Popular Science Monthly/Volume 76/May 1910/Sneezing, Sea-Sickness, Pain
|SNEEZING, SEA-SICKNESS, PAIN|
SOMETIME MASTER OF DOWNING COLLEGE, CAMBRIDGE
PHYSIOLOGISTS can not lay claim to a theory of pain. Even definition is difficult. The distress of bodily wants overlaps pain on the one side; fear, anxiety and similar mental states overlap it on the other. Excessive stimulation of certain organs of special sense, particularly those for touch, temperature and hearing, leads up to it. If an attempt be made to isolate, in thought, the effect in consciousness to which the term "pain" properly applies, it may be said to be the awareness of something amiss in some part of the body, irrespective of the testimony of either of the special senses. It is a modification of consciousness, not a part of its content. We have learned to associate the receipt of the modifying influence through particular nervous channels, with its provenance, just as we have learned to associate sensations of touch conveyed by particular nerve-fibers with the contact with external objects of particular regions of the skin; but such topognosis is no more innate in the one case than in the other. It is the product of self-investigation, and is based either upon the testimony of the eye or upon experiments in moving the hand to the spot. Hence in the case of organs which are out of sight and out of reach topographical guidance is unobtainable. Since we can have no knowledge of its seat, the pain is referred to some accessible part of the segment of the body in which it occurs. A gulp of very hot water, on reaching the closed sphincter muscle of the stomach—the valve which must open before it can pass from gullet into stomach—gives rise to pain which seems to have its seat in the skin over the lower end of the breast-bone. In the same way disease of the various viscera gives rise to pain and tenderness of areas of the skin of the segments of the body in which the nerves of the viscera join the spinal cord.
From a physiological standpoint "pain" and "sensation" are antithetical terms. Sensations inform. Pain is a state of consciousness which masks sensation. Sensations are transient. Their apparent prolongation is due to repetition. They are vibratory. Pain is a condition, slowly set up, slow to disappear. Even the briefest pain is long as compared with the constituent unit—a nerve wave—of sensation. It is of the very essence of sensation that it has quality or modality; the informing value of any given sensation depends upon its excluding all other forms of stimulation. The sensation of a bright red spot of light is not susceptible of confusion in place or quality with other visual sensations. Still less is it liable to be mistaken for a sensation of hearing or of taste. Pain has no modality. If it may be justly described as stabbing, aching, burning, it owes its individual character to the form of its onset and to its duration, and these in turn depend upon either the vascular condition of the part affected—the pumping of blood through the vessels of a tissue free to expand, or packed in a bony case—or they are due to the effect upon the inflamed or injured part of muscular contractions. If the injured part be inaccessible, pain has no "local sign." If it be on or near the surface of the body the pain felt in it has or seems to have a topographical meaning; but it is very doubtful whether the mind can localize the source of pain in the absence of evidence simultaneously afforded by the nervous apparatus of the sense of touch. Many instances are on record of disease or injury to the central nervous system resulting in complete loss of sensitiveness to pain, whilst sensitiveness to touch and pressure remained undiminished. But there are no recorded cases, so far as we are aware, of complete paralysis of the mechanisms of touch and of the recognition of heat, of cold and of pressure, with the retention of normal sensitiveness to pain. Such a condition, if it were established, would make it possible for an investigator to ascertain whether skin-pain, by itself and unsupported by collateral evidence, has a topographical meaning, or "local sign"; and whether the expression pain-spot may be legitimately used, as meaning a sounding spot in the midst of a dumb area, and not merely a focus of sensitiveness at which the weakest stimulus which can evoke pain is effective.
Dr. Henry Head caused the large cutaneous nerve of the thumb-side of the forearm and hand to be cut in his own arm, in order that he might study carefully the revival of sensations which follows on nerve repair. He found that, long before he regained the ability to distinguish degrees of warmth, to feel as separate the two points of a pair of compasses, or to recognize a touch with cotton-wool, he regained his power of recognizing stimulation by agents that do harm—hot things, cold things, pricking with a pin—but his power of localizing the spot injured was extremely vague. Trotter and Davies have made similar experiments in their own persons on a still more extensive scale and have confirmed and amplified Head's results.
Investigations with the aid of new histological methods has shown that the epithelial tissues are supplied with nerve-filaments in inconceivable abundance. It is probable that the conclusion is justified that every cell of the skin, of the mucous membranes, of the lining epithelium of the air-chambers in the lungs, of the pleural and peritoneal cavities, of the various glands, is connected with a nervous thread. It is certainly true also of every muscle-fiber in the walls of the alimentary tract, of ducts and of blood-vessels. By these filaments the cells of the body-surfaces both external and internal, the central nervous system and all motile organs, are bound together.
