Studies in the Osteopathic
The Nerve Centers: Volume
Louisa Burns, M.S., D.O., D.Sc.O.
THE METABOLISM OF NEURONS
In its physiological activities the neuron greatly resembles the other
cells of the body, and, indeed, all other living cells. The essential
facts of life are essentially the same everywhere.
in the case of other cells, the nutrition of the neuron is governed by
its nucleus. Any part of the protoplasm, including the prolongations
through their greatest extent, which is separated from the nucleus dies,
becomes degenerated, and is ultimately digested and absorbed. Under
certain conditions, the part of the cell which includes the nucleus may
send out a new growth which takes the place of the lost part. Thus
the nerve cell, the most complex and highly differentiated cell in the
human body, probably, has a power of regeneration which resembles that
of the parts of the lower animals. It is the only highly differentiated
tissue in the human body which has the power to replace a lost part.
specific function of the neuron is to receive the effects of certain environmental
circumstances, transform these into nerve impulses and ultimately transmit
these nerve impulses to the active tissues of the body in such a manner
as to initiate those movements, secretions, and positions of the parts
of the body which most efficiently preserve the life of the individual
and his race. Thus the neuron differs in its metabolism from that
of the muscle cell, whose only function is to shorten, or the gland cell,
whose only function is to form a specific secretion.
food of the neuron, like that of the other cells of the body, is provided
by the blood, and the lymph which arises from the blood. Each cell
is surrounded by its pericellular lymph space, from which it is fed and
into which its wastes are poured. The substances required for the
best nutrition of the nerve cells are those substances found in the blood
flowing through the body of a healthy person. This must include not
only the products of the absorption of good foods, well digested, of thorough
oxygenation, and thorough elimination of the waste products of metabolism,
but it must also include the products of the metabolism of the ductless
glands, the so-called internal secretions. The lack of the internal
secretions, the lack of any important class of food, or of sufficient oxygen
supply, must impair the nutrition of the neuron and cause abnormal functional
conditions of the nerve centers. The same disturbances may result
from abnormal circulatory conditions. Recovery from such conditions
must depend upon the recognition and correction of the fault. It
must be recognized that the activity of the nerve centers probably results
in the correction of many faults of living and of structural conditions
without the intermediation of any particular therapeutic measures.
Nothing but the most temporary relief can follow the use either of drugs
or of non-drug methods which increase or decrease neuron activity without
at the same time correcting the environmental conditions responsible for
activity of any nerve center, and of the nervous system as a whole, is
simply the sum of the activities of the constituent neurons. Any
neuron, properly nourished and properly stimulated, performs its proper
duties, so long as its structure remains normal. It seems certain,
then, that any nerve center must perform its normal duties if its structural
relations are normal, if it is properly nourished, and if it receives the
stimuli normal to it. The entire nervous system is but the sum of
its parts; if these be normal, its activity must be normal.
nerve cell has for its specific function the receipt of nerve impulses
and their transmission in a more or less modified form to other nerve cells
or to the active structures of the body, such as muscles, gland cells,
and the like. During the intervals of this activity the nerve cell
is supposed to be at rest. During rest the cell body becomes of homogeneous
appearance, the vacuoles disappear from protoplasm and nucleus, the nucleus
becomes round and centrally placed, the tigroid masses become of large
size, of angular outline and capable of staining deeply. During long
periods of forced activity the nucleus shrinks and later becomes vacuolated,
becomes eccentrically placed, its outline becomes of ragged appearance,
and it takes the nuclear stains more deeply than do the rested nuclei.
The protoplasm becomes shrunken also, and vacuolated; the tigroid masses
become smaller, less angular in outline, and finally disappear; the protoplasm
takes a pale, uniform blue tint with the stains used for the demonstration
of the tigroid masses.
functional effects of fatigue upon the neuron are not less marked.
The first effect of fatigue upon the neuron is that of a stimulant.
