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Let us test this hypothesis by some practical examples。 The first is an incident that accidentally occurred in our laboratory during experiments on fear which were performed as follows： A keen； snappy fox terrier was completely muzzled by winding a broad strip of adhesive plaster around his jaw so as to include all but the nostrils。 When this aggressive little terrier and the rabbit found themselves in close quarters each animal became completely governed by instinct； the rabbit crouched in fear； while the terrier； with all the ancestral assurance of seizing his prey； rushed； upon the rabbit； his muzzle always glancing off and his attack ending in awkward failure。
This experiment was repeated many times and each time provoked the serious…minded scientific visitors who witnessed it to laughter。 Why？ Because the spectacle of a savage little terrier rushing upon an innocent rabbit as if to mangle it integrated the body of the onlooker with a strong desire to exert muscular action to prevent the cruelty。 This integration caused a conversion of the potential energy in the brain…cells into kinetic energy； and there resulted a discharge into the blood…stream of activating internal secretions for the purpose of producing muscular action。 Instantly and unexpectedly the danger passed and the preparation for muscular action intended for use in the protection of the rabbit was not needed。 This fuel was consumed by the neutral muscular action of laughter； which thus afforded relief。
A common example of the same nature is that encountered on the street when a pedestrian slips on a banana peel and； just as he is about to tumble； recovers his equilibrium。 The onlookers secure relief from the integration to run to his rescue by laughing。 On the other hand； should the same pedestrian fall and fracture his skull the motor integration of the onlookers would be consumed by rendering physical assistancehence there would be no laughter。 In children almost any unexpected phenomenon； such as a sudden 〃booing〃 from behind a door； is attended by laughter； and in like manner the kinetic reaction produced by the innumerable threats of danger which are suddenly averted； a breach of the conventions； a sudden relief from acute nervous tension； a surpriseindeed； any excitant to which there is no predetermined method of giving a physical response may be neutralized by the excitation of the mechanism of laughter。
In the same way the laughter excited by jokes may be explained。 An analysis of a joke shows that it is composed of two parts the first； in which is presented a subject that acts as a stimulus to action； and the second； in which the story turns suddenly so that the stimulus to action is unexpectedly withdrawn。 The subject matter of the joke affects each hearer according to the type of stimuli that commonly excites that individual。 Hence it is that a joke may convulse one person while it bores another； and so there are jokes of the classes； bankers' jokes； politicians' jokes； the jokes of professional men； of the plebeian； of the artist； etc。 If the joke fails； the integration products of the excitation may be used in physical resentment （Fig。 32）。
Another type of laughter is that associated with the ticklish points of certain parts of the body； the soles of the feet and certain parts of the trunk and of the abdomen。 The excitation of the ticklish receptors； like pain； compels self…defensive motor acts。 This response is of phylogenetic origin； and may be awakened only by stimuli which are too light to be painful。 In this connection it is of interest to note that a superficial； insect…like contact with the skin rarely provokes laughter； and that the tickling of the nasal； oral； and pulmonary tracts does not produce laughter。 The ticklish points that cause laughter are rather deeply placed； and a certain type of physical contact is required to constitute an adequate stimulus。 That is； the contact must arouse a phylogenetic association with a physical struggle or with physical exertion。 In the foot； the adequate stimuli for laughter are such contacts as resemble or suggest piercing by stones or rough objects。。 The intention of the one who tickles must be known； if his intention be playful； laughter results； whereas if injury be intended； then an effort toward escape or defense is excited； but no laughter。 If deep tickling of the ribs is known to be malicious； it will excite physical resentment and not laughter。 Self…tickling rarely causes laughter for the reason that auto…tickling can cause only a known degree of stimulation； so that there results no excessive integration which requires relief by the neutral muscular activity of laughter。 In fact； one never sees purposeful acts and laughter associated。 According to its severity； an isolated stimulus causes either an action or laughter。 The ticklish points in our bodies were probably developed as a means of defense against serious attacks and of escape from injurious contacts。
Anger； fear； and grief are also strong excitants and； therefore； are stimuli to motor activity。 It is obvious that whatever the excitant the physico…chemical action of the brain and the ductless glands cannot be reversedthe effect of the stimulus cannot be recalled； therefore either a purposeful muscular act or a neutralizing act must be performed or else the liberated energy must smoulder in the various parts of the body。
It is on this hypothesis that the origin and the purpose of laughter and crying may be understood。 Even a superficial analysis of the phenomena of both laughter and crying show them to be without any external motor purpose； the respiratory mechanism is intermittently stimulated and inhibited； and the shoulder and arm muscles； indeed； many muscles of the trunk and the extremities are； as far as any external design is concerned； purposelessly contracted and released until the kinetic energy mobilized by excitation is utilized。 During this time the facial expression gives the index to the mental state。
Crying； like laughter； is always preceded by a stimulation to some motor action which may or may not be performed （Figs。 33 and 34）。 If a mother is anxiously watching the course of a serious illness of her child and if； in caring for it； she is stimulated to the utmost to perform motor acts； she will continue in a state of motor tenseness until the child recovers or dies。 If relief is sudden； as in the crisis of pneumonia； and the mother is not exhausted； she will easily laugh if tired； she may cry。 If death occurs； the stimulus to motor acts is suddenly withdrawn and she then cries aloud； and performs many motor acts as a result of the intense stimulation to motor activity which is no longer needed in the physical care of her child。 With this clue we can find the explanation of many phenomena。 We can understand why laughter and crying are so frequently interchangeable； why they often blend and why either gives a sense of relief； we can understand why either laughter or crying can come only when the issue that causes the integration is determined； we can understand the extraordinary tendency to laughter that discloses the unspoken sentiments of love； we can understand the tears of the woman when she receives a proposal of marriage from the man she loves； we can understand why any averted circumstance； such as a threatened breach of the conventions； which would have led to embarrassment or humiliation； leads to a tendency to laughter； and why the recital of heroic deeds by association leads to tears； On the other hand； under the domination of acute diseases； of acute fear； or of great exhaustion； there is usually neither laughter nor crying because the nervous system is under the control of a dominating influence as a result of which the body is so exhausted that the excess of energy which alone can produce laughing or crying is lacking。
A remarkable study of the modification of laughter and crying by disease is found in that most interesting of diseasesexophthalmic goiter。 In this disease there is a low threshold to all stimuli。 That the very motor mechanism of which we have been speaking is involved； is shown by an enormous increase in its activity。 There is also an increase in the size of