THE BRAIN AND NERVOUS SYSTEM
A fine brain, or a good mind. These terms are often used
interchangeably, as if they stood for the same thing. Yet the brain is
material substance--so many cells and fibers, a pulpy protoplasmic mass
weighing some three pounds and shut away from the outside world in a
casket of bone. The mind is a spiritual thing--the sum of the processes
by which we think and feel and will, mastering our world and
accomplishing our destiny.
1. THE RELATIONS OF MIND AND BRAIN
INTERACTION OF MIND AND BRAIN.--How, then, come these two widely
different facts, mind and brain, to be so related in our speech? Why are
the terms so commonly interchanged?--It is because mind and brain are so
vitally related in their processes and so inseparably connected in their
work. No movement of our thought, no bit of sensation, no memory, no
feeling, no act of decision but is accompanied by its own particular
activity in the cells of the brain. It is this that the psychologist has
in mind when he says, _no psychosis without its corresponding neurosis_.
So far as our present existence is concerned, then, no mind ever works
except through some brain, and a brain without a mind becomes but a mass
of dead matter, so much clay. Mind and brain are perfectly adapted to
each other. Nor is this mere accident. For through the ages of man's
past history each has grown up and developed into its present state of
efficiency by working in conjunction with the other. Each has helped
form the other and determine its qualities. Not only is this true for
the race in its evolution, but for every individual as he passes from
infancy to maturity.
THE BRAIN AS THE MIND'S MACHINE.--In the first chapter we saw that the
brain does not create the mind, but that the mind works through the
brain. No one can believe that the brain secretes mind as the liver
secretes bile, or that it grinds it out as a mill does flour. Indeed,
just what their exact relation is has not yet been settled. Yet it is
easy to see that if the mind must use the brain as a machine and work
through it, then the mind must be subject to the limitations of its
machine, or, in other words, the mind cannot be better than the brain
through which it operates. A brain and nervous system that are poorly
developed or insufficiently nourished mean low grade of efficiency in
our mental processes, just as a poorly constructed or wrongly adjusted
motor means loss of power in applying the electric current to its work.
We will, then, look upon the mind and the brain as counterparts of each
other, each performing activities which correspond to activities in the
other, both inextricably bound together at least so far as this life is
concerned, and each getting its significance by its union with the
other. This view will lend interest to a brief study of the brain and
nervous system.
2. THE MIND'S DEPENDENCE ON THE EXTERNAL WORLD
But can we first see how in a general way the brain and nervous system
are primarily related to our thinking? Let us go back to the beginning
and consider the babe when it first opens its eyes on the scenes of its
new existence. What is in its mind? What does it think about? Nothing.
Imagine, if you can, a person born blind and deaf, and without the sense
of touch, taste, or smell. Let such a person live on for a year, for
five years, for a lifetime. What would he know? What ray of intelligence
would enter his mind? What would he think about? All would be dark to
his eyes, all silent to his ears, all tasteless to his mouth, all
odorless to his nostrils, all touchless to his skin. His mind would be a
blank. He would have no mind. He could not get started to think. He
could not get started to act. He would belong to a lower scale of life
than the tiny animal that floats with the waves and the tide in the
ocean without power to direct its own course. He would be but an inert
mass of flesh without sense or intelligence.
THE MIND AT BIRTH.--Yet this is the condition of the babe at birth. It
is born practically blind and deaf, without definite sense of taste or
smell. Born without anything to think about, and no way to get anything
to think about until the senses wake up and furnish some material from
the outside world. Born with all the mechanism of muscle and nerve ready
to perform the countless complex movements of arms and legs and body
which characterize every child, he could not successfully start these
activities without a message from the senses to set them going. At birth
the child probably has only the senses of contact and temperature
present with any degree of clearness; taste soon follows; vision of an
imperfect sort in a few days; hearing about the same time, and smell a
little later. The senses are waking up and beginning their acquaintance
with the outside world.
[Illustration: FIG. 5.--A NEURONE FROM A HUMAN SPINAL CORD. The central
portion represents the cell body. N, the nucleus; P, a pigmented or
colored spot; D, a dendrite, or relatively short fiber,--which branches
freely; A, an axon or long fiber, which branches but little.]
THE WORK OF THE SENSES.--And what a problem the senses have to solve! On
the one hand the great universe of sights and sounds, of tastes and
smells, of contacts and temperatures, and whatever else may belong to
the material world in which we live; and on the other hand the little
shapeless mass of gray and white pulpy matter called the brain,
incapable of sustaining its own shape, shut away in the darkness of a
bony case with no possibility of contact with the outside world, and
possessing no means of communicating with it except through the senses.
