EXPERIMENTS IN SECTION OF COVERED-EYED MEDUSÆ.
_Amount of Section which the Neuro-muscular Tissues of the
Medusæ will endure without suffering Loss of their Physiological
Continuity._
The extent to which the neuro-muscular tissues of the Medusæ may
be mutilated without undergoing destruction of their physiological
continuity is in the highest degree astonishing. For instance, to
begin with the covered-eyed Medusæ, I shall briefly state three modes
of section, the results of which serve to show in a striking manner
the fact in question.
[Illustration: Fig. 8.]
[Illustration: Fig. 9.]
The annexed woodcuts represent the umbrella of Aurelia aurita, with
its manubrium cut off at the base, and the under or concave surface
of the umbrella exposed to view, shewing in the centre the ovaries,
and radiating from them the branched system of nutrient tubes. The
umbrella when fully expanded, as here represented, is about the size
of a soup plate, and, as previously stated, all the marginal ganglia
are aggregated in the eight marginal bodies or lithocysts. Therefore
if the reader will imagine the first of the diagrams (Fig. 8) to
be overspread with a disc of muslin, the fibres and mesh of which
are finer than those of the finest and closest cobweb, and if he
will imagine the mesh of these fibres to start from these marginal
ganglia, he will gain a tolerably correct idea of the lowest nervous
system in the animal kingdom. Now, suppose that seven of these eight
ganglia are cut out, the remaining one then continues to supply its
rhythmical discharges to the muscular sheet of the bell, the result
being, at each discharge, two contraction waves, which start at the
same instant, one on each side of the ganglion, and which then course
with equal rapidity in opposite directions, and so meet at the point
of the disc which is opposite to the ganglion. Suppose, now, a number
of radial cuts are made in the disc according to such a plan as this
(Fig. 9), wherein every radial cut deeply overlaps those on either
side of it. The contraction waves which now originate from the
ganglion must either become blocked and cease to pass round the disc,
or they must zigzag round and round the tops of these overlapping
cuts. Now, remembering that the passage of these contraction waves
is presumably dependent on the nervous network progressively
distributing the ganglionic impulse to the muscular fibres, surely
we should expect that two or three overlapping cuts, by completely
severing all the nerve-fibres lying between them, ought to destroy
the functional continuity of these fibres, and so to block the
passage of the contraction wave. Yet this is not the case; for even
in a specimen of Aurelia so severely cut as the one here represented,
the contraction waves, starting from the ganglion, continued to
zigzag round and round the entire series of sections.
[Illustration: Fig. 10.]
The second mode of section to which I have alluded is as follows
(Fig. 10). The central circle (_x_) stands for an open space cut out
of the umbrella; the outer circle indicates the margin of the animal,
with all lithocysts save one (_l_) removed; and the median circular
line represents a cut. It will be seen that the effect of this cut is
almost completely to sever the mass of tissue at _z_ from the rest of
the umbrella, the only connection between them being the narrow neck
of tissue at _z_. Yet, in the case to which I refer, the contraction
waves emanating from _l_ passed in the directions represented by
the arrows without undergoing any appreciable loss of vigour. Upon
completing the circular cut at _z_, the ring of tissue (_y z_) became
totally paralyzed, while the outer circle, of course, continued
its contractions as before. Now, the neck of tissue at _z_ measured
only one-eighth of an inch across, while the ring of tissue (_y z_),
when cut through and straightened out upon the table, measured one
inch across and sixteen inches in length; that is to say, sixteen
square inches of tissue derived its impulse to vigorous contractions
through a channel one-eighth of an inch wide, notwithstanding that
the latter was situated at the furthest point of the circle from the
discharging lithocyst which the form of the section rendered possible.
[Illustration: Fig. 11.]
Lastly, the third mode of section is represented in the next cut.
