A human experiment in nerve division/Chapter 2

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It was recognized by experiments before the operation that the ordinary distractions of a busy life were fatal to the detachment required by the sensory tests we wished to apply. We therefore determined that the work should be carried out in St. John's College, Cambridge (Dr. Head wishes to take this opportunity of expressing his gratitude to the President and Fellows of St. John's College, Cambridge, for the generous hospitality extended to him during the five years he was their frequent guest.) The inner of a set of rooms on the top floor of the second court, belonging to Dr. Rivers, was devoted to these observations. Here, absolutely quiet and undisturbed, free from the petty worries of a busy life, H. gave himself over entirely to examination.

As a rule, he travelled to Cambridge on Saturday, after spending several hours in the out-patient department of the London Hospital. But, on Saturday evening, he was found to be in a condition of so great fatigue that no observations could be made until Sunday morning. If, therefore, it was necessary to carry out a long-continued series of tests, H. came to Cambridge on Friday night, returning to London on Monday morning. Occasionally longer periods could be devoted to these observations.

Between April 25, 1903, the date of the operation, and the last sitting with Dr. Rivers on December 13, 1907, 167 days were devoted to this investigation.

The greater part of the work was done in the morning. Then H. went for a walk or a ride, and in the summer occasionally spent the afternoon on the river. The time between 5 p.m. and 7 p.m. was commonly spent in control experiments on normal parts, or in amplifying the results obtained earlier in the day. During the evening, these were talked over whilst H. marked out the hand, and determined the position of the cold and heat-spots in preparation for further tests next day.

The plan of investigation was debated beforehand, and was frequently committed to paper the night before the testing began. But R. always varied this order to such an extent, that H. remained ignorant of the results until the close of the sitting. This was especially the case with the compass-records; for instance, during many months, H. purposely refrained from inquiring into the nature of R.'s series of observations on the phenomenon of "double ones."

Throughout the examination, R. recorded exactly the procedure and H.'s answer at the time. At the close of a series, whilst still ignorant of the actual tests applied by R., H. dictated a note commenting on his experiences. Even then he was not told the nature of R.'s manipulations, unless some new fact had appeared which demanded immediate consideration.

Under no circumstances was H. allowed to know at the time whether his answers were right or wrong. For if he was told he had answered wrongly, he was roused to an intense determination to do better, producing thus a mental condition which was found to be unfavourable for the appreciation of sensory stimuli. Knowing his answers had not been correct, he would catch at every accessory circumstance in his attempt to interpret his sensations.

H. always sat with his eyes closed throughout the examination, as he found that this produced in him the condition most favourable for sensory testing. He always answered more correctly to all tests which required no close introspection when he did not attempt to think of what was going on. He would sit with closed eyes, his head resting on the right hand and his attention wandering widely over internal images. He soon learnt to adopt at will this state of passivity, provided he was undisturbed. But a knock at the door, or the entry of the servant, would rouse him into a state in which he again began to interpret his sensations.

H.'s mental processes are based upon visual images to a remarkable degree. Every thought is in some way bound up with internal vision, and even numbers, the days of the week and abstract ideas, such as virtue and cowardice, are associated with images of varying tones of white and black. He cannot recall musical sounds, except by seeing the notes or attaching the sounds to words which are clearly visualized. He has no power of reproducing directly scents or cutaneous sensations. He knows that the scent of violets is pleasing, and recognizes it with ease whenever it is present; but he is unable to recall a scent or a tactile impression in the same way that he can project the memory-picture of an object once seen.

In all these points, he corresponds to the common group of strong visualizers who learn to depend so exclusively on visual images that all other less dominant faculties of sensory reproduction fall into disuse. Throughout this paper we shall frequently allude to the part played in H.'s answers by these vivid mental images.

