ nerve cell, the second a _motor_ nerve cell. Some idea of the

appearance of these cells can be gotten from the figures on page 126.

It will be seen that the cell body of the sensory cell appears to be off

on a little side branch. As a matter of fact, the branch is double, so

that when a nervous disturbance passing along from the sense organ comes

to the beginning of this branch it can pass up to the cell body and then

out from the cell body along the second part of the branch, and so along

the other part of the axon. This part of the axon is seen in the figure

to have several branches; these are really branches of protoplasm and

not separate axons coming off, as in the case of the nerve trunk. The

use of these branches we shall see in a moment. Also at the tips of

each branch there is a tiny feathering. We shall explain this

presently. Let us look first at the figure of the motor nerve cell. This

has a cell body and long axon, and, besides these, has a great many

short protoplasmic branches sticking out in all directions from the cell

body. Since a nervous disturbance to get from a sense organ to a muscle

has to pass over a sensory nerve cell, and also over a motor nerve cell,

evidently there will have to be some point at which it leaves the

sensory cell and gets into the motor. This is accomplished by having the

tiny feathering at the tip of the sensory cell interwoven with the fine

processes projecting from the body of the motor cell. This arrangement

we may call a nerve junction. In the whole body there are, of course,

millions of these nerve junctions.

[Illustration: MOTOR NERVE CELL

SENSORY NERVE CELL

(From Martin’s “Human Body”)]

We have just described the simplest arrangement of a nerve path from a

sense organ to a muscle; it consists of the sensory nerve cell, a nerve

junction, and a motor nerve cell. This arrangement will answer where the

sense organ and the muscle are in the same part of the body, but it may

happen that the sense organ is in one part of the body and the muscle is

in a distant part, as, for example, the eye and the muscles of the hand.

To make these distant connections there must be additional nerve cells,

and these we find in the body in the form of a kind of nerve cell that

serves as a connecting link between sensory cells and motor cells. It

may be called simply a connecting nerve cell. In appearance it is like

the motor nerve cell, except that it has many branches which do not

terminate in muscle fibers, but in fine feathering like that at the tips

of the sensory nerve cell. When these connecting cells are present in

the chain, the arrangement is as follows: from the sense organ the

sensory nerve cell will pass just as previously described, the

feathering at the tip will form a nerve junction with a connecting cell

instead of with a motor cell. This connecting cell also has feathering

at the tips of its branches, and these featherings will form a nerve

junction with another nerve cell. This may be a motor cell, in which

case the pathway is completed, or it may be another connecting cell,

which in turn may lead into motor cells or connecting cells.

The nervous system is made up, then, of chains of nerve cells. Now why

is this arrangement present? In very many cases sense organs and muscles

are side by side or within a short distance of each other; why does not

a single nerve cell reach directly from the sense organ to the muscle?

The

[Illustration: CONNECTING NERVE CELL

(From Martin’s “Human Body”)]

answer is simple, if we think for a moment of how the body works. The

information that comes in by way of particular sense organs cannot

always be used to arouse particular muscles to activity. Things that we

see will sometimes cause us to move one hand, sometimes another hand,

sometimes the legs, sometimes muscles of the head, and so on. This means

that the eye is able to make connection with a very large number of

different muscles. The same thing is true of the other sense organs. The

body could not possibly work as it does, if certain sense organs

connected with certain muscles and no others. As a matter of fact, it is

not an exaggeration to say that the proper working of the body requires

any sense organ to be able to make connection with any muscle. It might

be possible to do this by giving every sense organ as many sensory nerve

cells as there are muscles, but this would be as bunglesome as to

attempt to provide every business house in a large city with a separate

telephone wire to every other business house.

The arrangement of the nervous system is very much like that of a city

telephone system. The sensory nerve cells all lead into a part of the

nervous system to which is given the name of the _Central Nervous

System_, just as the telephone wires all lead into a “central exchange.”