Here seven of the marginal ganglia having been removed as before,
the eighth one was made the point of origin of a circumferential
section, which was then carried round and round the bell in the
form of a continuous spiral--the result, of course, being this long
ribbon-shaped strip of tissue with the ganglion at one end and the
remainder of the swimming-bell at the other. Well, as before, the
contraction-waves always originated at the ganglion; but now they
had to course all the way along the strip until they arrived at its
other extremity; and, as each wave arrived at that extremity, it
delivered its influence into the remainder of the swimming-bell,
which thereupon contracted. Now, in this experiment, when the spiral
strip is only made about half an inch broad, it may be made more than
a yard long before all the bell is used up in making the strip; and
as nothing can well be imagined as more destructive of the continuity
of a nerve-plexus than this spiral mode of section must be, we cannot
but regard it as a very remarkable fact that the nerve-plexus should
still continue to discharge its function. Indeed, so remarkable
does this fact appear, that to avoid accepting it we may well
feel inclined to resort to another hypothesis, namely, that these
contraction-waves do not depend for their passage on the nervous
network at all, but that they are of the nature of the muscle-waves,
or of the waves which we see in undifferentiated protoplasm,
where all parts of the mass being equally excitable and equally
contractile, however severely we cut the mass, as long as we do not
actually divide it, contraction-waves will pass throughout the whole
mass. But this very reasonable hypothesis of the contraction-waves in
the Medusæ being possibly nothing more than muscle-waves is negatived
by other facts, which I shall now proceed to state.
In the first place, if a number of experiments be tried in any of
the three modes of section above described, it will be found that
extreme variations are manifested as regards the degree of tolerance.
In the spiral mode of section, for instance, it will sometimes happen
that the contraction-wave will become blocked when the contractile
strip is only an inch long, while in other cases (as in the one
represented) the wave will continue to pass through a strip more
than thirty times that length; and between these two extremes there
are all possible grades of tolerance. Now it seems to me that if the
tissue through which these contraction-waves pass is supposed (as far
as they are concerned) to be of a functionally homogeneous nature,
no reason can be assigned why there should be such great differences
in the endurance of the tissue in different individual cases; while,
if we suppose that the passage of the contraction-waves is more or
less dependent on the functional activity of the nervous plexus which
we know from microscopical examination to be present, we encounter
no such difficulty; for it is almost to be expected that in some
cases it would happen that important nerves would soon be encountered
by the section, while in other cases it would happen that such
nerves would escape the section for a longer distance. It is indeed
incredible that any one nerve should happen to pursue a spiral course
twice or thrice round the umbrella, and at the same time happen to be
concentric with the course pursued by the section; but, as we shall
presently see, such an hypothesis as this is not necessary to account
for the facts.
Again, in the second place, strong evidence that the passage of
the contraction-waves is dependent on the functional activity
of the nervous plexus, and therefore that they are not merely
muscle-waves, is furnished by the fact that at whatever point in
a spiral strip which is being progressively elongated by section
the contraction-wave becomes blocked, the blocking is sure to
take place _completely_ and _exclusively_ at that point. Now, as
I have tried this experiment a great number of times, and always
tried it by carefully feeling the way round (_i.e._ only making a
very short continuation of the cut after the occurrence of each
contraction-wave, and so very precisely localizing the spot at
which the contraction-wave ceased to pass), I can scarcely doubt
that in every case the blocking is caused by the cutting through of
nerves.[13]
[13] In a highly interesting paper recently published by Dr. W.
H. Gaskell, F.R.S., on "The Innervation of the Heart" (_Journ.
of Physiol._, vol. iv. p. 43, _et seq._), it is shown that the
experiments in section thus far described yield strikingly
similar results when performed upon the heart of the tortoise and
the heart of the skate. Dr. Gaskell inclines to the belief that
in these cases the contraction-waves are merely muscle-waves.
There is one important fact, however, which even here seems to
me to indicate that the propagation of the wave is at least in
some measure dependent on nervous conduction. This fact is, that
after a contraction-wave has been blocked by the activity of a
spiral or other form of section, it may again be made to force a
passage under the influence of vagus stimulation. Moreover, in
a paper still more recently published by Drs. Brunton and Cash
on "Electrical Stimulation of the Frog's Heart" (_Proc. Roy.
Soc._, vol. xxxv., No. 227, p. 455, _et seq._) it is remarked,
"Another interesting consideration is, whether the stimulus
which each cavity of the heart transmits to the succeeding one
consists in the propagation of an actual muscular wave, or in
the propagation of an impulse along the nerves. The observations
of Gaskell have given very great importance to the muscular wave
occurring in each cavity of the heart of cold-blooded animals
as a stimulus to the contraction of the next succeeding cavity.
