Chapter VII The Elementary Structure of the Nervous System
112. The progress of science involves an ever-increasing Analysis. Investigation is more and more directed towards the separated details of the phenomena previously studied as events; the observed facts are resolved into their component factors, complex wholes into their simpler elements, the organism into organs and tissues. But while the analytical process is thus indispensable, it is, as I have often to insist, beset with an attendant danger, namely, that in drawing the attention away from one group of factors to fix it exclusively on another, there is a tendency to forget this artifice, and instead of restoring the factors provisionally left out of account, we attempt a reconstruction in oblivion of these omitted factors. Hence, instead of studying the properties of a tissue in all the elements of that tissue, and the functions of an organ in the anatomical connections of that organ, a single element of the tissue is made to replace the whole, and very soon the function of the organ is assigned to this particular element. The “superstition of the nerve-cell” is a striking illustration. The cell has usurped the place of the tissue, and has come to be credited with central functions; so that wherever anatomists have detected ganglionic cells, physiologists have not hesitated to place central functions. By such interpretations the heart and intestines, the glands and blood-vessels, have, erroneously, I think, their actions assigned to ganglionic cells.
It is unnecessary to point out the radical misconception which thus vitiates a great mass of anatomical exposition and physiological speculation. I only call the reader’s attention to the point at the outset of the brief survey we have now to make of what is known respecting the elementary structure of the nervous system.
Difficulties of the Investigation
113. So great and manifold are the difficulties of the search, that although hundreds of patient observers have during the last forty years been incessantly occupied with the elementary structure of the nervous system, very little has been finally established. Indeed, we may still repeat Lotze’s sarcasm, that “microscopic theories have an average of five years’ duration.” This need not damp our ardor, though it ought to check a too precipitate confidence. Nothing at the present moment needs more recognition by the student than that the statements confidently repeated in text-books and monographs are very often for the most part only ingenious guesses, in which Observation is to Imagination what the bread was to the sack in Falstaff’s tavern bill. Medical men and psychologists ought to be warned against founding theories of disease, or of mental processes, on such very insecure bases; and physiological students will do well to remember the large admixture of Hypothesis which every description of the nervous system now contains. Not that the potent aid of Hypothesis is to be undervalued; but its limits must be defined. It may be used as a finger-post, not as a foundation. It may suggest a direction in which truth may be sought; it cannot take the place of Observation. It may link together scattered facts; it must not take the place of a fact. We are glad of corks until we have learned to swim. We are glad of a suggestion which will for the nonce fill up the gaps left by observation, and hold the facts intelligibly together. And both as suggestion and colligation, Hypothesis is indispensable. Indeed, every discovery is a verified hypothesis; and there is no discovery until verification has been gained: up to this point it was a guess, which might have been erroneous—a torchbearer sent out to look for a missing child in one direction, while the child was wandering in another; only when he finds the child can we acknowledge that the torchbearer pursued the right path. Hypothesis satisfies the intellectual need of an explanation, but we must be wary lest we accept this fulfilment of a need as equivalent to an enlargement of knowledge; we must not accept explanation as demonstration, and suppose that because we can form a mental picture of the possible stages of an event, therefore this picture represents the actual stages. Let us be alert, forewarned against the tendency to seek evidence in support of a conclusion, instead of seeking to unfold the conclusion step by step from the evidence. To seek for evidence in support of a guess is very different from seeking it in support of a conclusion; which latter practice is like that of people asking advice, and only following it when it chimes in with their desires.
114. Is not the warning needed, when we find anatomists guided by certain “physiological postulates,” and consequently seeing only what these postulates demand? For example, there is the postulate of “isolated conduction,” which is said to require that every nerve-fibre should pursue its course singly from centre to periphery. Accordingly the fibres are described as unbranched. Whatever may be the demand of the postulate, or the felt necessity of the deduction, the fact is that nerve-fibres do branch off during their course at various points; nay, it is doubtful whether any lengthy fibre is unbranched. Other postulates demand what fact plainly denies. It is said to be “necessary” that every cell should have at least two fibres, and that sensory and motor nerves should be directly connected through their respective cells. These things cannot be seen, but they are described with unhesitating precision. Diagrams are published in which the sensory fibres pass into the cells of the posterior horn of the spinal cord, and these cells send off prolongations to the cells of the anterior horn, and thence the motor fibres pass out to the muscles: an absolutely impossible arrangement, according to our present data! Again, the postulate that nerve-force originates in the cells, and that nerve-functions depend on cells, required that the cells should be most abundant where the function was most energetic. Of course they were found most abundant in the required places—no notice whatever being taken of the facts which directly contradicted the deduction.
115. Among the serious obstacles to research we must reckon this tendency to substitute Imaginary Anatomy for Objective Anatomy. I am conscious of the tendency in myself, as I note it in others; and have constantly to struggle against it, though not perhaps always aware of it. Many a time have I had to relinquish plausible explanations, which would have supported my speculations could I but have believed that they represented the facts; but being unable to believe this, I had to remember that hypotheses and explanations appear and disappear—only the solid fact lives. If there is one lesson emphatically taught by Philosophy, it is the unwisdom of founding our conclusions on our desires rather than on the objective facts.
116. In the following pages a constantly critical attitude is preserved: this is simply to keep active the sense of how much is still needed to be done before a satisfactory theory of the nervous system can be worked out. The objective difficulties are greater than in any other department of Anatomy. The problem is to form a precise picture of what the organites are, and of how they are arranged in the living tissue; yet our present means of investigation involve as a preliminary that we should alter that arrangement, removing some elements of the tissue, and changing the state of others, without knowing what were their precise state and arrangement before the change. Place a piece of nerve-tissue under the microscope, without having subjected it to various mechanical and chemical operations, and you can see next to nothing of its structure. You must tear the parts asunder, and remove the fat and nerve-sap (plasmode) before you can see anything; you must coagulate the albumen, and otherwise chemically alter the substances before a thin section can be made; you must get rid of the tissues in which it is embedded, without knowing what are the connections thus destroyed. Living neurine has no greater consistence than cream, often no greater than oil. How, then, can thin sections be made until this viscid substance has been hardened by alcohol or acids? But substances thus acted on lose their constituent water, which can no more be removed without alteration of their structure, than it can be removed from certain salts without destruction of their special properties. Losing their water alone, they become deformed. They lose much more. Sometimes the loss can be estimated, as in the case of the hyaline substance investing the nucleus during the process of segmentation in embryonic cells, which may be seen to disappear when a weak solution of acid is applied.137 At other times we are unable to say what has disappeared. Under different modes of preparation very different appearances are observed, and anatomists are accordingly at variance. Yet unless some hardening method be adopted little can be seen! Stilling, who has given his life to the study, declares that no results are reliable which are obtained from the unprepared tissue, because the mechanical isolation of the elements destroys the textural arrangement.138 There is one method of hardening, and only one, which we can be certain does not chemically alter the structure, and that is the freezing method. The experiments of Dr. Weir Mitchell and Dr. Richardson prove this, because they prove that the brain of the living animal may be frozen and frozen again and again, yet recover its vital activity when thawed. Professor Rutherford has invented an admirable instrument for making sections of the frozen tissue, of any delicacy that may be required; but with the thinnest section there will still be certain difficulties of observation, unless the tissue has undergone a staining process. Whatever is seen, however, in the frozen tissue is to be accepted as normal.
117. Two points must be determined before reliance can be placed on observations of tissues chemically acted on: First, we must prove that the forms now visible existed before the preparation—the chemical action merely unveiling them; secondly, we must estimate the part played by the elements which have been removed in order to make the rest visible. We know, for example, that the nucleus often exists in the cell, though an acid may be needed to make it visible. We also know that cells which during life are quite free from visible granules are distinctly granulated after death, even without external chemical action. Imagine the explanation of a steam-engine to be attempted by first taking it to pieces, and examining these pieces, with no account of the coals and steam which had previously been removed in order to facilitate the examination. When we know the part played by coals and steam, we may disregard these items of the active machine. So when we know the part played by water, fat, amorphous substance, and plasmode, we may describe nerve-tissue without taking these into account.
