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.
[Illustration:
Fig. 16.--_Nerve-cell from anterior horn of spinal cord (man),
magnified 150 diameters._ _a_, cell process unbranched passing into
or joining an axis cylinder, the other processes are branched; _b_,
pigment. The nucleus and nucleolus are visible.
]
[Illustration: Fig. 17.--_Nerve-cell from the anterior gray substance
of the spinal cord of a calf magnified 600._ _a_, the axis cylinder;
_b_, the branched process. The neuroplasm is represented as distinctly
fibrillated, with granular substance interspersed. Nucleus and
nucleolus very distinct.]
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.
[Illustration: Fig. 18.--_Supposed union of two nerve-cells and a
fibre._ The processes subdivide into a minute network, in which the
fibre also loses itself.]
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.
[Illustration:
Fig. 19.--_Anastomosing nerve-cells_ (after Gratiolet). _a_, body
of the cell; _c_, process of uniting two cells; _d_, branching
process.
]
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.
[Illustration:
Fig. 20.--_a_, axis cylinder formed by the fibrils of the cell
contents, and at _a’_ assuming the medullary sheath; _b_, naked
axis cylinder from spinal cord.
]
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 sheath[154]--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.
[Illustration:
Fig. 21.--_Nerve-fibres from the white substance of the cerebrum._
_a_, _a_, _a_, the medullar contents pressed out of the tube as
irregular drops.
]
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.
[Illustration: Fig. 22.--_Cells, fibres, and amorphous substance from
the ganglion of a mollusc_ (after Bucholtz).]
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.
[Illustration:
Fig. 23--_Fibres from the auditory nerve._ _a_, the axis cylinder;
_b_, the cellular enlargement; _c_, the medullary sheath.
]
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.
[Illustration: Fig. 24.--_Embryonic nerve-cells._]
[Illustration: Fig. 25.--_Embryonic nerve-fibres._]
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
investigator[172] 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 remarks[183] 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.
[Illustration: Fig. 26.--_Nerve-cells with processes terminating in
neuroglia._]
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.