From this central nervous system or “exchange” all the motor nerve cells

extend to the muscles. There is one important difference between the

arrangement of the nervous system and that of the telephone exchange;

namely, that the nervous system is a “one-way” system. As we all know, a

telephone instrument can be used either for sending or receiving and the

same wires conduct the messages in both directions. This is not true of

the nervous system. The messages from the sense organs pass into the

center by way of the sensory nerves and out from the center by way of

the motor nerves. The central nervous system operates as an “exchange,”

connections can be made from any sensory cell to any of the motor cells.

Since this ability of the central nervous system to make connections

here and there within itself is about the most important of all our

nervous activities, as we shall see shortly, we must try to form an idea

of how it is done. If we look again at the figures of the sensory and

connecting nerve cells, we shall note that both kinds are branched; the

tip of every branch makes a nerve junction. This means that every

sensory cell, for example, has as many outlets as it has branches. If

every one of these outlets were to communicate directly with a motor

nerve cell, the number of connections that

[Illustration: DIAGRAM OF NERVE CELL CONNECTIONS

_S C_, spinal cord; _S_, sense organ; _M_, muscle; _a_, sensory nerve

cell; _b_, connecting nerve cell; _c_, motor nerve cell]

this sensory cell could make would depend on the number of its branches;

but, as a matter of fact, most of the branches from the sensory cells

make nerve junctions with connecting cells, and not with motor cells

directly. These connecting cells in turn are branched and many of these

branches lead again to connecting cells, so we see that the number of

connections that a single sensory cell can make quickly becomes very

large. The arrangement is shown in the accompanying diagram. Not only

does one sensory cell have in this way the possibility of nerve

connection with all the muscles, but the reverse is also true; namely,

that every muscle has the possibility of being connected with every

sense organ. This means that there must be a number of nerve junctions

connecting with the cell body of each of the motor nerve cells. If we

look back at the diagram of the motor nerve cell, we shall see that it

has a great many tiny branches leading off it. These are numerous enough

to enable the feathery tips of a great many sensory or connecting nerve

cells to interweave with them, and so enable any motor cell to be acted

upon from a great many different directions.

The central nervous system, which is the place where all these

much-branched pathways are and where all the nerve junctions are

located, is made up of two chief parts, the brain and the spinal cord.

The brain is inside the skull and the spinal cord is an extension down

the back. It lies in the tunnel made up of the arches of the bones of

the vertebral column as described in Chapter VI. The cables which

contain the axons of both sensory and motor nerve cells extend from the

brain or from the spinal cord out to the different parts of the body

where the sense organs and the muscles are located. They start as large

nerve trunks which divide and subdivide as they get farther and farther

away from the central nervous system. The large nerve trunks are

arranged in pairs--those that spring from the brain are called _cranial_

nerves; those that spring from the spinal cord are called _spinal_

nerves. The cranial nerves, with the exception of one, lead only to

points in the head or neck, and so are short; the spinal nerves on the

other hand reach to the various parts of the trunk or down the arms and

legs, so that some of these may be three feet long or more. An idea of

the arrangement of the nervous system is given in the accompanying

figure.

A good way to realize the actual working of the nervous system is to

take a particular action and follow it through. Suppose a barefooted boy

steps on a sharp thorn. The thorn arouses some of the sense organs in

the sole of the foot. These in turn start a disturbance in the sensory

nerve cells which pass up the leg to the lower end of the spinal cord in

the small of the back. Within the spinal cord the sensory cells branch

and the disturbance set up by the prick of the thorn spreads all over

these branches to their tips. Some of the nerve junctions thus affected

lead to muscles which will cause the foot to be jerked up; others

communicate with connecting nerve cells which extend all the way up the

spinal cord and into the brain and make the boy aware of the fact that

he has stepped on the thorn; still others may make connection with

muscles which would cause him to sit down and look at the bottom of his

foot; still others may lead to the vocal muscles and the tear glands,

causing the boy to cry. Of course these are not the only possible nerve

connections; every muscle in the body might theoretically be aroused to

action as the result of the stepping on the thorn. As a matter of fact,

there is a condition in which this will happen. The drug strychnine has

such an effect upon the nervous system that the stimulation of any sense

organ actually does arouse all

[Illustration: DIAGRAM ILLUSTRATING THE GENERAL ARRANGEMENT OF THE

NERVOUS SYSTEM

(From Martin’s “Human Body”)]

the muscles in the body, giving what is called a convulsion. There are

some other poisons which may act similarly. Convulsions are not at all

uncommon in young children. The way in which a convulsion is produced is

by the spreading of the disturbance all over from a single sense organ.