118. “You have convinced me,” said Rasselas to Imlac, “that it is impossible to be a poet.” My readers may, perhaps, infer from this enumeration of the difficulties that a knowledge of the minute anatomy of the nervous system is impossible. Not so; but a knowledge of these difficulties should impress us with the necessity for a vigilant scepticism, and the search after new methods. If the difficulties are fairly faced, they may be finally overcome. What we must resign ourselves to at present is the conviction that our knowledge is not sufficiently accurate to be employed as a basis of deduction in the explanation of physiological and psychological processes.139
119. Having said so much, let me add that there are some positive materials, and these yearly receive additions. The organites are described with a general agreement as to their composition and structure—although there is much that is hypothetical even here. Neurine is known under two aspects: the amorphous and the figured. The figured, which is the better known, comprises cells of different kinds, fibres and fibrils. The amorphous, more generally called Neuroglia, or nerve-cement, is less understood, and is indeed by many authorities excluded altogether from the nerve-tissue proper, and relegated to the class of connective tissues.
The Nerve-Cell
120. It is unfortunate that the term nerve-cell is applied to organites of very variable structure. Nerve-cell is a generic term of which the species are many; under it are designated organites in different stages—as infancy, childhood, and manhood are all included under Man. Most commonly by nerve-cell is understood the ganglionic corpuscle, conspicuous in its size and its prolongations, such as it appears in the great centres, and in ganglia. It also designates smaller different organites, sometimes called “nuclei” (Kerne), sometimes grains (Körner). There would be advantage in designating the earlier stages as neuroblasts, reserving the word cells for the more developed forms. Such a distinction would facilitate the discussion of whether nerve-fibres had or had not their origin in cells; because while I, for one, see very coercive evidence against the accepted notion that all the fibres have their origin in the processes of ganglionic corpuscles, I see no reason to doubt that both fibres and corpuscles have their origin in neuroblasts. Of this anon.
The cell is a composite organite, the primary element being a microscopic mass of protoplasm, or what may more conveniently be termed neuroplasm. It appears as finely granulated and striated or fibrillated substance on a hyaline ground, with water, fat, and diffused pigment in varying quantities. The cell contains a nucleus, and nucleolus—sometimes two. Like other animal cells, it sometimes has a distinct cell-wall, sometimes not. Its size and shape are variable: sometimes distinctly visible to the naked eye, generally visible only under the microscope.140 It is round, oval, pyramidal, club-shaped, pear-shaped, or many-cornered. It has one, two, three, or many outgrowths called “processes,” and according to the processes it is known as unipolar, bipolar, and multipolar. When there are no processes the cell is called apolar. Some idea of these processes may be formed if they are likened to the pseudopodia of Amœbæ and Foraminifera. Compare Fig. 16, a nerve-cell, figured by Gerlach, with Fig. 17, one highly magnified, in which Max Schultze’s hypothesis is represented.
121. Such is a general description of the nerve-cell as it is seen in various places, and under various modes of preparation. How much is due to preparation we cannot positively say. While we always discover fibrine in the blood after it is withdrawn from the vessels, we know that fibrine as such does not exist in the circulating blood. And if neurine is a semi-liquid substance, we may doubt whether in the living cell it is fibrillated. Doubts have been thrown even on the normal existence of the granular substance, which has been attributed to coagulation. Thus we know that the nucleus of the white blood-corpuscle appears perfectly homogeneous until subjected to heat, yet at a certain temperature (86° F.) it assumes the aspect of a fine network. Haeckel observed the hyaline substance of the neurine in crayfish become troubled and changed directly any fluid except its own blood-serum came in contact with it. Leydig noticed the transparent ganglion of a living Daphnia become darker and darker as the animal died; and I saw something like this, after prolonged struggles of a Daphnia to escape from a thread in which its leg was entangled. Charles Robin, indeed, asserts that the passage from the hyaline to the finely granulated state is a characteristic of the dying cell.141 On the other hand, it should be noted that Max Schultze describes a fibrillated appearance in cells just removed from the living animal, and placed in serum.
When, therefore, one observer describes the neuroplasm as being clear as water, another as finely granular, and a third as fibrillated, we must conclude that the observations refer to cells, 1°, under different states of vitalization, or, 2°, under different modes of preparation. On the first head we note that some nerve-cells are so perishable that Trinchese declares he could find no cells in the ganglia of a cuttlefish which had been dead twenty-four hours, although they were abundant in one recently killed.142 On the second head we note that the changes wrought by modes of preparation cannot be left out of consideration. Auerbach notices that the cells and fibres apparent in the plexus myentericus after an acid has been applied, cannot be detected before that application—nothing is visible but a pale gelatinous network, with here and there knots of a paler hue; and I remember my surprise on examining the fresh spinal cord of a duck-embryo, and finding no trace of cells such as I had that very morning seen in the cord of a chick of earlier date, but which had been soaked in weak bichromate of potash. Now we have excellent grounds for believing that in both cases these organites were present, and that it was the reagent which disclosed their presence in the chick; and so in other cases we must ask whether the forms which appear under a given mode of preparation are simply unmasked, or are in truth produced by the reagent? This question we can rarely answer.
If one of the very large cells be taken from the ganglion of a living mollusc, and be gently pressed till it bursts, the discharged contents will be seen to be of a hyaline viscid substance, with fine granules but no trace of fibres. Yet we must not rashly generalize from this, and declare that in the vertebrate cells the substance is not also fibrillated. As a good deal of speculation rests on the assumption of the fibrillated cell-contents, I have thought it worth while to note the uncertainty which hovers round it.
122. Among the uncertainties must be reckoned the question as to the cell-processes. The existence of apolar and unipolar cells is flatly denied by many writers, who assert that the appearances are due to the fragility of the processes. Fragile the processes are, and evidence of their having been broken off meet us in every preparation; but the denial of apolar and unipolar cells seems to me only an example of the tendency to substitute hypothesis for observation (§ 114). The “postulate” which some seem to regard as a “necessity of thought” that every nerve-cell shall have at least two fibres, one ingoing, the other outgoing, is allowed to override the plain evidence.143 It originated in the fact first noticed by Wagner and Charles Robin that certain cells in the spinal ganglia of fishes are bipolar. The fact was rapidly generalized, in spite of its not being verified in other places; the generalization was accepted because (by a strange process of reasoning running counter to all physiological knowledge) it was thought to furnish an elementary illustration of the reflex process. As the centre had its ingoing and outgoing nerve, so the cell was held to be a centre “writ small,” and required its two fibres, No one paused to ask, how a cell placed in the track of an ingoing nerve could fulfil this office of a reflex centre; no one supposed that the portion of the sensory fibre which continued its course, after the interruption of the cell, was a motor fibre.
What does Observation teach? It teaches that at first all nerve-cells are apolar. Even in the cortex of the cerebrum, where (unless we include the nuclei and grain-like corpuscles under cells) all the cells are finally multipolar, there is not one which has a process, up to the seventh or eighth day of incubation (in the chick); from that day, and onwards, cells with one process appear; later on, cells with two, and later still, with three. By this time all the apolar cells have disappeared. They may therefore be regarded as cells in their infancy. However that may be, we must accept the fact that apolar cells exist; whether they can co-operate in neural functions, is a question which must be decided after the mode of operation of cells is placed beyond a doubt.
123. If apolar cells are embryonic forms of cells which afterwards become multipolar, this interpretation will not suffice for the unipolar cells. They are not only abundant, but are mature forms in some organs, and in some animals; though in some organs they may truly be regarded as embryonic. Thus in the human embryo up to the fourth month all the cells of the spinal cord are said to be unipolar,144 later on they become multipolar. But in birds, rabbits, dogs, and even man, the cells in the spinal ganglia are mainly (if not wholly) unipolar;145 nor is there any difficulty in observing the same fact in the œsophageal ganglia of molluscs (see Fig. 22).
Such are the observations. They have indeed been forced into agreement with the bipolar postulate, by the assumption that the single process branches into two, one afferent, the other efferent.146 But before making observation thus pliant to suit hypothesis, it would be well to look more closely into the evidence for the hypothesis itself. For my own part, I fail to see the justification of the postulate; whereas the existence of unipolar cells is an observation which has been amply verified.
