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the pyogenic infections。 The pyogenic infections and the exanthemata constitute the great majority of infections and are the basis of the discussion which follows。
I will state one of my principal conclusions first； _i。 e_。； that the only types of infection that are associated with pain are those 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； and； further； as a striking corollary； that the type of infection that may cause muscular action when it attacks one region of the body may cause no such action when it attacks another region。
The primary； and perhaps the most striking； difference between the painless exanthemata and the painful pyogenic infections is that in the case of the exanthemata the protective response of the body is a chemical one；the formation of antibodies in the blood； which usually produce permanent immunity；while the response to the pyogenic infections is largely phagocytic。 In the pyogenic infections； in order to protect the remainder of the body； which； of course； enjoys no immunity； every possible barrier against the spread of the infection is thrown about the local point of infection。 How are these barriers formed？ First； lymph is poured out； then the part is fixed by the continuous contraction of the neighboring muscles and by the inhibition of those muscles that； in the course of their ordinary function； would by their contractions spread the infection。 Wherever there is protective muscular rigidity there is also pain。 On the other hand； in pyogenic infections in the substance of the liver； in the substance of the kidney； within the brain； in the retroperitoneal space； in the lobes of the lung； in the chambers of the heart and in the blood…vessels of the chest and the abdomen； in all locations in which muscular contractions can in no way assist in localizing the disease； pyogenic infections produce no muscular rigidity and no pain。 Apparently； therefore； only those infections are painful which are associated with a protective muscular contraction。 This explains why tuberculosis of the hip is painful； while tuberculosis of the lung is painless。
There is a third type of pain which modifies muscular action in a curious way。 We have already stated that local pain serves an adaptive purpose。 In this light let us now consider headache。 Headache is one of the commonest initiatory symptoms of the various infections； especially of those infections which are accompanied by no local pain and by no local muscular action。 In peritonitis； cholecystitis； pleurisy； arthritis； appendicitis； salpingitis； child…birth； in obstructions of the intestinal and the genito…urinary tract； in short； in those acute processes in which the local symptoms are powerful enough to govern the individual as a whole；to make him lie down and keep quiet； refuse food and possibly reject what is already in the stomach； in all these conditions there is rarely a headache； but in the diseases in which local pain is absent； such as the exanthemata； typhoid fever； and auto…intoxication； which have no dominating local disturbances to act as policemen to put the individual to bed and to make him refuse food that he may be in the most favorable position to combat the oncoming disease； in such cases in which these masterful and beneficent local influences are absent we postulate that headache has been evolved to perform this important service。
On the hypothesis that it is good for the individual who is acutely stricken by a disease or who is poisoned by autointoxication to rest and fast； and that the muscular system obeys the imperial command of pain； and in view of the fact that the brain is not only in constant touch with the conditions of every part of the body but that it is also the controlling organ of the body； one would expect that in these diseases the major pain whose purpose it is to govern general muscular action would be located in the head and there we find it。 How curious and yet how intelligible is the fact that； though a headache may be induced by even a slight auto…intoxication； an abscess may exist within the brain without causing pain。 When an obliterative endarteritis is threatening a leg with anemic gangrene； or when one lies too long in the same position on a hard bed； there is threatening injury from local anemia； and as a result there is acute pain； but when the obliterative endarteritis threatens anemia of the brain； or when an embolism or thrombosis has produced anemia of the brain； there may be no accompanying pain。 The probable explanation of the pain which results in the first instance and the lack of pain in the second is that in the former muscular action constitutes a self…protective response； but in the other it does not。 Diseases and injuries of the brain are notoriously difficult to diagnosticate。 This may well be because it has always been so well protected by the skull that there have been evolved within it few tell…tale self…protective responses； so that in the presence of injury and disease within itself the brain remains remarkably silent。 It should occasion no surprise that there are in the brain no receptors； the mechanical stimulation of which can cause pain； because its bony covering has always prevented the adaptive implantation within it of contact pain receptors。 Dr。 Frazier tells me that in the course of his operations on the brains of unanesthetized patients he is able to explore the entire brain freely and without pain。 From my own experience I am able to confirm Dr。 Frazier's observation。 In addition； the two…stage operation for the excision of the Gasserian ganglion provides an observation of extraordinary interest。 If at the first seance the ganglion is exposed； but is not disturbed except by the iodoform gauze packing； then on the following day the gauze may be removed； the ganglion picked up； and its branches and root excised without anesthesia and without pain。 The same statement and explanation may be made regarding the distribution of pain receptors for physical contact within the parenchyma of the liver； the gall…bladder； the abdominal viscera； the spleen； the heart； the lungs； the retroperitoneal tissue； the deep tissue of the back； the vertebrae； and in certain portions of the spinal cord。 Just what is the distribution of the receptors for heat and for cold I am unable to state； but this much we do know； that without anesthesia the intestines may be cauterized freely without the least pain resulting； and in animals the cauterization of the brain causes no demonstrable change in the circulatory or respiratory reactions。 It is probable therefore that the distribution of the pain receptors for physical contact and for heat are limited to those parts of the body that have been exposed to injurious contacts with environment。
Of special significance is the pain which is due to cold； which increases muscular tone and produces shivering。 The general increase in muscular tone produces an interesting postural phenomenon： the limbs are flexed and the body bent forward； a position which probably is due to the fact that the flexors are stronger than the extensors。 As muscular action is al