What we have, then, in the nervous system is an arrangement whereby

under special conditions a nervous disturbance can pass from any sense

organ to any or all of the muscles, but under ordinary conditions the

disturbance spreads to a particular muscle or group of muscles only. It

is evident that the nervous system would be of no use at all, if this

latter arrangement did not exist. In order for our muscles to serve us,

they must act in obedience to information brought in by the sense

organs, and this can happen only when certain groups of muscles work in

accordance with the information brought in by certain sense organs or

groups of sense organs. We explain the behavior of the central nervous

system by saying that there are preferred pathways through it, or, to

put it in a slightly different way, when a nervous disturbance spreads

over any nerve cell it extends equally over all parts of it, but does

not pass with equal ease over all the nerve junctions. Some of the nerve

junctions allow the disturbance to pass more readily than others, and it

is this difference in the ease of passing the nerve junctions that

determines which pathway the disturbance shall follow. We know no other

means by which this picking out of particular paths from the huge number

of possible paths could be accomplished.

What we are describing now is the simple foundation on which all our

nervous activities rest. For that reason we are not taking up at this

point the working of the brain, but only the direct connections between

sense organs and muscles. In some very low animals the whole nervous

system is made up of such simple connections.

[Illustration: DIAGRAM ILLUSTRATING A REFLEX

An object _O_ suddenly appears in front of the eye. Its image, formed on

the retina _R_, a sense organ, starts impulses along the fibers of the

sensory nerve cell _s_, which in turn stimulate the motor nerve cells

_m_. These in turn stimulate the appropriate muscles of the eyelid,

compelling a wink. (After Hough and Sedgwick, “The Human Mechanism.”)]

A nervous activity which consists of the passage of a disturbance from a

sense organ to a muscle is called a _reflex_. We have many examples in

ourselves; if we inadvertently touch something hot, the hand is jerked

away. Tickling the soles of the feet in one who is asleep will cause

them to be drawn up; irritation in the throat causes us to cough, or in

the nose to sneeze; the flashing of a bright light into the eye compels

us to wink; all these are examples of the direct passage of nervous

disturbances from sense organs to muscles. In every case the path is

from the sense organ over the sensory nerve cell to some point in the

central nervous system, then either by a direct nerve junction to a

motor nerve cell or over one or more connecting nerve cells to a motor

nerve cell and so to the appropriate muscles. These reflex actions

follow the arousing of the sense organ with no more delay than is

required for the passage of the disturbance over the nervous pathway.

There is a delay of a small fraction of a second at every nerve

junction, which makes some reflexes slower in their action than others.

For example, there is a reflex known as the knee jerk; this is an

outward kick which results from a sharp blow on the front of the leg

just below the knee. The kick follows so closely after the blow that

there cannot be more than one or at the most two nerve junctions in the

pathway. The reflex of winking, on the other hand, takes several times

longer, although the eye is much nearer the central nervous system than

is the place on the leg which is struck in arousing the knee jerk. Since

the actual length of nerve to be passed by the disturbance is much

shorter in the winking than in the knee jerk, while the time for the

reflex is a good deal longer, we conclude that the nerve pathway which

is used in arousing winking contains a great many more nerve junctions,

and therefore includes a great many more connecting cells than does the

path for the knee jerk. In this particular case the reason why the

pathway of winking contains so many connecting cells is that it is a

brain pathway, while that for the knee jerk includes only the lower end

of the spinal cord, where the arrangement of nerve cells is very much

simpler.