Our observations appear to us to show that, while this in an
important factor, it is not the only one in the transmission of
stimuli.... We consider that stimuli are also propagated from one
chamber of the heart to another through nervous channels: thus we
find that irritation of the venus sinus will sometimes produce
simultaneous contractions of the auricle and ventricle, instead
of the ventricular beat succeeding the auricular in the ordinary
way. This we think is hardly consistent with the hypothesis, that
a stimulus consists of the propagation of a muscular wave only
from the auricle to the ventricle."
[Illustration: Fig. 12.]
But, lastly, the strongest evidence in favour of this view as
afforded by the following observations. At the beginning of this
treatise I stated that the distinguishing function of nerve consists
in its power of conducting stimuli to a distance, irrespective of
the passage of a contraction-wave; and I may here add that when
a stimulus so conducted reaches a ganglion, or nerve-centre, it
causes the ganglion to discharge by so-called "reflex action."
Now, this distinguishing function of nerve can plainly be proved
to be present in the Medusæ. For instance, take such a section of
Aurelia as this one (Fig. 12), wherein the bell has been cut into
the form of a continuous parallelogram of tissue with the ovaries
and a single remaining ganglion at one end. (The cuts interposed in
the parallelogram may, for the present, be disregarded.) Now, if
the end marked _a_ of the neuro-muscular sheet most remote from
the ganglion be gently brushed with a camel's hair brush--_i.e._
too gently to start a responsive contraction-wave--the ganglion at
the other end will shortly afterwards discharge, as shown by its
starting a contraction-wave at its own end of the parallelogram,
_b_; thus proving that the stimulus caused by brushing the tissue at
the other end, _a_, must have been conducted all the way along the
parallelogram to the terminal ganglion, _b_, so causing the terminal
ganglion to discharge by reflex action. Indeed, in many cases, the
passage of this nervous wave of stimulation admits of being _seen_.
For the numberless tentacles which fringe the margin of Aurelia are
more highly excitable than is the general contractile tissue of the
bell; so that on brushing the end _a_ of the parallelogram remote
from the ganglion, the tentacles at this end respond to the stimulus
by a contraction, then those next in the series do the same, and so
on--a wave of contraction being thus set up in the tentacular fringe,
the passage of which is determined by the passage of the nervous wave
of stimulation in the superjacent nervous network. This tentacular
wave is in the illustration represented as having traversed nearly
half the whole distance to the terminal ganglion, and when it reaches
that ganglion it will cause it to discharge by reflex action, so
giving rise to a visible wave of muscular contraction passing in the
direction _b a_, opposite to that which the nervous or tentacular
wave had previously pursued. Now this tentacular wave, being an
optical expression of a passage of a wave of stimulation, is a sight
as beautiful as it is unique; and it affords a first-rate opportunity
of settling this all-important question, namely, Will this conductile
or nervous function prove itself as tolerant towards a section of
the tissue as the contractile or muscular function has already
proved itself to be? For, if so, we shall gain nothing on the side
of simplicity by assuming that the _contraction_-waves are merely
muscle-waves, so long as the _conduction_ or _undoubtedly nervous_
waves are equally able to pass round sections interposed in their
path. Briefly, then, I find that the nervous waves of stimulation
are quite as able to pass round these interposed sections as are the
waves of contraction. Thus, for instance, in this specimen (Fig. 12),
the tentacular wave of stimulation continued to pass as before, even
after I had submitted the parallelogram of tissue to the tremendously
severe form of section which is represented in the illustration;
and this fact, in my opinion, is one of the most important that has
been brought to light in the whole range of invertebrate physiology.
For what does it prove? It proves that the distinguishing function
of nerve, where it first appears upon the scene of life, admits of
being performed vicariously to almost any extent by all parts of
the same tissue-mass. If we revert to our old illustration of the
muslin as representing the nerve-plexus, it is clear that, however
much we choose to cut the sheet of muslin with such radial or spiral
sections as are represented in the illustrations, one could always
trace the threads of the muslin with a needle round and round the
disc, without once interrupting the continuity of the tracing; for
on coming to the end of a divided thread, one could always double
back on it and choose another thread which might be running in the
required direction. And this is what we are now compelled to believe
takes place in the fibres of this nervous network, if we assume that
these visible fibres are the only conductile elements which are
present. Whenever a stimulus wave reaches a cut, we must conclude
that it doubles back and passes into the neighbouring fibres, and so
on, time after time, till it succeeds in passing round and round any
number of overlapping cuts.
