按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
e 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is not exhausted； she will easily laugh if tired； she may cry。 If death occurs； the stimulus to motor acts is suddenly withdrawn and she then cries aloud； and performs many motor acts as a result of the intense stimulation to motor activity which is no longer needed in the physical care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 certain at least of the activating glandsthe thyroid and the adrenals are enlarged and overactive and the glycogen…producing function of the liver is stimulated。 The most striking phenomenon of this disease； however； is the remarkable lowering of the brain thresholds to stimuli。 In other words； in Graves' disease the nervous system and the activating glands the entire motor mechanismare in an exalted state of activity。
If this be true； then these patients should exhibit behavior precisely contrary to that of those suffering from acute infection； that is； they should be constantly clearing their systems of these superabundant energizing materials by crying or laughing； and this is precisely what happensthe flood…gates of tears are open much of the time in Graves' diseasea disease of the emotions。
We have already interpreted pain as a phenomenon of motor activity。 When pain does not lead to muscular activity it therefore frequently leads to crying or to moaning； just as tickling； which is equally an incentive to motor activity； results in laughter if it does not find full expression in action。
From the foregoing we infer that pain； the intense motor response to tickling； and emotional excitation are all primitive biologic reactions for the good of the individual； and that all have their origin in the operation of the great laws of evolution。 If to this inference we add the physiologic dictum that the nervous system always acts as a whole； and that it can respond to but one stimulus at a time； we can easily understand that while diverse causes may integrate the nervous system for a specific action； if the cause be suddenly removed； then the result of the integration of the nervous system may be； not a specific action； but an undesigned muscular action； such as crying or laughter。 Hence it is that laughter and crying may be evoked by diverse exciting causes。 The intensity of the laughter or of the crying depends upon the intensity of the stimulus and the dynamic state of the individual。
The linking together of these apparently widely separated phenomena by the simple law of the discharge of energy by association perhaps explains the association of an abnormal tendency to tears with an abnormally low threshold for pain （Fig。 36）。 In the neurasthenic； tears and pain are produced with abnormal facility。 Hence it is that； if a patient about to undergo a surgical operation is in a state of fear and dread before the operation； the threshold to all stimuli is lowered； and this lowered threshold will continue throughout the operation； even under inhalation anesthesia； because the stimulus produced by cutting sensitive tissue is transmitted to the brain just as readily as if the patient were not anesthetized。 In like manner； the brain may be sensitized by the administration of large doses of thyroid extract prior to operation； the threshold to injury in such a case continuing to be low to traumatic stimuli even under anesthesia。 Under the sensitizing influences of thyroid extract or of Graves' disease the effect of an injury； of an operation； or of emotional excitation is heightened。 The extent to which the threshold to pain or to any other excitant is affected by Graves' disease is illustrated by the almost fatal reaction which I once saw result from the mere pricking with a hypodermic needle of a patient with this disease。 As the result of a visit from a friend； the pulse…rate of a victim of this disease may increase twenty beats and his temperature rise markedly。 I have seen the mere suggestion of an operation produce collapse。 As the brain is thus remarkably sensitized in Graves' disease； we find that in these patients laughter； crying； emotional disturbances； and surgical shock are produced with remarkable facility。
I hope that even this admittedly crude and imperfect consideration of this subject will suggest the possibility of establishing a practical viewpoint as to the origin and purpose of pain； of tickling； and of such expressions of emotion as laughter and crying； and that it may help us to understand their significance in health and in disease。
THE RELATION BETWEEN THE PHYS