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purposes were achieved： （1） Self…defense against or escape from enemies； （2） the acquisition of food； and （3） procreation； and these were virtually the only purposes for which nervous energy was discharged。 In its last analysis； in a biologic sense； this statement holds true of man today。 Disregarding for the present the expenditure of energy for procuring food and for procreation； let us consider the discharge of energy for self…preservation。 The mechanisms for self…defense which we now possess were developed in the course of vast periods of time through innumerable intermediary stages from those possessed by the lowest forms of life。 One would suppose； therefore； that we must now be in possession of mechanisms which still discharge energy on adequate stimulation； but which are not suited to our present needs。 We shall point out some examples of such unnecessary mechanisms。 As Sherrington has stated； our skin； in which are implanted many receptors for receiving specific stimuli which are transmitted to the brain； is interposed between ourselves and the environment in which we are immersed。 When these stimuli reach the brain； there is a specific response； principally in the form of muscular action。 Now； each receptor can be adequately stimulated only by the particular factor or factors in the environment which created the necessity for the existence of that receptor。 Thus there have arisen receptors for touch； for temperature； for pain； etc。 The receptors for pain have been designated _nociceptors_ （nocuous or harmful） by Sherrington。
On the basis of natural selection； nociceptors could have developed in only those regions of the body which have been exposed to injury during long periods of time。 On this ground the finger； because it is exposed； should have many nociceptors； while the brain； though the most important organ of the body； should have no nociceptors because； during a vast period of time； it has been protected by a skull。 Realizing that this point is a crucial one； Dr。 Sloan and I made a series of careful experiments。 The cerebral hemispheres of dogs were exposed by removing the skull and dura under ether and local anesthesia。 Then various portions of the hemispheres were slowly but completely destroyed by rubbing them with pieces of gauze。 In some instances a hemisphere was destroyed by burning。 In no case was there more than a slight response of the centers governing circulation and respiration； and no morphologic change was noted in an histologic study of the brain…cells of the uninjured hemisphere。 The experiment was as completely negative as were the experiments on the 〃spinal dog。〃 Clinically I have confirmed these experimental findings when I have explored the brains of conscious patients with a probe to determine the presence of brain tumors。 Such explorations elicited neither pain nor any evidence of altered physiologic functions。 The brain； therefore； contains no mechanism no nociceptorsthe direct stimulation of which can cause a discharge of nervous energy in a self…defensive action。 That is to say； direct injury of the brain can cause no purposeful nerve…muscular action； while direct injury of the finger does cause purposeful nerve…muscular action。 In like manner； the deeper portions of the spinal region have been sheltered from trauma and they； too； show but little power of causing a discharge of nervous energy on receiving trauma。 The various tissues and organs of the body are differently endowed with injury receptorsthe nociceptors of Sherrington。 The abdomen and chest when traumatized stand first in their facility for causing the discharge of nervous energy； _i。 e_。； THEY STAND FIRST IN SHOCK PRODUCTION。 Then follow the extremities； the neck； and the back。 It is an interesting fact also that different types of trauma elicit different responses as far as the consequent discharge of energy is concerned。
Because it is such a commonplace observation； one scarcely realizes the importance of the fact that clean…cut wounds inflicted by a razor…like knife cause the least reaction； while a tearing； crushing trauma causes the greatest response。 It is a suggestive fact that the greatest shock is produced by any technic which imitates the methods of attack and of slaughter used by the carnivora。 _*In the course of evolution； injuries thus produced may well have been the predominating type of traumata to which our progenitors were subjected_。 In one particular respect there is an analogy between the response to trauma of some parts of the body of the individuals of a species susceptible to shock and the response to trauma of the individuals in certain other great divisions of the animal kingdom。 Natural selection has protected the crustaceans against their enemies by protective armor； _e。 g_。； the turtle and the armadillo； to the birds； it has given sharp eyes and wings； as； for instance； the wild goose to another speciesthe skunkit has given a noisome odor for its protection。 The turtle； protected by its armor against trauma； is in a very similar position to that of the sheltered brain of man and； like the brain； the turtle does not respond to trauma by an especially active self…protective nerve…muscular response； but merely withdraws its head and legs within the armored protection。 It is proverbially difficult to exhaust or to kill this animal by trauma。 The brain and other phylogenetically sheltered parts likewise give no exhausting self…protective nerve…muscular response to trauma。 The skunk is quite effectively protected from violence by its peculiar odor。 This is indicated not only by the protective value of the odor itself； but also by the fact that the skunk has no efficient nerve…muscular mechanism for escape or defense； it can neither run fast nor can it climb a tree。 Moreover； in encounters it shows no fear and backs rather than runs。 The armadillo rolls itself into a ball for defense。 On these premises we should conclude that the turtle； the armadillo； and the skunk have fewer nociceptors than has a dog or man； and that they would show less response to trauma。 In two carefully conducted experiments on skunks and two on armadillos （an insufficient number） the energy discharged in response to severe and protracted trauma of the abdominal viscera was very much less than in similar experiments on dogs； opossums； pigs； sheep； and rabbits。 It was indeed relatively difficult to exhaust the skunks and armadillos by trauma。 These experiments are too few to be conclusive； but they are of some value and furnish an excellent lead。 It seems more than a coincidence that proneness to fear； distribution of nociceptors； and susceptibility to shock go hand…in…hand in these comparative observations （Figs。 6； 7； and 8）。
The discharge of energy caused by an adequate mechanical stimulation of the nociceptors is best explained in accordance with the law of phylogenetic association。 That is； injuries awaken those reflex actions which by natural selection have been developed for the purpose of self…protection。 Adequate stimulation of the nociceptors for pain is not the only means by which a discharge of nervous energy is caused。 Nervous energy may be discharged also by adequate stimulation of the various ticklish regions of the body； the entire skin surface of the body contains delicate ticklish receptors。 These receptors are closely related to the nociceptors for pain； and their adequate stimulation by an insect…like touch causes a discharge of energy；a nerve…muscular reaction； resembling that developed for the purpose of brushing off insects。 This reflex is similar to the scratch reflex in the dog。 The discharge of energy is almost wholly independent of the will and is a self…protective action in the same sense as is the response to pain stimuli。 The ear in man and in animals is acutely ticklish； the adequate stimulus being any foreign body； especially a buzzing； insect…like contact。 The discharge of nervous energy in horses and in cattle on adequate stimulation of the ticklish receptors of the ear is so extraordinary that in the course of evolution it must have been of great importance to the safety of the animal。 A similar ticklish zone guards the nasal chambers； the discharge of energy here taking a form which effectively dislodges the foreign body。 The larynx is exquisitely ticklish； and； in response to any adequate stimulus； energy is discharged in the production of a vigorous cough。 The mouth and pharynx have active receptors which cause the rejection of noxious substances。 The conjunctival reflex； though not classed as ticklish； is a most efficient self…protective reflex。 I assume that there is no doubt as to the relation between the adequate stimuli and the nerve…muscular response of the various ticklish receptors of the surface of the skin； of the ear； the nose； the eye； and the larynx。 These mechanisms were developed by natural selection as protective measures against the intrusion of insects and foreign bodies into regions of great importance。 The discharge of energy in these instances is in accordance with the laws of inheritance and association。 The other ticklish points which are capable of discharging vast amounts of energy are the lateral chest…wall； the abdomen； the loins； the nec