And yet this universe of external things must be brought into
communication with the seemingly insignificant but really wonderful
brain, else the mind could never be. Here we discover, then, the two
great factors which first require our study if we would understand the
growth of the mind--_the material world without, and the brain within_.
For it is the action and interaction of these which lie at the bottom of
the mind's development. Let us first look a little more closely at the
brain and the accompanying nervous system.
3. STRUCTURAL ELEMENTS OF THE NERVOUS SYSTEM
It will help in understanding both the structure and the working of the
nervous system to keep in mind that it contains _but one fundamental
unit of structure_. This is the neurone. Just as the house is built up
by adding brick upon brick, so brain, cord, nerves and organs of sense
are formed by the union of numberless neurones.
[Illustration: FIG. 6.--Neurones in different stages of development,
from _a_ to _e_. In _a_, the elementary cell body alone is present; in
_c_, a dendrite is shown projecting upward and an axon downward.--After
DONALDSON.]
THE NEURONE.--What, then, is a neurone? What is its structure, its
function, how does it act? A neurone is _a protoplasmic cell, with its
outgrowing fibers_. The cell part of the neurone is of a variety of
shapes, triangular, pyramidal, cylindrical, and irregular. The cells
vary in size from 1/250 to 1/3500 of an inch in diameter. In general the
function of the cell is thought to be to generate the nervous energy
responsible for our consciousness--sensation, memory, reasoning, feeling
and all the rest, and for our movements. The cell also provides for the
nutrition of the fibers.
[Illustration: FIG. 7.--Longitudinal (a) and Transverse (b) section of
nerve fiber. The heavy border represents the medullary, or enveloping
sheath, which becomes thicker in the larger fibers.--After DONALDSON.]
NEURONE FIBERS.--The neurone fibers are of two kinds, _dendrites_ and
_axons_. The dendrites are comparatively large in diameter, branch
freely, like the branches of a tree, and extend but a relatively short
distance from the parent cell. Axons are slender, and branch but little,
and then approximately at right angles. They reach a much greater
distance from the cell body than the dendrites. Neurones vary greatly in
length. Some of those found in the spinal cord and brain are not more
than 1/12 of an inch long, while others which reach from the extremities
to the cord, measure several feet. Both dendrites and axons are of
diameter so small as to be invisible except under the microscope.
NEUROGLIA.--Out of this simple structural element, the neurone, the
entire nervous system is built. True, the neurones are held in place,
and perhaps insulated, by a kind of soft cement called _neuroglia_. But
this seems to possess no strictly nervous function. The number of the
microscopic neurones required to make up the mass of the brain, cord and
peripheral nervous system is far beyond our mental grasp. It is computed
that the brain and cord contain some 3,000 millions of them.
COMPLEXITY OF THE BRAIN.--Something of the complexity of the brain
structure can best be understood by an illustration. Professor Stratton
estimates that if we were to make a model of the human brain, using for
the neurone fibers wires so small as to be barely visible to the eye, in
order to find room for all the wires the model would need to be the size
of a city block on the base and correspondingly high. Imagine a
telephone system of this complexity operating from one switch-board!
"GRAY" AND "WHITE" MATTER.--The "gray matter" of the brain and cord is
made up of nerve cells and their dendrites, and the terminations of
axons, which enter from the adjoining white matter. A part of the mass
of gray matter also consists of the neuroglia which surrounds the nerve
cells and fibers, and a network of blood vessels. The "white matter" of
the central system consists chiefly of axons with their enveloping or
medullary, sheath and neuroglia. The white matter contains no nerve
cells or dendrites. The difference in color of the gray and the white
matter is caused chiefly by the fact that in the gray masses the
medullary sheath, which is white, is lacking, thus revealing the ashen
gray of the nerve threads. In the white masses the medullary sheath is
present.
4. GROSS STRUCTURE OF THE NERVOUS SYSTEM
DIVISIONS OF THE NERVOUS SYSTEM.--The nervous system may be considered
in two divisions: (1) The _central_ system, which consists of the brain
and spinal cord, and (2) the _peripheral_ system, which comprises the
sensory and motor neurones connecting the periphery and the internal
organs with the central system and the specialized end-organs of the
senses. The _sympathetic_ system, which is found as a double chain of
nerve connections joining the roots of sensory and motor nerves just
outside the spinal column, does not seem to be directly related to
consciousness and so will not be discussed here. A brief description of
the nervous system will help us better to understand how its parts all
work together in so wonderful a way to accomplish their great result.