124. Bipolar cells abound; multipolar cells are still more abundant; and these are the cells found in the gray substance of the neural axis. Deiters, in his epoch-making work,147 propounded an hypothetic schema which has been widely accepted. Finding that the large cells in the anterior horn of the spinal cord gave off processes of different kinds, one branched, the other unbranched, he held that the latter process was the origin of the axis cylinder of a nerve-fibre, whereas the branched process was protoplasm which divided and subdivided, and formed the connection between one cell and another. Gerlach has modified this by supposing that the minute fibrils of the branching process reunite and form an axis cylinder (Fig. 18). There is no doubt that some processes terminate in a fine network; and there is a probability (not more) that the unbranched process is always continuous with the axis cylinder of a motor nerve, as we know it sometimes is with that of a dark-bordered fibre in the white substances. This, though probable, is, however, very far from having been demonstrated. Once or twice Kölliker, Max Schultze, and Gerlach have followed this unbranched process as far as the root of a motor nerve; and they infer that although it could not be traced further, yet it did really join an axis cylinder there. In support Of this inference came the observations of Koschennikoff,148 that in the cerebrum and cerebellum, processes were twice seen continuous with dark-bordered nerve-fibres. But the extreme rarity of such observations amid thousands of cells is itself a ground for hesitation in accepting a generalized interpretation, the more so since we have Henle’s observation of the similar entrance of a branched process into the root.149 Now it must be remembered that the branched process is by no anatomist at present regarded as the origin of the axis cylinder; so that if it can enter the root without being the origin of a nerve-fibre, we are not entitled to assume that the entrance of the unbranched process has any other significance (on this head compare § 145), especially when we reflect that no trustworthy observer now professes to have followed a nerve-fibre of the posterior root right into a multipolar cell. Figures, indeed, have been published which show this, and much else; but such figures are diagrams, not copies of what is seen. They belong to Imaginary Anatomy.150 The relation of the cell-process to the nerve-fibre will be discussed anon.
125. A word in passing on the contradictory assertions respecting the anastomosis of nerve-cells. That the gray substance forms a continuum of some kind is certain from the continuity of propagation of a stimulus. But it is by no means certain that one cell is directly united to its neighbor by a cell-process. Eminent authorities assert that such direct union never takes place; others, that it is a rare and insignificant fact; others, that it is constant, and “demanded by physiological postulates.” I will not, in the presence of distinct affirmations, venture to deny that such appearances as are presented in Fig. 19 may occasionally be observed; the more so as I have myself seen perhaps half a dozen somewhat similar cases; but it is the opinion of Deiters and Kölliker that all such appearances are illusory.151 Granting that such connections occur, we cannot grant this to be the normal mode; especially now the more probable supposition is that the connection is normally established by means of the delicate ramifications of the branching processes.
Imaginary Anatomy has not been content with the cells of the anterior horn being thus united together, to admit of united action, but has gone further, and supposed that the cells of the posterior horn, besides being thus united, send off processes which unite them with the cells of the anterior horn—and thus a pathway is formed for the transmission of a sensory impression, and its conversion into a motor impulse. What will the reader say when informed that not only has no eye ever beheld such a pathway, but that the first step—the direct union of the sensory nerve-fibre with a cell in the posterior horn—is confessedly not visible?
126. The foregoing criticisms will perhaps disturb the reader who has been accustomed to theorize on the data given in text-books; but he may henceforward be more cautious in accepting such data as premises for deduction, and will look with suspicion on the many theories which have arisen on so unstable a basis. When we reflect how completely the modern views of the nervous system, and the physiological, pathological, and psychological explanations based on these views, are dominated by the current notions of the nerve-cell, it is of the last importance that we should fairly face the fact that at present our knowledge even of the structure of the nerve-cell is extremely imperfect; and our knowledge of the part it plays—its anatomical relations and its functional relations—is little more than guesswork!
The Nerves
127. We now pass to the second order of organites; and here our exposition will be less troubled by hesitations, for although there is still much to be learned about the structure and connections of the nerve-fibres, there is also a solid foundation of accurate knowledge.
A nerve is a bundle of fibres within a membranous envelope supplied with blood-vessels. Each fibre has also its separate sheath, having annular constrictions at various intervals. It is more correctly named by many French anatomists a nerve-tube rather than a nerve-fibre; but if we continue to use the term fibre, we must reserve it for those organites which have a membranous sheath, and thereby distinguish it from the more delicate fibril which has none.
The nerve tube or fibre is thus constituted: within the sheath lies a central band of neuroplasm identical with the neuroplasm of nerve-cells, and known as the axis cylinder; surrounding this band is an envelope of whitish substance, variously styled myeline, medullary sheath, and white substance of Schwann: it is closely similar to the chief constituent of the yolk of egg, and to its presence is due the whitish color of the fibres, which in its absence are grayish. The axis cylinder must be understood as the primary and essential element, because not only are there nerve-fibrils destitute both of sheath and myeline yet fulfilling the office of Neurility, but at their terminations, both in centres and in muscles, the nerve-fibres always lose sheath and myeline, to preserve only the neuroplasmic threads of which the axis cylinder is said to be composed. In the lowest fishes, in the invertebrates, and in the so-called sympathetic fibres of vertebrates, there is either no myeline, or it is not separated from the neuroplasm.
128. Nerve-fibres are of two kinds—1°. The dark-bordered or medullary fibres, which have both sheath and myeline, as in the peripheral system; or only myeline, without the sheath, as in the central system. 2°. The non-medullary fibres, which have the sheath, without appreciable myeline—such are the fibres of the olfactory, and the pale fibres of the sympathetic.
Nerve-fibrils are neuroplasmic threads of extreme delicacy, visible only under high magnifying powers (700–800), which abound in the centres, where they form networks. The fibrils also form the terminations of the fibres. Many fibrils are supposed to be condensed in one axis cylinder. This is represented by Max Schultze in Figs. 17 and 20.
129. As may readily be imagined, the semi-liquid nature of the neuroplasm throws almost insuperable difficulties in the way of accurately determining whether the axis cylinder in the living nerve is fibrillated or not; whether, indeed, any of the aspects it presents in our preparations are normal. Authorities are not even agreed as to whether it is a pre-existent solid band of homogeneous substance, or a bundle of primitive fibrils, or a product of coagulation.152 Rudanowsky’s observations on frozen nerves convinced him that the cylinder is a tubule with liquid contents.153 My own investigations of the nerves of insects and molluscs incline me to the view of Dr. Schmidt of New Orleans, namely, that the cylinder axis consists of minute granules arranged in rows and united by a homogeneous interfibrillar substance, thus forming a bundle of granular fibrils enclosed in a delicate sheath154—in other words, a streak of neuroplasm which has a fibrillar disposition of its granules. We ought to expect great varieties in such streaks of neuroplasm; and it is quite conceivable that in the Rays and the Torpedo there are axis cylinders which are single fibrils, and others which are bundles, with finely granulated interfibrillar substance.155
The fibres often present a varicose aspect, as represented in Fig. 21. It is, however, so rarely observed in the fresh tissue, that many writers regard it (as well as the double contour) as the product of preparation.156 It is, indeed, always visible after the application of water.
We need say no more at present respecting the structure of nerve-fibres, except to point out that we have here an organite not less complex than the cell.
The Neuroglia
130. Besides cells and fibres, there is the amorphous substance, which constitutes a great part of the central tissue, and also enters largely into the peripheral tissue. It consists of finely granular substance, and a network of excessively delicate fibrils, with nuclei interspersed. Its character is at present sub judice. Some writers hold it to be nervous, the majority hold it to be simply one of the many forms of connective tissue: hence its name neuroglia, or nerve-cement.
In the convolutions of the frozen brain Walther finds the cells and fibres imbedded in a structureless semi-fluid substance wholly free from granules; the granules only appear there when cells have been crushed. It is to this substance he attributes the fluctuation of the living brain under the touch, like that of a mature abscess; the solidity which is felt after death is due to the coagulation of this substance. Unhappily we have no means of determining whether the network visible under other modes of investigation is present, although invisible, in this substance. The neuroglia, as it appears in hardened tissues, must therefore be described with this doubt in our minds.
