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obably is due to the fact that the flexors are stronger than the extensors。 As muscular action is always accompanied by heat production； the purpose of the muscular contraction and the shivering is quite certainly caused by cold to assist in the maintenance of the normal body temperature。
We have now discussed many of the causes of pain and in each instance we have found an associated muscular action which apparently serves some adaptive purpose （Figs。 24 and 25）。 If we assume that pain exists for the purpose of stimulating muscular reactions； we may well inquire what part of the nervous are is the site of the sensation of painthe nerve…endings； the trunk； or the brain？ Does pain result from physical contact with the nerve…endings； with the physical act of transmitting an impression along the nerve trunk； or with the process within the brain…cells by which energy is released to cause a motor act？
It seems most probable that the site of the pain is in the brain…cells。 If this be so； then what is the physical process by which the phenomena of pain are produced？ The one hypothesis that can be tested experimentally is that pain is a phenomenon resulting from the rapid discharge of energy in the brain…cells。 If this be true； then if every pain receptor of the body were equally stimulated in such a manner that
｛illust。 caption = FIG。 25。FEAR AND AGONY。 〃Amid this dread exuberance of woe ran naked spirits wing'd with horrid fear。〃 Dante's 〃Inferno；〃 Canto XXIV； lines 89； 90。 all the stimuli reached the brain…cells simultaneously； the cells would find themselves in equilibrium and no motor act would be performed。 But if all the pain receptors of the body but one were equally stimulated； and this one stimu…lated harder than the rest； then the latter would gain possession of the final common path； the sensation of pain would be felt； and a muscular contraction would result。 It is well known that when a greater pain is thrown into competition with a lesser one； the lesser is completely submerged。 In this manner the school…boy initiates the novice into the mystery of the painless plucking of hair。 The simultaneous； but severe application of the boot to the blindfolded victim takes complete and exclusive possession of the final common path and the hair is painlessly plucked through the triumph of the boot stimulus over the hair stimulus in the struggle for the possession of the final common path。 Another argument in favor of this hypothesis that pain is an accompaniment of the release of energy in the brain… cells is found in the fact that painless stimuli received through the special senses may completely submerge the painful stimuli of physical injury； for although the stimuli to motor action； which are received through the senses of sight； hearing； and smell； cause even more powerful motor action than those caused by physical contact stimuli； yet they are not accompanied by pain。 Examples of this triumph of stimulation of the special senses over contact stimulation are frequently seen in persons obsessed by anger or fear； and to a less degree in those obsessed by sexual emotion。 In the fury of battle the soldier may not perceive his wound until the emotional excitation is wearing away； when the sensation of warm blood on the skin may first attract his attention。 Religious fanatics are said to feel no pain when they subject themselves to self…injury。 Now； since both psychic and mechanical stimuli cause motor action by the excitation ofprecisely the same mechanism in the brain； and since the more rapid release of energy from psychic stimuli submerges the physical stimuli and prevents pain； it would seem that pain must be a phenomenon which is associated with the process of releasing energy by the brain…cells。 Were physical injury inflicted in a quiescent state equal to that inflicted in the emotional state； great pain and intense muscular action would be experienced。 Now the emotions are as purely motor excitants as is pain。 The dynamic result is the same the principal difference being the greater suddenness and the absolute specificity of the pain stimuli as compared with the more complex and less peremptory stimuli of the emotions。 A further evidence that pain is a product of the release of brain…cell energy is the probability that if one could pierce the skin at many points on a limb in such a manner that antagonistic points only were equally and simultaneously stimulated； then an equilibrium in the governing brain… cells would be established and neither pain nor motion would follow。 An absolute test of this assumption cannot be made but it is supported by the obtainable evidence。 We will now turn to a new viewpoint； a practical as well as a fascinating one； which can best be illustrated by two case histories： A man； seventy…eight years old； whose chief complaint was obstinate constipation； was admitted to the medical ward of the Lakeside Hospital several years ago。 The abdomen was but slightly distended； there was no fever； no increased leukocytosis； no muscular rigidity； and but slight general tenderness。 He claimed to have lost in weight and strength during the several months previous to his admission。 A tentative diagnosis of malignant tumor of the large intestine was made； but free movements weresecured rather easily； and we abandoned the idea of an exploratory operation。 The patient gradually failed and died without a definite diagnosis having been made by either the medical or the surgical service。 At autopsy there was found a wide…spread peritonitis arising from a perforated appendix。 A child； several years old； was taken ill with some indefinite disease。 A number of the ablest medical and surgical consultants of a leading medical center thoroughly and repeatedly investigated the case。 Although they could make no definite diagnosis they all agreed that the trouble surely could not be appendicitis because there was neither muscular rigidity nor tenderness。 The autopsy showed a gangrenous appendix and general peritonitis。 How can these apparently anomalous cases be explained？ These two cases are illustrations of the same principle that underlies the freedom from pain which results from the use of narcotics and anesthetics； the same principle that explains the fact that cholecystitis may occur in the aged without any other local symptoms than the presence of a mass and perhaps very slight tenderness； and that accounts in general for the lack of well…expressed disease phenomena in senility and in infancy。 The reason why the aged； the very young； and the subjects of general paresis show but few symptoms of disease is that in senility the brain is deteriorated； while in infancy the brain is so undeveloped that the mechanism of association is inactive； hence pain and tenderness； which are among the oldest of the associations； are wanting。 Senility and infancy are by nature normally narcotized。 The senile are passing through the twilight into the night； while infants are traversing through the dawn into the day。 Hence it is that the diagnosis of injury and disease in the extremes oflife is beset by especial difficulties； since the entire body is as silent as are the brain； the pericardium； the mediastinum； and other