Superimposed on this basal system are the various specialized systems of nerves which originate in organs in which their ends are so modified and so enveloped as to render them sensitive in the highest degree to one particular order of stimulus, whether of smell, sight, taste, hearing, touch, heat, cold, pressure or traction, and inaccessible to stimuli of every other class. These nerves, with the chains of neurones which link them to the muscles, via the spinal cord and brain, stand out as a pattern on the basal system, like the pattern formed of thicker fibers and coarser knots on a sheet of lace.
In a book recently published, "The Body at Work," I have endeavored to present a picture of the nervous system and its activities, which, although not original in any of its details, is new in their grouping and in its comprehensiveness. It is based upon the teaching that the two great functions of the nervous system, notwithstanding that they grade one into the other, must, for purposes of analysis and description, be considered apart. By the basal system of protopathic nerves all the cells of the body, with the exception of those of the connective tissues, bones, tendons and so forth, are bound together into a continuous inseparable whole. No change can occur in the nutritive condition of any part of the skin or of an internal epithelium without the induction of a nutritive change in the central nervous system and thence, onward, in the plain muscle-fibers of arteries and other structures of the segment of the body in which the inducing change occurs. As contrasted with the influence which spreads through this basal system the "impulses" which travel up the nerves of special sense are peculiar in kind or, at any rate, in intensity. In order that they may overcome the resistance of a chain of neurones they have a certain potential, and progress in pulsations or waves.
Pain is explained as due to the setting up in a particular segment of the axial nervous system of a focus "pain-conditioned" sympathetically with the injured tissues. Consciousness of pain depends upon the direction of attention to impulses which ascend through the pain-conditioned segment from end-organs of nerves of special sense. If the seat of injury be the skin it is through the specialized nerves of the injured spot that modified impulses reach the cortex of the brain. If the seat of injury be an internal organ no effect is produced in consciousness until the pain agitation of the spinal cord has become sufficiently intense, and sufficiently wide-spread, to modify impulses which ascend to the cortex from skin areas of the segment in which the viscus is situate. The pain in angina pectoris is felt on the left side of the breast bone at its lower end. This shows that the nerves of the aorta have their centers in the same region of the spinal cord as the cutaneous nerves of this area on the surface of the chest.
To give an illustration of the difference of mechanism of pain and of sensation. In a railway station lavatory I recently observed a man who absent-mindedly placed his fingers on a free-standing iron stove to ascertain whether it was hot. It was a frosty morning in November. Obviously, the stranger had a strong prejudice that the station-master would not have thought it necessary to order a fire to be lighted so early in the winter. As nearly as I could estimate, three seconds elapsed between the touching of the stove and the ejaculation which announced with unnecessary emphasis that the man had obtained the information he desired. Had he, expecting to find the iron hot, directed his attention to the modification of his skin sensation he would have withdrawn his fingers in one seventh of a second.
One of the characteristics of incipient pain is exaltation of reflex actions. I can not by any effort of will prevent my muscles from withdrawing my hand from hot iron (although the resolute withdrawal of attention from pain-modified sensations and the forcing of a conviction that it does not exist, has in certain cases a remarkable effect in suppressing pain). Equally characteristic of established pain is the inhibition of action. A whitlow abolishes all temptation to shake the finger.
Physiologists can not investigate the phenomena of pain, although they make elaborate studies of its threshold value and of the distribution of "pain spots." It would take us too far were we to consider the evidence of a degree of specialization in the protopathic nerves of the skin which is held by some to justify the use of the expression "pain-nerves," and of the allied question of neuronic conduction of incipient or threshold pain along pain-tracts in the spinal cord.
The chief interest of the hypothesis of structural continuity through the protopathic nervous system, with its corollary of sympathetic nutritional change, lies in the explanation which it affords of the influence upon reflex action of the establishment of a pain-condition in the axial nervous system in circumstances in which, consciousness not being affected, there is no "pain."
Pain-condition which inhibits reflexes due to impulses which start in the damaged organ or skin area, greatly increases in many instances the conductivity of the portion of the nervous system which it affects for impulses which do not come from the damaged part. Such a reinforced reflex is the attack of sneezing to which many persons, most monkeys, and some breeds of dogs are subjected when the eye is stimulated by a bright light. When the gaze is directed towards a bright cloud, excessive stimulation of the retina sets up a pain-condition in the mid-brain. In the progress of evolution this portion of the cerebrospinal axis has undergone great changes. Its sensory nerves with their protopathic constituents have been drawn backwards into the great bundle of the fifth nerve, which joins the hind-brain, whilst the nerve from the retina has established a secondary connection with the mid-brain. The mid-brain receives in consequence the protopathic nerves of the eye. But the nose being the real tip of the body and anterior to the eye the sensory fibers of the skin which lines it although bound up with the fifth nerve, extend forwards within the cerebrospinal axis to the very front of the mid-brain. A remarkable state of affairs is thus established. The mid-brain receives the protopathic nerves of the eye and the root-fibers of the sensory nerves of the nostrils. When excessive stimulation of the retina by bright light sets up a pain-condition in the mid-brain the every-moment impulses ascending from the nostril acquire undue importance. It is as if they had been increased in intensity by a pinch of snuff. Their urgency causes the reflex by which irritating substances are expelled.