THE CENTRAL SYSTEM.--In the brain we easily distinguish three major
divisions--the _cerebrum_, the _cerebellum_ and the _medulla oblongata_.
The medulla is but the enlarged upper part of the cord where it connects
with the brain. It is about an inch and a quarter long, and is composed
of both medullated and unmedullated fibers--that is of both "white" and
"gray" matter. In the medulla, the unmedullated neurones which comprise
the center of the cord are passing to the outside, and the medullated to
the inside, thus taking the positions they occupy in the cerebrum. Here
also the neurones are crossing, or changing sides, so that those which
pass up the right side of the cord finally connect with the left side of
the brain, and vice versa.
THE CEREBELLUM.--Lying just back of the medulla and at the rear part of
the base of the cerebrum is the cerebellum, or "little brain,"
approximately as large as the fist, and composed of a complex
arrangement of white and gray matter. Fibers from the spinal cord enter
this mass, and others emerge and pass on into the cerebrum, while its
two halves also are connected with each other by means of cross fibers.
[Illustration: FIG. 8.--View of the under side of the brain. B, basis of
the crura; P, pons; Mo, medulla oblongata; Ce, cerebellum; Sc, spinal
cord.]
THE CEREBRUM.--The cerebrum occupies all the upper part of the skull
from the front to the rear. It is divided symmetrically into two
hemispheres, the right and the left. These hemispheres are connected
with each other by a small bridge of fibers called the _corpus
callosum_. Each hemisphere is furrowed and ridged with convolutions, an
arrangement which allows greater surface for the distribution of the
gray cellular matter over it. Besides these irregularities of surface,
each hemisphere is marked also by two deep clefts or _fissures_--the
fissure of Rolando, extending from the middle upper part of the
hemisphere downward and forward, passing a little in front of the ear
and stopping on a level with the upper part of it; and the fissure of
Sylvius, beginning at the base of the brain somewhat in front of the
ear and extending upward and backward at an acute angle with the base
of the hemisphere.
[Illustration: FIG. 9.--Diagrammatic side view of brain, showing
cerebellum (CB) and medulla oblongata (MO). F' F'' F''' are placed on
the first, second, and third frontal convolutions, respectively; AF, on
the ascending frontal; AP, on the ascending parietal; M, on the
marginal; A, on the angular. T' T'' T''' are placed on the first,
second, and third temporal convolutions. R-R marks the fissure of
Rolando; S-S, the fissure of Sylvius; PO, the parieto-occipital
fissure.]
The surface of each hemisphere may be thought of as mapped out into four
lobes: The frontal lobe, which includes the front part of the hemisphere
and extends back to the fissure of Rolando and down to the fissure of
Sylvius; the parietal lobe, which lies back of the fissure of Rolando
and above that of Sylvius and extends back to the occipital lobe; the
occipital lobe, which includes the extreme rear portion of the
hemisphere; and the temporal lobe, which lies below the fissure of
Sylvius and extends back to the occipital lobe.
THE CORTEX.--The gray matter of the hemispheres, unlike that of the
cord, lies on the surface. This gray exterior portion of the cerebrum is
called the _cortex_, and varies from one-twelfth to one-eighth of an
inch in thickness. The cortex is the seat of all consciousness and of
the control of voluntary movement.
[Illustration: FIG. 10.--Different aspects of sections of the spinal
cord and of the roots of the spinal nerves from the cervical region: 1,
different views of anterior median fissure; 2, posterior fissure; 3,
anterior lateral depression for anterior roots; 4, posterior lateral
depression for posterior roots; 5 and 6, anterior and posterior roots,
respectively; 7, complete spinal nerve, formed by the union of the
anterior and posterior roots.]
THE SPINAL CORD.--The spinal cord proceeds from the base of the brain
downward about eighteen inches through a canal provided for it in the
vertebræ of the spinal column. It is composed of white matter on the
outside, and gray matter within. A deep fissure on the anterior side and
another on the posterior cleave the cord nearly in twain, resembling the
brain in this particular. The gray matter on the interior is in the form
of two crescents connected by a narrow bar.
The _peripheral_ nervous system consists of thirty-one pairs of
_nerves_, with their end-organs, branching off from the cord, and twelve
pairs that have their roots in the brain. Branches of these forty-three
pairs of nerves reach to every part of the periphery of the body and to
all the internal organs.
[Illustration: FIG. 11.--The projection fibers of the brain. I-IX, the
first nine pairs of cranial nerves.]