He was able to reproduce the image of a thing seen with such accuracy that it could be searched for details, at first unnoticed. But this was not the case with any other sensory impression. As soon as the stimulus was removed, he retained so much only as had been noticed at the time; for he was unable to reproduce any sensory images, except those of vision. This peculiarity common to most of those who visualize strongly leads to the following difficulty in testing sensation. Suppose that H. was returning correct answers to stimuli of different kinds applied to the affected area; the one was said to cause a sensation of touch, another appeared hot, and a third seemed to be cold. If, after withdrawing the cold object, R. inquired unexpectedly concerning the nature of the referred sensation, H. was frequently puzzled. Had the question referred to the qualities of a thing seen, H. would have recalled the visual image, examined it carefully and answered accordingly. But as he could not recall any tactile or thermal image of the cold object, he was unable to answer a question relating to some sensory quality to which his attention had not been directed at the time. A prick or other painful stimulus, however intense, evoked a sensation that could not be reproduced.

In the same way when tested with the compasses, H. might answer " two " rightly or wrongly ; but if, after the points were withdrawn, he was asked unexpectedly whether they seemed close together or far apart, he was unable to give an opinion. This occurred when the test was applied over both normal and abnormal parts, provided the compasses were suitably adapted.

Again, some unexpected feature in the sensation might arouse H.'s attention, but inability to reproduce the sensory image greatly hindered its introspective study. It was often necessary to repeat the stimulus several times before H. could appreciate each separate aspect of the sensation.

Now, sense-organs, and particularly those of the skin, do not react in an exactly similar way to every repetition of the stimulus. As this is particularly the case with those of the protopathic system, H.'s inability to recapture the features of a somatic sensation in the form of a reproduced image considerably hindered his introspective analysis. For this reason, every record dictated by H. was the direct result of introspection exercised during the period occupied by the stimulus. We have laid stress on this peculiarity, because the majority of persons in this country seem to belong to the group of those who depend on visual images, and approximate, at any rate as far as somatic sensations are concerned, to the condition of H.

Since H. was at the same time collaborator and patient, we took unusual precautions to avoid the possibility of suggestion. No questions were asked until the termination of a series of tests; for we found it was scarcely possible, in the long run, to ask even simple questions without giving a suggestion either for or against the right answer. Sounds and movements, that would have conveyed no information to an ordinary person, would disturb H.'s judgment in a case requiring fine discrimination. The clinking of ice against the glass, the removal of the kettle from the hob, tended to prejudice his answers and destroyed that negative attitude of attention essential for such experiments. R. was therefore particularly careful to make all his preparations beforehand; the iced tubes were filled and jugs of hot and cold water ranged within easy reach of his hand, so that water of the temperature required might be mixed silently.

Towards the end of a series of observations with finer tests over an area of defective sensibility, H. would frequently become uncertain in his answers, because he had forgotten his sensations with the coarser forms of the same stimuli. He might, for instance, speak of contact with the neutral tube as warmth. But occasional unexpected stimulation with a tube at 38° C. would at once correct this tendency, and throughout the further observations the neutral tube would be recognized with certainty. After a long series of " double ones," the application of the compasses widely separated so as to produce a definite sensation of two points frequently produced a similar steadying effect.