If we examine a bit of central gray substance where the cells and fibres are sparse, we see, under a low power, a network of fibrils in the meshes of which lie nerve-cells. Under very high powers we see outside these cells another network of excessively fine fibrils embedded in a granular ground substance, having somewhat the aspect of hoar-frost, according to Boll. It is supposed that the first network is formed by the ultimate ramifications of the nerve-cell processes, and that the second is formed by ramifications of the processes of connective cells. In this granular, gelatinous, fibrillar substance nuclei appear, together with small multipolar cells not distinguishable from nerve-cells except in being so much smaller. These nuclei are more abundant in the tissue of young animals, and more abundant in the cerebellum than in the cerebrum. The granular aspect predominates the fresher the specimen, though there is always a network of fibrils; so that some regard the granules as the result of a resolution of the fibrils, others regard the fibrils as the linear crystallization (so to speak) of the granules.157
131. Such is the aspect of the neuroglia. I dare not venture to formulate an opinion on the histological question whether this amorphous substance is neural, or partly neural and partly connective (a substance which is potentially both, according to Deiters and Henle), or wholly connective. The question is not at present to be answered decisively, because what is known as connective tissue has also the three forms of multipolar cells, fibrils, and amorphous substance; nor is there any decisive mark by which these elements in the one can be distinguished from elements in the other. The physical and chemical composition of Neuroglia and Neuroplasm are as closely allied as their morphological structure. And although in the later stages of development the two tissues are markedly distinguishable, in the early stages every effort has failed to furnish a decisive indication.158 Connective tissue is dissolved by solutions which leave nerve-tissue intact. Can we employ this as a decisive test? No, for if we soak a section of the spinal cord in one of these solutions, the pia mater and the membranous septa which ramify from it between the cells and fibres disappear, leaving all the rest unaltered. This proves that Neuroglia is at any rate chemically different from ordinary connective tissue, and more allied to the nervous. As to the staining process, so much relied on, nothing requires greater caution in its employment. Stieda found that the same parts were sometimes stained and sometimes not; and Mauthner observed that in some cells both contents and nucleolus were stained, while the nucleus remained clear, in other cells the contents remained clear; and some of the axis cylinders were stained, the others not.159 Lister found that the connective tissue between the fibres of the sciatic nerve, as well as the pia mater, were stained like the axis cylinders;160 and in one of my notes there is the record of both (supposed) connective cells and nerve-cells being stained alike, while the nerve-fibres and the (supposed) connective fibres were unstained. Whence I conclude that the supposition as to the nature of the one group being different from that of the other was untenable, if the staining test is to be held decisive.
132. The histological question is raised into undue importance because it is supposed to carry with it physiological consequences which would deprive the neuroglia of active co-operation in neural processes, reducing it to the insignificant position of a mechanical support. I cannot but regard this as due to the mistaken tendency of analytical interpretation, which somewhat arbitrarily fastens on one element in a complex of elements, and assigns that one as the sole agent. Whether we call the neuroglia connective or neural, it plays an essential part in all neural processes, probably a more important part than even the nerve-cells, which usurp exclusive attention! To overlook it, or to assign it a merely mechanical office, seems to me as unphysiological as to overlook blood-serum, and recognize the corpuscles as the only nutrient elements. The notion of the neuroglia being a mere vehicle of support for the blood-vessels arises from not distinguishing between the alimental and instrumental offices. In the function of a limb, bone is a co-operant. In the function of a centre, connective tissue is a co-operant; so that even if we acknowledge neuroglia to be a special form of connective tissue, it is an agent in neural processes; what its agency is, will be hereafter considered.
Following Bidder and Kupffer, the Dorpat school proclaimed the whole of the gray substance of the posterior half of the spinal cord to be connective tissue; and Blessig maintained that the whole of the retina, except the optic fibres, was connective tissue.161 Even those anatomists who regarded this as exaggerated, admitted that connective tissue largely enters into the gray substance, especially if the granular ground substance be reckoned as connective, the nerve-cells being very sparse in the posterior region. Be it so. Let us admit that the gray matter of the frog’s spinal cord is mainly composed of neuroglia, in which a very few multipolar nerve-cells are embedded. What must our conclusion be? Why, that since this spinal cord is proved to be a centre of energetic and manifold reflex actions—even to the extent of forcing many investigators to attribute sensation and volition to it—this is proof that connective tissue does the work of nerve-tissue, and that the neuroglia is more important than nerve-cells!
Three hypotheses are maintainable—1°. The neuroglia is the amorphous ground-substance of undeveloped tissue (neuroplasm) out of which the cells and fibres of nerve-tissue and connective tissue are evolved. 2°. It is the product of dissolved nerve cells and fibres. 3°. It is the undeveloped stage of connective tissue. For physiological purposes we may adopt any one of these views, provided we keep firm hold of the fact that the neuroglia is an essential element, and in the centres a dominant element. To make this clear, however, we must inquire more closely into the relations of the three elements, nerve-cells, fibres, and neuroglia.
The Relations of the Organites
133. In enumerating among the obstacles to research the tendency to substitute hypothetic deductions in place of objective facts, I had specially in my mind the wide-reaching influence of the reigning theories of the nerve-cell. Had we a solidly established theory of the cell, equivalent, say, to our theory of gas-pressure, we should still need caution in allowing it to override exact observation; but insecure as our data are, and hypothetical as are the inferences respecting the part played by the cell, the reliance placed on deductions from such premises is nothing less than superstition. Science will take a new start when the whole question is reinvestigated on a preliminary setting aside of all that has been precipitately accepted respecting the office of the cell. This exercise of the imagination, even should the reigning theories subsequently be confirmed, would not fail to bring many neglected facts into their rightful place.
I am old enough to remember when the cell held a very subordinate position in Neurology, and now my meditations have led me to return, if not to the old views of the cell, at least to something like the old estimate of its relative importance. Its existence was first brought prominently forward by Ehrenberg in 1834, who described its presence in the sympathetic ganglia; and by Remak in 1837, who described it in the spinal ganglia. For some time afterwards the ganglia and centres were said to contain irregular masses of vesicular matter which were looked on as investing the fibres; what their office was, did not appear. But there rapidly arose the belief that the cells were minute batteries in which “nerve-force” was developed, the fibres serving merely as conductors. Once started on this track, Hypothesis had free way, and a sort of fetichistic deification of the cell invested it with miraculous powers. In many works of repute we meet with statements which may fitly take their place beside the equally grave statements made by savages respecting the hidden virtues of sticks and stones. We find the nerve-cells credited with “metabolic powers,” which enable them to “spiritualize impressions, and materialize ideas,” to transform sensations into movements, and elaborate sensations into thoughts; not only have they this “remarkable aptitude of metabolic local action,” they can also “act at a distance.”162 The savage believes that one pebble will cure diseases, and another render him victorious in war; and there are physiologists who believe that one nerve-cell has sensibility, another motricity, a third instinct, a fourth emotion, a fifth reflexion: they do not say this in so many words, but they assign to cells which differ only in size and shape, specific qualities. They describe sensational, emotional, ideational, sympathetic, reflex, and motor-cells; nay, Schröder van der Kolk goes so far as to specify hunger-cells and thirst-cells.163 With what grace can these writers laugh at Scholasticism?
134. The hypothesis of the nerve-cell as the fountain of nerve-force is supported by the gratuitous hypothesis of cell-substance having greater chemical tension and molecular instability than nerve-fibre. No evidence has been furnished for this; indeed the only experimental evidence bearing on this point, if it has any force, seems directly adverse to the hypothesis. I allude to the experiments of Wundt, which show that the faint stimulus capable of moving a muscle when applied directly to its nerve, must be increased if the excitation has to pass through the cells by stimulation of the sensory nerve.164 Wundt interprets this as proving that the cells retard every impulse, whereby they are enabled to store up latent force. The cells have thus the office of locks in a canal, which cause the shallow stream to deepen at particular places. I do not regard this interpretation as satisfactory; but the fact at any rate seems to prove that so far from the cells manifesting greater instability than the fibres, they manifest less.
135. The hypothesis of nerve-force being developed in the ganglia, gradually assumed a more precise expression when the nerve-cells were regarded as the only important elements of a ganglion. It has become the foundation-stone of Neurology, therefore very particular care should be taken to make sure that this foundation rests on clear and indisputable evidence. Instead of that, there is absolutely no evidence on which it can rest; and there is much evidence decidedly opposed to it. Neither structure nor experiment points out the cells as the chief agents in neural processes. Let us consider these.
Fig. 22 shows the contents of a molluscan ganglion which has been teased out with needles.
The cells are seen to vary in size, but in all there is a rim of neuroplasm surrounding the large nucleus, and from this neuroplasm the fibre is seen to be a prolongation. The dotted substance in the centre is the neuroglia. Except in the possession of a nucleus, there is obviously here no essential difference in the structure of cell and fibre.