Some persons merely feel a tickling in the nose when they look at a bright light, but do not sneeze. This phenomenon is extremely interesting. It proves that stimuli "adequate" to impress nerve-endings are not necessarily "adequate" to arouse consciousness. External forces incessantly press the button with sufficient energy to make contact. At each pressure a bell rings in the chamber of consciousness, but, if it is to attract attention, it must ring more loudly. The stimuli which gave rise to a tickling feeling were not originated nor intensified by the light which fell upon the retina. The mid-brain through which impulses passed to reach the cortex was rendered more conductile.
In normal conditions no pain results from stimulation of the retina, however severe; because the nerve-fibers which convey visual impulses from this highly specialized sense-organ, are connected, not with the mid-brain, but with the optic thalamus and the occipital cortex. It would stultify so highly specialized a sense, were its news admixed with, or modified by any influence or information not directly connected with its proper function.
Sea-sickness is another illustration of the effect upon reflex action of central agitation due to impulses which do not appear in consciousness however voluminous the sensations may be of which they are the indirect cause. The nerve which is concerned with the adjustment of the position of the body is a constituent of the auditory nerve. It comes from the semicircular canals. Never under any circumstances do the impulses which originate in these organs of orientation enter consciousness; but when a ship begins to roll, or worse, to heave, they churn up the gray matter of the hind-brain until its conductivity is so affected as to demonstrate their urgency beyond misunderstanding. Root-fibers of the vagal nerve traverse the hind-brain much in the same way as root-fibers of the fifth nerve traverse the mid-brain. Habitual, every-moment impulses ascending from the stomach by the vagal nerve, for the routine regulation of its purely domestic functions, acquire, when the hind-brain is pain-conditioned by impulses from the semicircular canals, a terrifying import, causing the explosion of numberless motor neurones. The stomach sneezes, with the zealous support of muscles of the throat, chest and abdomen. In the early stages of the malady the same cause leads, no doubt, to reflex derangement of the secretion of the stomach involving nausea. Disturbances of vision add to the victim's discomposure, and in some small degree precipitate the stomachic catastrophe; but the effective cause is, I take it, the agitation, by impulses from the semicircular canals, of the gray matter of the hind-brain.
Hiccough, again, is an exaggerated reflex due to increased conductivity of gray matter. A child takes a cold drink, or he rapidly fills his stomach with insufficiently masticated food. The ends of the vagal nerve in the stomach are irritated. They convey an influence which sets up the pain-condition in a portion of the gray matter through which nerve-fibers from the lungs extend their roots towards the nucleus of the phrenic nerve. The diaphragm sneezes.
It would carry us beyond the proper sphere of this journal were we to consider the phenomena of inhibition of some reflexes and exaggeration of others which the modification of the normal conductivity of gray matter due to the establishment of pain-conditioned foci, brings about in hysteria, angina pectoris and many other morbid conditions.
The pain of headache is as truly "referred" as is the pain of angina pectoris, although it must be assigned to a different category. Medical men tell their patients that their headaches are in their scalps and not within their skulls. The patient finds it difficult to understand how this can be, when there is nothing the matter with his scalp; but agrees with his doctor that, were it otherwise, it would be impossible to explain the beneficial effect of a cold wet rag. Again as in sea-sickness the vagal nerve is at the bottom of the mischief. Indeed, in many persons, intolerable headache takes the place of sickness on the sea. Impulses ascending the vagus agitate the gray matter of the hind-brain. Into this pain-conditioned gray matter the nerves of the scalp pour a constant stream of impulses. Myriads of fibers connecting the scalp with the brain twang ceaselessly with messages to which, under normal circumstances, consciousness gives no heed—until a draught of cold air or the tickling of a fly's feet accentuates a certain group. Let the gray matter through which they pass be pain-conditioned, the vibrations traveling to the cortex from innumerable spots on the surface of the head produce a widely diffused dull ache which has no sensational quality, because no particular group of nerve-endings is being especially stimulated by external force. Vascular changes in the scalp due to the same cause, the exaggeration of impulses during their transit of the gray matter of the hind-brain, making believe that the scalp is injured and needs more blood, react upon the nerve-endings increasing the illusion of injury. The pain is no illusion. It is impossible to decide whether vascular changes are the first effect of vagal agitation and therefore the immediate cause of pain or whether they are merely subsidiary results of the exaggeration of sensory impulses from the scalp. A cold compress by constricting the blood-vessels reduces the din of the multitudinous messages shouted into consciousness by the sense-organs of the scalp. If completely successful it subdues their chorus to its habitual murmur, too faint to secure attention. Long continued, and therefore damaging, contraction of the muscles which move the eye-ball, and particularly its elevator, sends through their protopathic nerves a stream of influence to the mid-brain, with which these nerves are connected, which causes frontal headache in just the same way as the influence which ascends the stomach nerve.
