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o 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 mass 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 sexual excitement the mimicry is as obvious。 The emotions； then； are the preparations for phylogenetic activities。 If the activities are consummated； the fuelglycogenand 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 sexual excitement wants only possession。
All the lesser emotionsworry； jealousy； envy； grief； disappointment； expectationall 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 anestheticsthe state of anoci…association （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 constitute 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 associative memory resides； but while associative 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 consciousnesspain。
Like the other phenomena of life； pain was undoubtedly evolved for a particular purposesurely 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 associated 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 associated 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； cystitis； cholecystitis； 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； anem