Each phylogenetic and each ontogenetic experience by an indirect method develops its own mechanism of adaptation in the brain; and the brain threshold is raised or lowered to stimuli by the strength and frequency of repet.i.tion of the experience.

Thus through the innumerable symbols supplied by environment the distance ceptors drive this or that animal according to the type of brain pattern and the particular state of threshold which has been developed in that animal by its phylogenetic and ontogenetic experiences.

The brain pattern depends upon his phylogeny, the state of threshold upon his ontogeny. Each BRAIN PATTERN is created by some particular element in the environment to which an adaptation has been made for the good of the species. The _*state of threshold_ depends upon the effect made upon the individual by his personal contacts with that particular element in his environment. The presence of that element produces in the individual an a.s.sociative recall of the adaptation of his species--that is, the brain pattern developed by his phylogeny becomes energized to make a specific response.

The intensity of the response depends upon the state of threshold-- that is, upon the a.s.sociative recall of the individual's own experience--his ontogeny.

If the full history of the species and of the individual could be known in every detail, then every detail of that individual's conduct in health and disease could be predicted.

Reaction to environment is the basis of conduct, of moral standards, of manners and conventions, of work and play, of love and hate, of protection and murder, of governing and being governed, in fact, of all the reactions between human beings--of the entire web of life.

To quote Sherrington once more: "Environment drives the brain, the brain drives the various organs of the body."

By what means are these adaptations made? What is the mechanism through which adequate responses are made to the stimuli received by the ceptors?

We postulate that in the brain there are innumerable patterns each the mechanism for the performance of a single kind of action, and that the brain-cells supply the energy--electric or otherwise-- by which the act is performed; that the energy stored in the brain-cells is in some unknown manner released by the force which activates the brain pattern; and that through an unknown property of these brain patterns each stimulus causes such a change that the next stimulus of the same kind pa.s.ses with greater facility.

Each separate motor action presumably has its own mechanism-- brain pattern--which is activated by but one ceptor and by that ceptor only when physical force of a certain intensity and rate of motion is applied. This is true both of the visible contacts affecting the nociceptors and of the invisible contacts by those intangible forces which affect the distance ceptors.

For example, each variation in speed of the light-producing waves of ether causes a specific reaction in the brain.

For one speed of ether waves the reaction is the perception of the color blue; for another, yellow; for another, violet.

Changes in the speed of air waves meet with specific response in the brain patterns tuned to receive impressions through the aural nerves, and so we distinguish differences in sound pitch.

If we can realize the infinite delicacy of the mechanisms adapted to these infinitesimal variations in the speed and intensity of invisible and intangible stimuli, it will not be difficult to conceive the variations of brain patterns which render possible the specific responses to the coa.r.s.er contacts of visible environment.

Each brain pattern is adapted for but one type of motion, and so the specific stimuli of the innumerable ceptors play each upon its own brain pattern only. In addition, each brain pattern can react to stimuli applied only within certain limits.

Too bright a light blinds; too loud a sound deafens. No mechanism is adapted for waves of light above or below a certain rate of speed, although this range varies in different individuals and in different species according to the training of the individual and the need of the species.

We have already referred to the fact that there is no receptive mechanism adapted to the stimuli from the _x_-ray, from the high-speed bullet, from electricity. So, too, there are innumerable forces in nature which can excite in man no adaptive response, since there exist in man no brain patterns tuned to their waves, as in the case of certain ethereal and radioactive forces.

On this mechanistic basis the emotions may be explained as activations of the entire motor mechanism for fighting, for escaping, for copulating.

The sight of an enemy stimulates in the brain those patterns formed by the previous experiences of the individual with that enemy, and also the experiences of the race whenever an enemy had to be met and overcome.

Each of these many brain patterns in turn activates that part of the body through which lies the path of its own adaptive response-- those parts including the special energizing or activating organs.