Since fatigue is really due to the accumulation of an excess of the waste
products of metabolism in the blood, it is probable that it is these more
or less acid products which irritate the nervous tissues. This increased
irritability usually leads to increased activity, the reflexes are increased,
voluntary action is increased, the subject is apt to be conscious of a
feeling of well being and of great ability for work. If the real
condition is recognized and a short rest is taken, the later more serious
symptoms may be avoided. If the nerve cells are kept in activity,
a later stage of decreased irritability is found. The reflexes usually
remain increased for a long time, but the more complex neurons in which
the higher coordinations are made become unable to act in any normal manner.
The condition at this time is that of the neurasthenic. The same
sequence of symptoms may be produced by an accumulation of the wastes
of metabolism in the blood without any overactivity of the nervous system,
by abnormal eliminating organs, by wrong habits of living, eating, breathing,
working, etc., or by bony or other structural malpositions which interfere
with the circulation of the blood or the activity of the organs of elimination.
used in the effort to avoid fatigue add to the disturbance, partly by increasing
neuron activity beyond normal limits, partly by adding to the poisonous
substances in the blood and lymph.
more complex coordinations, those concerned in the activity of the whole
body, the emotional and logical and volitional reactions, are governed
by correspondingly complex nerve centers. The individual neurons
of these centers are also more complex; each center includes neurons of
greater variety of form, and the relations of these to one another are
most intricate. These are thus more easily affect ed by abnormal
conditions than are the simpler neurons of the lower and less complex centers.
Thus the reflexes and the autonomic activities may remain normal for a
long time after the reasoning and memory are less efficient, the loss of
these may leave the emotional centers excessively excitable, and so on.
It is the higher centers, as a rule almost without exception, which are
most seriously injured by excessive fatigue, by the drug addictions, and
by the autointoxications.
however, extracellular poisons seem to have a selective effect upon the
lower neurons. Thus certain poisons act in a typical manner.
Metabolism of Neurons
In locomotor ataxia the long sensory fibers are first affected. These
carry the impulses concerned in the appreciation of touch and of muscular
effort, and the impulses thus carried are concerned in the cerebellar activities
of coordination and equilibration. The effect of the poison upon
the long, more highly developed fibers is doubtless due in part to the
fact that the fibers extend farther from the nucleus than do shorter fibers,
and in part to the fact that the cells representing the highest development,
the greatest differentiation, are most easily affected by abnormal environmental
conditions. The impulses of pain and temperature are carried by the
spinothalmic and the anterior ascending cerebellar tract, and also by short
fibers through the spinal gray matter. These thus retain their function
and structure in a fairly normal manner under decidedly abnormal conditions.
relative frequency of lateral sclerosis, affecting the long fibers of the
highly developed pyramidal cells, also illustrates the same principle.
resting cell is stimulated to increased activity by changes in the environment.
No nerve cells act independently, nor is their activity a matter of chance
or whim. There is no change in the functional activity of the neuron
except as this is initiated by changes in its environment, or in its structural
the most part, the change which initiates increased functional activity
on the part of a neuron is the impulse sent to it from some other neuron.
By far the larger number of all nerve cells in the body are dependent upon
other nerve cells for their stimulation. It is by means of the almost
infinite variety of numbers of nerve cells which may be affected by comparatively
few sensory impulses that we have the inconceivably complex reactions to
simple original stimuli.
The sensory neurons of the first order, alone, receive impulses from extra-somatic
sources. All of these are affected by changes in the environment
of the body, or by changes in the condition of the body. These sensory
neurons are stimulated by remarkably small amounts of external variations.
The amount of light, for example, which is amply sufficient to arouse perfectly
plain images upon the retina is so very small that it is practically impossible
for us to imagine that it could be of any physiological influence whatever.
Also, the amount of energy displayed in the sound of a bell, the mass of
the air thrown into vibration by that slight motion, and the extremely
small fraction of the vibrations which reach the listening ear, seems impossibly
small. The same conditions apply in the case of smell and taste.
