Natural History Review/Series 2/Volume 1/Number 2/The Sensory and Motor Functions of Nerves
XVIII.—The Sensory and Motor Functions of Nerves.
By G. H. Lewes.
When once a doctrine has been generally accepted, and confidently taught, it necessarily calls forth a strong conservative principle of resistance against every effort to change it. That very reluctance to change of mental attitude which made the public deaf to the voice of the original teacher, now serves to close the ears of the public against the opponents of that teacher. Bell had trouble enough to get his discovery of the sensory and motor nerves accepted; but now that the contest has long been ended, and Bell is crowned victor, all the conservatism which embittered his efforts is employed to maintain his triumph. Not only is he declared victor, but "victory along the whole line" is claimed, and his errors are consecrated with his truths.
I have alreadjr paid my small but hearty tribute to Bell's genius, and to the unimpeachable validity of his anatomical discovery; but, conceiving that he had founded on that discovery a physiological induction which was erroneous, I laid before the British Association,[1] and the public,[2] certain critical strictures, the purport of which was to show that there was no essential distinction between the two nerves: both being sensory and motor, though in varying degrees. In these strictures there may be a fundamental error; and if so, I should be glad to see it pointed out. The discussion is one which cannot be without service; and if any champion of Bell's doctrine will do me the honour to descend into the arena, he may be assured that the harder he hits (without foul blows), the more welcome he will be.
Ever since the time of Galen, it has been suspected that there were "nerves of motion," and "nerves of sensation." Latterly we have had "nerves of secretion." The question to be settled is: Are these nerves different in kind? or are they the same in kind, but differentin function, or use? Flexor, extensor, and sphincter muscles are not held to be different kinds, but the same kind applied to different uses; the peculiar property of muscles, Contractility, is found in all three, but this property is put to different uses, when the anatomical connexions of the muscles differ. In like manner, as I conceive, the nerves which are distributed to sensory surfaces, to muscles, and to glands, are all of the same kind, and have the same properties; but differ in their uses, as they are different in their anatomical connexions.
The champion of Bell must prove, first, that the motor nerve is of such a structure as to be incapable of transmitting a stimulus to a sensitive centre; or he must prove, secondly, that its anatomical distribution is such that no sensitive centre can be reached by it. One of these two conclusions must be established. No other alternative is possible. Let us examine both points.
It is admitted that motor and sensory nerves are of the same structure. The conclusion, therefore, that they must be identical in property seemed to me self-evident, the more so as, to guard against superficial objections, I added the qualification, "under similar conditions." If any one chooses to maintain that two substances identical in structure, under identical conditions, can have different properties, I must leave him to the indulgence of his "right of private judgment;" meanwhile, the axiom remains, that identity of structure implies identity of property. There is, indeed, another avenue of attack. It has been said, "You do not know that the two nerves are identical; there may be differences important, yet too minute for appreciation at present. Of two bars of steel, one may have a magnetic property, and the other none, yet you cannot on mere inspection detect any difference in their structure. Of two tissues, one may be dead, and the other living, yet you cannot detect a difference." We do not know that two muscles, or two secreting cells, may not be so different in structure as to have different properties; but until this is proved, we are not warranted in assuming it. All we do know of the nerves points to their identity: they have the same physical, electrical, and chemical properties; and, till the contrary be proved, we must assume them to be identical in all their properties. The two bars of steel have precisely similar properties, according to their similarities of structure; but, over and above these, one of them possesses a magnetic property, in consequence of its having been submitted to peculiar conditions; under the like conditions, the second bar of steel would be magnetic. The same may be said of dead and living tissues; they have in common, structure and property, and as long as they are under similar conditions there will be no difference between them; but under the group of conditions known as "life" and " death," there will of course be differences.
Cut off the leg of a frog, and resect its sciatic nerve, the muscles will for some time retain their property (Contractility), and will contract on being stimulated. The nerve also retains its property (Neurility), and on being stimulated, it will excite the muscles to contraction. We know that muscles will contract without the intervention of nerves, and that glands will secrete without the intervention of nerves. It is also probable that the sensorium may be stimulated without the direct intervention of nerves. But it is not less certain that the ordinary stimulus which awakens the activity of muscles, glands, and nerve-centres, is the stimulus of nerves. How this is effected we cannot say. What the peculiar property of the nerves may be, baffles science. It may be electricity; it may be a correlation of that force; it may be a special "nerve-force," something sui generis. To avoid every hypothesis, and yet to secure a specific name, I proposed the term Neurility, as corresponding with the terms Sensibility and Contractility; the term, having met with some acceptance, may be used throughout this paper.
