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 a.s.surance 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 pa.s.sed 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 a.s.sistance--hence 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 surprise--indeed, 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 a.n.a.lysis 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a stimulus to action, and the second, in which the story turns suddenly so that the stimulus to action is unexpectedly withdrawn.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 cla.s.ses, 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 a.s.sociated 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 const.i.tute an adequate stimulus. That is, the contact must arouse a phylogenetic a.s.sociation 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 a.s.sociated.

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 reversed--the 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 a.n.a.lysis 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 circ.u.mstance, such as a threatened breach of the conventions, which would have led to embarra.s.sment or humiliation, leads to a tendency to laughter; and why the recital of heroic deeds by a.s.sociation 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 diseases--exophthalmic 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 glands--the 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 mechanism--are 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 happens--the flood-gates of tears are open much of the time in Graves' disease--a 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 a.s.sociation perhaps explains the a.s.sociation 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 ill.u.s.trated by the almost fatal reaction which I once saw result from the mere p.r.i.c.king 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 PHYSICAL STATE OF THE BRAIN-CELLS AND BRAIN FUNCTIONS--EXPERIMENTAL AND CLINICAL[*]

[*] Address before The American Philosophical Society, April 18, 1913.

The brain in all animals (including man) is but the clearing-house for reactions to environment, for animals are essentially motor or neuromotor mechanisms, composed of many parts, it is true, but integrated by the nervous system. Throughout the phylogenetic history of the race the stimuli of environment have driven this mechanism, whose seat of power--the battery--is the brain.

Since all normal life depends upon the response of the brain to the daily stimuli, we should expect in health, as well as in disease, to find modifications of the functions and the physical state of the component parts of this central battery-- the brain-cells. Although we must believe, then, that every reaction to stimuli, however slight, produces a corresponding change in the brain-cells, yet there are certain normal, that is, non-diseased, conditions which produce especially striking changes.

The cell changes due to the emotions, for example, are so similar, and in extreme conditions approach so closely to the changes produced by disease, that it is impossible to say where the normal ceases and the abnormal begins.

In view of the similarity of brain-cell changes it is not strange that in the clinic as well as in daily life, we are confronted constantly by outward manifestations which are so nearly identical that the true underlying cause of the condition in any individual case is too often overlooked or misunderstood.

In our laboratory experiments and in our clinical observations we have found that exhaustion produced by intense emotion, prolonged physical exertion, insomnia, intense fear, certain toxemias, hemorrhage, and the condition commonly denominated surgical shock, produce similar outward manifestations and identical brain-cell changes.

It is, therefore, the purpose of this paper to present the definite results of laboratory researches which show certain relations between alterations in brain functions and physical changes in the brain-cells.

Fear.--Our experiments have shown that the brain-cell changes due to fear may be divided into two stages: First, that of hyperchromatism-- stimulation; second, that of hypochromatism--exhaustion (Figs. 5 and 13). Hyperchromatism was shown only in the presence of the activating stimuli or within a very short time after they had been received.

This state gradually changed until the period of maximum exhaustion was reached--about six hours later. Then a process of reconstruction began and continued until the normal state was again reached.

Fatigue.--Fatigue from overexertion produced in the brain-cells like changes to those produced by fear, these changes being proportional to the amount of exertion (Fig. 4). In the extreme stage of exhaustion from this cause we found that the total quant.i.ty of Nissl substance was enormously reduced.

When the exertion was too greatly prolonged, it took weeks or months for the cells to be restored to their normal condition.

We have proved, therefore, that in exhaustion resulting from emotion or from physical work a certain number of the brain-cells are permanently lost. This is the probable explanation of the fact that an athlete or a race-horse trained to the point of highest efficiency can reach his maximum record but once in his life.

Under certain conditions, however, it is possible that, though some chromatin is forever lost, the remainder may be so remarkably developed that for a time at least it will compensate for that which is gone.

Hemorrhage.--The loss of blood from any cause, if sufficient to reduce the blood-pressure, will occasion a change in the brain-cells, provided that the period of hypotension lasts for more than five minutes.

This time limit is a safeguard against permanent injury from the temporary hypotension which causes one to faint.

If the hemorrhage be long continued and the blood-pressure be low, there will be a permanent loss of some of the brain-cells. This explains why an individual who has suffered from a prolonged hemorrhage will never again be restored to his original powers.

Drugs.--According to their effect upon the brain-cells, drugs may be divided into three cla.s.ses: First, those that stimulate the brain-cells to increased activity, as strychnin (Fig. 37); second, those that chemically destroy the brain-cells, as alcohol and iodoform (Figs. 38 and 39); third, those that suspend the functions of the cells without damaging them, as nitrous oxid, ether, morphin.

Our experiments have shown that the brain-cell changes induced by drugs of the first cla.s.s are precisely the same as the cycle of changes produced by the emotions and by physical activity.

We have found that strychnin, according to the dosage, causes convulsions ending in exhaustion and death; excitation followed by la.s.situde; stimulation without notable after-results; or

{ill.u.s.t. caption = A, Section of Cerebellum of Normal Dog. C, Section of Cerebellum of Dog after Repeated Doses of Strychnin. FIG. 37.-- BRAIN-CELLS SHOWING STAGE OF HYPERCHROMATISM FOLLOWED BY CHROMATOLYSIS RESULTING FROM THE CONTINUATION OF THE STIMULUS. (Camera lucida drawings.)increased mental tone; while the brain-cells accurately display these physiologic alterations in proportional hyperchromatism in the active stages, and proportional chromatolysis in the stages of reaction. The biologic and therapeutic application of this fact is as obvious as it is important.

In our experiments, alcohol in large and repeated dosage caused marked morphologic changes in the brain-cells which went as far even as the destruction of some of the cells (Fig. 39). Ether, on the other hand, even after five hours of administration, produced no observable destructive changes in the brain-cells.

The effect of iodoform was peculiarly interesting, as it was the only drug that produced a rise of temperature. Its observed effect upon the brain-cells was that of wide-spread destruction.

Infections.--In every observation regarding the effect of pyogenic infections on dogs and on man we found that they caused definite and demonstrable lesions in certain cells of the nervous system, the most marked changes being in the cortex and the cerebellum (Fig. 40). For example, in fatal infections resulting from bowel obstruction, in peritonitis, and in osteomyelitis, the real lesion is in the brain-cells. We may, therefore, reasonably conclude that the la.s.situde, the diminished mental power, the excitability, irritability, restlessness, delirium, and unconsciousness which may be a.s.sociated with acute infections, are due to physical changes in the brain-cells.