At first, we marked out both the affected area of the forearm and back of the hand into squares of approximately 1 cm. But we found the larger area on the forearm unmanageable, especially during the period when sensation changed rapidly. We therefore confined our attention, for the more minute investigations, mainly to a portion of the radial half of the back of the hand 5 cm. in every direction. This was marked out as follows: the hand was laid flat on the table and a line drawn along the axis of the third metacarpal bone. This corresponded almost exactly to the boundary between the normal and affected parts of the back of the hand. As base line, we used a fold of skin over the wrist which was prolonged backwards to meet the longitudinal line at a right angle. From this point, the line along the metacarpal was divided into seven portions, each 1 cm. in length. From the seventh division a line was drawn in the direction of the thumb at right angles to the longitudinal border. This was divided into five portions of 1 cm. in length. By drawing longitudinal lines from each of these points parallel to the axis of the third metacarpal, and by crossing these by five transverse lines parallel to the distal base line, a series of 1 cm. squares, twenty-five in number, could be constructed. On most occasions, we used these squares only as shown on fig. 17, but at times the whole of the back of the hand was marked out into centimetre squares (fig. 21). This was a somewhat tiresome procedure and materially disturbed the sensibility of the affected area. Such marking should never be undertaken shortly before sensory tests are employed. Moreover, it is extremely difficult, even with the greatest care, to ensure the exact correspondence of the squares on different occasions. The skin on the back of the hand is extraordinarily flexible, and any change in the position of the fingers modifies the size and appearance of the squares to a remarkable degree. After the base lines had been settled, we therefore attempted to mark out the squares with the hand exactly in the position adopted in the photographs. Occasionally, these squares were not allowed to become erased for long periods {e.g., from January 28 to March 12, 1906), so that, whatever its faults, we might be certain we were photographing the same field (Wo would suggest that if this experiment is repeated it would be wise to tattoo certain fixed points on the skin before the operation. This would ensure that each square always occupied exactly the same area.)

any mechanical disturbance of the hairs excites both protopathic and epicritic sensibility. Gentle stroking with cotton wool is not a specific stimulus to hair-clad parts, as far as the peripheral nervous system is concerned. Since the whole of the area affected by our experiment was covered with long hairs, we have always used this test, both before and after shaving. It is no easy task to shave the forearm and back of the hand so cleanly that no stumps project above the surface of the skin. But throughout the protopathic stage, H. soon learnt to recognize by the widespread tingling character of the reaction, that one or more-hairs had not been completely shaved. This was invariably confirmed by the use of a magnifying glass of low power, and these hairs were then removed by the direct reapplication of a sharp razor.

We never tested any part immediately after shaving, on account of the material diminution in sensibility it produced. Usually the part was cold-lathered, shaved and then carefully cleansed in hot water. After an interval of an hour or more, the tests with cotton wool were reapplied. The hand could never be shaved over-night, as by morning the hairs were found to have grown sufficiently to vitiate the experiment. These precautions were wasteful of time, and the presence of the hairs was often necessary for the series of observations upon which we were engaged; we therefore learnt to depend upon von Frey's method of testing cutaneous tactile sensibility. Before the operation, Professor von Frey presented us with eight graduated hairs set in handles, which we have used constantly in this research. We cannot sufficiently express our gratitude for his liberality and for the kindly interest he has shown throughout this work.

Each of these graduated hairs for testing tactile sensibility will be spoken of by the number in the first column of the following table. As we have used the same hairs on every occasion, the actual pressure exerted by each is alone necessary for comparing our results. For completeness, however, we also give the measured radii of the elliptical section of each hair. Now it is obvious that from these measurements we can arrive at flie pressure per unit area, if the force exerted in bending the hair is divided by its total area in mm.2. The result is expressed in grm./mm.2. But von Frey states that this is not a correct method of comparing hairs which are used as graduated tactile stimuli. For this purpose he divides the pressure in milligrammes by the radius of a circle of the same area as the elliptical section of the hair. The result expressed in grm./mm. will be found in the last column of the following table :—


Towards the end of our research, we received a second set of hairs from Professor von Frey which were useful in measuring the punctate pressure capable of producing cutaneous pain. These so-called " painhairs " exercise considerably greater pressure than those used for testing cutaneous tactile sensibility, and are graduated by calculating the pressure per unit area. Thus, we shall speak of the tactile hairs as No. 5, No. 8, &c, whilst the so-called "pain-hairs" will be known by the pressure they exert per unit area.