In the course of investigation, it appeared to me that many of the vexed questions of nerve-physiology would rapidly receive answers, if the perplexing ambiguities of phraseology were to give place to a more rigorous nomenclature. For example, it is difficult to come to an understanding respecting the motor and sensory nerves, so long as we continue to talk as if we believed that "motility" resides in the spinal chord, and that the posterior roots are "sensitive." Motor force no more resides in the spinal chord, than explosive force resides in the lighted match; the motor-force is in the muscles, the explosive force is in the gunpowder; and when eminent physiologists are at great pains to detect the "seat of motility" (siège de la motricité) in the grey matter of the chord, they are perplexing a subject already difficult enough. I do not assert that competent physiologists ever believe that the seat of motility is elsewhere than in the muscles; what they mean is, doubtless, that the centre, from which the stimulus issues which will excite the muscles, is in the spinal chord. But how easily the ambiguous language leads to ambiguity of conception may be seen in a hundred examples; and it may be, to a great extent, avoided by rigorously demarcating the phenomena of Sensibility, Neurility, and Contractility, as the actions of three different organs: nerve-centres, nerves, and muscles.
Müller puts this question:—"Is the nervous principle, or force of the motor fibres, different in its quality from that of the sensitive fibres? or are what are here called the motor and sensitive principles, actions of the same nervous principle, differing only indirection—being centrifugal in the motor, and centripetal in the sensitive fibres?" Put into the language of the essay, this question will run thus:—Are there two Neurilities, one motor, and the other sensory (with the possibility of a third—the secretory)? Or does the Neurility in each nerve act only in one direction, from a centre along the motor nerve; to the centre along a sensory nerve?
That there are two Neurilities is extremely improbable, nor is there a shadow of evidence in its favour. Neither motions nor sensations belong to the nerves themselves, but to the muscles and centres, stimulated by the nerves. It is only in the looseness of unsystematized phraseology, that we speak of "sensitive impressions" being "conveyed to the brain;" and of "motor-impulses" being "conveyed to the muscles;" it is the stimulated nerves which excite the activity of brain and muscles, as the spark excites the explosive activity of gunpowder. We do not need three kinds of Contractility for flexors, extensors, and sphincters; nor do we need three kinds of Neurility for muscles, centres, and glands. One property serves for the three functions. The differences in the functions do not depend on the organs themselves, but on the connexion of these organs with others; the same organ (nerve) which, in connexion with a muscle, produces motion, in connexion with a gland would produce secretion.
The idea of different Neurilities must, therefore, be rejected. The two nerves having similar structure must have similar properties; but these properties may be put to different uses. My critic in the British and Foreign Medical Review seems to have wholly misunderstood me; and thinks that had I been "longer engaged in the study of physiology," I should be "less inclined to rest upon an apparent similarity of structure as justifying an inference of identity of property." Perhaps his longer study will enable him to enlighten me on this point; at present my conviction is, that if the similarity were only apparent, it would amply justify the inference; whereas, if the similarity were real, and not apparent only, it would carry a demonstration. My critic seems to think otherwise; and he is kind enough to say that my "dogmatism on this point, indeed, is absolutely confounding to those who have been accustomed to look with marvel at the diversity of operations performed by elementary parts which present no appreciable structural differences." One naturally feels a little perplexed at having confounded others by one's dogmatism, when the point in question is so excessively simple as the discrimination between properties and uses. I would, therefore, submit that the operations performed by means of chain cables, tenpenny nails, marling spikes, and grappling irons, though various enough, are not generally held as evidence that the iron of which they are all composed has different properties in each. I never denied that different nerves had different functions, but only that they had different properties. If any one conceives that the anterior roots send forth nerves having a Neurility as widely opposed to that of the nerves issuing from the posterior roots as Motion is to Sensation, let his evidence be produced. If he conceives that the anterior nerves will only act in one direction, and the posterior in another and contrary direction, so that the motor nerve cannot excite a centre, and the sensory cannot excite a muscle or a gland, let him produce his evidence. Meanwhile, I will suggest the evidence against such a notion.