It is very evident that the amount of the initial stimulus can bear no
mathematical relation to the amount of nerve impulses aroused by it, and
still less to the sum of the reflex actions initiated by the effects of
the sensory impulses upon the nerve centers. Even more inexplicable
is the nature of the effects of these sensory impulses upon the later reactions,
as they occur through lives modified by the processes of associative memory.
nerve cells affected by the impulses from the sensory neurons on the first
order transmit these impulses, modified or not modified, to other groups
of nerve cells, and these to others, and so on. Ultimately these
impulses arouse motor impulses, and thus affect the life history of the
individual. Practically all motor impulses are thus initiated, and
practically all sensory impulses terminate by initiating motor impulses.
are certain nerve cells, however, which are stimulated by the changes in
their immediate environment, apart from the effects of the impulses from
other cells, or apparently apart from these impulses. The nerve cells
which are especially recognized in this connection are those of the cardiac
and respiratory centers in the medulla. The nerve cells of these
centers are stimulated to increased activity by the presence of an excess
of carbon dioxid in the blood, and their activity is decreased by an excess
of oxygen in the blood, or, rather, in the lymph which immediately bathes
the cells’ bodies.
a certain extent all of the nerve cells of the body are affected by their
immediate environment. Not any of the nerve cells act quite in their
normal manner in the lack of food or oxygen, or in the presence of an excess
of the toxic products of metabolism or of any other poisons, whether produced
within the body or used as drug or stimulant.
The Liminal Value
liminal value, threshold value, and neuron threshold are all terms which
are used to express the relative amount of stimulation necessary to affect
the activity of the neuron in a perceptible manner. Amounts of stimulation
which do not initiate the nerve impulse are called “submiminal.”
Stimuli which are submiminal may affect the activity of the neuron in some
way, since the repetition of submiminal stimuli at frequent intervals may
ultimately cause the discharge of a nerve impulse. This condition
is called the “summation of stimuli.” The periodical discharge of
impulses from certain nerve centers is probably due to the summation of
the submiminal, or inefficient stimuli. The epileptic fit may be
due to summation of abnormal stimuli.
amount of stimulation necessary to cause the discharge of the nerve impulse
by any given neuron is the liminal value of that neuron. The amount
of stimulation necessary to cause the discharge of nerve impulses by any
nerve center is the liminal value of that nerve center. As the irritability
of any cell or any center increases, its liminal value decreases.
It is evident that the liminal value of any neuron or any center may, under
abnormal conditions, be either too high or too low, and that the normal
activity of the nervous system as a whole or in any of its parts depends
upon the existence of the normal liminal value of each of its constituent
liminal value of any neuron or any center may be lowered normally by short
periods of rest, by frequent stimulation, by the presence of normal nutritive
conditions, and the normal elimination of the wastes of metabolism.
Normally, the liminal value is raised by long periods of rest, by lack
certain abnormal conditions the liminal value may become too high or too
poisons, as strychnine, quinine, alcohol, caffeine, etc., and the products
of the body metabolism in general, present in the blood in small amounts,
lower the liminal value of the neurons. Slightly increased temperature,
slightly increased blood pressure, and slight degrees of fatigue, all lower
the liminal value of the neurons. Thus the excitement and the increased
reflexes and the stimulating effects of these conditions.
amounts of the poisons mentioned and others which will occur to every one,
greater increase of temperature, greater increase of blood pressure, all
raise the liminal value of the neurons to an abnormal extent. Thus
is produced the paralysis of the nerve centers, the inertia of mental activities,
and the loss of reflexes associated with the more pronounced degrees of
poisoning, or of fevers, or of very high blood pressure.
is thus seen that the very things which increase neuron activity when used
in small amounts, cause the decrease of cell energy and ultimately the
destruction of the cells when used in greater amounts. It must be
recognized that these methods of increasing or decreasing the liminal value
are abnormal—nothing can be added to the normal environment of the neuron
which increases its energy output without at the same time lessening its
real value as an efficient part of the nervous system. Any stimulant
beyond the normal blood, and the normal stimulation from normally-aroused
sensory impulses, must injure the neurons affected, and ultimately lessen
the efficiency of the nervous system as a whole.
