Studies in the Osteopathic Sciences
The Nerve Centers: Volume 2
Louisa Burns, M.S., D.O., D.Sc.O.
            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.

            As 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.

            The 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.


            The 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 the disturbance.

            The 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.


            The 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.

            The 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.

            Drugs 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.

            The 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.

            Occasionally, 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.

            The relative frequency of lateral sclerosis, affecting the long fibers of the highly developed pyramidal cells, also illustrates the same principle.

Neuron Activity

            The 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 relations.

            For 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.

            The 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.

            There 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.

            To 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

            The 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.

            The 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 neurons.

            The 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 of stimulation.

            Under certain abnormal conditions the liminal value may become too high or too low.

            Certain 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.

            Larger 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.

            It 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

            The 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.

            While 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 discordant factors.

            The 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

            The 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.

            For 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.

            The occurrence of similar waves of negativity in sensory nerves has aided in the investigation of certain physiological problems associated with these nerves also.

            The 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.

            By 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.

Non-electric Phenomena
            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.

            The 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 current.

            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.

            The 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

            Under normal conditions, nerve impulses may be initiated under most diverse conditions.

            Sound 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.

            These 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.

            Within 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.

            The 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 justifies.


            The 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.

            The 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.

            The 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

            According 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.

            Stimulation 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.

            If 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.

            The 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.

            On 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.

            If 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.

            Should 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

            The 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 nature.

            The 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.

            Substances 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 whatever.

            Intrinsically, 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.

            The actual 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.