It has been proved by Schiff, and others, that the nerve will conduct both ways; not only will it conduct electricity, it will conduct its own proper stimulus. In other words, it has been shown experimentally that Neurility will act both in the centripetal and centrifugal directions. I will now call attention to a still more striking fact, one which has strangely enough been overlooked, probably because investigators were seeking only the phenomena of sensation and motion; a fact which disproves the fundamental position of the established doctrine that a sensory nerve conducts only to a centre, never from it. Let any one follow the distribution of the Fifth Pair. Of the three trunks, into which this nerve is divided as it issues from the posterior root, two are called sensory, and the third is called "mixed," because, after its emergence from the Gasserian ganglion, it is joined by the nerve from the motor root. No fibres whatever from this anterior (motor) root join the two first trunks; and these two trunks are, therefore, considered on every ground of anatomy and experiment to be purely sensory. Now, I think it demonstrable by anatomy and experiment that these so-called sensory nerves have the distinguishing characters of motor nerves; that is to say, one of these nerves can be proved to transmit Neurility from the centre to an organ; and the other will not transmit a "sensitive impression" to its centre.
The first trunk is the ophthalmic. Among the parts it supplies there is one deserving particular notice—the lachrymal gland. This is the secreting organ, which is innervated solely from a branch of the ophthalmic, and a twig of the superior maxillary—that is to say, from the two purely "sensory" trunks. Yet that these nerves have a part to play in the mechanism of secretion is proved beyond a doubt by the great diminution of the secretion which follows division of the trunk. It is true that division of the trunk does not wholly suspend the secretion; but that is because the influence of a nerve upon the gland is only that of a stimulus. Let the part played by the nerves be never so small, the fact that some influence over the secretion is exercised by them, proves that they transmit a stimulus from the centre to the organ—they act centrifugally; which is precisely the character claimed for a motor nerve. What the nature of the influence may be which nerves exercise on glands is still a mystery; nor is it necessary for the present argument that anything more than the fact of a transmitted stimulus be admitted; but that fact is conclusive. All the argument needs is that a sensory nerve will act centrifugally; that proved, it follows that, if properly connected with a muscle, it would act upon the muscle as it acts upon the gland, viz., it would stimulate it.
Müller seems to have been on the point of adopting this view, but was held back by another consideration. "The affection of the nervus lachrymalis," he says, "under the influence of certain passions and ideas, is apparently an instance of the transmission of nervous influence in a centrifugal direction in a decidedly sensitive nerve; and this would be decisive proof that sensitive nerves can propagate nervous action in the centrifugal direction, if it were certain that the lachrymal nerve is not, like other branches of the fifth, accompanied by branches of the sympathetic. But it is probable that the lachrymal nerve receives grey fibres."[3] It is to be regretted that this great physiologist did not pursue the investigation, and assure himself of the actual facts. Had he done so, I believe he would have seen that no grey fibres mingle with this lachrymal branch. I have sought in vain for any connexion between the sympathetic and this branch; and Hirschfeld states that it is only the filaments of the sympathetic which accompany the artery in the gland, to which the secretion may be due, after division of the fifth. "La branche lachrymale du nerf ophthahnique de Willis, et un filet lachrymale de la branche orbitaire du nerf maxillair supérieur, se distribuent dans la glande lachrymale et tiennent en grande partie sous leur dépendance la sécrétion des larmes; car celle-ci diminue considérablement après la section de la cinquième paire, mais sans cesser, toutefois, complétement. Ce qui a fait supposer que les filets du grand sympathétique qui accompagnent les artères de la glande lachrymale avaient aussi une certaine influence sur la sécrétion."[4] Observe that not only has the presence of the grey fibres in the lachrymal nerve to be demonstrated as a fact, but I think their presence might be admitted without damage to my argument; for an examination of the connexion which does exist between the sympathetic and the fifth pair, will show that division of the fifth would not interfere with the action of the sympathetic filaments joining it from the carotid. Granting, therefore, that one part of the nervous stimulus reaches the gland through the sympathetic, we have still the greater part reaching it through the lachrymal nerve. In other words, a sensory nerve acts centrifugally.
The second point to which I referred, in the functions of the fifth, is the "insensibility" of the nasal branch; but this must be noticed presently, in connexion with the analogous "insensibility" of the motor nerves.