It is important for us to get clearly in mind the working of the

reflexes in order to be able to understand the more complex nervous

actions which will be described in the next chapter. We need to remember

that the ordinary way of starting nervous disturbances is from the sense

organs. With a few exceptions, to be described later, whenever a

nervous action occurs anywhere in our bodies it can be traced back,

although often very indirectly, to the sense organ from which the

disturbance originally came. The part played in this by the brain and by

what we call our mental processes will be described in the next chapter.

Before going on to that topic we have a word to say about nervous

fatigue. We mentioned the fact in Chapter VII that much of our actual

feeling of fatigue is nervous rather than muscular. Not as much is known

about nervous fatigue as about the fatigue that comes on when the

muscles are overworked. One thing that seems pretty evident is that the

place where the fatigue actually is located is in the very delicate

nerve junctions. These junctions offer some resistance to the passing of

nervous disturbances over them, and if they are compelled to submit too

often to this passing they appear to offer still more resistance; in

other words, to become fatigued. We must remember that the nerve

junctions are exceedingly delicate things, consisting, as they do, of

the interweaving of the almost inconceivably tiny featherings at the

tips of sensory or connecting nerve cells with the equally tiny

featherlike branches from the cell bodies of connecting or motor nerve

cells. It is likely, also, that on account of their delicacy they are

easily affected by the waste products that may be circulating about in

the blood stream from the active muscles. In either case, the way to

recover from nervous fatigue is simply by resting. It is not hard for

the delicate nerve junctions to throw off fatigue if given a chance. The

way to give them this chance is not to use them. As we shall see in the

next chapter, mental processes are made up of nervous disturbances

passing here and there in the brain. If we allow ourselves to be

occupied too continuously with the same lines of thought, we are

evidently sending nervous disturbances over the same nerve junctions

over and over again. In order to give those nerve junctions a chance to

rest, what we have to do is to think about something entirely different.

The word that best expresses what we have in mind is “diversion.” In the

strict sense diversion means a turning aside from what we have been

doing to something different, and that is the best way to allow the

brain to rest. The man who takes his business home with him, and dwells

on it during the hours that are supposed to be set aside for rest, may

be able to achieve more for the moment than if he were really to rest,

although even that is doubtful; but in the long run there is no doubt

that continuous efficiency depends on allowing the fatigued nerve

junctions ample opportunity to recover, which means that the thoughts

must be directed into entirely different channels.

CHAPTER XI

THE BRAIN AND COMPLEX NERVOUS ACTIONS

We have just seen that the underlying arrangement of the nervous system

is one which makes communication possible between any sense organ in the

body and any muscle; also that in the working of the nervous system

there are certain paths from given sense organs to particular muscles

which we may call the selected paths, and over which nervous

disturbances easily pass. These paths work out in such a way that the

muscles that are thrown into activity as the result of the arousing of

any sense organ are those most likely to serve the needs of the animal.

There are a number of respects in which a nervous system, having no more

than we have thus far described, would fall short of meeting the

requirements of any animal except perhaps the very simplest. The thing

that is missing that is of most importance, perhaps, is the ability to

perform actions in response to information received at some past time;

in other words, the ability to profit by experience. As the nervous

system has been described up to this point, only immediate effects upon

the sense organs are transmitted to the muscles to throw them into

activity. As a matter of fact, we know that we ourselves, as well as

many of the higher animals, are able to profit by past experience. That

is to say, we can make our muscles move in accordance with information

received at some time in the past. There must be in the nervous system,

then, some arrangement for doing this, and our present task is to

describe it. In the chapter on the distance sense organs considerable

emphasis was placed on the fact that the advantage of having distance

sense organs is to allow time for a choice among the various responses

that the animal might make to the information received by the sense

organs. It is this opportunity for choice that lies at the foundation of

the higher nervous activities that we are now beginning to describe. As

soon as the element of choice enters, the nervous disturbance cannot

pass from the sense organ to the muscle in the least possible time. In

order for there to be a choice there must be a delay while the selection

of the muscle to be aroused to activity is being made. We have, then, a

new feature in connection with the operation of distance sense organs in

that the nervous disturbance can come to a stop at some point in the

course of its progress from sense organs to muscles. Every animal that

has distance sense organs shows this feature of possible delay in the

movement of the nervous disturbance over the nervous pathway, and in

every one the particular part of the nervous system in which this occurs

is known as the brain. In fact no other nerve cells except some of the

nerve cells of the brain have this ability of stopping the nervous

disturbance and holding it for a time before sending it on.

