Part 5

In young women exophthalmic goiter often follows in the wake of a disappointment in love; in women, too, it frequently follows the illnesses of children or parents during which they have had to endure the double strain of worry and of constant care. Since such strains usually fall most heavily upon women, they are the most frequent victims of this disease. Now, whatever the exciting cause of exophthalmic goiter, whether it be unusual business worry, disappointment in love, a tragedy, or the illness of a loved one, the symptoms are alike and closely resemble the phenomena of one of the great primitive emotions. How could disappointment in love play a role in the causation of Graves' disease? If the hypothesis which has been presented as an explanation of the genesis and the phenomena of fear be correct, then that hypothesis explains also the emotion of love. If fear be a phylogenetic physical defense or escape which does not result in muscular action, then love is a phylogenetic conjugation without physical action. The quickened pulse, the leaping heart, the accelerated respiration, the sighing, the glowing eye, the crimson cheek, and many other phenomena are merely phylogenetic recapitulations of ancestral acts. The thyroid gland is believed to participate in such physical activities. Hence it may well follow that the disappointed maiden who is intensely integrated for a youth will, at every thought of him, be subjected by phylogenetic association to a specific stimulation analogous to that which attended the ancestral consummation. Moreover, a happy marriage has many times been followed by a cure of the exophthalmic goiter which appeared in the wake of such an experience.

The victims of Graves' disease present a counterpart of emotional exhaustion. That the emotions in Graves' disease are abnormally acute is illustrated by my personal observation of the death of a subject of this disease from fear alone. Whatever the exciting cause of this disease, the symptoms are the same; just as in fear, the phenomena are the same whatever the exciting cause.

Figures 12 and 16 show the resemblance between the outward appearances of a patient with Graves' disease and of a person obsessed by fear. Fear and Graves' disease have the following phenomena in common: Increased heart-beat, increased respiration, rising temperature, muscular tremors, protruding eyes, loss in weight; Cannon has found an increased amount of adrenalin in the blood in fear and Frankel in Graves' disease; increased blood-pressure; muscular weakness; digestive disturbances; impaired nervous control; hypersusceptibility to stimuli; in protracted intense fear the brain-cells show marked physical changes, and in Graves' disease analogous changes are seen (Figs. 13 C and 15). In Graves' disease there seems to be a composite picture of an intense expression of the great primitive emotions. If Graves' disease be a disease of the great primitive emotions, or rather of the whole motor mechanism, how is the constant flow of stimulation of this complicated mechanism supplied? It would seem that there must be secreted in excessive amount some substance that activates the motor mechanism. The nervous system in Graves' disease is hypersusceptible to stimuli and to thyroid extract. It might follow that even a normal amount of thyroid secretion would lead to excessive stimulation of the hypersusceptible motor mechanism.

This condition of excessive motor activity and hyperexcitability may endure for years. What is the source of this pathologic excitation? The following facts may give a clue. In suitable cases of Graves' disease, if the thyroid secretion is sufficiently diminished by a removal of a part of the gland or by interrupting the nerve and the blood supply, the phenomena of the disease are diminished immediately, and in favorable cases the patient is restored to approximately the normal condition. The heart action slows, the respiratory rate falls, the restlessness diminishes, digestive disturbances disappear, tremors decrease, there is a rapid increase in the body weight, and the patient gradually resumes his normal state. On the other hand, if for a period of time extract of the thyroid gland is administered to a normal individual in excessive dosage, there will develop nervousness, palpitation of the heart, sweating, loss of weight, slight protrusion of the eyes, indigestion; in short, most of the phenomena of Graves' disease and of the strong emotions will be produced artificially (Figs. 15 and 23). When the administration of the thyroid extract is discontinued, these phenomena may disappear. On the other hand, when there is too little or no thyroid gland, the individual becomes dull, stupid, and emotionless, though he may be irritable; while if a sufficient amount of thyroid extract be given to such a patient he may be brought back to his normal condition.

Hence we see that the phenomena of the emotions may within certain limits be increased, diminished, or abolished by increasing, diminishing, or totally excluding the secretion of the thyroid gland.

Graves' disease may be increased by giving thyroid extract and by fear. It may be diminished by removing a part of the gland, or by interrupting the blood and nerve supply, or by complete rest. In addition, at some stage of Graves' disease there is an increase in the size and in the number of the secreting cells. These facts regarding the normal and the pathologic supply of thyroid secretion point to this gland as one of the sources of the energizing substance or substances, by means of which the motor phenomena of animals are executed and their emotions expressed.