Laboratory experiments show that in an animal driven strongly by emotion the following changes may be seen: (1) A mobilization of the energy-giving compound in the brain-cells, evidenced by a primary increase of the Nissl substance and a later disappearance of this substance and the deterioration of the cells (Figs. 5 and 13); (2) increased output of adrenalin (Cannon), of thyroid secretion, of glycogen, and an increase of the power of oxidation in the muscles; (3) accelerated circulation and respiration with increased body temperature; (4) altered metabolism. All these are adaptations to increase the motor efficiency of the mechanism. In addition, we find an inhibition of the functions of every organ and tissue that consumes energy, but does not contribute directly to motor efficiency.

The mouth becomes dry; the gastric and pancreatic secretions are lessened or are completely inhibited; peristaltic action stops.

The obvious purpose of all these activations and inhibitions is to ma.s.s every atom of energy upon the muscles that are conducting the defense or attack.

So strong is the influence of phylogenetic experience that though an enemy to-day may not be met by actual physical attack, yet the decks are cleared for action, as it were, and the weapons made ready, the body as a result being shaken and exhausted.

The type of emotion is plainly declared by the activation of the muscles which would be used if the appropriate physical action were consummated. In anger the teeth are set, the fists are clenched, the posture is rigid; in fear the muscles collapse, the joints tremble, and the running mechanism is activated for flight; in s.e.xual excitement the mimicry is as obvious.

The emotions, then, are the preparations for phylogenetic activities.

If the activities are consummated, the fuel--glycogen--and the activating secretions from the thyroid, the adrenals, the hypophysis are consumed.

In the activation without action, these products must be eliminated as waste products and so a heavy strain is put upon the organs of elimination. It is obvious that the body under emotion might be clarified by active muscular exercise, but the subject of the emotion is so strongly integrated thereby that it is difficult for him to engage in diverting, clarifying exertion. The person in anger does not want to be saved from the ill effects of his own emotion; he wants only to fight; the person in fear wants only to escape; the person under s.e.xual excitement wants only possession.

All the lesser emotions--worry, jealousy, envy, grief, disappointment, expectation--all these influence the body in this manner, the consequences depending upon the intensity of the emotion and its protraction.

Chronic emotional stimulation, therefore, may fatigue or exhaust the brain and may cause cardiovascular disease, indigestion, Graves'

disease, diabetes, and insanity even.

The effect of the emotions upon the body mechanism may be compared to that produced upon the mechanism of an automobile if its engines are kept running at full speed while the machine is stationary.

The whole machine will be shaken and weakened, the batteries and weakest parts being the first to become impaired and destroyed, the length of usefulness of the automobile being correspondingly limited.

We have shown that the effects upon the body mechanism of the action of the various ceptors is in relation to the response made by the brain to the stimuli received. What is this power of response on the part of the brain but CONSCIOUSNESS? If this is so, then consciousness itself is a reaction to environment, and its intensity must vary with the state of the brain and with the environmental stimuli.

If the brain-cells are in the state of highest efficiency, if their energy has not been drawn upon, then consciousness is at its height; if the brain is fatigued, that is, if the energy stored in the cells has been exhausted to any degree, then the intensity of consciousness is diminished. So degrees of consciousness vary from the height maintained by cells in full vigor through the stages of fatigue to sleep, to the deeper unconsciousness secured by the administration of inhalation anesthetics, to that complete unconsciousness of the environment which is secured by blocking the advent to the brain of all impressions from both distance and contact ceptors, by the use of both local and inhalation anesthetics--the state of anoci-a.s.sociation (Fig. 14).

Animals and man may be so exhausted as to be only semi-conscious.

While a brain perfectly refreshed by a long sleep cannot immediately sleep again, the exhausted brain and the refreshed brain when subjected to equal stimuli will rise to unequal heights of consciousness.

The nature of the physical basis of consciousness has been sought in experiments on rabbits which were kept awake from one hundred to one hundred and nine hours. At the end of this time they were in a state of extreme exhaustion and seemed semi-conscious. If the wakefulness had been further prolonged, this state of semi-consciousness would have steadily changed until it culminated in the permanent unconsciousness of death.

An examination of the brain-cells of these animals showed physical changes identical with those produced by exhaustion from other causes, such as prolonged physical exertion or emotional strain (Figs. 45 and 46). After one hundred hours of wakefulness the rabbits were allowed a long period of sleep. All the brain-cells were restored except those that had been in a state of complete exhaustion.