This is, no doubt, as I have already observed, a very remarkable
fact; but it becomes still more so when we have regard to the
histological researches of Professor Schäfer on the structural
character of this nerve-plexus. For these researches have shown that
the nerve-fibres which so thickly overspread the muscular sheet of
Aurelia do not constitute a true plexus, but that each fibre is
comparatively short and nowhere joins with any of the other fibres;
that is to say, although the constituent fibres of the network
cross and recross one another in all directions--sometimes, indeed,
twisting round one another like the strands of a rope--they can
never be actually seen to join, but remain anatomically insulated
throughout their length. So that the simile by which I have
represented this nervous network--the simile, namely, of a sheet
of muslin overspreading the whole of the muscular sheet--is, as a
simile, even more accurate than has hitherto appeared; for just as
in a piece of muslin the constituent threads, although frequently
meeting one another, never actually coalesce, so in the nervous
network of Aurelia, the constituent fibres, although frequently in
contact, never actually unite.
Now, if it is a remarkable fact that in a fully differentiated
nervous network the constituent fibres are not improbably capable
of vicarious action to almost any extent, much more remarkable does
this fact become when we find that no two of these constituent
nerve-fibres are histologically continuous with one another. Indeed,
it seems to me we have here a fact as startling as it is novel. There
can scarcely be any doubt that _some_ influence is communicated from
a stimulated fibre _a_ to the adjacent fibre _b_ at the point where
these fibres come into close apposition. But what the nature of the
process may be whereby a disturbance in the excitable protoplasm of
_a_ sets up a sympathetic disturbance in the anatomically separate
protoplasm of _b_, supposing it to be really such--this is a question
concerning which it would as yet be premature to speculate. But I
think it may be well for physiologists to keep awake to the fact that
a process of this kind probably takes place in the case of these
nerve-fibres. For it thus becomes a possibility which ought not to
be overlooked, that in the fibres of the spinal cord, and in ganglia
generally, where histologists have hitherto been unable to trace any
anatomical or structural continuity between cells and fibres, which
must nevertheless be supposed to possess physiological or functional
continuity--it thus becomes a possibility that in these cases no such
anatomical continuity exists, but that the physiological continuity
is maintained by some such process of physiological induction as
probably takes place among the nerve-fibres of Aurelia.[14]
[14] That it can scarcely be _electrical induction_ would seem
to be shown by the fact that such effects can only be produced
on nerves by strong currents, and also by the fact that the
saline tissues of the swimming-bell must short-circuit any feeble
electrical currents as soon as they are generated.
I have now to detail another fact of a very puzzling nature, but
one which is certainly of importance. When the spiral section
is performed on Aurelia aurita, and when, as a consequence, the
contraction-waves which traverse the elongating strip become at some
point suddenly blocked, if the section be stopped at this point it
not unfrequently happens that after a time the blocking suddenly
ceases, the contraction-waves again passing from the strip into
the umbrella as freely as they did before the section reached the
point at which the blocking occurred. The time required for this
restoration of physiological continuity is very variable, the limits
being from a few seconds to an hour or more; usually, however, it
is from two to four minutes. This process of re-establishing the
physiological connections, although rapid, is not so instantaneous
as is that of their destruction by section. In general it requires
the passage of several contraction-waves before the barrier to the
passage of succeeding waves is completely thrown down. The first
wave which effects a passage appears to have nearly all its force
expended in overcoming the barrier, the residue being only sufficient
to cause a very feeble, and sometimes almost imperceptible,
contraction of the umbrella. The next wave, however, passes across
the barrier with more facility, so that the resulting contraction
of the umbrella is more decided. The third wave, again, causes a
still more pronounced contraction of the umbrella; and so on with
all succeeding waves, until every trace of the previous blocking has
disappeared. When this is the case, it generally happens that the
strip will again admit of being elongated for a short distance before
a blocking of the contraction-waves again supervenes. Sometimes it
will be found that this second blockage will also be overcome, and
that the strip will then admit of being still further elongated
without the passage of the waves being obstructed; and so on
occasionally for three or four stages.