PainHairs.jpg Throughout the greater part of this research, we have attempted to measure roughly the force applied in pricking the skin by means of an instrument we have called the " algesimeter." It consists essentially of a sharp needle attached by a flexible joint to a rigid rod. This is weighted and slides freely through two supports placed 10 cm. apart projecting horizontally from a vertical brass bar. When the needle is brought into contact with the skin, the full pressure of the weight on the rod would be exerted on its point, were it not for a fine counteracting spring. This spring attached to the rod and to the upper brass support exactly balances the weight, and the needle exerts no pressure. But, if the instrument is pressed on the skin, this spring is no longer completely extended and the weight exerts a pressure in proportion to the amount, to which it is no longer counterbalanced by the coiled spring. This is read off on a scale attached to the bar, that unites the two guiding arms of the instrument.

Six divisions of this scale correspond to the pressure of 1 grm. ; but readings below 15 are of little value as the weight hardly comes into action owing to the friction of the rod. Thus the corrected readings are as follows:—

25 scale divisions ... ... = 5 grm

31 scale divisions ... ... = 6 grm

37 scale divisions ... ... = 7 grm

43 scale divisions ... ... = 8 grm

49 scale divisions ... ... = 9 grm

No such apparatus can give anything but approximate readings. For the moment at which pain is produced by the needle depends on the rapidity with which it is applied. If it is placed on the normal skin with the slightest jar, pain is produced at once, before the spring has begun to be affected. Moreover, the sensation of a point merges gradually into the pain of a prick over normal parts. Throughout the stage of recovering sensibility to prick, no pain was produced until considerable pressure was applied, when the characteristic pain made its appearance. Thus, the readings of the algesimeter formed a useful record of the progress of returning protopathic sensibility within the affected area; but we do not recommend it for general use on the normal skin.

All our earlier observations on the pressure necessary to produce deep pain were made by means of Cattell's pressure algometer. More recently we have used the modified form described in the paper by Head and Thompson.

The conditions of our experiment enabled us to investigate thermal sensibility with unusual completeness. Not only was the part explored with tubes containing hot and cold water, but we carefully studied the distribution of the heat- and cold-spots, especially on the back of the hand.

In our preliminary investigations before the operation, we found that different parts of the back of the normal hand were not equally sensitive to thermal stimuli. The skin over the knuckles responded badly, and over the long tendons sensibility seems to be less than over the parts between. After the operation, we avoided these situations as much as possible, especially when employing the finer thermal tests.

Many difficulties surround the testing of sensibility to heat and cold, particularly when dealing with intermediate degrees. A glass tube cannot be employed for finer observations, owing to the great difference between the temperature of its external and internal surfaces. We have, therefore, throughout used flat-bottomed silver tubes with a diameter of 1'25 cm. These tubes were filled with broken ice, or with water at the temperature desired, and contained a thermometer. They were never warmed or cooled from without. When used for testing sensibility to heat, several tubes ranged in a wooden stand were filled with water at temperatures considerably higher than those we wished to use for testing; from these, a tube was selected as soon as it had sunk to the temperature required. These silver tubes lose their heat so rapidly that it is possible to use the same one for a short series of tests only.

With such precautions, few difficulties attend the testing of sensibility to the more extreme degrees of heat and cold. But occasionally, when the part is sensitive to painful stimuli but insensitive to heat, a tube at 50° C. and above may be said to be hot, solely on account of the peculiar pain produced. H. soon learnt to differentiate this pain, due to a hot object, from a true sensation of heat; he frequently said, " Any ordinary patient would have called such stimuli hot, because the pain produced is of a kind associated in daily life with the action of hot bodies only. Further, a patient is told to say if he feels heat, cold, or a touch. Given, then, that he knows his thermal sensibility is being tested, he would certainly call the sensation I.experience ' hot.' "

Occasionally, contact with a neutral tube would cause an indeterminate and somewhat tingling sensation over the affected area; this was frequently said to be warm, and was one of the greatest difficulties against which we had to contend.

Fewer difficulties surround the testing of sensibility to cold; but it must be remembered that a silver tube always seems colder than one of glass containing water at the same temperature, on account of the greater rapidity with which it abstracts heat. Cold stimuli, more particularly those used for testing cold-spots, are liable to cause a vaso-constriction, shown by a blanching of the skin. This condition is peculiarly unfavourable for all sensory tests.