Now compare this with Fig. 23, representing three fibres from the auditory nerve.
Here the cell substance, as Max Schultze remarks, “is a continuation of the axis cylinder, and encloses the nucleus. The medulla commonly ceases at the point where the axis enters the cell, to reappear at its exit; but it sometimes stretches across the cell to enclose it also: so that such a ganglion cell is in truth simply the nucleated portion of the cylinder axis.”165 There are many places in which fibres are thus found with cells inserted in their course as swellings: in the spinal ganglia of fishes these are called bipolar cells; they are sometimes met with even in the cerebellum; but oftener in peripheral nerves, where they are mostly small masses of granular neuroplasm from which usually a branching of the fibre takes place. The point to which attention is called is that in some cases, if not in all, the nerve-fibre is structurally continuous with the cell contents. The two organites—fibre and cell—differ only as regards the nucleus and pigment. Haeckel, who affirms that in the crayfish (Astacus fluviatilis) he never saw a cell which did not continue as a fibre, thinks there is always a marked separation of the granular substance from its “hyaline protoplasm,” and that only this latter forms the axis cylinder. But although my observations agree with this as a general fact, I have seen even in crayfish the granular substance prolonged into the axis cylinder; and in other animals the granular substance is frequently discernible.
Indeed it may be said that anatomists are now tolerably unanimous as to the axis cylinder being identical with the protoplasmic cell substance. If this be so, we have only to recall the principle of identity of property accompanying identity of structure, to conclude that whatever properties we assign to the cells (unless we restrict these to the nucleus and pigment) we must assign to the axis cylinders. We can therefore no longer entertain the hypothesis of the cells being the fountains or reservoirs of Neurility; the less so when we reflect that cells do not form the hundredth part of nerve-tissue: for even the gray substance bears but a small proportion to the white; and of the gray substance, Henle estimates that one half is fibrous, the rest is partly cellular, partly amorphous. Those who derive Neurility from the cells, forget that although the organism begins as a cell, and for some weeks consists mainly of cells, yet from this time onwards there is an ever-increasing preponderance of cell-derivatives—fibres, tubes, and amorphous substance—and corresponding with this is the ever-increasing power and complexity of the organism.
136. From another point of view we must reject the hypothesis. Not only does the evidence which points to the essential continuity in structure of nerve cell and fibre discredit the notion of their physiological diversity, but it is further supported by the fact that although the whole nervous system is structurally continuous, an immense mass of nerve-fibres have no immediate connection with ganglionic cells:—neither springing from nor terminating in such cells, their activity cannot be assigned to them. To many readers this statement will be startling. They have been so accustomed to hear that every fibre begins or terminates in a cell, that a doubt thrown on it will sound paradoxical. But there is an equivoque here which must be got rid of. When it is said that every fibre has its “origin” in a cell, this may be true if origin mean its point of departure in evolution, for “cells” are the early forms of all organites; but although every organite is at first a cell, and in this sense a nerve-fibre must be said to originate in a cell, we must guard against the equivoque which arises from calling the highly differentiated organite, usually designated ganglionic cell, by the same name as its starting-point. On this ground I suggest the term neuroblast, in lieu of nerve-cell, for the earlier stages in the evolution of cell and fibre. Both Embryology and Anatomy seem to show that cell and fibre are organites differentiated from identical neuroblasts, with a somewhat varying history, so that in their final stages the cell and fibre have conspicuous differences in form with an underlying identity; just as a male and female organism starting from identical ova, and having essential characters in common, are yet in other characters conspicuously unlike. The multipolar cell is not necessarily the origin of a nerve-fibre, although it is probable that some short fibres have their origin in the prolongations of cells. Although the latter point has not, I think, been satisfactorily established, except in the invertebrata, I see no reason whatever to doubt its probability; what seems the least reconcilable with the evidence is the notion that all fibres arise as prolongations from ganglionic cells, instead of arising independently as differentiations from neuroblasts. The reader will observe that my objection to the current view is purely anatomical; for the current view would suit my physiological interpretations equally well, and would be equally irreconcilable with the hypothesis of the cell as the source of Neurility, so long as the identity of structure in the axis cylinder and cell contents is undisputed.
137. The evidence at present stands thus: There are numerous multipolar cells which have no traceable connection with nerve-fibres; and fibres which have no direct connection with multipolar cells. By the first I do not mean the disputed apolar cells, I mean cells in the gray substance of the centres which send off processes that subdivide and terminate as fibrils in the network of the Neuroglia (Figs. 16, 18). It is indeed generally assumed that these have each one process—the axis-cylinder process—which is prolonged as a nerve-fibre; nor would it be prudent to assert that such is never the case; though it would be difficult to distinguish between a fibre which had united with a process and a fibre which was a prolongation of a process, in both cases the neuroplasm being identical. I only urge that the assumption is grounded not on anatomical evidence, but on a supposed necessary postulate. All that can be demonstrated is that some processes terminate in excessively fine fibrils; and occasionally in thousands of specimens processes have been traced into dark-bordered fibres. It is true that they often present appearances which have led to the inference that they did so terminate—appearances so deceptive that Golgi and Arndt independently record observations of unbranched processes having the aspect of axis cylinders being prolonged to a considerable distance (600 μ in one case), yet these were found to terminate not in a dark-bordered fibre, but in a network of fibrils.166
138. While it is thus doubtful whether dark-bordered fibres are always immediately connected with cells, it is demonstrable that multitudes of fibres have only an indirect connection with cells, being developed as outgrowths from other fibres. Dr. Beale considers that in each such outgrowths have their origin in small neuroplasmic masses (his “germinal matter”). That is another question. The fact here to be insisted on is that we often find groups of cells with only two or three fibres, and groups of fibres where very few cells exist. Schröder van der Kolk says that in a sturgeon (Accipenser sturio) weighing 120 pounds he found the spinal cord scarcely thicker than that of a frog; the muscles of this fish are enormous, and its motor nerves abundant; yet these nerves entered the cord by roots no thicker than a pig’s bristle; and in the very little gray matter of the cord there was only a cell here and there found after long search. Are we to suppose that these rare cells were the origins of all the motor and sensory nerves? A similar want of correspondence may be noticed elsewhere. Thus in the spinal cord of the Lamprey my preparations show very few cells in any of the sections, and numerous sections show none at all. Stieda counted only eight to ten cells in each horn of some osseous fishes, except at the places where the spinal roots emerged. In the eel and cod he found parts of the cord quite free from cells, and in other parts found two, three, never more than ten. In birds he counted from twenty-five to thirty. Particular attention is called to this fact of the eel’s cord being thus deficient, because every one knows the energetic reflex action of that cord, each separate segment of which responds to peripheral stimulation.
It may indeed be urged that these few cells were the origin of all the fibres, the latter having multiplied by the well-known process of subdivision; and in support of this view the fact may be cited of the colossal fibres of the electric fishes, each of which divides into five-and-twenty fibres, and in the electric eel each fibre is said by Max Schultze to divide into a million of fibrils. But I interpret this fact otherwise. It seems to me to prove nothing more than that the neuroplasm has differentiated into few cells and many fibres. And my opinion is grounded on the evidence of Development, presently to be adduced. If we find (and this we do find) fibres making their appearance anywhere before multipolar cells appear, the question is settled.
139. Dr. Beale regards the large caudate cells of the centres as different organites from the oval and pyriform cells, and thinks they are probably stations through which fibres having different origins merely pass, and change their directions; and Max Schultze says that no single fibril has been found to have a central origin; every fibril arises at the periphery, and passes through a cell, which is thus crossed by different fibrils.167 (Comp. Fig. 17.)
The teaching of Development is on this point of supreme importance. Unhappily there has not yet been a sufficient collection of systematic observations to enable us to speak very confidently as to the successive stages, but some negative evidence there is. The changes take place with great rapidity, and the earliest stages have hardly been observed at all. Although for several successive years I watched the development of tadpoles, the difficulties were so great, and the appearances so perplexing, that the only benefit I derived was that of being able the better to understand the more successful investigations of others. Four or five days after fecundation is the earliest period of which I have any recorded observation; at this period the cerebral substance appeared as a finely granular matter, having numerous lines of segmentation marking it off into somewhat spherical and oval masses, interspersed with large granules and fat globules. Here and there hyaline substance appeared between the segments. Similar observations have since been recorded by Charles Robin in the earliest stages of the Triton.168 He says that when the external gills presented their first indications, nuclei appeared, each surrounded by a rim of hyaline substance, from which a pale filament was prolonged at one end, sometimes one at both ends, and this filament subdivided as it grew in length until it had all the appearance of an axis cylinder. This, however, he says, is a striation, not a fibrillation; he refuses to admit that the axis cylinder is a bundle of fibrils. He further notices the simultaneous appearance of amorphous substance; and as this is several days before there is any trace of a pia mater, or proper connective tissue, he urges this among the many considerations which should prevent the identification of neuroglia with connective tissue.