A single seance of sleep served to restore some of the cells, but those which had undergone extreme changes required prolonged rest.

These experiments give us a definite physical basis for explaining the cost to the body mechanism of maintaining the conscious state.

We have stated that the brain-cell changes produced by prolonged consciousness are identical with those produced by physical exertion and by emotional strain. Rest, then, and especially sleep, is needed to restore the physical state of the brain-cells which have been impaired, and as the brain-cells const.i.tute the central battery of the body mechanism, their restoration is essential for the maintenance of normal vitality.

In ordinary parlance, by consciousness we mean the activity of that part of the brain in which a.s.sociative memory resides, but while a.s.sociative memory is suspended the activities of the brain as a whole are by no means suspended; the respiratory and circulatory centers are active, as are those centers which maintain muscular tone.

This is shown by the muscular response to external stimuli made by the normal person in sleep; by the occasional activation of motor patterns which may break through into consciousness causing dreams; and finally by the responses of the motor mechanism made to the injuring stimuli of an operation on a patient under inhalation anesthesia only.

Direct proof of the mechanistic action of many of life's phenomena is lacking, but the proof is definite and final of the part that the brain-cells play in maintaining consciousness; of the fact that the degree of consciousness and mental efficiency depends upon the physical state of the brain-cells; and finally that efficiency may be restored by sleep, provided that exhaustion of the cells has not progressed too far.

In this greatest phenomenon of life, then, the mechanistic theory is in harmony with the facts.

Perhaps no more convincing proof of our thesis that the body is a mechanism developed and adapted to its purposes by environment can be secured than by a study of that most constant manifestation of consciousness--pain.

Like the other phenomena of life, pain was undoubtedly evolved for a particular purpose--surely for the good of the individual.

Like fear and worry, it frequently is injurious. What then may be its purpose?

We postulate that pain is a result of contact ceptor stimulation for the purpose of securing protective muscular activity.

This postulate applies to all kinds of pain, whatever their cause-- whether physical injury, pyogenic infection, the obstruction of hollow viscera, childbirth, etc.

All forms of pain are a.s.sociated with muscular action, and as in every other stimulation of the ceptors, each kind of pain is specific to the causative stimuli. The child puts his hand in the fire; physical injury pain results, and the appropriate muscular response is elicited. If pressure is prolonged on some parts of the body, anemia of the parts may result, with a corresponding discomfort or pain, requiring muscular action for relief.

When the rays of the sun strike directly upon the retina, light pain causes an immediate protective action, so too in the evacuation of the intestine and the urinary bladder as normal acts, and in overcoming obstruction of these tracts, discomfort or pain compel the required muscular actions. This view of pain as a stimulation to motor action explains why only certain types of infection are a.s.sociated with pain; namely, those types in which the infection may be spread by muscular action or those in which the fixation of parts by continued muscular rigidity is an advantage.

As a further remarkable proof of the marvelous adaptation of the body mechanism to meet varying environmental conditions, we find that just as nociceptors have been implanted in only those parts of the body which have been subject to nocuous contacts, so a type of infection which causes muscular action in one part of the body may cause none when it attacks another.

This postulate gives us the key to the pain-muscular phenomena of peritonitis, pleurisy, cyst.i.tis, cholecyst.i.tis, etc., as well as to the pain-muscular phenomena in obstructions of the hollow viscera.

If pain is a part of a muscular response and occurs only as a result of contact ceptor stimulation by physical injury, infection, anemia, or obstruction, we may well inquire which part of the nerve mechanism is the site of the phenomenon of pain.

Is it the nerve-ending, the nerve-trunk, or the brain? That is, is pain a.s.sociated with the physical contact with the nerve-ending, or with the physical act of transmission along the nerve-trunk, or with the change of brain-cell substance by means of which the motor-producing energy is released?

We postulate that the pain is a.s.sociated with the discharge of energy from the brain-cells. If this be true, then if every nociceptor in the body were equally stimulated in such a manner that all the stimuli should reach the brain-cells simultaneously, then the cells would find themselves in equilibrium and no motor act would be performed.