The Nature of the Nerve Impulse
term “impulse,” which has been applied to the excitation which passes from
the cell body over the axon of a neuron, or which passes toward the cell
body from a sensory nerve ending, owes its chief merit as a name to the
fact that it expresses nothing of the nature of this excitation.
In other words, while in most terminology the naming of anything should
be done by applying some term which indicates the nature of the thing named;
in this case the merit of the name given lies in the fact that it makes
no attempt to even suggest the character of the thing named. And
this is good, because the real nature of that which passes over the nerve
fiber, which causes in muscles contraction, in glands secretion, in cortical
nerve cells the changes which affect consciousness, in other nerve cells
other action, leading to yet further stimulation—the nature of this thing
which produces these variable changes in the cells affected by it, yet
remains an inexplicable mystery. So, since there is as yet no adequate
conception of the nature of the thing named, the term “impulse,” which
means only “something impelled” or “sent,” is most fortunately applied.
it is true that we know nothing of the real nature of the nerve impulse,
we have determined some facts which seem to govern its action. A
consideration of these data may be given some attention, though it must
be clearly understood that these studies are very imperfect, and that the
investigations in progress may at any time cause our view to be altogether
changed. Very much careful study needs to be done in this field before
we may decide any one of many questions now in dispute.
Direction of the Nerve Impulse
The facts which are known to be true in regard to the passage of the nerve
impulse over a nerve fiber are many, yet the significance of these facts,
and their application to the functions of the nerve fibers and the impulses
transmitted are so various, and in some cases so contradictory, that one
must necessarily doubt whether any real harmony can exist between so many
nerve impulse passes over any given nerve fiber always in the same direction.
Under experimental conditions, such as the direct stimulation of the nerve
trunk, the impulse may be caused to pass in both directions, but this probably
never occurs in the unmutilated body.
electrical phenomena associated with the passage of a nerve impulse over
a nerve fiber are of interest in this connection. When a nerve impulse
traverses a nerve trunk, there is produced in the fiber a change in its
electrical condition—a wave of negativity which passes at the same rate
and in the same direction as the wave of nerve impulse. This can
be demonstrated absolutely for motor and sensory nerves, and also for the
non-medullated nerves. So constant is this wave of negativity, and
so correctly related to the passage of the nerve impulse, that its
occurrence has been made a criterion for the nerve impulse itself, in those
cases in which the nerve impulse itself is not easily or not possibly determined.
example, if a nerve trunk be cut, or be removed from the body, and be stimulated
midway in its course, there is produced a wave of negativity which travels
both peripherally toward the muscle, which is caused to contract by the
associated nerve impulse, and there is also produced a wave of negativity
which travels centrally--that is, in the direction opposite to that traversed
by the impulses normally carried by the nerve experimented upon.
The occurrence of this wave of negativity is held as evidence that under
this condition the nerve impulse can be carried in both directions.
occurrence of similar waves of negativity in sensory nerves has aided in
the investigation of certain physiological problems associated with these
manner in which the passage of nerve impulses over the fibers is affected
by electrical reactions is also of interest. If the electrodes carrying
the continuous current are placed upon any nerve, there is thus produced,
at the time of the making of connection, and again at the time of breaking
the connection, the muscular contraction which should ensue upon the normal
stimulation of that nerve. During the continuous passage of the unvarying
current from the non-polarizable electrodes through the nerve, there
is not produced any muscular contraction. But during this time there
is produced in the nerve trunk certain modifications of its activity.