The same series of phenomena may be shown in another way. If
a contractile strip of tolerable length be obtained, with the
waves passing freely from one end to the other, and if a series
of parallel and equidistant cuts be made along one side of the
strip, in a direction at right angles to the length, and each cut
extending two-thirds of the breadth of the strip, the chances are
in favour of the contraction-waves being wholly unaffected by the
sections, however numerous these may be. But now, if another series
of parallel and equidistant cuts of the same length as the first
ones, and alternating with them, be made along the other side
of the contractile strip, the result is, of course, a number of
interdigitating cuts; and it is easy to see that by beginning with a
few such cuts and progressively increasing their number, a point must
somewhere be reached at which one portion will become physiologically
separated from the rest. The amount of such section, however, which
contractile strips will sometimes endure is truly surprising. I
have seen such a strip twenty inches long by one and a half inches
wide with ten such cuts along each side, and the contraction-waves
passing without impediment from end to end. But what I wish more
especially to observe just now is, that by progressively increasing
the number of such interdigitating cuts up to the point at which the
contraction-wave is blocked, and then leaving the tissue to recover
itself, in many cases it will be observed that the blocking is sooner
or later overcome; that on then adding more interdigitating cuts the
blocking again supervenes; but that in time it may again be overcome,
and so on. It is, however, comparatively rare to find cases in which
blocking is overcome twice or thrice in succession.
Section is not the only way in which blocking of waves may be caused
in contractile strips. I find that pressure, even though very gentle,
exerted on any part of a strip causes a blocking of the waves at that
part, even after the pressure has been removed. If the pressure has
been long continued, after its removal the blocking will probably
be permanent; but if the pressure has been only of short duration,
the blocking will most likely be transitory. Even the slight strains
caused by handling a contractile strip in the air are generally
followed by a decrease in the rate of the waves, and sometimes by
their being completely blocked. Other methods by which the passage of
waves in contractile strips admits of being blocked will be alluded
to farther on.
Now, in all these cases of temporary blocking we must conclude that
when the contraction-waves succeed in at last forcing a passage,
some structural change has taken place in the tissue at the
region of injury, corresponding with the functional change of the
re-establishment of physiological continuity. The waves previously
stopped at a certain point of section or otherwise, after beating for
a time on the physiological barrier, are at last able to throw down
the barrier, and thenceforward to proceed on their way unhindered.
What, then, is the nature of the structural change which has taken
place?
In the early days of this research, before the presence of a
nerve-plexus had been proved histologically, I argued in favour of
such a plexus on the grounds furnished by many of the foregoing
experiments; and at a lecture given in the Royal Institution I
ventured to say that if a careful investigation of the histology of
these tissues should fail to show the plexus which the result of
those experiments required me to assume, we should still be compelled
to suppose that the plexus was present, although not sufficiently
differentiated to admit of being seen. I further ventured to
suggest that in this event the facts just stated might be taken to
substantiate the theory of Mr. Herbert Spencer on the genesis of
nerve-tissue in general. This theory is that which supposes incipient
conductile tissues, or rudimentary nerve-fibres, to be differentiated
from the surrounding contractile tissues, or homogeneous protoplasm,
by a process of integration which is due simply to use; so that just
as water continually widens and deepens the channel through which
it flows, so molecular or nervous waves of stimulation, by always
flowing through the same tissue-tracts, tend ever more and more to
excavate for themselves functionally differentiated lines of passage.
Such being Mr. Spencer's theory, I applied it hypothetically to the
above facts in the words which I may here quote.
"As the successive waves beat rhythmically on the area of
obstruction, more or less of the molecular disturbances must every
time be equalized through these lines of discharge, which from the
first have been almost sufficient to maintain the physiological
continuity of the tissue. Therefore, according to the hypothesis,
every wave that is blocked imposes upon these particular lines of
discharge a much higher degree of functional activity than they were
ever before required to exercise; and this greater activity causing
in its turn greater permeability, a point will sooner or later arrive
at which these lines of discharge, from having been _almost_, become
_quite_ able to draft off sufficient molecular motion, or stimulating
influence, to carry on the contraction-waves beyond the areas of
previous blocking. In such instances, of course, we should expect
to find what I always observed to be the case, viz. that the first
contraction-wave which passes the barrier is only very feeble, the
next stronger, the next still stronger, and so on, according as the
new passage becomes more and more permeable by use, until at last
the contraction-waves pour over the original barrier without any
perceptible diminution of their force. In some cases, by exploring
with graduated stimuli and needle-point terminals, I was able to
ascertain the precise line through which this eruption of stimulating
influence had taken place."