In a very young embryo of a mole (I could not determine its age) the cortex of the hemispheres showed granular amorphous substance, in which were embedded spherical masses of somewhat paler color, which had no nuclei, and were therefore not cells. Besides these, there were nucleated masses (apolar cells, therefore) and more developed cells, unipolar, bipolar, and tripolar. Not a trace of a nerve-fibre was visible. In agreement with this are the observations of Masius and Van Lair, who cut out a portion of the spinal cord in a frog, and observed the regenerated tissue after the lapse of a month. It contained apolar, bipolar, and multipolar cells, together with “corpuscles without processes, for the most part larger than the cells, and appearing to be mere agglomerations of granules,”—these latter I suppose to have been what I describe as segmentations of the undeveloped substance. Gray fibres, with a few varicose fibres, also appeared.169
140. The admirable investigations of Franz Boll have given these observations a new significance. He finds in the cerebral substance of the chick on the third or fourth day of incubation a well-marked separation between the neuroglia and nerve-tissue proper. Fig. 24, A, represents three nerve-cells, each with its nucleus and nucleolus, and each surrounded with its layer of neuroplasm. The other four masses he regards as nuclei of connective tissue. Three days later the distinction between the two is more marked (Fig. 24, B). Not only have the nerve-cells acquired an increase of neuroplasm, they also present indications of their future processes, which at the twelfth day are varicose (Fig. 24, C). (All this while the connective corpuscles remain unchanged.) Although Boll was unable to trace one of these processes into nerve-fibres, he has little doubt that they do ultimately become (unite with?) axis cylinders.
It is difficult to reconcile such observations with the hypothesis of the cells being simply points of reunion of fibrils. We see here multipolar cells before any fibrils appear. Respecting the development of the white substance, i. e. the nerve-fibres, Boll remarks that in the corpus callosum of the chick the first differentiation resembles that of the gray substance.
The polygonal and spindle-shaped cells represented in Fig. 25, A, are respectively starting-points of connective and neural tissues. The spindle-shaped cells elongate, and rapidly become bipolar. This is supposed to result in the whole cell becoming transformed into a fibre, the nucleus and nucleolus vanishing; but the transformation is so rapid that he confesses that he was unable to trace its stages; all that can positively be asserted is that one or two days after the appearance presented in Fig. 25, B, the aspect changes to that of fibrils. The columns of polygonal cells between which run these fibrils, he regards as the connective corpuscles described by several anatomists in the white substance both of brain and cord, and which are sometimes declared to be multipolar nerve-cells.170
141. Dr. Schmidt’s observations on the human embryo were of course on tissue at a very much later stage. According to him, the fibrils of the axis cylinders are formed by the linear disposition and consolidation of elementary granules. The fibrils thus formed are separated by interfibrillar granules which in time become fibrils. Not earlier than three months and a half does the formation of individual axis cylinders begin by the aggregation of these fibrils into minute bundles, which are subsequently surrounded by a delicate sheath.171
142. With respect to the transition of the spindle-shaped cells into fibrils, since there is a gap in the observations of Boll, and since those of Schmidt are subsequent to the disappearance of the cells, and in both cases all trace of nucleus has disappeared, I suggest that we have here an analogy with what Weismann has recorded of the metamorphoses of insects. In the very remarkable memoir of that investigator172 it is shown that the metamorphoses do not take place by a gradual modification of the existing organs and tissues, but by a resolution of these into their elements, and a reconstruction of their elements into tissues and organs. The muscles, nerves, tracheæ, and alimentary canal, undergo what may be called a fatty degeneration, and pass thence into a mere blastema. It is out of these ruins of the old tissues that the new tissues are reconstructed. On the fourth day the body of the pupa is filled with a fluid mass—a plasma composed of blood and dissolved tissues. The subsequent development is thus in all essential respects a repetition of that which originally took place in the ovum.173
Two points are especially noticeable: First, that in this resolved mass of granules and fat globules there quickly appear large globular masses which develop a fine membrane, and subsequently nuclei. A glance at the figure 51 of Weismann’s plates reveals the close resemblance to the earliest stages of nerve-cells; and the whole process recalls the regeneration of nerves and nerve-centres after their fatty degeneration.
Secondly, the nerves reappear in their proper places in the new muscles, and this at a time when the nerve-centres are still unformed; so that the whole peripheral system is completely rebuilt in absolute independence of the central system. The idea, therefore, that nerve-fibres are the products of ganglia must be relinquished. This idea is further discountenanced by Boll’s observations, which show that the fibre-cells are from the first different from the ganglionic cells; and by the observations of Foster and Balfour, that “fibres are present in the white substance on the third day of incubation”; whereas cell processes do not appear until the eighth day. Foster and Balfour are inclined to believe “that even on the seventh day it is not possible to trace any connection between the cells and fibres.” In the later stages, the connection is perhaps established.174
143. We may, I think, conclude from all this that in the higher vertebrates the white substance of brain and cord is not the direct product of the gray substance; in other words, that here nerve-fibres, even if subsequently in connection with the ganglionic cells, have an independent origin. They may grow towards and blend with cell processes; they are not prolongations of those processes. They may be identical in structure and property, as one muscle is identical with another, but one is not the parent of the other.
144. Sigmund Mayer emphatically declares that in no instance has he traced a cell process developed into a dark-bordered nerve-fibre. The process, he says, may often be traced for a certain distance alongside of a fibre; but it then suddenly ceases, whereas the fibre is seen continuing its course unaltered. Still more conclusive is the evidence afforded by nerves having only very few fibres (2–4 sometimes in the frog), which have, nevertheless, a liberal supply of cells, visible without preparation. Valentin counted twenty-four cells in a nerve which had but two fibres.175 Now although it is possible to explain the presence of numerous fibres with rare cells either as due to subdivisions of fibres, or to the fibres having cells elsewhere for their origin, it is not thus that we can explain the presence of numerous cells which have no fibres developed from their processes.
145. With regard to this observation of the cell process running alongside of the fibre, the recent researches of Ranvier may throw some light on it. He describes the cells in the spinal ganglia as all unipolar; each single process pursues a more or less winding course as a fibril, often blending with others, till it reaches one of the fibres from the sensory root. It blends with this fibre at the annular constriction of the fibre, becoming here incorporated with it, so that a T-shaped fibre is the result.176 If this should be confirmed, it would reconcile many observations; but it would greatly disturb all current interpretations. Ranvier remarks that it is no longer tenable to suppose that the ganglionic cell is a centre, sensory or motor, receiving the excitation or sending forth a motor impulse; for if the fibril issuing from a cell becomes laterally soldered to a nerve-fibre, there is no possibility of saying in which direction this cell receives the excitation, nor in which it transmits the impulse.
146. We have seen good reason to conclude that the essential element of the nerve—the axis cylinder—is the same substance as the neuroplasm which forms the essential element of the cell. At any rate, we are quite certain that the cell process is neuroplasm. On this ground there is no difficulty in understanding that a cell process may sometimes be drawn out into an axis cylinder (as indeed we see to be the case in the invertebrata and electric fishes); while again in numerous other cases the nerve-fibre has an independent origin, being, in short, a differentiation from the neuroplasm which has become a fibre instead of a cell. It is clear from the observations of Rouget on Development, and of Sigmund Mayer on Regeneration, that fibres, nuclei, and cells become differentiated from the same neuroplasm, those portions which are not converted into fibres remaining first as lumps of neuroplasm, then acquiring a nucleus, and some of these passing into cells. I mean that between fibres, nuclei, and cells there are only morphological differences in an identical neuroplasm.177 If this is in any degree true, it will not only explain how fresh fibres may be developed in the course of fibres, branching from them as from trunks, and branchlets from branchlets, twigs from branchlets, the same conditions of growth being present throughout; it will also completely modify the notion of any physiological distinction between cell and fibre greater than can be assigned to the morphological differences. We shall then no longer suppose that the cell is the fountain whence the fibre draws its nutrition and its “force”; and this will be equally the case even if we admit that a cell is, so to speak, the germ from which a whole plexus of fibres was evolved, for no one will pretend that the “force” of an organism is directly derived from the ovum, or that the ovum nourishes the organism.