Anger is similar to fear in origin and, like fear, is an integration and stimulation of the motor mechanism and its accessories. Animals which have no natural weapons for attack experience neither fear nor anger, while the animals which have weapons for attack express anger principally by energizing the muscles used in attack. Although, as has already been stated, the efficiency of the hands of man has largely supplanted the use of the teeth, he still shows his teeth in anger and so gives support to the theory that this emotion is of remote ancestral origin and proves the great persistence of phylogenetic association. On this conception we can understand why it is that a patient consumed by worry--which to me signifies interrupted stimulation, a state of alternation between hope and fear--suffers so many bodily impairments and diseases even. This hypothesis explains the slow dying of animals in captivity. It explains the grave digestive and metabolic disturbances which appear under any nerve strain, especially under the strain of fear, and the great benefits of confidence and hope; it explains the nervousness, loss of weight, indigestion--in short, the comprehensive physical changes that are wrought by fear and by sexual love and hate. On this hypothesis we can understand the physical influence of one individual over the body and personality of another; and of the infinite factors in environment that, through phylogenetic association, play a role in the functions of many of our organs. It is because under the uncompromising law of survival of the fittest we were evolved as motor beings that we do not possess any organs or faculties which have not served our progenitors in accomplishing their survival in the relentless struggle of organic forms with one another. We are now, as we were then, essentially motor beings, and the only way in which we can meet the dangers in our environment is by a motor response. Such a motor response implies the integration of our entire being for action, this integration involving the activity of certain glands, such as the adrenals (Cannon), the thyroid, the liver, etc., which throw into the blood-stream substances which help to form energy, but which, if no muscular action ensues, are harmful elements in the blood. While this motor preparation is going on, the entire digestive tract is inhibited. It thus becomes clear why an emotion is more harmful than action.

Any agency that can sufficiently inspire faith,--dispel worry,-- whether that agency be mystical, human, or divine, will at once stop the body-wide stimulations and inhibitions which cause lesions which are as truly physical as is a fracture. The striking benefits of good luck, success, and happiness; of a change of scene; of hunting and fishing; of optimistic and helpful friends, are at once explained by this hypothesis. One can also understand the difference between the broken body and spirits of an animal in captivity and its buoyant return to its normal condition when freed.

But time will not permit me to follow this tempting lead, which has been introduced for another purpose--the proposal of a remedy.

Worries either are or are not groundless. Of those that have a basis, many are exaggerated. It has occurred to me to utilize as an antidote an appeal to the same great law that originally excited the instinctive involuntary reaction known as fear-- the law of self-preservation.

I have found that if an intelligent patient who is suffering from fear can be made to see so plainly as to become firmly convinced that his brain, his various organs, indeed his whole being, could be physically damaged by fear, that this same instinct of self-preservation will, to the extent of his conviction, banish fear. It is hurling a threatened active militant danger, whose injurious influences are both certain and known, against an uncertain, perhaps a fancied, one. In other words, fear itself is an injury which when recognized is instinctively avoided. In a similar manner anger may be softened or banished by an appeal to the stronger self-preserving instinct aroused by the fear of physical damage, such as the physical injury of brain-cells. This playing of one primitive instinct against another is comparable to the effect produced upon two men who are quarreling when a more powerful enemy of both comes threateningly on the scene.

The acute fear of a surgical operation may be banished by the use of certain drugs that depress the associational power of the brain and so minimize the effect of the preparations that usually inspire fear. If, in addition, the entire field of operation is blocked by local anesthesia so that the associational centers are not awakened, the patient will pass through the operation unscathed.

The phylogenetic origin of fear is injury, hence injury and fear cause the same phenomena. In their quality and in their phenomena psychic shock and traumatic shock are the same. The perception of danger by the special senses in the sound of the opening gun of a battle, or in the sight of a venomous snake, is phylogenetically the same and causes the same effects upon the entire body as an operation under anesthesia or a physical combat in that each drives the motor mechanism. The use of local anesthetics in the operative field prevents nerve-currents from the seat of injury from reaching the brain and there integrating the entire body for a self-defensive struggle. The result, even though a part of the brain is asleep and the muscles paralyzed, is the same as that produced by the interception of the terrifying sound of the gun, or of the sight of the dangerous reptile, since the stimulation of the motor mechanism is prevented.

By both the positive and the negative evidence we are forced to believe that the emotions are primitive instinctive reactions which represent ancestral acts; and that they therefore utilize the complicated motor mechanism which has been developed by the forces of evolution as that best adapted to fit the individual for his struggle with his environment or for procreation.