It will help in understanding the peripheral system to remember that a
_nerve_ consists of a bundle of neurone fibers each wrapped in its
medullary sheath and sheath of Schwann. Around this bundle of neurones,
that is around the nerve, is still another wrapping, silvery-white,
called the neurilemma. The number of fibers going to make up a nerve
varies from about 5,000 to 100,000. Nerves can easily be identified in a
piece of lean beef, or even at the edge of a serious gash in one's own
flesh!
Bundles of sensory fibers constituting a sensory nerve root enter the
spinal cord on the posterior side through holes in the vertebræ. Similar
bundles of motor fibers in the form of a motor nerve root emerge from
the cord at the same level. Soon after their emergence from the cord,
these two nerves are wrapped together in the same sheath and proceed in
this way to the periphery of the body, where the sensory nerve usually
ends in a specialized _end-organ_ fitted to respond to some certain
stimulus from the outside world. The motor nerve ends in minute
filaments in the muscular organ which it governs. Both sensory and motor
nerves connect with fibers of like kind in the cord and these in turn
with the cortex, thus giving every part of the periphery direct
connection with the cortex.
[Illustration: FIG. 12.--Schematic diagram showing association fibers
connecting cortical centers with each other.--After JAMES and STARR.]
The _end-organs_ of the sensory nerves are nerve masses, some of them,
as the taste buds of the tongue, relatively simple; and others, as the
eye or ear, very complex. They are all alike in one particular; namely,
that each is fitted for its own particular work and can do no other.
Thus the eye is the end-organ of sight, and is a wonderfully complex
arrangement of nerve structure combined with refracting media, and
arranged to respond to the rapid ether waves of light. The ear has for
its essential part the specialized endings of the auditory nerve, and is
fitted to respond to the waves carried to it in the air, giving the
sensation of sound. The end-organs of touch, found in greatest
perfection in the finger tips, are of several kinds, all very
complicated in structure. And so on with each of the senses. Each
particular sense has some form of end-organ specially adapted to respond
to the kind of stimulus upon which its sensation depends, and each is
insensible to the stimuli of the others, much as the receiver of a
telephone will respond to the tones of our voice, but not to the touch
of our fingers as will the telegraph instrument, and _vice versa_. Thus
the eye is not affected by sounds, nor touch by light. Yet by means of
all the senses together we are able to come in contact with the material
world in a variety of ways.
5. LOCALIZATION OF FUNCTION IN THE NERVOUS SYSTEM
DIVISION OF LABOR.--Division of labor is the law in the organic world as
in the industrial. Animals of the lowest type, such as the amoeba, do
not have separate organs for respiration, digestion, assimilation,
elimination, etc., the one tissue performing all of these functions. But
in the higher forms each organ not only has its own specific work, but
even within the same organ each part has its own particular function
assigned. Thus we have seen that the two parts of the neurone probably
perform different functions, the cells generating energy and the fibers
transmitting it.
It will not seem strange, then, that there is also a division of labor
in the cellular matter itself in the nervous system. For example, the
little masses of ganglia which are distributed at intervals along the
nerves are probably for the purpose of reënforcing the nerve current,
much as the battery cells in the local telegraph office reënforce the
current from the central office. The cellular matter in the spinal cord
and lower parts of the brain has a very important work to perform in
receiving messages from the senses and responding to them in directing
the simpler reflex acts and movements which we learn to execute without
our consciousness being called upon, thus leaving the mind free from
these petty things to busy itself in higher ways. The cellular matter of
the cortex performs the highest functions of all, for through its
activity we have consciousness.
[Illustration: FIG. 13.--Side view of left hemisphere of human brain,
showing the principal localized areas.]
The gray matter of the cerebellum, the medulla, and the cord may receive
impressions from the senses and respond to them with movements, but
their response is in all cases wholly automatic and unconscious. A
person whose hemispheres had been injured in such a way as to interfere
with the activity of the cortex might still continue to perform most if
not all of the habitual movements of his life, but they would be
mechanical and not intelligent. He would lack all higher consciousness.
It is through the activity of this thin covering of cellular matter of
the cerebrum, the _cortex_, that our minds operate; here are received
stimuli from the different senses, and here sensations are experienced.
Here all our movements which are consciously directed have their origin.
And here all our thinking, feeling, and willing are done.
DIVISION OF LABOR IN THE CORTEX.--Nor does the division of labor in the
nervous system end with this assignment of work. The cortex itself
probably works essentially as a unit, yet it is through a shifting of
tensions from one area to another that it acts, now giving us a
sensation, now directing a movement, and now thinking a thought or
feeling an emotion. Localization of function is the rule here also.