One thing about the information that comes in through the distance sense

organs is that much of it does not require an immediate activity. For

example, a hungry fox, seeing a fowl roosting in an accessible place,

might pounce upon it at once, but if he happened at the moment to be

either fully satisfied or to be carrying a fowl which he had captured a

moment ago, there would be no occasion for him to seize this one; rather

would it be desirable for him to set it aside in his mind to be captured

in the future. In the case used in the illustration the nervous action

goes on in the beginning just as we have been picturing it; that is, the

sense organ is aroused and this in turn starts a disturbance over the

nerve path; when it comes to the brain, however, it does not go on from

there to cause immediate muscular activity, but instead is held in the

brain and can cause activity at some future time, as for example if the

fox, after taking the fowl which he is carrying home to his young, comes

back in search of the one that he saw roosting. We are perfectly

familiar with this ability to stop nervous disturbances; we know it

under the name of _memory_, although we may not have thought of memory

in exactly this sense. If we consider what memory really is, we shall

see that it is just what we have been describing, namely the stopping of

nervous disturbances that come in from the sense organs and holding

them, so that they may in turn set up at some future time a nervous

disturbance that shall produce activity. If we try to picture what

actually goes on in the nerve cells where this memory is located, we can

say only that when the nervous disturbance strikes upon these cells, it

does not pass on through at once leaving them very much as they were

before, but stops in them and brings about in them some kind of

permanent change. An interesting thing about memory is its persistence.

We may act upon memories that we have held for years. More than one

person moving away from a particular town early in life, and going back

to it in later years has remembered things he saw in childhood well

enough to find his way to them again.

Since memory is registered in the brain cells as a permanent or fairly

permanent change, it can become the source of nervous disturbance over

and over again. This in fact is one of its greatest advantages, because

when we have once learned a thing, we can make use of it a great many

times, and do not have to have the sense organs freshly aroused every

time it is desirable to use this particular bit of information.

In order that the brain cells may receive and store up memories, it is

evidently necessary that nerve paths from the sense organs should lead

into the brain; so, if we go back to the sensory nerve cells, we shall

find that in nearly every one of them, if not in every one, one of the

branches makes a nerve junction with a connecting cell, which either

extends directly into the brain, or links with other connecting cells

which do. This is true not only of the distance sense organs, but also

of the contact sense organs and to some extent of those inside the body.

The path by which the brain is reached from the sense organs that are

located in the body is over the sensory nerve cells to the spinal cord,

and along the spinal cord by way of connecting nerve cells to the brain.

All the distance sense organs are located in the head, so their nerve

paths lead into the brain directly over cranial nerves; for the eye, the

optic nerve; for the ear, the auditory nerve; and for the organ of

smell, the olfactory nerve. One very interesting fact about the

connection of the sense organs with the brain is that the nerve paths in

every case cross from the left-hand

[Illustration: DIAGRAM OF NERVE PATHWAYS

_B_, brain; _S. C._, spinal cord]

side to the right-hand side, and vice versa, so that all the sense

organs in the left half of the body have their connection with the right

half of the brain, and those in the right half of the body with the left

half of the brain. We know of no reason why this should be so. It is

merely an interesting fact. A diagram showing the path from a sense

organ in the body and from one of the head senses to the brain is given

in the accompanying figure.

[Illustration: DIAGRAM ILLUSTRATING THE GENERAL RELATIONSHIPS OF THE

PARTS OF THE BRAIN

_A_, cerebrum; _B_, cerebellum; _b_, _C_, _D_, brain stem. (From

Martin’s “Human Body”)]

The brain in man and in all the higher animals is a very complicated

organ made up of a number of different parts. To simplify the

description as much as possible we shall omit all account of smaller

subdivisions and speak only of the most important parts. These are three

in number, named the _cerebrum_, the _cerebellum_, and the _brain stem_.