If there is any difference between sensory and motor nerves, it is not a difference of kind, but of use. Each nerve is capable of serving either function, provided it be properly distributed. If nerves are distributed through the substance of muscles, they will be motor—if distributed through glands, they will be secretory—if distributed to the surfaces, they will be sensory. There will probably be little objection raised to this statement. But we must go farther, and ask whether the skin-nerve is ever motor, and whether the muscle-nerve is ever sensory? To answer this, we must first settle one or two points of physiology and anatomy. A nerve is sensory because it stimulates the Sensibility of its Centre, and not because its termination is in the skin. It is not the nerve which is sensitive, bat the centre. Stimuli, which reach the nerve through the skin, affect the centre. It is to the centre, therefore, that we must look. So much for physiology; now for anatomy. "There is no difference," says Dr. Todd, "between a motor and sensory nerve as regards structure. We can attribute the difference of endowment of the fibres to no other cause but to the nature of their peripheral and central connexions. The same nervous force is propagated by the fibres of each kind; but whether that force is to excite motion or sensation, must depend on the connexion of the fibres with muscles in the one case, and with the centre of sensation in the other."[5] The principle here laid down is irreversible; but it is an error to suppose that only one of the nerves is in connexion with the centre of sensation. I pointed out the fact, which had been universally disregarded, that the anterior (motor) roots were quite as unmistakeably in anatomical connexion with the ganglionic substance of the spinal chord, as the posterior roots were; and the conclusion seemed irresistible, that if one nerve in connexion with a centre will stimulate the activity of that centre, another nerve precisely analogous in structure, and endowed with an analogous property (propagating the same nervous force), if also in connexion with that centre, must also stimulate its activity. Modern microscopic researches have rendered the direct connexion of the anterior roots with the ganglionic substance, a fact beyond dispute. To resist the conclusion I have drawn, it. will be necessary to prove: 1st. That the ganglionic substance with which the anterior roots is connected has not the same property as the ganglionic substance of the posterior roots; or, 2nd. That nerves are only capable of stimulating in one direction. But it has been proved by Schiff that there is no difference between the properties of the anterior and posterior horus. And it has also been proved that nerves conduct in both ways.
If, therefore, Sensibility is the property of Nerve-centres awakened by the stimulus of Neurility—if both nerves are in direct anatomical connexion with their centres—and if there be not two different kinds of Neurility, acting in very different ways upon the centre—there is no alternative but to accept the conclusion that both nerves have a sensory function.
In vindicating the essential similarity of the two sets of nerves, we are not overlooking their specific diversity. The functions of various nerves, that is to say, the uses they serve in the mechanism, depend upon their anatomical connexions. A nerve that is not distributed to a muscle cannot be expected to have a motor function; a nerve that is not distributed to a gland cannot be expected to have a secretory function; a nerve that is not distributed to an organ of sense cannot be expected to have a function of special Sensation.
Bell's discovery that the anterior roots ministered to motion, and the posterior to sensation, may be interpreted thus: the anterior nerves are muscle-nerves, the posterior are skin-nerves. My critic in the British and Foreign demurs to this. " It is known to every anatomist," he says, "that it is a pure assumption on the part of Mr. Lewes to assert that the fibres of the anterior roots are distributed exclusively to the muscles, and those of the posterior roots exclusively to the skin. How, we would ask, is it possible anatomically to separate the fibres of the anterior and the posterior roots in any nerve of mixed endowments? and on what basis, save that of physiological experiment, can any positive statement be made as to their peripheral distribution?" An experiment will convince my critic that it is not only "possible" to separate the fibres, but that my "assumption" is easily demonstrated. Let him divide the anterior roots of the nerves supplying one of the extremities of a frog, and he will find that all the fibres in the muscles of that extremity degenerate, but none of those distributed over the sheath or to the skin. A more absolute proof could not be required. I would further remark that it is not enough for a nerve to pass through or along a muscle, its filaments must terminate in the substance of the muscle, if its function is to be motor.
My critic is also inaccurate in stating that I attribute the difference of function of the two nerves entirely to their peripheral distribution. He should have said mainly; the difference in their central distribution is insisted on, as one of the reasons why the muscular sensations differ from the skin sensations. Both nerves are directly connected with the spinal chord, and "both must, therefore, have a similar functional relation to it." The critic should not have passed over the emphatic sentence of the next paragraph,—"Observe, I say the relation is similar, not the same. It requires but a moderate acquaintance with microscopic anatomy to be aware that the anterior and posterior roots differ in their distribution over the spinal chord; indeed, it is partly on this difference that I explain the different forms of Sensibility excited by each root. But, underlying this diversity, there is a fundamental agreement. Hence they may be called similar, though not the same. The form of sensibility excited by the anterior root is as unlike the form of sensibility excited by the posterior, as the sensation of sound is unlike the sensation of light, which are nevertheless similar, in being both sensations."