I have now to state the effect upon this hypothesis which in my
opinion has been produced by the histological proof that the plexus
in question is composed of fully differentiated nerves. Briefly,
then, I think that the hypothesis still holds to the extent of
being the only one available whereby to explain the facts; but
there is this great difference, viz. that the hypothesis need not
now be applied to the genesis of nerve-tissue out of comparatively
undifferentiated contractile tissue, but rather to the increasing of
the functional activity of already well-differentiated nerve-tissue.
In other words, we have not now to suppose that nerve-tissue is
formed _de novo_ in the region of blocking; but, in my opinion,
we still have to suppose that the nerve-fibres which were already
there have their functional capabilities so far improved by the
greater demand imposed upon them, that whereas at first they were
not able, eventually they became able to draft off enough molecular
disturbance to carry on a stimulus adequate to cause a muscular
contraction. It will be observed that the difference thus expressed
is one of considerable importance; for now the facts cease to lend
any countenance to Mr. Spencer's theory touching the formation
of nerves out of protoplasm, or other contractile material. They
continue, however, to countenance his views touching the improvement
of functional capacity which nerve-fibres, when already formed,
undergo by use; and this, which is in itself an important matter, is
the point with which I was mainly concerned in the lecture of the
Royal Institution just alluded to. For, as I then observed, in this
theory of nerve-fibres becoming more and more functionally developed
by use, we probably have a physical explanation, which is as full and
complete as such an explanation can ever be, of the genesis of mind.
"For from the time that intelligence first dawned upon the scene of
life, whenever a new relation had to be established in the region of
mind, it could only be so established in virtue of some new line of
discharge being excavated through the substance of the brain. The
more often this relation had to be repeated in the mind, the more
often would this discharge require to take place in the brain, and
so the more easy would every repetition of the process become....
Thus it is, according to the theory, that there is always a precise
proportion between the constancy with which any relations have
been joined together during the history of intelligence, and the
difficulty which intelligence now experiences in trying to conceive
of such relations as disjoined. Thus it is that, even during the
history of an individual intelligence, 'practice makes perfect,' by
frequently repeating the needful stimulations along the same lines
of cerebral discharge, so rendering the latter ever more and more
permeable by use. Thus it is that a child learns its lessons by
frequently repeating them; and thus it is that all our knowledge is
accumulated."[15]
[15] I have associated the above theory of nerve-genesis with the
name of Mr. Spencer, because it occupies so prominent a place in
his "Principles of Psychology." But from what I have said in the
text, I think it is clear that the theory, as presented by Mr.
Spencer, consists of two essentially distinct hypotheses--the
one relating to the formation of nerve-tissue out of protoplasm,
and the other to the increase of functional capacity in a
nerve-fibre by use (a third hypothesis of Mr. Spencer relating to
the formation of ganglion-tissue does not here concern us). The
latter hypothesis, however, ought not to be associated with Mr.
Spencer's name without explaining that it has likewise occurred
to other writers, the first of which, so far as I can ascertain,
was Lamarck, who says, "Dans toute action, le fluide des nerfs
qui la provoque, subit un mouvement de déplacement qui y donne
lieu. Or, lorsque cette action a été plusieurs fois répétée, il
n'est pas douteux que le fluide qui l'a exécutée, ne se soit
frayé une route, qui lui devient alors d'autant plus facile à
parcourir, qu'il l'a effectivement plus souvent franchie, et
qu'il n'ait lui-même une aptitude plus grand à suivre cette route
frayée que celles qui le sont moins." ("Phil. Zool.," tom ii. pp.
318-19.)