If the current be an ascending one—that is, if the negative electrode be
placed nearest the muscle—then there is produced in the nerve trunk below
the negative electrode a condition called catelectrotonus, in which the
excitability of the nerve is increase. At the same time, above the
positive electrode, there is produced a condition called anelectrotonus,
in which the excitability of the nerve is decreased. The part of
the nerve trunk which lies between the two electrodes is called the interpolar
section; the area nearest the anode is in a condition of anelectrotonus,
the part nearest the cathode is in a condition of catelectrotonus.
If the current be a weak one the catelectrotonic area is larger; if the
current be strong, the anelectrotonic area may occupy nearly the whole
of the interpolar space. Indeed, a very strong current may cause
an anelectrotonic condition of the whole nerve subject to the experiment.
several investigators the phenomena associated with electrotonus have been
produced by various models of wires in paraffine, with weak currents of
electricity passing in various directions through them. These imitations
indicate that the nerve impulse follows many of the laws governing the
electrical current, and also that electricity is associated with nervous
activity. If these conditions were recognized, and no others, the
conclusion would be fairly just that in dealing with the nerve impulse
we are dealing with some more complex manifestation of electricity.
But this is not the case—there are other no less important phenomena to
be considered in this study.
Nerve impulses are not able to pass over the nerve trunk which is kept
at a temperature a little above freezing. A test tube filled with
ice water, for example, forms a very efficient block for experimental purposes.
nerve impulse can not pass over the nerve trunk which is passed through
a vessel containing carbon dioxide, or hydrogen, or nitrogen, or any other
gas, to the exclusion of oxygen. Since oxygen is an essential factor
in the passage of the nerve impulse, there must be some chemical action
associated with the passing of the nerve impulse which is not to be considered
in the discussion of the passing of an electrical current. Chloroform,
ether, and other poisons efficiently block the passage of the nerve impulse.
None of these can be considered as affecting the passage of the electrical
The nerve fibers seem not to be subject to fatigue, even after very long
stimulation. Halliburton and Brodie stimulated the splenic nerves
of a dog for nine hours with an induced current, and upon the removal of
block (a tube of ice water) the contractions of splenic muscle again occurred.
No investigator has been able to demonstrate fatigue in nerves, except
as their temperature should be greatly lowered.
presence of carbon dioxide has not been certainly shown in the passage
of nerve impulses over nerve fibers. It has not been shown that any
rise of temperature is produced by the passage of the nerve impulse.
Initiation of the Nerve Impulse
normal conditions, nerve impulses may be initiated under most diverse conditions.
waves cause the stimulation of the endings of the auditory waves, and this
stimulation is qualitatively and quantitatively dependent upon the rate,
and force, and combinations of the exciting vibrations. Light waves
excite the rods and cones, and this stimulation is qualitatively dependent
upon the stimulus. But in the case of color perception, the differences
in vibratory rate affect the visual apparatus in such a way as to cause
sensations which are qualitatively different, as red and yellow and blue,
to result from the mere speed differences of the vibratory rates.
In the case of taste, we have similar phenomena. Substances which
are chemically and physically closely related, such as sugar and starch,
have no relationship in taste, necessarily, while there may be great similarity
in the tastes of substances whose chemical and physical relations are not
at all alike. This is shown in the similar tastes of saccharine and
sugar, of picric acid and quinine. Also, in the sense of smell there
is not necessarily any qualitative relationship between substances which
affect the sense in similar manners.
facts are elemental, and do not depend upon any deceit in sensation.
A certain deceit is shown in many ways, as in the hungry feeling associated
with dyspepsia, or the false judgments of sight, etc.; but in dealing with
the simpler primary sensations we must realize that there is no essential
relationship between the effect produced in consciousness or in reflex
actions and the real nature of the thing which gives origin to the nerve
impulses. In other words, there is no accounting for nerve impulses
in terms of the original stimulation, except as these relations become
known to us by experience and correlation during life.
the nervous system, upon the receipt of sensory impulses of whatever origin,
there is aroused in other cells the physiological change associated with
the passage of the nerve impulse. The sensory neuron of the second
order is caused to initiate the nerve impulse by the effects produced upon
it by the sensory neuron of the first order, and the nerve impulse thus
initiated causes the stimulation of yet higher neurons, and thus the impulse
is carried, through devious pathways, to the reflex and conscious centers.