147. At this stage of the discussion it is needful to consider a point which will spontaneously occur to every instructed reader, I mean the interesting fact discovered by Dr. Waller, that when a sensory root was divided, the portion which was still in connection with the ganglion remained unaltered, whereas the portion which was only in connection with the spinal cord degenerated; and vice versa, when a motor root was divided, the portion connected with the cord remained unaltered, the portion severed from the cord degenerated. The observation has been frequently confirmed, and the conclusion drawn has been that the cells in the ganglion of the posterior root are the nutritive centres of posterior nerves, the cells in the anterior horn of the cord being the nutritive centres of the anterior nerves. Another interpretation is however needed, the more so because the fact is not constant.178 True of some nerves, it is not true of others. Vulpian found that when he cut out a portion of the lingual nerve, and transplanted it by grafting under the skin of the groin, where of course it was entirely removed from all ganglionic influence, it degenerated, but it also regenerated. Pathological observations convinced Meissner that the ganglia are wholly destitute of an influence on the nutrition of the vagus; and Schiff proved experimentally that other ganglia were equally inoperative, since motor nerves could be separated from the spinal cord without degeneration.179 Not however to insist on this, nor on the other facts of regeneration, in the absence of ganglionic influence, let us remark that Dr. Waller’s examples would not be conclusive unless the teaching of Embryology could be disproved. That nerves degenerate when separated from ganglia is a fact; but it is also a fact that muscles degenerate when separated from a nerve-centre; yet we do not suppose the nerve-centre to nourish the muscles. And against the fact that the sensory nerve remains unaltered only in that portion which is connected with the ganglion, we must oppose the observations of Kölliker and Schwalbe,180 who affirm that none of the fibres which enter the posterior columns of the spinal cord have any direct connection with the cells of the ganglion on the posterior root. The cells of this ganglion they declare to be unipolar (in the higher vertebrates), and the fibres in connection with these cells are not those which pass to the cord, but all of them pass to the periphery. According to Ranvier, the fibres from the cells join the fibres of the posterior root. Schwalbe found that if the spinal nerve be firmly grasped and steadily drawn, it will often be pulled from its sheath, and the ganglion laid bare;181 in this ganglion all the cells are found undisturbed, which could not be the case had fibres from those cells entered the cord, since the traction would necessarily have disturbed them.
Recapitulation
148. At the opening of this chapter mention was made of the besetting sin of the analytical tendency, namely, to disregard the elements which provisionally had been set aside, and not restore them in the reconstruction of a synthetical explanation. Familiar experiences tell us that a stimulus applied to the skin is followed by a muscular movement, or a glandular secretion; sometimes this takes place without any conscious sensation; sometimes we are distinctly conscious of the stimulus; and sometimes we consciously will the movement. These facts the physiologist tries to unravel, and to trace the complicated processes involved. The neurologist of course confines himself exclusively to the neural processes; all the other processes are provisionally left out of account. But not only so: the analytical tendency is carried further, and even in the neural process the organs are neglected for the sake of the nervous tissue, and the nervous tissue for the sake of the nerve-cell. The consequence has been that we have an explanation offered us which runs thus:—
149. The nerve-cell is the supreme element, the origin of the nerve-fibre, and the fountain of nerve-force. The cells are connected one with another by means of fibres, and with muscles, glands, and centres also by means of fibres, which are merely channels for the nerve-force. A stimulus at the surface is carried by a sensory fibre to a cell in the centre; from that point it is carried by another fibre to another cell; and from that by a third fibre to a muscle: a reflex contraction results. This is the elementary “nervous arc.” But this arc has also higher arcs with which it is in connection: the sensory cell besides sending a fibre directly to a motor cell, also sends one upwards to the cerebral centres; and here again there is a nervous arc, so that the cerebral centre sends down an impulse on the motor cells, and the contraction which results is due to a volitional impulse. The transmission of the stimulation which in the first case was purely physical, becomes in the latter case psychical. The sensory impression is in one cell transformed into a sensation, in another cell into an idea, in a third cell into a volition.
150. This course is described with a precision and a confidence which induces the inexperienced reader to suppose that it is the transcript of actual observation. I venture to say that it is imaginary from beginning to end. I do not affirm that no such course is pursued, I only say no such course was ever demonstrated, but that at every stage the requisite facts of observation are either incomplete or contradictory. First, be it noted that the actions to be explained are never the actions of organs so simple as the description sets forth. It is not by single fibres and cells that the stimulus is effected, but by complex nerves and complex centres. Only by a diagrammatic artifice can the fibre represent the nerve, and the cell the centre. In reality the cells of the centre (supposing them to be the only agents) act in groups, and Anatomy should therefore show them to be mutually united in groups—which is what no Anatomy has succeeded in showing, unless the Neuroglia be called upon. Secondly, be it noted that the current scheme of the relations between cells and fibres is one founded on physiological postulates, not on observation. Thirdly, much of what is actually observed is very doubtful, because we do not know whether the appearances are normal, or due to modes of preparation and post-mortem changes. We cannot at present say, for instance, whether the fibrillated appearance of cell contents and axis cylinder represents the living structure or not. We may either suppose that the neuroplasmic pulp splits longitudinally into fibres, or that neuroplasmic threads resolve themselves into a homogeneous pulp—the axis cylinder may be a condensation of many fibrils, or the fibrils may be a resolution of the substance.
151. Let us contrast step by step the Imaginary Anatomy found in the text-books with the Objective Anatomy as at present disclosed by the researches of all the chief workers. Imaginary Anatomy assumes that the sensory fibre passes from a surface into the cells of the posterior horn of the spinal cord. Objective Anatomy sees the fibre pass into the gray substance, but declares that no direct entrance of a fibre into a cell is there visible.
Imaginary Anatomy assumes that from the sensory cells of the gray substance pass fibres in connection with the motor cells of the anterior horn, thus forming a direct channel through which the excitation of a sensory cell is transmitted to a motor cell. Objective Anatomy fails to discover any such direct channel—no such fibres are demonstrable.
Imaginary Anatomy assumes that from the motor cells issue fibres which descend to the muscles and glands, and carry there the motor impulses and the “mandates of the will.” Objective Anatomy fails to find at the utmost more than a probability that these cells are continued as fibres, a probability which is founded on the rare facts of cell processes having been seen extending into the roots of the nerves, and of a cell process having occasionally been seen elsewhere continuous with a dark-bordered fibre. Granting, however, that this probability represents the fact, we have thus only one part of the “nervous arc” which can be said to have been verified.
Imaginary Anatomy further assumes that this nervous arc is connected with cerebral centres by means of fibres going upwards from the posterior cells, and fibres descending downwards to the anterior cells. Objective Anatomy sees nothing of the kind. It sees fibres entering the gray substance, and there lost to view in a mass of granular substance, fibrils, neuroblasts, and cells. There may be uninterrupted fibres passing upwards and downwards; but it is impossible to see them. And if we are told that physiological interpretations demand such a structure, we may fairly ask if this, and this only, is the structure which is adequate to the propagation of excitation? Now it seems to me that another kind of structure, and one more closely agreeing with what is observed, better answers the demands of Physiology. This will be more evident after the Laws of Nervous Action have been expounded in the succeeding chapter. Meanwhile we may remark that the arrangement of cells and fibres which is imagined as the mechanism of propagation and reflexion is absolutely irreconcilable with the teaching of Experiment: for the spinal cord may be cut through anywhere, without destruction of the transmission of sensory and motor excitations, provided only a small portion of gray substance be left to establish the continuity of the axis. Divide all the substance of the posterior half in one place, and all the substance of the anterior half in another, yet so long as there is a portion of gray substance left as a bridge between the lower and upper segments, the transmission of sensory and motor excitations will take place.