Their location with reference to one another is shown herewith.

As the figure shows, the _cerebrum_ is the main part of the brain. It in

fact is the part we ordinarily think of, when we have the brain in mind.

The brain stem is really the upper extension of the spinal cord within

the head. It is a very important part of the brain, because all the

nerve pathways in from the sense organs, and from the brain out to the

muscles, pass through it. Besides that it has some activities of its

own, which will be described a little later. The _cerebellum_ is a

subdivision of the brain which plays a very important part, but is not

concerned in those complicated nervous actions which make up our mental

processes. All these are conducted wholly in the cerebrum. In fact, this

section of the brain is the only part which has the ability to stop the

progress of nervous disturbances; the property of memory, which was

described a moment ago, is found only in the cerebrum. Since the

cerebrum is the seat of memory and of our mental life in general, it is

both the most interesting and the most important part of the whole body,

and a very large amount of study has been given to it. Many years ago

attempts were made to show that the brain is subdivided into a number of

parts, each of which has control over certain mental characteristics.

Nothing very important came of these attempts, although they gave rise

to the false science of phrenology, which has been widely exploited by

fakers. Notwithstanding the failure of these early attempts at locating

particular mental activities in particular parts of the brain, recent

studies have shown that there is something of the sort, although it is

not at all what the phrenology charts would indicate. It has been shown,

however, that the various sense organs do make connections with

particular parts of the cerebrum. Comparing the pathways over which the

nervous disturbances pass to railroad tracks these places in the brain

are often spoken of as the “arrival platforms” of the various senses.

Thus the arrival platform for the sense of

[Illustration: THE BRAIN FROM THE LEFT SIDE

_Cb_, cerebrum; _Cbl_, cerebellum; _BS_, brain stem. (From Martin’s

“Human Body”)]

sight is at the back of the cerebrum down at its lower margin; the

arrival platform for the senses of touch, pain, and the like are just

about at the top of the brain. The arrival platform for the sense of

hearing is down at the side. The location of these arrival platforms has

been worked out by studying the effects of brain diseases. It has been

found, for example, that persons who are blind, although there is

nothing the matter with their eyes, are so because of a disease of the

lower back part of the cerebrum, and persons who are deaf, even though

their ears are perfect, are so because of diseases in the parts of the

cerebrum at the sides. The only way to explain these findings is by

supposing that the arrival platforms of these senses are located in the

parts that are found diseased. The cells which stop the nervous

disturbances coming in from the sense organs and so serve as the seat of

memories of sensations are in the arrival platforms, so that when we

recall how something looked, for example, the nerve cells which are

active in this recollection are those at the lower back part of the

cerebrum. All these arrival platforms taken together occupy only a very

small fraction of the whole of the cerebrum, so that evidently it has

more to do than simply to register these memories.

One additional thing we shall look for in the cerebrum is a pathway from

the brain to the muscles whereby the memories that are stored in the

brain can make themselves effective in arousing the muscles to activity.

There is such a pathway, and it is one of the best marked of all the

nerve pathways of the body. It is made up of nerve cells which start

just about at the top of the cerebrum, a little in front of the middle

and pass down into the brain stem; these cross over from the side in

which they started to the opposite side; and go on down the spinal cord

to make connection by nerve junctions directly with the motor nerve

cells. Thus from the brain to the muscles the nerve pathway is made up

of just two nerve cells. Since this pathway crosses over from one side

to the other in the brain stem, the whole right half of the body, both

sensory and motor, connects with the left half of the brain, and vice

versa. The part of the cerebrum in which this nerve pathway starts is

called the _motor area_ of the brain.

