Popular Science Monthly/Volume 21/June 1882/Sir Charles Bell and Physiological Experimentation
By Dr. WILLIAM B. CARPENTER.
IT has been repeatedly urged, by the opponents of physiological experimentation, that Sir Charles Bell in his later life declared that his physiological discoveries had been really made by anatomy only, and that he had only made experiments for the satisfaction of others; and a quotation to this effect has been lately brought prominently forward by Mrs. Dr. A. Kingsford, in order to set in the most unfavorable light what she characterizes as the needless, fruitless, and barbarous experiments of Magendie on the same subject.
As it is probable that the vivisection question will be again brought before Parliament, I think it important that the public should be informed of the real history of the discoveries with which Sir Charles Bell is commonly credited; that history having been most erroneously narrated by his brother-in-law, Mr. A. Shaw (who may be presumed to have written with Bell's sanction and authority), and its errors, though fully exposed at the time (during Bell's life), having been repeated and even exaggerated by the most recent of his biographers.
The great discovery ordinarily attributed to Sir Charles Bell is that of the distinctness of the motor and sensory nerve-fibers; as shown by the separate existence of motor and sensory endowments, (1) in the anterior and posterior roots of the spinal nerves, in whose trunks these two orders of fibers are bound up together; and (2) in certain nerves of the head, some of which are motor only, while others are sensory only. These doctrines, according to Mr. A. Shaw, had been conceived as far back as 1809; and were then embodied in a tract which Bell printed for private distribution among his friends, under the title "Idea of a New Anatomy of the Brain." In support of this statement Mr. Shaw cited certain passages from Bell's very scarce tract, which, read in the light of subsequent events, seemed an adequate justification of it. But, unluckily for the credit of both, a copy of the tract had found its way into the possession of a certain Mr. Alexander Walker, who had claims of his own to advance; and he re-printed it in full in a thin volume (now before me) published anonymously in 1839, under the title of "Documents and Dates of Modern Discoveries in the Nervous System."
I well remember the sensation which was produced at the time, among those who took an interest in the subject, by this publication; from which it plainly appeared that the fundamental conception enunciated in this "Idea" had gone no further than this—"that the nerves of sense, the nerves of motion, and the vital nerves, are distinct throughout their whole course, though they seem sometimes united in one bundle; and that they depend for their attributes on the organs of the brain to which they are severally attached"; while, in carrying out this conception, Bell, misled by his anatomy, had gone altogether wrong.
This doctrine was by no means new. It had been known from a very early period that our limbs can only feel or move (I use these words in their ordinary sense) by virtue of the nerve-trunks which connect their skin and muscles with the spinal cord, and through it with the brain. And although, when a limb is paralyzed, it is usually deprived at the same time of feeling and of motion, yet as cases were occasionally observed in which motion was lost without feeling, or (more rarely) feeling was lost without motion, the idea arose that two distinct sets of fibers may be bound up in the same trunks; one for feeling and the other for motion—or, as we should now express it more scientifically, one set conducting impressions made on the sensory surfaces toward the central sensorium, while the other transmits nerve-force from the motor centers of the nervous system to the muscles which it stimulates to contraction. This idea found distinct expression in the writings of certain ancient medical authors; and cropped up from time to time in modern medical literature, some writers approving it, while others dissented from it. And it was formally advanced in 1809 by Mr. Alexander Walker, who, in a paper entitled "New Anatomy and Physiology of the Brain in Particular, and of the Nervous System in General," published in the "Archives of Universal Science" for July in that year, argued that "medullary action" (or, as we should now say, a nerve-current) "commences in the organs of sense; passes, in a general manner, to the spinal marrow, by the anterior fasciculi of the spinal nerves, which are, therefore, nerves of sensation, and ascends through the anterior columns of the spinal marrow, to the hemispheres of the cerebrum," in which he located the sensorium commune. Thence he traced his "medullary action" downward and backward into the cerebellum, which he supposed to be the center of volition; from this "it descends through the posterior columns of the spinal marrow, and expands through the posterior fasciculi of all the nerves, which are, therefore, the nerves of volition, toward the muscular system."