152. In other essential respects we have to note that the anatomical evidence for the current interpretations is absolutely deficient or contradictory. There is no adequate warrant for the assumption that all nerves have their origin in ganglia, all fibres in cells. Such evidence as at present exists is against that supposition, and in favor of the supposition that both cell and fibre are differentiations of a common neuroplasm, sometimes directly, sometimes indirectly continuous. Fibres, and plexuses of fibres, interspersed with cells irregularly distributed—now singly, now in small groups, now in larger and larger groups—constitute the figured elements of nerve-tissue; and even if we set aside the amorphous substance as indifferent or subordinate, we have still no ground for assigning the supremacy, much less the sole significance, to the cells. The grounds of this denial have been amply furnished in our exposition. For, let it be granted that nerve-cells are the origins of the fibres and the sources of their nutrition—a point which is eminently disputable—this would in no sense help the physiological hypothesis of the cell as the fountain of Neurility. If the fibre is simply the cell-contents drawn out longitudinally, if its essential element is identical with the essential element of the cell, then we can no more ascribe to the cell the exclusive property of Neurility than we can draw a lump of lead out into a wire, and then ascribe different properties to the thin end and the thick end. But on this point it is needless to speculate, since we have experimental evidence proving that the nerve-fibre has its Neurility even when separated from the cell, or even from the ganglion.
153. It is possible—I do not see sufficient evidence for a stronger assertion—that the cells are the nutritive sources of the fibres. They may represent the alimental rather than the instrumental activities of nervous life. (Compare Problem I. § 42.) My contention is that in any case they are not the supreme elements of the active tissue, and in no sense can they be considered as organs. Only confusion of ideas could for a moment permit such language, or could assign central functions to cells which are elements of tissue. If the cell be credited with such powers anywhere, it must be credited with them everywhere. Now I ask what conceivable central function can be ascribed to a cell which terminates the fibre in a peripheral ganglion, or which is merely an enlargement in the course of a fibre in a nerve-bundle? Besides the facts already adduced, let attention be called to this: If a nerve-bundle from the submucosa of the intestine be examined, there appear among the fibres many nuclei (neuroblasts), and occasionally cells, unipolar and bipolar. These cells—if we may trust the observations of Rouget on the earliest development of nerves, and of Sigmund Mayer on regenerated nerves—are simply more advanced stages of evolution of the neuroblasts; but whatever their genesis may be, there can be nothing in the nature of a central function assigned to them.
154. It may be asked, What part can we assign to cells in neural actions if they are apolar, unipolar, and even when multipolar, isolated from each other, and from fibres? I confess that I have no answer ready, not even an hypothesis. Until some rational interpretation of the cell be given we must be content to hold an answer in suspense. What I would urge is that we are precipitate in assuming that the anatomical connection between one element and another must necessarily be that of a fibre. In a semi-fluid substance, such as neurine, continuity may be perfect without solid fibres: the amorphous substance and the plasmode may as well transmit waves of molecular motion from one part of the tissue to another, and therefore from cell to cell, or from cell to fibre, as a figured substance may. When the posterior root enters the gray substance of the cord, there is no more necessity for its fibres passing directly into the cells of that gray substance, in order to excite their activity, than there is for a wire to pass from the bell to the ear of the servant, who hears the vibrations of the bell through the pulsations of the intervening air upon her tympanum. Look at the structure of the retina, or the cerebellum, and you will find that the ganglionic cells which have processes passing in a direction contrary to that whence the stimulus arrives, have none where continuity of fibre and cell would be indispensable on the current hypothesis. Light stimulates the rods and cones, but there are no nerve-fibres, hitherto discovered, passing from these to the ganglionic cells; instead of that there is a ground-substance thickly interspersed with granules and nuclei. From the cells we see processes issue; to the cells none are seen arriving. So with the cerebellum. The large cells send their processes upwards to the surface; but downwards towards the white substance the processes are lost in the granular layer, which most histologists regard as connective tissue.
155. A mere glance at nervous tissue in any part will show that cells are far from forming the principal constituents. In the epidermis or a gland the cell is obviously the chief element, forming the bulk of the tissue, and being the characteristic agent. In nerve-tissue, as in connective tissue, the reverse is the case. We must therefore cease to regard the cell as having the importance now attached to it, and must rather throw the emphasis on the fibres and neuroglia.
156. Before quitting this subject let a word be said on the amazing classification which has attained wide acceptance (although rejected by the most eminent authorities), founded on the size of the cells—the large multipolar cells being specified as motor, the smaller cells as sensory, while those of an intermediate size are sympathetic. I forbear to dwell on the development of this notion which specifies sensational, ideational, and emotional cells, because this does not pretend to have a basis in observation; whereas there are anatomical facts which give a certain superficial plausibility to the original classification. The conception is profoundly unphysiological; yet, if the anatomical evidence were constant, one might give it another interpretation. The evidence is, however, not constant. Large cells are found in regions assigned to sensory nerves, and small cells in motor regions. In the spinal cord of the tortoise Stieda declares that the so-called motor cells are limited to the cervical and lumbar enlargements; all the rest of the motor region being absolutely destitute of them.182 Again look at the cells of the retina—no one will assign motor functions to them—yet they are the same as those of the cerebellum and the anterior horns of the spinal cord. (It is worth a passing mention that the structure of the nervous parts of the retina more closely resembles that of the cerebellum than of the cerebrum.)
157. While our knowledge of the cell is thus far indeed from having the precision which the text-books display, and in no sense warrants the current physiological interpretations, our knowledge of fibres and neuroglia is also too incomplete for theoretic purposes. We know that the axis cylinder is the essential element; but we are still at a loss what part is to be assigned to the medullary sheath. There is indeed a popular hypothesis which pronounces it to be the means of insulating the fibre, and thus preserving the isolated conduction of nerve-force. Being of a fatty nature, this insulating office was readily suggested in agreement with the assumption that Neurility was Electricity. Now, without discussing whether Neurility is or is not Electricity, even admitting the former to be satisfactorily proved, I must remark that the admission still leaves the medullary sheath incapable of fulfilling the supposed office, since not only is there no such sheath in most of the invertebrates and in the sympathetic nerves of vertebrates, but even in those nerves which have the sheath it is precisely in places where the insulation would be most needed—namely, just before the terminations of the fibres in muscles and in centres—that the sheath is absent. This is as if we tried to conduct water through a pipe which fell short at both ends—before it left the cistern, and before it reached the spot to be watered. If there is a tendency in Neurility to spread wherever it is not insulated by a medullary sheath, then before reaching the centres and the muscles, it must, on the insulating hypothesis, dribble away!
158. The facts expressed in the “law of isolated conduction” are important, and are difficult of explanation; but it is obvious that they cannot be referred to the presence of the medullary sheath. Nor indeed will any insight into the propagation of stimulation through the central axis be intelligible until we have reformed our anatomical theories, and taken the Neuroglia into account. The theory which connects every fibre directly with a cell, and every cell with another by anastomosis—even were it demonstrated—would not explain the law of isolated conduction. Butzke cogently remarks183 that such a disposition of the elements should render all neural paths invariable; whereas the fact is that they are very variable. We learn to perform actions, and then we unlearn them; the paths are traversed now in one direction, now in another. Fluctuation is the characteristic of central combinations. And for this fluctuating combination of elements a corresponding diversity is required in the possible channels. This seems to be furnished by the network of the Neuroglia. See the representation copied from Butzke’s plate, and note how the cell-process blends with the meshes of the Neuroglia. Is it fanciful to regard this network of fibrils as having somewhat the relation of capillaries to blood-vessels? Did we not experimentally know that the capillaries are terminal blood-vessels, we should not suspect it from mere examination of the structure.
159. Having insisted that our knowledge is insufficient for any explanation of the “law of isolated conduction,” I can only suggest a path of research which may lead to some result. What we know is that some stimulations are propagated from one end of the cerebro-spinal axis to the other in definitely restricted paths, while others are irradiated along many paths. In the succeeding chapter this will be more fully considered; what we have here to note is that the manifold irradiations of a stimulation have an anatomical substratum in the manifold sub-divisions of the network of fibrils and the amorphous substance in which they penetrate.
160. In conclusion, I would say, let no one place a too great confidence in the reigning doctrines respecting the elementary structure of the nervous system, but accept every statement as a “working hypothesis” which has its value in so far as it links together verified facts, or suggests new research, but is wholly without value in so far as it is made a basis of deductions not otherwise verified. Hypotheses are indispensable to research, but they must be accompanied by vigilant scepticism. Imagination is only an enemy to Science when Scepticism is asleep.