The mechanism by which the motor acts are performed and the mechanism by which the emotions are expressed are one and the same. These acts in their infinite complexity are suggested by association-- phylogenetic association. When our progenitors came in contact with any exciting element in their environment, action ensued then and there. There was much action--little restraint or emotion. Civilized man is really in auto-captivity. He is subjected to innumerable stimulations, but custom and convention frequently prevent physical action. When these stimulations are sufficiently strong but no action ensues, the reaction constitutes an emotion. A phylogenetic fight is anger; a phylogenetic flight is fear; a phylogenetic copulation is sexual love, and so one finds in this conception an underlying principle which may be the key to an understanding of the emotions and of certain diseases.

PAIN, LAUGHTER, AND CRYING[*]

PAIN

Pain, like other phenomena, was probably evolved for a particular purpose-- surely for the good of the individual; like fear and worry, it frequently is injurious. What then may be its purpose?

We postulate that pain is one of the phenomena which result from a stimulation to motor action. When a barefoot boy steps on a sharp stone it is important that the injuring contact be released as quickly as possible; and therefore physical injury pain results and impels the required action. Anemia of the soft parts at the points of pressure results from prolonged sitting or lying in one position, and as a result pain compels a muscular action that shifts the damaging pressure--this is the pain of anemia; when the rays of the blazing sun shine directly upon the retina, pain immediately causes a protective muscular action--the lid is closed, the head turns away--this is light pain; when standing too close to a blazing fire the excessive heat causes a pain which results in the protective muscular action of moving away--this is heat pain; when the urinary bladder is acutely overdistended the resultant pain induces voluntary as well as involuntary muscular contraction-- this is evacuation pain; associated with defecation is a characteristic warning pain, and an active pain which induces the required muscular action--this, like the pain accompanying micturition, is an evacuation pain; in obstruction of the urinary passages and of the large and the small intestine the pain is exaggerated, as is the accompanying muscular contraction--this is a pathologic evacuation pain; when the fetus reaches full term and labor is to begin, it is heralded by pain which is associated with rhythmic contractions of the uterine muscle; later, many other muscles take part in the birth and pain is associated with all these muscular contractions--these are labor pains; when a foreign body, be it ever so small, falls upon the conjunctiva or cornea there results what is perhaps the acutest pain known, and quick and active muscular action follows--this is special contact pain. Special pain receptors are placed in certain parts of the nose, the pharynx, and the larynx, the stimulation of which causes special motor acts, such as sneezing, hawking, coughing. Curiously vague pains are associated with the protective motor act of vomiting and with the sexual motor acts--these may be termed nausea pains and pleasure pains. We now see, therefore, that against the injurious physical contacts of environment, against heat and cold, against damaging sunlight, against local anemia when resting or sleeping, the body is protected by virtue of the muscular action which results from pain. Then, too, for the emptying of the pregnant uterus, for the evacuation of the intestine and of the urinary bladder as normal acts, and for the overcoming of obstructions in these tracts, pain compels the required muscular actions, For passing gall-stones and urinary calculi, urgent motor stimuli are awakened by pain. For each of these diversified pains the consequent muscular action is specific in type, distribution, and intensity. This statement is so commonplace that we are apt to miss the significance and the wonder of it. It is probable that every nerve-ending in the skin and every type of stimulation represents a separate motor pattern, the adequate stimulation of which causes always the same response.

Let us pass on to the discussion of another and perhaps even more interesting type of pain, that associated with infection. Not all kinds of infection are painful; and in those infections that may be associated with pain there is pain only when certain regions of the body are involved. Among the infections that are not associated with pain are scarlet fever, typhoid fever, measles, malaria, whooping-cough, typhus fever, and syphilis in its early stages. The infections that are usually, though not always, associated with pain are 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 always accompanied by heat production, the purpose of the muscular contraction and the shivering is quite certainly caused by cold to assist in the maintenance of the normal body temperature.

We have now discussed many of the causes of pain and in each instance we have found an associated muscular action which apparently serves some adaptive purpose (Figs. 24 and 25). If we assume that pain exists for the purpose of stimulating muscular reactions, we may well inquire what part of the nervous are is the site of the sensation of pain--the nerve-endings, the trunk, or the brain? Does pain result from physical contact with the nerve-endings, with the physical act of transmitting an impression along the nerve trunk, or with the process within the brain-cells by which energy is released to cause a motor act?