Thus, then, it is clearly Mr. Alexander Walker who must be credited with the first promulgation of the idea of the functional distinctness of the anterior and posterior roots of the spinal nerves, in virtue of what he supposed to be their connections with the cerebrum and the cerebellum respectively: but, working out this idea under a wrong conception of the relative functions of the two brain-centers, he was led to regard the anterior roots as sensory, and the posterior as motor; and, as he neither submitted nor proposed to submit this erroneous doctrine to the test of experiment, it fell unheeded to the ground.
Now, those who only know the history of Bell's work either directly or indirectly through Mr. A. Shaw's first account of it, will be considerably surprised to learn that (whether or not he was acquainted with Walker's speculations) he pursued, in the first instance, precisely the same anatomical track; and that, through his having followed this under the guidance of another wrong preconception as to the functions of the cerebellum (which had not at that date been elucidated by experiment), the physiological conclusion at which he arrived was even further from the truth than that of his predecessor.
A distinguished Edinburgh professor of the last century, Dr. Robert Whytt, who had studied with great care what he termed the "vital and involuntary motions" of the body, had argued with considerable ingenuity that, while the cerebrum is the center of sensation and the originator of voluntary motion, the cerebellum is the organ of such "vital and involuntary motions" as the action of the heart and the muscular walls of the alimentary canal, together with the movements of respiration. Now, Bell, brought up in the Edinburgh school, and commencing his investigations under the influence of this prepossession, was led by it in an entirely wrong direction; for the whole argument of his "Idea" is to the effect that the anterior roots of the spinal nerves minister both to sensation and voluntary motion, in virtue of their connection with the cerebrum, while the posterior roots "govern the operation of the viscera necessary to the continuance of life," in virtue of their connection with the cerebellum. He did institute experiments, indeed, both on the columns of the spinal cord and on the roots of the spinal nerves; but, under the influence of his anatomical preconception, he entirely missed the true meaning of their results, and deemed them to be confirmatory of his erroneous views:
"Experiment I.—I opened the spine, and pricked and injured the posterior filaments of the nerves; no motion of the muscles followed. I then touched the anterior division; immediately the parts were convulsed."
"Experiment II.—I now destroyed the posterior part of the spinal marrow by the point of a needle; no convulsive movement followed. I injured the anterior part, and the animal was convulsed."
The experiments thus narrated by Bell in a letter to his brother, dated March 2, 1810, have been cited as proving that he had thus early attributed motor functions to the anterior roots, and sensory to the posterior. But the inference which he himself drew from them at the time was altogether different:
"It is almost superfluous to say that the part of the spinal marrow having sensibility [i. e., the anterior column] comes from the cerebrum; the posterior and insensible port belongs to the cerebellum."
Thus, although on the track of a great physiological discovery, Bell allowed himself to be completely diverted from it by his anatomical preconception. Of the true functional relations of the two sets of nerve-roots, there is not the remotest hint in this "Idea."
None the less, however, do I recognize in it what (to my mind) constitutes the real basis of Bell's claim to the elucidation of the meaning of the double origin of the spinal nerves. "Considering," he said, "that the spinal nerves have a double root, and being of opinion that the properties of the nerves are derived from their connections with the parts of the brain, I thought that I had an opportunity of putting my opinion to the test of experiment, and of proving at the same time that nerves of different endowments were in the same cord and held together by the same sheath." This was, unquestionably, one of the most fertile suggestions that the insight of a man of genius has ever put forth for the guidance of physiological inquiry; and, even if Bell had never himself pursued it further, he would clearly be entitled to a very large share of any discoveries that others might make by working upon it. It seems, however, as if the unsatisfactory character of the results he obtained and his dislike to experimentation upon living animals turned his thoughts in a different direction; and he applied himself for some years to the study of the nerves of the face, on the peculiarities of whose anatomical distribution he seems to have long pondered, with the idea that these might furnish him with the key of which he was in search.