It is true that nerve cells may be caused to act by local conditions, such
as the character of the blood flowing through the centers. In cases
of tumors and other local conditions, also, the nerve cells may be directly
stimulated. But for the most part the stimulation of the cells of
the central nervous system depends upon the effect of the impulses from
the sensory nerves, and from cells in relation to the sensory nerves.
motor nerves carry the impulses, however produced, to the structures with
which they are related. The effects produced by these impulses are
almost as varied as are the stimuli which initiate the sensory impulses
in the first place. Yet there is no more reason to impute differences
in the nature of the impulses concerned to the motor than to the sensory
nerves. It seems to be the function of the structures which the nerve
impulses reach to transmit these into the activity peculiar to their own
structure. The gland secretes its own juices, the muscle contracts
in its own manner, the cells are modified in their physiological condition
by the nerve impulses in the manner in which their own peculiar function
nerve impulse, then, represents an infinitesimal amount of energy.
Its apparent activity is chiefly due to its causing the use of energy by
other structures. It is associated with the production of electricity,
but it is not electricity as we now understand electricity. It is
associated with certain of the phenomena of metabolism, but it fails in
other aspects of metabolic action as we find this action in other physiological
activities. Nerve impulses must be essentially alike, yet the manner
of their transmission, and even more the nature of their effects in consciousness,
differ very widely.
nerve impulse travels at very different rates, always much slower than
does electricity. It is fastest in the higher animals, as a rule
not without exceptions. Its rate is modified by disease, and by the
physiological conditions of the structures which transmit it. Its
production by the nerve cell is subject to practically infinite variation,
both in different cells and in the same cell at different times.
transmission of nerve impulses changes the cells affected in their physiological
condition. The cells which today transmit an impulse of a certain
origin and effect, tomorrow are somewhat more easily affected by similar
conditions. This effect upon the condition of the nerve cell is permanent,
and is probably the source of memory, as it is certainly the important
consideration in the formation of habit.
Specific Nerve Energies
to the “Doctrine of Specific Nerve Energies” of Johannes Muller, each sensory
neuron arouses in consciousness its own quality of sensation and no other.
For example, the specific energy of each neuron in the chain leading from
the ganglion spirale to the auditory area in the cerebral cortex is the
sensation of sound; the specific energy in each neuron concerned in carrying
impulses of touch is touch, and so on. There are certain facts of
clinical and experimental evidence which support this view.
of “cold spots” by the electric current give the sensation of cold, the
same stimulation of another skin area may give the sensation of heat, of
another area the sensation of touch, etc., while if the same electrodes
are placed upon the zygoma or the mastoid, sensations of sound result;
if they are placed upon the frontal bone, lights flash before the eyes;
placed upon different parts of the tongue, different sensations of touch,
pain and taste are perceived.
any part of the brain be diseased or injured in such a manner as to stimulate
the nerve cells of the cortex, sensations arise in consciousness which
are the specific energies of the parts affected.
aura of certain forms of epilepsy is frequently the specific energy of
the part of the brain affected. This is so well recognized that surgical
measures for the relief of this condition are usually successful in the
sense of determining the nature of the disease, though not so often in
the sense of securing recovery from the attacks. These facts seem
to support the view that each neuron carries its own qualitative impulses
and none other.
the other hand, it is noted that the stimulation of nerve trunks, as, for
example, a blow upon the ulnar nerve, never produces in consciousness exactly
the same sensations as those aroused by stimulation of the sensory nerve
endings in the fingers. Also, while stimulation of the optic and
auditory nerves by other agents than light or sound is followed by conscious
sensations of light or sound, these sensations are merely those of flashes
of light and of snapping or rumbling noises, but that exact sights or sounds
of things can not be produced in consciousness, either by abnormal stimulation
or as the effect of disease or injury. The appearance of a tree,
for example, or the notes of a song are not to be produced in consciousness
by any experimental stimulation of nerve trunks. Disease of the memory
areas of the cortex may, however, be associated with the reproduction of
sights, sounds, and other sensations long since experienced.