We have now seen how nervous disturbances from the various sense organs

can come into the brain and be registered there as memories. We have

also seen that there is a nerve pathway by which nervous disturbances

starting in the motor area can pass out to the muscles, and arouse them

to activity. We have left to see how the connections are made between

the incoming and the outgoing disturbances, or in

[Illustration: DIAGRAM OF MOTOR PATHWAY

_B_, brain; _S. C._, spinal cord; _M_, muscle]

other words to see how the memories that are registered in the brain act

upon the outgoing pathways. We shall expect to find connecting nerve

cells reaching across from the various arrival platforms to the motor

area, and such connecting cells exist, but they are not simple and

direct connections for a reason which we shall now try to make clear.

We all know from our own experience that our memories are never the pure

registering of a single sense. What we mean is that the sight of

something by itself or the sound of something by itself never remains as

a separate memory, but is always worked in with some other memories from

some of the other sense organs. This putting together of the simple

sensory memories began in earliest childhood, long before we were old

enough to think about our mental processes, and see how they are carried

on, so that unless our attention has been called to it, we have probably

failed to realize how complicated our simplest memories are. A good

example of this is in the experience of a baby with its mother’s voice.

So far as the baby is concerned, the voice is an influence affecting the

organ of hearing, arousing nervous disturbances which pass to the

arrival platform for hearing in the brain and are registered there as

memories, but it does not take the infant long to learn that some other

memories that have come to it by way of other sense organs belong with

this particular memory and always go with it. For example there is the

sight of the mother, or the feel of the mother’s face and hands, all

these are influences affecting different sense organs and registering in

the child’s brain in different arrival platforms. Yet within his brain

they become fused into a single composite memory of the mother, and

after this fusion has once occurred, the arousing of any part of the

memory brings up the whole of it, so that the child may hear the

mother’s voice from the next room, but the memory that will be aroused

as the result will be not simply of the voice but of the mother as a

whole. This is an illustration of how our pure sense memories are fused

into complex memories. After one of these memory complexes is once

started, we add to it any time any sense organ is acted upon by anything

that has relationship to the complex. For example, the child’s idea of

the mother at first is a very simple one made up of a few sensory

impressions, but as time goes on and more and more sensory impressions

of the mother are received, the child’s idea of her becomes more and

more complex. This process of memory fusion is called association or

sometimes association of ideas. Strictly it is an association of

memories and this is the method by which all our mental activities are

carried on. The thought process consists of putting together various

memories in various ways and so building up associations of different

kinds. Of course adults of wide experience can form associations which

are made of literally hundreds or perhaps thousands of separate memories

of sense impressions. We receive sense impressions at the rate of

hundreds every day, and very many of these, perhaps all of them, are

registered as memories and are fitted into their proper associations.

The nervous machinery for carrying on different processes of association

consists of nerve cells of the brain. These are all of the kind known as

connecting cells. It will be remembered that the connecting cells may

have many nerve junctions leading into them and they in turn are much

branched so that many nerve junctions can lead out from them. The

connecting cells of the brain are richer in these respects than those in

any other part of the body, and the cells of the human brain than the

brain of any of the lower animals. We suppose that the associations are

formed by the passage of nervous disturbances from the arrival

platforms where the sense memories are registered over various

connecting nerve cells to a common meeting point in some cell or in some

group of cells, where the associated memories are all brought together

into a single memory complex. It is supposed that the large areas of the

brain which are not taken up either by the arrival platforms or by the

motor areas are the regions in which these associations take place.