The cold-spots were sought for with fine copper rods of about 1 mm. in diameter. These were placed in a glass containing• broken ice; on removal, each rod was carefully wiped and, after its flat base had been applied to the skin, was returned again to the ice. We found it best for H. to mark out the cold-spots over night, independently of any previous diagram or photograph. Next morning, they were tested by R., who confirmed or rejected these observations. After the spots had been charted, the result was compared with previous photographs, and if any spots of the first order did not appear, a further examination was made. At the end of the series of sittings, a photograph was taken of the final results (figs. 17 and 18). Occasionally, the back of the hand was protected by a glove, and the marks preserved for several weeks or even months. For the discovery of heat-spots, we used a simple method which, as far as we can discover, has not been described before. We chose a " soldering iron " consisting of a large copper block fixed to an iron rod let into a wooden handle. This block, about 3 in. (7'5 cm.) in length and 1 in. (2'5 cm.) across every face, we cut down to a pyramidal point. The apex of the pyramid was flat and 1 mm. square. Into the copper block we bored a circular shaft, passing obliquely downwards in the direction of the point. This was of sufficient size to contain the bulb of a thermometer, just under 1 cm. in diameter.

Two of these irons were placed in a jug containing hot water. When sufficiently heated, one of them was removed and dried; the thermometer was placed in the cavity and the instrument laid on a cloth, until the required temperature was recorded. It was then held firmly in the hand like a large pen, and lightly applied, vertically, to the surface of the skin. So large a block of copper retains its heat for a considerable time, and the thermometer gives a sufficient indication of its temperature. This should lie between 50° C. and 40° C, preferably at about 45° C. Higher temperatures cause distinct pain, which complicates the observations; a temperature below 40° C. fails to stimulate most of the heat-spots.

A low external temperature greatly increased the difficulty in discovering both heat- and cold-spots; and in the winter, when the affected hand seemed numb and cold, previous immersion in warm water greatly facilitated their determination. To test the power of discriminating two points we used a pair of carpenter's compasses, the points of which had been ground down until they gave no sensation of sharpness (Most of tho instruments, called testhesiometers, used for this purpose are provided with points so sharp that they arc wholly useless. The sharp ivory points of Spearman's instrument should be replaced by blunt ones.)

For recording our observations, we used the method suggested by McDougall. The compasspoints were set at a certain distance from one another ; they were then applied to the part to be tested in such a way that sometimes two points, sometimes one point only, touched the skin. The stimuli followed one another in an entirely irregular order, but so that, ultimately, H. had been touched ten times with one point, ten times with two points. Each correct answer was marked with a stroke, whereas a mistake was recorded by a cross. Thus if he answered " one," when touched with two points, a. cross was placed below the line; if one point had been called " two," the cross was marked above. By this method it was at once obvious in how many instances he had answered correctly among the ten single and ten double stimuli. The answers whether right or wrong were ranged in strict sequence above and below the horizontal line.

Perfect appreciation of the compass-points at a distance of 4 cm. would be represented thus—


If, however, H. was unable to differentiate the two points, answering " one " to every stimulation, the record would stand—


Less complete failure would be represented by some such formula as—


In the following chapters these records are translated into the number of answers which were right (R.) or wrong (W.) for the sake of simplicity.

Observations made before the operation showed that the threshold for the flexor aspect of the left forearm lay at about 3'5 cm., when the compass-points were applied in the longitudinal axis of the limb (throughout this research all compass-tests have been applied in tho longitudinal axis of the limb.) Over the radial half of the back of the hand, under similar conditions, it lay between 15 cm. and 2 cm.