the doctrine of specific nerve energies be true, it becomes necessary to
determine whether the differences in the sensations around in consciousness
by different sensory nerve stimulations is due to essential differences
in the structure of the neuron, or to its characteristic end organs, or
to the relations and connections of the cortical cells. That there
are very great differences in the structure of the end organs is evident.
But the fact that pathological stimulation of the cortex produces the same
class of sensations as those aroused by the normal stimulation of the same
area indicates that the determining factors in the production of the various
sensations are not found in peripheral sensory neurons alone; unless, indeed,
we ascribe to habit and education the association of the various sensations
with the stimulation of the corresponding cortical areas. Were this
true—that is, if there be no primary difference between the form of sensation
aroused in consciousness by the stimulation of the cells of the temporal
lobe and those produced by the stimulation of cells in the occipital lobe,
it is difficult to see how the associations ever became formed in the first
place. On the other hand, it is certainly no less difficult to conceive
of the enormous number of differences in the metabolism of the various
neurons which would be necessary if every form of sensation were produced
each by its own special form of nerve impulse.
the doctrine of specific nerve energies hold true for sensory neurons,
it should hold true for motor neurons as well. It is well known that
the stimulation of the nerve to a muscle causes the muscle to contract,
while the same stimulation of the nerve to a gland initiates the characteristic
secretion of the gland. The mode of action of the so-called inhibitors
suggests one of the most puzzling problems in physiology.
Nature of Sensory Impulses
sensory impulses, in themselves, are probably simply nerve impulses, not
to be distinguished qualitatively from the other nerve impulses.
The changes in the environmental conditions affecting the more or less
specialized endings of the sensory neurons initiate the nerve impulse.
This, being transmitted over the sensory neurons of the first, second and
higher orders, initiates certain reflexes, and, in many cases, reaches
the cortical neurons and arouses consciousness of a more or less specific
lack of any qualitative relationship between the sensations in consciousness
and the qualities of the objects in the external world is very difficult
of comprehension. It is true that in some cases there seems to be
actual correlation between the sensations in consciousness and the thing
which causes the sensation, as in the case of sound. Here the differences
in vibration rate are associated in consciousness with changes in tone
which are qualitatively relative to the changes in vibration rates.
The sensory impulses aroused by changes of pressure, by resistance to effort,
also give rise in consciousness to sensations which probably bear a qualitative
relation to the things perceived. But the sensations aroused in consciousness
by changes in the light vibrations are qualitatively different; it is not
possible to think of green as being merely a “faster” shade of red, nor
can we think of yellow as a “slow” shade of blue.
which are alike chemically do not necessarily taste or smell alike; sugar
tastes not at all like starch, with which it is chemically related, but
very much like saccharine, with which it has practically no chemical relationship
heat is a matter of vibration rate. Nothing in our consciousness
has ever been subjected to the stimulus of an object without heat.
We know, then, not the lack of heat, but only degrees of heat. Yet
in consciousness are two qualitatively distinct sensations, that of heat
and that of cold. That is, from certain degrees of heat we receive
sensations of warmth, and from other degrees of heat we receive sensations
totally different in quality, and capable of arousing both reflex and conscious
actions of a totally different quality.
conditions of the external world are, then, not to be exactly perceived by our
mentality under any conditions, except as we determine them by various scientific
and mathematical methods. What we do perceive is the relation between
the environmental changes and our own reaction to those changes.
In the biological sense those changes in the external world which affect us
at all affect us in such a manner as to initiate those reactions most adapted
to the preservation of the life of the individual and the race. Actual
truth as a mathematical proposition is as far from the biological concept as
is the preservation of the lives of the weaklings.