I endeavoured to prove by experiment that it was through the anterior nerves that the "muscular sense" was excited. The evidence cannot be reproduced here; but, perhaps, for the sake of argument, the reader will admit the point as proved, and we may then show that the one objection which is always raised against the sensory function of the anterior nerves falls to the ground. When both anterior and posterior roots are divided, an irritation of the central ends of the anterior, produces none of the ordinary signs of sensation; but the irritation of the posterior produces unequivocal signs of pain. This is held to be conclusive against the sensory functions of the anterior root. But is it so? On the supposition that the anterior root serves the muscular sense, we have no right to expect anything than what we find. The muscular sensations are as special as those of sight or hearing, and every special sense responds only in its special form: the optic nerve, when irritated, produces sensation of light, but no pain; the auditory nerve a sensation of sound, but none of temperature, light, or pain. In like manner, the irritation of a muscle-nerve will produce the sensations habitually produced by that nerve, which are not those of pain. My assertion that muscular sensations are not those of pain has been scornfully rejected, and a reference made to the agonies of cramp. But cramp, I must maintain, with Schiff and others, is not a muscle-pain; it is produced by pressure on the nerves of the muscle-sheath, or of some neighbouring nerve.[6] And I have proved that no sign of pain is elicited by burning or pricking the muscle itself.
Now, if we are justified in attributing muscular sensibility to the anterior nerves, it is obvious that these nerves, when irritated, can only excite muscular sensations—no others. It is further obvious that the signs of such sensations must be very different from those of other sensations. Irritation of the root can only produce that sensation which precedes or accompanies adjustment of the muscles, or one of the vague diffusive sensations which the muscles contribute to the general consciousness. The direct response to such a sensation would be an adjustment of the muscles to which the particular nerves were sent; but this cannot take place, because the connexion between the nerves and the muscles is cut off. None, therefore, of the ordinary signs could be manifested. But is this a proof that the muscle-nerves are not sensory? To say so, would be to say that the optic nerve is not sensory, because it may be divided without the animal's manifesting any sign.
A few paragraphs back, allusion was made to the fact, that one of the nerves of the fifth pair has an "insensible branch"—will not transmit sensitive impressions (as the phrase is usually understood) more than a motor nerve will. Claude Bernard says: "Examinant chez le chien le nerf nasopalatin qui va à la membrane muqueuse du nez nous avons eté très surpris de le trouver en apparence complétement insensible, tandis que la branche principale, la sous orbitaire, nous offrait tous les signes d'une sensibilité vive." Here are two branches of the same nerve (both sensory), yet one of them completely without response to the stimulus which excited the others. Did Bernard thence conclude that the naso-palatine was not sensory? By no means. "Cette insensibilité d'un rameau appartenant à la cinquiéme paire porterait à penser qu'elle renferme des filet de sensibilité speciale; Majendie ayant prouvé que les nerfs de sensations speciales sont complétement insensibles aux irritations mécaniques."[7]
That all nerves may be, and that most are, double in function, the muscle-nerve being predominantly motor, because distributed to motor organs, whereas the skin-nerve is only distributed to the minute muscles of the skin, may be inferred from the anatomy of the invertebrata, in whom no double roots exist. Let us examine the ventral chord of a bee. From each ganglion one nerve-trunk issues, to supply both skin and muscles of each side. The first time I made a preparation of the bee's nervous system, I was forcibly arrested by this unity of motor and sensory nerves. But as I then believed in the classical doctrine, the explanation quickly suggested itself that, in the bee, there had not yet taken place that specialization into motor and sensory, which was found in vertebrata. True enough: but what is the specialization? Is it the introduction of a new kind of nerve, or only the assignment of one set of nerves to muscles, and of another to the skin? Clearly the latter; for there is nothing in the nature of the nerve tissue itself to prevent its serving both functions, as we see in the animal which has only one nerve for both: this nerve is not, as in vertebrata, split up into two, having two different origins, and two different peripheral terminations, but is one nerve, with one origin, sending off branches, here to muscles, and there to surfaces.
In his memoir on the Haliotis, M. Lacaze-Duthiers notices that the optic nerve has two kinds of branches, "les uns, que l'on pourrait nommer tégumentaires, et les autres oculaires proprement dits. Les premiers se distribuent aux téguments et aux tissus contractiles de nature musculaire qui forment les parois du tubercle; évidement ils apportent et la sensibilité et la motilité à ce support de l'organe de la vision." He notices as remarkable, that from the very trunk of a nerve of special sense, branches are given off, which are nerves of general sensibility and nerves of motion. But he contents himself with the supposition that there may be sensitive and motor fibres in this trunk, separate at their origin, though combined together in the trunk. This is, however, irreconcileable with microscopic observation of the molluscan nervous system; for when there are fibres, they are nothing but linear arrangements of the granular mass filling the neurilemma, which enter the ganglion together, and not from separate parts corresponding to anterior and posterior horns.
I will not extend this paper further, by any attempt to assign more definitely the functions of the nerves. The question at issue is: Are we justified in denying a sensory function to the anterior nerves, and a motor function to the posterior nerves? Is the difference between them one of property, or of function?