Certain areas of the cortex are devoted chiefly to sensations, others to
motor impulses, and others to higher thought activities, yet in such a
way that all work together in perfect harmony, each reënforcing the
other and making its work significant. Thus the front portion of the
cortex seems to be devoted to the higher thought activities; the region
on both sides of the fissure of Rolando, to motor activities; and the
rear and lower parts to sensory activities; and all are bound together
and made to work together by the association fibers of the brain.
In the case of the higher thought activities, it is not probable that
one section of the frontal lobes of the cortex is set apart for
thinking, one for feeling, and one for willing, etc., but rather that
the whole frontal part of the cortex is concerned in each. In the motor
and sensory areas, however, the case is different; for here a still
further division of labor occurs. For example, in the motor region one
small area seems connected with movements of the head, one with the arm,
one with the leg, one with the face, and another with the organs of
speech; likewise in the sensory region, one area is devoted to vision,
one to hearing, one to taste and smell, and one to touch, etc. We must
bear in mind, however, that these regions are not mapped out as
accurately as are the boundaries of our states--that no part of the
brain is restricted wholly to either sensory or motor nerves, and that
no part works by itself independently of the rest of the brain. We name
a tract from the predominance of nerves which end there, or from the
chief functions which the area performs. The motor localization seems to
be the most perfect. Indeed, experimentation on the brains of monkeys
has been successful in mapping out motor areas so accurately that such
small centers as those connected with the bending of one particular leg
or the flexing of a thumb have been located. Yet each area of the cortex
is so connected with every other area by the millions of association
fibers that the whole brain is capable of working together as a unit,
thus unifying and harmonizing our thoughts, emotions, and acts.
6. FORMS OF SENSORY STIMULI
Let us next inquire how this mechanism of the nervous system is acted
upon in such a way as to give us sensations. In order to understand
this, we must first know that all forms of matter are composed of minute
atoms which are in constant motion, and by imparting this motion to the
air or the ether which surrounds them, are constantly radiating energy
in the form of minute waves throughout space. These waves, or
radiations, are incredibly rapid in some instances and rather slow in
others. In sending out its energy in the form of these waves, the
physical world is doing its part to permit us to form its acquaintance.
The end-organs of the sensory nerves must meet this advance half-way,
and be so constructed as to be affected by the different forms of energy
which are constantly beating upon them.
[Illustration: FIG. 14.--The prism's analysis of a bundle of light rays.
On the right are shown the relation of vibration rates to temperature
stimuli, to light and to chemical stimuli. The rates are given in
billions per second.--After WITMER.]
THE END-ORGANS AND THEIR RESPONSE TO STIMULI.--Thus the radiations of
ether from the sun, our chief source of light, are so rapid that
billions of them enter the eye in a second of time, and the retina is of
such a nature that its nerve cells are thrown into activity by these
waves; the impulse is carried over the optic nerve to the occipital lobe
of the cortex, and the sensation of sight is the result. The different
colors also, from the red of the spectrum to the violet, are the result
of different vibration rates in the waves of ether which strike the
retina; and in order to perceive color, the retina must be able to
respond to the particular vibration rate which represents each color.
Likewise in the sense of touch the end-organs are fitted to respond to
very rapid vibrations, and it is possible that the different qualities
of touch are produced by different vibration rates in the atoms of the
object we are touching. When we reach the ear, we have the organ which
responds to the lowest vibration rate of all, for we can detect a sound
made by an object which is vibrating from twenty to thirty times a
second. The highest vibration rate which will affect the ear is some
forty thousand per second.
Thus it is seen that there are great gaps in the different rates to
which our senses are fitted to respond--a sudden drop from billions in
the case of the eye to millions in touch, and to thousands or even tens
in hearing. This makes one wonder whether there are not many things in
nature which man has never discovered simply because he has not the
sense mechanism enabling him to become conscious of their existence.
There are undoubtedly "more things in heaven and earth than are dreamt
of in our philosophy."
DEPENDENCE OF THE MIND ON THE SENSES.--Only as the senses bring in the
material, has the mind anything with which to build. Thus have the
senses to act as messengers between the great outside world and the
brain; to be the servants who shall stand at the doorways of the
body--the eyes, the ears, the finger tips--each ready to receive its
particular kind of impulse from nature and send it along the right path
to the part of the cortex where it belongs, so that the mind can say, "A
sight," "A sound," or "A touch." Thus does the mind come to know the
universe of the senses. Thus does it get the material out of which
memory, imagination, and thought begin. Thus and only thus does the mind
secure the crude material from which the finished superstructure is
finally built.