_Rate of Transmission of Stimuli._
The rate at which contraction-waves traverse spiral strips of Aurelia
is variable. It is largely determined by the length and width of
the strip; so that the best form of strip to use for the purpose
of ascertaining the _maximum_ rate is one which I shall call the
circular strip. A circular strip is obtained by first cutting out
the central bodies (_i.e._ manubrium and ovaries), and then, with a
single radial cut, converting the animal from the form of an open
ring to that of a continuous band. I distinguish this by the name
"circular" band or strip, because the two ends tend to preserve their
original relative positions, so giving the strip more or less of a
circular form. Such a strip has the advantage of presenting all the
contractile tissue of the swimming-bell in one continuous band of
the greatest possible width, and is therefore the form of strip that
yields the _maximum_ rate at which contraction-waves are able to
pass. The reason why the _maximum_ rate should be the one sought for
is because this is the rate which must most nearly approximate the
natural rate of contraction-waves in the unmutilated animal. This
rate, at the temperature of the sea and with vigorous specimens, I
find to be eighteen inches per second.
In a circular strip the rate of the waves is uniform over the whole
extent of the strip; so that the time of their transit from one
point to another varies directly as the length of the strip. But
on now narrowing such a strip, although the rate is thus slowed,
the relation between the narrowing and the slowing is not nearly so
precise as to admit of our saying that the rate varies inversely as
the width. The following figure will serve to show the proportional
extent to which the passage of contraction-waves is retarded by
narrowing the area through which they pass:--
[Illustration: Fig. 13.
Time from end to end of a circular strip ...
Time after width has been reduced to one-half ...
Time after width has been reduced to one-quarter ...
Time after width has been reduced to one-eighth ...
Fig. 14.
In such experiments it generally happens, as here represented,
that reducing the width of a circular strip by one-half produces
no effect, or only a slight effect, on the rate, while further
narrowing to the degree mentioned produces a conspicuous
effect. I may also state that if, as occasionally happens, the
immediate effect of narrowing a circular strip to one-half is to
temporarily block the contraction-waves, when the latter again
force their passage, their rate is slower than it was before. It
seems as if the more pervious tissue tracts having been destroyed
by the section, the less pervious ones, though still able to
convey the contraction-wave, are not able to convey it so rapidly
as were the more pervious tracts.]
[Illustration: Fig. 14.]
In order to ascertain whether certain zones of the circular
contractile sheet in all individuals habitually convey more of
the contractile influence than do other zones, I tried a number
of experiments in the following form of section. Having made a
circular strip, I removed all the lithocysts save one, and then cut
the strip as represented in Fig. 14. On now stimulating the end
_a_, or on watching the lithocyst there discharge, the resulting
contraction-wave would be observed to bifurcate at _b_, and then pass
on as two separate waves through the zones, _b_ _c_, _b_ _d_. Now,
as these two waves were started at the same instant of time, they
ran, as it were, a race in the two zones, and in this way the eye
could judge with perfect ease which wave occupied the shortest time
in reaching its destination. This experiment could be varied by again
bisecting each of these two zones, thus making four zones in all, and
four waves to run in each race. A number of experiments of this kind
showed me that there is no constancy in the relative conductivity
of the same zones in different individuals. In some instances, the
waves occupy less time in passing through the zone _b_ _c_ than in
passing through the zone _b_ _d_; in other instances, the time in the
two zones is equal; and, lastly, the converse of the first-mentioned
case is of equally frequent occurrence. Very often the waves become
blocked in _b_ _c_, while they continue to pass in _b_ _d_, and
_vice versâ_. Now, all these various cases are what we might expect
to occur, in view of the variable points at which contraction-waves
become blocked in spiral strips, etc.; for if the contractile tissues
are not functionally homogeneous, and if the relatively pervious
conductile tracts are not constant as to their position in different
individuals, the results I have just described are the only ones
that could be yielded by the experiments in question. Considering,
however, that in these experiments the central zones are not so long
as the peripheral zones, I think it may fairly be said that the
conductile power of the latter is greater than that of the former;
for, otherwise, the above experiments ought to yield a large majority
of races won by the waves that course through the central zones,
and this is not the case. Indeed, it is surprising how often the
race is, as it were, neck and neck, thus showing that the relative
conductivity of all the zones is precisely adjusted to their relative
lengths; and forasmuch as in the unmutilated animal this adjustment
must clearly serve the purpose of securing to the contraction-wave
a passage of uniform rate over the whole radius of the umbrella,
I doubt not that, if it were possible to perform the race-course
section without interrupting any of the lines of conduction-tissue,
neck and neck races would be of invariable occurrence.