It seems most probable that the site of the pain is in the brain-cells. If this be so, then what is the physical process by which the phenomena of pain are produced? The one hypothesis that can be tested experimentally is that pain is a phenomenon resulting from the rapid discharge of energy in the brain-cells. If this be true, then if every pain receptor of the body were equally stimulated in such a manner that

{illust. caption = FIG. 25.--FEAR AND AGONY. "Amid this dread exuberance of woe ran naked spirits wing'd with horrid fear."-- Dante's "Inferno," Canto XXIV, lines 89, 90. all the stimuli reached the brain-cells simultaneously, the cells would find themselves in equilibrium and no motor act would be performed. But if all the pain receptors of the body but one were equally stimulated, and this one stimu-lated harder than the rest, then the latter would gain possession of the final common path, the sensation of pain would be felt, and a muscular contraction would result. It is well known that when a greater pain is thrown into competition with a lesser one, the lesser is completely submerged. In this manner the school-boy initiates the novice into the mystery of the painless plucking of hair. The simultaneous, but severe application of the boot to the blindfolded victim takes complete and exclusive possession of the final common path and the hair is painlessly plucked through the triumph of the boot stimulus over the hair stimulus in the struggle for the possession of the final common path. Another argument in favor of this hypothesis that pain is an accompaniment of the release of energy in the brain- cells is found in the fact that painless stimuli received through the special senses may completely submerge the painful stimuli of physical injury; for although the stimuli to motor action, which are received through the senses of sight, hearing, and smell, cause even more powerful motor action than those caused by physical contact stimuli, yet they are not accompanied by pain. Examples of this triumph of stimulation of the special senses over contact stimulation are frequently seen in persons obsessed by anger or fear, and to a less degree in those obsessed by sexual emotion. In the fury of battle the soldier may not perceive his wound until the emotional excitation is wearing away, when the sensation of warm blood on the skin may first attract his attention. Religious fanatics are said to feel no pain when they subject themselves to self-injury. Now, since both psychic and mechanical stimuli cause motor action by the excitation ofprecisely the same mechanism in the brain, and since the more rapid release of energy from psychic stimuli submerges the physical stimuli and prevents pain, it would seem that pain must be a phenomenon which is associated with the process of releasing energy by the brain-cells. Were physical injury inflicted in a quiescent state equal to that inflicted in the emotional state, great pain and intense muscular action would be experienced. Now the emotions are as purely motor excitants as is pain. The dynamic result is the same the principal difference being the greater suddenness and the absolute specificity of the pain stimuli as compared with the more complex and less peremptory stimuli of the emotions. A further evidence that pain is a product of the release of brain-cell energy is the probability that if one could pierce the skin at many points on a limb in such a manner that antagonistic points only were equally and simultaneously stimulated, then an equilibrium in the governing brain- cells would be established and neither pain nor motion would follow. An absolute test of this assumption cannot be made but it is supported by the obtainable evidence. We will now turn to a new viewpoint, a practical as well as a fascinating one, which can best be illustrated by two case histories: A man, seventy-eight years old, whose chief complaint was obstinate constipation, was admitted to the medical ward of the Lakeside Hospital several years ago. The abdomen was but slightly distended; there was no fever, no increased leukocytosis, no muscular rigidity, and but slight general tenderness. He claimed to have lost in weight and strength during the several months previous to his admission. A tentative diagnosis of malignant tumor of the large intestine was made, but free movements weresecured rather easily, and we abandoned the idea of an exploratory operation. The patient gradually failed and died without a definite diagnosis having been made by either the medical or the surgical service. At autopsy there was found a wide-spread peritonitis arising from a perforated appendix. A child, several years old, was taken ill with some indefinite disease. A number of the ablest medical and surgical consultants of a leading medical center thoroughly and repeatedly investigated the case. Although they could make no definite diagnosis they all agreed that the trouble surely could not be appendicitis because there was neither muscular rigidity nor tenderness. The autopsy showed a gangrenous appendix and general peritonitis. How can these apparently anomalous cases be explained? These two cases are illustrations of the same principle that underlies the freedom from pain which results from the use of narcotics and anesthetics, the same principle that explains the fact that cholecystitis may occur in the aged without any other local symptoms than the presence of a mass and perhaps very slight tenderness; and that accounts in general for the lack of well-expressed disease phenomena in senility and in infancy. The reason why the aged, the very young, and the subjects of general paresis show but few symptoms of disease is that in senility the brain is deteriorated, while in infancy the brain is so undeveloped that the mechanism of association is inactive, hence