Would doctors be more logical if they said that the headache was in the hind-brain—the region which contains the agitated gray matter which gives pain-value to the impulses from the scalp—or if they said it was in the stomach? The irritation of nerve-endings in the stomach is the origin of the trouble, seeing that it sets up the pain-condition in the hind-brain.
Psychologists base their science upon conspicuous sensations—sensations of sufficient prominence to stand out in the field of consciousness. They can do no otherwise. But the terminology in which they have expressed the results of their analysis hampers physiology. In the process of conduction a physiologist can distinguish no stages intermediate between stimulation and muscular response. In reactions to which consciousness is adjunct, as judged by self-feeling, or, when outside oneself by attribution of self-feeling, the nerve-current may be termed a sensation, and sensations may or may not provoke attention. Nothing is gained by classifying the sequence of events into stimulation, passage of impulse, sensation, perception. Such terms are machinomorphic. The nervous mechanism is infinitely vibrating. "I always hear my clock stop" can have but one meaning. Every tick of the clock produces an answering vibration of the auditory nerve, however little attention be given to the message; and attention carries with it the idea of something which attends. Pain, as pictured in this essay, is the interpretation which the ego gives of hitherto unperceived sensations when they are increased in volume without definition. Pain is developed when impulses, without informing attributes, are raised in urgency to the level of attention.
The passage of a gall-stone from the gall-bladder to the intestine is the cause of intense pain, "referred," in the first instance, to the skin which overlies the liver. Yet the gall-bladder is insensitive. As surgeons have long been aware, the liver, stomach and other viscera may be cut, burned, scarified, without arousing pain. Laying stress on these two well-known facts, (1) the insensitiveness of the viscera and (2) their liability to become the source of referred pains, James Mackenzie has defined pain as "a disagreeable sensation due to stimulation of some portion of the cerebrospinal nervous system and referred to the peripheral distribution in the body wall of cerebrospinal sensory nerves." When a viscus is the seat of origin of pain the impulses which ascend its sympathetic nerves excite the centers of sensory nerves in the spinal cord.
The theory which I have attempted to outline in this article is laid on the same basis, somewhat broadened. All pain is "referred"—to the right spot, if its source be in the skin; because the skin is elaborately supplied with place-defining nerves—to an organ or part, skin, muscle, joint, which the ego, during the progress of self-investigation, has discovered in the same segment of the body, if its source be in a viscus.
The body is permeated with a felt-work of nerves, unprovided with specialized nerve-endings, conveying no definite information, and in consequence without precise distribution in the seat of consciousness. This non-specialized system which binds the various parts of the body together is the mechanism through which the caliber of blood-vessels, erection of hairs, secretion of glands, contraction of the walls of ducts and of the intestines, and many other domestic adjustments are effected. It is also the medium through which the gray matter of the cerebrospinal axis is affected sympathetically with damage to the tissues. The resultant altered conductivity of the gray matter leads to modification •of the only kind of impulses with which consciousness is concerned—impulses which inform. We infer that the damage which is giving:rise to a feeling of pain is in the part from which the modified impulses 'come.
When attempting to formulate the theory of pain it is necessary to discard the prejudice that there need be a proportional relation between the intensity of pain and the magnitude of the physiological changes which condition it. A heavy blow hurts more than a light one. Yet a change which could not be detected by any piece of apparatus in use in a physiological laboratory, if it affect the nerve-tissue of a tooth, may give rise to more pain than is caused by a crushed limb.
Another prejudice, from which it is difficult to shake free, attributes to the mind an innate knowledge of the topography of the body; an innate knowledge, that is to say, of the distribution of its news-agents, the sensory endings of nerves.
Thirdly, it is necessary to remember, when investigating the machine, that the machine is the man. It is not sufficient to design a scheme of telephone wires requiring for its use a listening ear at its center, the brain. The ear is a part of the machine. There is no need to picture a system of pain nerves, carrying news of damage to an attentive mind. A departure from the normal in the functioning of the sensory apparatus is pain.