Bell, as is well known, had considerable artistic ability; and one of the earliest of his publications was his very valuable "Anatomy of Expression," in which he pointed out how close is the relation between many of the muscular movements by which the emotions are expressed and those concerned in respiration. Still, as it would seem, under the "dominant idea" of a special set of nerves for the "vital and involuntary motions," he assigned this special motor function to the seventh pair, which arises by a single root, and supplies the muscles of the face generally; while he supposed the fifth pair, which arises (like the spinal nerves) by a double root, to be the nerve of ordinary (or voluntary) motion for the muscles of the face generally, as well as of sensation for its sensory surfaces. The analogy of the fifth pair to the spinal nerves (which was no new idea) seemed to him to be further indicated by the existence of a "ganglion" upon its larger root, corresponding with that which is seen on the posterior roots of the spinal nerves. Following up this train of reasoning, he instituted experiments with the view of determining what function the fifth pair had in virtue of its double root, which the seventh pair had not. And as he found that division of the seventh pair, while partially paralyzing the muscles of the face, did not in any perceptible degree impair its sensibility, while section of either of the three divisions of the fifth pair destroys the sensibility of the part of the face it supplies, he came to the conclusion that the sensory endowments of the fifth pair are due to its possession of a double root; a conclusion which he strengthened by the consideration that the third, fourth, and sixth nerves—which, being distributed exclusively to the muscles of the eyeball, can not be supposed to have any but motor endowments—all arise by single roots.
In this way, Bell was led to assign to the two roots of the spinal nerves the same double function which he attributed to the two roots of the fifth pair of nerves of the head; and thence to assign the sensory function to the posterior roots, because, like the second root of the fifth, they bore ganglia before uniting with the motor roots. Now, to say that Bell, by this train of reasoning, discovered the motor and sensory functions of the anterior and posterior roots of the spinal nerves, is utterly preposterous. He had not even truly determined (as the event proved) the true functions of the fifth and seventh nerves of the head. And the extension of his conclusions regarding the double roots of the fifth pair, to the spinal nerves generally, had rather the character of a happy guess than of a logical sequence. No scientific physiologist at the present time would think himself justified in putting forward such an extension as more than a suggestion, to be confirmed or negatived by experimental evidence. And let it not be forgotten, moreover, that it was experiment alone which afforded Bell any reason whatever for attributing a sensory function to the gangliated root of the fifth pair; and that, without this basis, the question of the spinal nerves remained exactly in the condition in which he had taken it up.
It is, indeed, not a little curious that in the two memoirs (1821 and 1822) in which Bell presented to the Royal Society the results of his investigations into the fifth and seventh nerves of the head, the present doctrine of the spinal nerves is nowhere explicitly stated. These memoirs can scarcely, indeed, be read in any other sense; and "A Manual of Anatomy," published by Mr. John Shaw (another brother-in-law) in 1821, contains a tolerably clear intimation of it. Moreover, Mr. J. Shaw, having visited Paris in 1821, and having repeated to Magendie the experiments on the fifth and seventh nerves which he had made with Sir C. Bell, further pointed out to him (as appears from Magendie's own narration) the analogy of the fifth to the spinal nerves, and attributed to the double roots of these "regular" nerves this double function of motion and sensation.
It was at this point that Magendie took up the experimental inquiry, both as to the roots of the spinal nerves and the functions of the fifth and seventh nerves of the head; and it will be convenient to dispose of the latter in the first instance. He showed that the second of the three divisions of the fifth pair is a nerve of sensation only; so that the part of the face which it supplies (between the eyes and the upper lip) depends for its motor action on the seventh pair, which he regarded as the ordinary motor nerve of the face, ministering to its voluntary movements, as well as to those of expression and respiration. These corrections (which were confirmed by other experimenters) were not only accepted by Sir C. Bell, but were appropriated by him as his own; the reprints of the two memoirs just referred to being altered in successive editions of his "Nervous System of the Human Body," by omission, addition, and variation, not only without any acknowledgment of the source of the correction, but without the least intimation of a change. It is clear, therefore, that although he shrank from making experiments himself, he was ready enough to profit by those of others.
On testing experimentally Bell's idea of the functions of the anterior and posterior roots of the spinal nerves, and varying his experiments in every way he could think of, Magendie was only able to arrive at this general conclusion—that the anterior roots are more especially motor, and the posterior more especially sensory. For he could not get over the fact that irritation of the anterior roots in the living animal called forth signs of pain, and that irritation of the posterior roots called forth movements. The repetition of the same experiments by others gave no more conclusive results; until, in 1831, Johann Müller (afterward the celebrated Berlin professor) was able, by a very carefully devised method of experimentation upon frogs, to show that, for these animals at least, Bell's doctrine was correct. And it was by the extension of the same method to warm-blooded animals, and by the light of the new ideas then dawning as to the "reflex function" of the spinal cord (which up to this time had been generally looked on as a bundle of nerves), that the truth of Bell's doctrine came at last to be fully established. For the movements called forth, by irritation of the posterior roots were found to be due, not to the direct transmission of motor impulses from them to the muscles, but to the transmission of a motor nerve-current through the anterior roots, in response to the stimulation given to the spinal cord itself by the irritation of the posterior; while, on the other hand, it was made clear that the indications of pain given when the anterior roots are irritated, are due to the presence, in those roots, of sensory filaments derived from the posterior, which pass inward at the point of junction between the two. But for the well-devised and carefully executed experiments by which these difficulties were cleared up, the whole matter would have remained in the state of uncertainty in which I well remember it to have been, when I first entered on the study of the subject, previously to Müller's experiments.