Of all sensory tests, the results obtained with the compasses are most influenced by accessory conditions. A stranger entering the room, or anything that disturbed the quiet state of internal absorption, would at ouce diminish the accuracy of H.'s answers. On one occasion, R.'s servant entered our workroom in the middle of an almost perfect series of answers : they at once became less accurate—


After his withdrawal H. again began to answer as before—


Any profound cooling of the skin, or even the occurrence of a pilomotor reflex, greatly diminishes the accuracy of the answers to compass stimulation. When the coat is removed, and the sleeve is rolled up, "goose-skin " is frequently produced ; testing should not be begun until this has entirely passed away.

We always began a series of tests with the compass-points widely separated from one another (9 cm.). Not uncommonly the records considerably improved as the distance was gradually diminished, and were frequently better at 7 cm. than at 9 cm.

This well-known phenomenon seemed, in H.'s case, to be associated with the increasing detachment of his attention from the procedure of testing. The following series of records obtained from the abnormal area on the left forearm are a good instance of such improvement.


The improvement at 4 cm. was associated with complete wandering of attention from the manipulations. When at the close R. asked whether there was anything to say about these observations, H. could have quite believed that nothing had been done. He was thinking about a book he had been reading and was completely absorbed, until recalled by R.'s question.

Occasionally, especially after exercise in the open air, this condition of detachment would pass into sleep. We noticed that the answers seemed to improve up to the point at which H. ceased to reply and therefore made several observations on the effect of somnolence on the compass-records. On October 26, 1907, H. fell asleep at the close of the following record—


an unusually good formula for the affected forearm. He was wakened and after a short interval it was found that the same distance of 4 cm. was completely below the threshold ; every double stimulation was said to be " one." He was allowed again to settle himself in the armchair and R. continued to test him with the points of the compasses at the same distance. With the return of the somnolent state the records improved; the total sixty stimulations gave the formula—


but of these the first and second twenty obtained when H. was more nearly asleep were better than the last series.


This sleepy condition which is so favourable for results with compasses is one that requires absolute freedom from all external appeal to responsible action. It is a condition which H. has never succeeded in producing surrounded by the multifarious interruptions of home.

Conversely, concentration on the details of the compass-test greatly diminished the accuracy of H.'s answers. During a large number of examinations, directed towards elucidation of the phenomenon of " double ones," H. was asked to state whether the two sensations seemed to be far apart and, if possible, to indicate the position of the two spots. This required much concentration of attention on the details of testing and considerably raised the threshold.

To measure the power of appreciating roughness, we used Graham Brown's ajsthesiometer. This instrument consists of a smooth metal surface from which small cylinders can be made to project for a measurable distance. In the pattern we used, a complete turn of the screw caused each of the six small cylinders to project 05 mm. As each complete turn was divided into ten equal parts, each of these divisions represented a projection of 005 mm.

The smooth surface was passed across the skin, and then the cylinders were made to project until a sense of roughness or " raking " was produced. In every case, the most careful comparative observations were made over similar parts of the sound limb.

For some of our observations on localization, we used the method of Victor Henri. H. was given a life-sized photograph of the back of his hand and was told to mark on it the spot stimulated. But for the majority of our observations, we depended on verbal description of its position or on pointing with the index of the right hand. H.'s strong powers of visualization rapidly led to the development of what may be called a visual map of the affected area. He had but to close his eyes to see a picture of his hand with the affected area marked upon it as clearly as in a photograph. As soon as a spot was stimulated, he saw its position on this map and at once described the neighbouring landmarks. He could even give approximate measurements; for instance, he would say that the point stimulated lay in " the interosseous space about 1 in. from the head of the first metacarpal." Occasionally, he was allowed to point with the index finger of the right hand; but, since this in itself acts as a stimulus, it should be rarely permitted and should be reserved for special occasions.

Unfortunately, the affected area on the back of the hand was too small for the satisfactory determination of the relative shape of different objects applied to the skin. Squares, triangles, and circles of wax or of india-rubber which lay well within its borders could not be distinguished with certainty over similar parts of the normal hand.

Deep sensibility was not disturbed by the operation. The vibrations of a tuning-fork were appreciated from the beginning, and this test played no further part in our investigations.