But if all the pain nerve ceptors but one were equally stimulated, and this one more strongly stimulated than the rest, then this one would gain possession of the final common path--would cause a muscular action and the sensation of pain.

It is well known that when a greater pain or stimulus is thrown into compet.i.tion with a lesser one, the lesser is submerged.

Of this fact the school-boy makes use when he initiates the novice into the mystery of the painless pulling of hair.

The simultaneous but severe application of the boot to the blindfolded victim takes complete but exclusive possession of the final common path and the hair is painlessly plucked as a result of the triumph of the boot stimulus over the pull on the hair in the struggle for the final common path.

Persons who have survived a sudden, complete exposure to superheated steam, or whose bodies have been enwrapped in flame, testify that they have felt no pain. As this absence of pain may be due to the fact that the emotion of fear gained the final common path, to the exclusion of all other stimuli, we are trying by experimentation to discover the effects of simultaneous painful stimulation of all parts of the body.

The data already in hand, and the experiments now in progress, in which anesthetized animals are subjected to powerful stimuli applied to certain parts of the body only, or simultaneously to all parts of the body, lead us to believe that in the former case the brain-cells become stimulated or hyperchromatic, while in the latter case no brain-cell changes occur. We believe that our experiments will prove that an equal and simultaneous stimulation of all parts of the body leaves the brain-cells in a state of equilibrium.

Our theory of pain will then be well sustained, not only by common observation, but by experimental proof, and so the mechanistic view will be found in complete harmony with another important reaction.

We have stated that when a number of contact stimuli act simultaneously, the strongest stimulus will gain possession of the final common path-- the path of action. When, however, stimuli of the distance ceptors compete with stimuli of the contact ceptors, the contact-ceptor stimuli often secure the common path, not because they are stronger or more important, but because they are immediate and urgent.

In many instances, however, the distance-ceptor stimuli are strong, have the advantage of a lowered threshold, and therefore compete successfully with the immediate and present stimuli of the contact ceptors. In such cases we have the interesting phenomenon of physical injury without resultant pain or muscular response.

The distance-ceptor stimuli which may thus triumph over even powerful contact-ceptor stimuli are those causing strong emotions--as great anger in fighting; great fear in a battle; intense s.e.xual excitement.

Dr. Livingstone has testified to his complete unconsciousness to pain during his struggle with a lion; although he was torn by teeth and claws, his fear overcame all other impressions.

By frequently repeated stimulation the Dervish secures a low threshold to the emotions caused by the thought of G.o.d or the devil, and his emotional excitement is increased by the presence of others under the same stimulation; emotion, therefore, secures the final common path and he is unconscious of pain when he lashes, cuts, and bruises his body. The phenomena of hysteria may be explained on this basis, as may the unconsciousness of pa.s.sing events in a person in the midst of a great and overwhelming grief.

By constant practice the student may secure the final common path for such impressions as are derived from the stimuli offered by the subject of his study, and so he will be oblivious of his surroundings.

Concentration is but another name for a final common path secured by the repet.i.tion and summation of certain stimuli.

If our premises are sustained, then we can recognize in man no will, no ego, no possibility for spontaneous action, for every action must be a response to the stimuli of contact or distance ceptors, or to their recall through a.s.sociative memory. Memory is awakened by symbols which represent any of the objects or forces a.s.sociated with the act recalled.

Spoken and written words, pictures, sounds, may stimulate the brain patterns formed by previous stimulation of the distance ceptors; while touch, pain, temperature, pressure, may recall previous contact-ceptor stimuli. Memory depends in part upon the adequacy of the symbol, and in part upon the state of the threshold.

If one has ever been attacked by a snake, the threshold to any symbol which could recall that attack would be low; the later recall of anything a.s.sociated with the bite or its results would produce in memory a recapitulation of the whole scene, while even harmless snakes would thereafter be greeted with a shudder.

On the other hand, in a child the threshold is low to the desire for the possession of any new and strange object; in a child, therefore, to whom a snake is merely an unusual and fascinating object, there is aroused only curiosity and the desire for the possession of a new plaything.