Having myself been afterward Sir Charles Bell's pupil (in surgery) both in London and Edinburgh, I can testify from personal knowledge that he himself never admitted that his discoveries needed any confirmation whatever; but was always strong in the conviction, not only that he had himself given all needful evidence of them, but that nothing more remained to be done in the physiology of the nervous system. It is not a little significant of his attitude of mind on this subject, that he used to declare his complete inability to understand "what Marshall Hall was driving at"; the doctrine of reflex action independently of sensation being altogether "beyond his comprehension." As this last doctrine, which forms the basis of modern neurology, is one which anatomy could scarcely even suggest, and which nothing but experiment can demonstrate, I hope that Sir C. Bell's opinion of the all-sufficiency of the study of anatomy for the advancement of physiological science may henceforth be appreciated at its true worthlessness. For I have shown, first, that Sir Charles Bell, trusting to anatomy for his guidance, went altogether wrong in the first instance; secondly, that it was by experiment on the nerves of the face that he was led into the right track; thirdly, that in regard to these, through placing too much trust in his anatomical preconceptions, and insufficiently testing them by further experiments, he was led into mistakes which were only corrected by the experiments of Magendie; and, fourthly, that the most important discovery with which he is usually credited—that of the motor and sensory functions of the anterior and posterior roots of the spinal nerves respectively—was only established in the true scientific sense by the experiments of others working on his lines. Those experiments might have issued, for any real proof ever given by Bell to the contrary, in establishing some other doctrine of the spinal nerve-roots than that to which he had been led by his study of the nerves of the face—such, for example, as that of Alexander Walker, or that of his own first "Idea."
These assertions are not now made for the first time, with the view (as might be urged) of lowering Sir Charles Bell's credit, and thereby-weakening the force of the testimony borne by him in regard to the uselessness of experimentation as a means of physiological discovery. Forty-two years ago, the history I have now sketched (which was then a matter of contemporary knowledge) was told in detail in the leading medical "Quarterly"; the misrepresentations of Mr. A. Shaw as to Sir C. Bell's "Idea" of 1811 were fully exposed; and Bell himself was distinctly charged with having altered what professed to be exact reprints of his papers in the "Philosophical Transactions," in order to make them square with the corrections supplied by the experiments of Magendie. To those charges, so far as I am aware, no reply was ever made, either by Mr. A. Shaw or Sir C. Bell; but a new and more correct history, including a reprint of Bell's "Idea," was given by Mr. A. Shaw nearly thirty years later in the "Journal of Anatomy and Physiology" (vol. iii, 1869). Further, in Professor Vulpian's "Leçons sur la Physiologie du Système Nerveux" (Paris, 1866), the history is narrated in terms almost identical with my own, omitting only the reference I have supplied to Magendie's first knowledge of Bell's views, but inserting several of the altered passages in Bell's papers. And, finally, the venerable Professor Milne-Edwards, in his admirable "Leçons sur la Physiologie et l'Anatomie Comparée" (tome xi, pp. 361, 362), has given a most true and just appreciation of the respective shares which Bell and Magendie had in this great discovery.
I have never admitted the truth of the well-worn adage, "A little knowledge is a dangerous thing"; because every one who studies any subject whatever must begin with "a little knowledge," and only by its possession can know where and how to obtain more.
But "a little knowledge" is dangerous when it leads its possessor to imagine that he (or she) knows all about the subject; and is doubly dangerous when it is taught as the whole truth to others. And this is exactly what Mrs. Dr. A. Kingsford has done, in her desire to excite a prejudice against physiological experimentation; fastening eagerly upon Sir Charles Bell's depreciation of it, without taking any trouble to ascertain historically what that depreciation is worth.—Fortnightly Review.