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ly 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 neck； and the soles of the feet。 The type of adequate stimuli of the soles of the feet； the distribution of the ticklish points upon them； and the associated response； leave no doubt that these ticklish points were long ago established as a means of protection from injury。 Under present conditions they are of little value to man。
The adequate stimulus for the ticklish points of the ribs； the loins； the abdomen； and the neck is deep isolated pressure； probably the most adequate being pressure by a tooth…shaped body。 The response to tickling in these regions is actively and obviously self…defensive。 The horse discharges energy in the form of a kick； the dog wriggles and makes a counter…bite； the man makes efforts at defense and escape。
There is strong evidence that the deep ticklish points of the body were developed through vast periods of fighting with teeth and claws （Fig。 9）。 Even puppies at play bite each other in their ticklish points and thus give a recapitulation of their ancestral battles and of the real battles to come （Fig。 10）。 The mere fact that animals fight effectively in the dark and always according to the habit of their species supports the belief that the fighting of animals is not an intellectual but a reflex process。 There are no rules which govern the conduct of a fight between animals。 The events follow each other with such kaleidoscopic rapidity that the process is but a series of automatic stimulations and physiologic reactions。 Whatever their significance； therefore； it is certain that man did not come either accidentally or without purpose into possession of the deep ticklish regions of his chest and abdomen。 Should any one doubt the vast power that adequate stimulation of these regions possesses in causing the discharge of energy； let him be bound hand and foot and vigorously tickled for an hour。 What would happen？ He would be as completely exhausted as though he had experienced a major surgical operation or had run a Marathon race。
A close analogy to the reflex process in the fighting of animals is shown in the role played by the sexual receptors in conjugation。 Adequate stimulation of either of these two distinct groups of receptors； the sexual and the noci； causes specific behavior the one toward embrace； the other toward repulsion。 Again； one of the most peremptory causes of the discharge of energy is that due to an attempt to obstruct forcibly the mouth and the nose so that asphyxia is threatened。 Under such conditions neither friend nor foe is trusted； and a desperate struggle for air ensues。 It will be readily granted that the reactions to prevent suffocation were established for the purpose of self…preservation； but the discharge of nerve…muscular energy to this particular end is no more specific and no more shows adaptive qualities than do the preceding examples。 Even the proposal to bind one down hand and foot excites resentment； a feeling originally suggested by the need for self…preservation。 No patient views with equanimity the application of shackles as a preparation for anesthesia。
We have now considered some of the causes of those discharges of nervous energy which result from various types of harmful physical contact； and have referred to the analogous； though antithetical； response to the stimulation of the sexual receptors。 The response to the adequate stimuli of each of the several receptors is a discharge of nerve…muscular energy of a specific type； that is； there is one type of response for the ear； one for the larynx； one for the pharynx； another for the nose； another for the eye； another for the deep ticklish points of the chest and the abdomen； quite another for the delicate tickling of the skin； and still another type of response to sexual stimuli。
According to Sherrington； a given receptor has a low threshold for only one； its own specific stimulus； and a high threshold for all others； that is； the doors that guard the nerve…paths to the brain are opened only when the proper password is received。 According to Sherrington's law； the individual as a whole responds to but one stimulus at a time； that is； only one stimulus occupies the nerve…paths which carry the impulses as a result of which acts are performed； _i。 e_。； the final common path。 As soon as a stronger stimulus reaches the brain it dispossesses whatever other stimulus is then occupying the final common path the path of action。 The various receptors have a definite order of precedence over each other （Sherrington）。 For example； the impulse from the delicate ticklish points of the skin； whose adequate stimulus is an insect…like contact； could not successfully compete for the final common path with the stimulus of a nociceptor。 The stimulus of a fly on the nose would be at once superseded by the crushing of a finger。 In quick succession do the various receptors （Sherrington） occupy the final common path； but each stimulus is for the time the sole possessor； hence the nervous system is integrated （connected） to act as a whole。 Each individual at every moment of life has a limited amount of dischar