There is one particular group of associations that are so interesting as

to call for special mention. These are the associations concerned with

language. We are so in the habit of using language that we are likely

never to have thought of it in its real meaning as a part of our mental

activity. We know that we have two kinds of language, spoken and

written, and that spoken language consists of certain sounds, and

written language of certain visual symbols. Two of the distance senses

then are concerned, sight for written language, hearing for spoken, and

language itself consists simply of sense impressions coming in through

one or the other of these sensory channels. The important feature of

language is that mankind has selected arbitrarily certain sounds or

certain written symbols to stand for particular things. When a child is

learning to understand what is said, what he is really doing is fitting

a particular set of arbitrary sense impressions into their proper places

in his associations. We can illustrate this by the same example that was

used a moment ago. The child becomes thoroughly familiar with its

mother, so far as sense impressions are concerned that come directly

from her. In course of time it adds to the associations thus formed an

additional one made up of the sound of the word “mother.” Of course

there is no particular reason why this sound should have that meaning

rather than any other. The proof is that different languages have

different sounds which stand for the same thing. After this sound has

once been selected and learned, it becomes as much a part of the idea of

mother as any other of the associations concerned, and thereafter,

whenever that particular sound strikes upon the child’s ear, the

association of mother is aroused. Precisely the same sort of thing is

true of written language. Arbitrarily selected symbols act through the

sense of sight to arouse nervous disturbances, which are built into

particular associations, and here again it makes no particular

difference what the symbol is; one will do as well as another, provided

a number of people have agreed to use that symbol to stand for the same

thing. One very useful feature of language is that we can make a single

word stand for a complex group of sensory impressions or even

associations. To illustrate, the word physiology, whether spoken or

written, is in itself simply a sound or a visible symbol, but it stands

for a highly complex group of associations in the human mind. In this

respect language is a kind of shorthand.

It is hard to overestimate the importance of language to the human race.

This is because of the enormous extension it gives to the ability to

profit by experience. The lower animals are able to profit by

experience, but the only experience that they can get is that which

comes to them individually or to another animal which they may observe.

Human beings, on the other hand, may profit by the experience of their

contemporaries through learning of them by word of mouth, or may profit

by the experience of present or past generations as recorded in

writing. It is not too much to say that the progress of civilization

depends on our ability to profit by the experience of past generations

as made known to us in writing. We are further advanced than our remote

ancestors, not because we are actually superior to them in mental power,

but because we have a much larger background of experience than they

had, and this background depends altogether on our ability to use

language.

The complex nervous activities that have been described up to this point

are purely a matter of the association of memories, or what we speak of

as intellectual processes. Besides these our mental life includes what

we call the feelings or emotions, and these make up so large a part of

our mental life, that we cannot leave the consideration of the brain

without saying a word about them. For convenience we may subdivide the

feelings into the two classes of agreeable and disagreeable. If we try

to get at the meaning of these, we can perceive that the agreeable

feelings are fundamentally associated with one of two things: either a

condition in which bodily well-being is assured, or conditions

associated with the perpetuation of the race. An example of this latter

is the pleasure that parents find in the care of children. The

disagreeable feelings are fundamentally concerned with the immediate

preservation of the body from injury. Thus fear and anger are two of our

most disagreeable emotions, and both of these are aroused ordinarily

only when danger threatens or when bodily well-being is otherwise

interfered with. One other disagreeable emotion, worry or anxiety, is

interesting in that it applies to ill-being that is expected in the

future, and while it is well to be on guard against trouble, this

emotion is really for the most part futile, because the expected

ill-being commonly fails to materialize. Not only do the feelings share

with the intellectual processes in making up our mental life as a whole,

but they also show themselves in certain bodily changes, which are so

important that we shall return to them at greater length in the next

chapter.

The feelings as well as the intellectual processes are made up of

nervous disturbances passing over the brain cells. We do not know

whether those which are concerned with the emotions are in different

parts of the brain from those that are concerned with the simple

associations or not. There are some differences between the two kinds of

nervous activity; particularly is this true of the vividness of the

memories concerned. Every intellectual process, presumably, results in

the formation of some associations that were not previously present, and

the memory of the new associations should be and usually is reasonably

sharp. Emotions on the other hand, although they may be remembered, do

not register themselves in such a way that a new emotion is aroused

every time the old one is remembered. It is probably true that for a

while after any strong emotion the recollection of it will tend to bring

up the emotional state again, but in course of time there is a dulling,

so that although we may remember clearly the occasion of the emotion,

the feelings are no longer actively aroused.

CHAPTER XII

SOME SPECIAL NERVOUS ACTIONS; SMOOTH MUSCLE AND GLAND CONTROL