pain and tenderness, which are among the oldest of the associations, are wanting. Senility and infancy are by nature normally narcotized. The senile are passing through the twilight into the night; while infants are traversing through the dawn into the day. Hence it is that the diagnosis of injury and disease in the extremes oflife is beset by especial difficulties, since the entire body is as silent as are the brain, the pericardium, the mediastinum, and other symptomless areas. For the same reason, when a patient who is seriously ill with a painful disease turns upon the physician a glowing eye and an eager face, and remarks how comfortable he feels, then the end is near. This is a brilliant and fateful clinical mirage. When one reflects on the vast amount of evidence as to the origin and the purpose of pain, he is forced to conclude that pain is a phenomenon of motor stimulation, and that its principal role is the protection of the individual against the gross and the microscopic enemies in his environment. The benefits of pain are especially manifested in the urgent muscular actions by means of which the body moves away from physical injury; obstructions of the hollow viscera are overcome; rest is compelled in the acute infections-- the infected points are held rigidly quiet, the muscles of the abdomen are fixed, and harmful peristalsis is arrested in peritonitis; while there is absolutely no pain in the diseases or injuries which affect those regions of the body in which in the course of evolution no pain receptors were placed, or in those diseases in which muscular inhibition or contraction is of no help. In a biologic sense pain is closely associated with the emotional stimuli, for both pain and the emotions incite motor activity for the good of the individual. The frequent occurrence of post-operative and post- traumatic pain is accounted for by the fact that the operation or the injury has lowered the threshold of the brain- cells to trauma; the brain and not the local sensitive field is the site of the pain. I have found that, by blockingthe field of operation with local anesthesia, post-operative pain is diminished; that is, since the local anesthesia prevents the strong stimuli of the trauma from reaching the brain, its threshold is not lowered. There is a close resemblance between the phenomena of pain habit, of education, of physical training, of love and of hate. In education, in pain habit, in all emotional relations, a low brain- cell threshold is established which facilitates the reception of specific stimuli; all these processes are motor acts, or are symbolic of motor acts, and we may be trained to perceive misfortune and pain as readily as we are trained to perceive mathematical formulae or moral precepts. In each and every case, readiness of perception depends, as it seems to me, upon a modified state of the brain-cells, their threshold especially, the final degree of perception possible in any individual being perhaps based on the type of potential molecules of which the brain is built. We must believe also that every impression is permanent, as only thus could an individual animal or a man be fitted by his own experience for life's battles. LAUGHTER AND CRYING What is laughter? What is its probable origin, its distribution, and its purpose? Laughter is an involuntary rhythmic contraction of certain respiratory muscles, usually accompanied by certain vocal sounds. It is a motor act of the respiratory apparatus primarily, although if intense it may involve not only the extraordinary muscles of respiration, but most of the muscles of the body. There are many degrees of laughter, from the mere brightening of the eyes, a fleeting smile, tittering andgiggling, to hysteric and convulsive laughter. Under certain circumstances, laughter may be so intense and so long continued that it leads to considerable exhaustion. The formation of tears is sometimes associated with laughter. When integrated with laughter, the nervous system can perform no other function. Crying is closely associated with laughter, and in children especially laughter and crying are readily interchanged. We postulate that laughter and weeping serve a useful purpose. According to Darwin, only man and monkeys laugh (Fig. 26); other animals exhibit certain types of facial expression accompanying various emotions, but laughter in the sense in which that word is commonly used is probably an attribute of the primates only, although it is probable that many animals find substitutes for laughter. The proneness of man to laughter is modified by age, sex, training, mental state, health, and by many other factors. Healthy, happy children are especially prone to laughter, while disease, strong emotions, fatigue, and age diminish laughter. Women laugh more than do men. The healthy, happy maturing young woman perhaps laughs most, especially when she is slightly embarrassed. What causes laughter? Good news, high spirits, tickling, hearing and seeing others laugh; droll stories; flashes of wit; passages of humor; averted injury; threatened breach of the conventions; and numerous other causes might be added. It is obvious that laughter may be produced by diverse influences, many of which are so unlike each other that it would at first sight seem improbable that a single general principle underlies all. Before presenting a hypothesis which harmonizes most of the facts, and which mayoffer an explanation of the origin and purpose of laughter, let us return for a moment to some previous considerations-- that man is essentially a motor being; that all his responses to the physical forces of his environment are motor; {illust. caption = FIG. 26.--LAUGHING CHIMPANZEE. "Mike," the clever chimpanzee in the London Zoo, evidently enjoys a joke as well as any one else. (Photo by Underwood and Underwood, N. Y.)}