Interdigitating cuts, as might be expected, prolong the time of
contraction-waves in their passage through the tissue in which
the cuts are interposed. For example, in a spiral strip measuring
twenty-six inches in length, the time required for the passage of
a contraction-wave from one end to the other is represented by the
line _a_ _b_ in the annexed woodcut. But after twenty interdigitating
cuts had been interposed, ten on each side of the strip, the time
increased to _c_ _d_, the line _e_ _f_ representing one second. And
more severe forms of section are, of course, attended with a still
more retarding influence.
[Illustration: Fig. 15.]
The effects of temperature on the rate of contraction-waves are
very striking. For instance, in a rather narrow strip measuring
twenty-eight inches long and one and a half inches wide, the
following variations in rate occurred:--
-------------------------+-------------------------------
Temperature of water. | Time occupied in passage of
| contractile waves.
-------------------------+-------------------------------
26° | 4 seconds.
32° | 3 seconds.
42° | 2-2/5 seconds.
65° | 2 seconds.
75° | 1-3/5 seconds.
85° | Blocked.
Or, adopting again the graphic method of statement, these variations
may be represented as follows:--
[Illustration: Fig. 16.
26°......
32°......
42°......
65°......
75°......
85°......
One second ...]
Submitting a contractile strip to slight strains has also the
effect of retarding the rate of the waves while they pass through
the portions of the strip which have been submitted to strain. The
method of straining which I adopted was to pass my finger below
the strip, and then, by raising my hand, to bring a portion of
the strip slightly above the level of the water. The irritable or
contractile surface was kept uppermost, and therefore suffered a
gentle strain; for the weight of the tissue on either side of the
finger made the upper surface somewhat convex. By passing the finger
all the way along the strip in this way, the latter might be gently
strained throughout its entire length, the degree of straining
being determined by the height out of the water to which the tissue
was raised. Of course, if the strip is too greatly strained, the
contraction-waves become blocked altogether, as described above;
but shortly before this degree of straining was reached, I could
generally observe that the rate of the waves was diminished. To give
one instance, a contractile strip measuring twenty-two inches had
the rate of its waves taken before and after straining of the kind
described. The result was as follows:--
[Illustration: Fig. 17. Before straining ...
After straining ...
One second ... ]
Immediately after severe handling of this kind, the retardation
of contraction-waves, is sometimes even more marked than here
represented; but I think this may be partly due to shock, for on
giving the tissue a little while to recover, the rate of the waves
becomes slightly increased.
Anæsthetics likewise have the effect of slowing the rate of
contraction-waves before blocking them. Taking, for instance, the
case of chloroform, a narrow spiral strip between one and two feet
long was immersed in sea-water containing a large dose of the
anæsthetic; the observations being taken at six seconds' intervals,
the following were the results:--
[Illustration: Fig 18.
Normal water ...
Six seconds after transference to chloroform
Six seconds later ...
Six seconds later ...
Six seconds later ...
Six seconds later ...
One second ... ]
In such experiments, the recovery of the normal rate in unpoisoned
water is gradual. Taking, for instance, the case of a spiral strip
in morphia (Fig. 19), it will be seen that the original rate did not
fully return. Some substances, however, exert a more marked permanent
effect of this kind than do weak solutions of morphia. Here, for
instance, is an experiment with alcohol (see Fig. 20).
[Illustration: Fig. 19
In normal water ...
Quarter of an hour after exposed to morphia ...
One minute after strengthening dose ...
Four minutes later, and just before blocking of wave ...
Fifteen seconds later, wave continuing blocked ...
Immediately after passage of wave on restoration to normal
sea-water ...
Four minutes later ...
Quarter of an hour later ...
An hour later ...
One second ... ]
[Illustration: Fig. 20
In normal water ...
Quarter of an hour after exposure to weak dose ...
Two minutes after strengthening of dose ...
Five minutes later, and just before blocking of wave ...
Fifteen seconds later, wave continuing blocked ...
Immediately after passage of wave on restoration to normal
sea-water ...
An hour later ...
One second ... ]
From these experiments, however, it must not be definitely concluded
that it is the anæsthesiating property of such substances which
exerts this slowing and blocking influence on contraction-waves,
for I find that almost any foreign substance, whether or not an
anæsthetic, will do the same. That nitrite of amyl, caffein, etc.,
should do so, one would not be very surprised to hear; but it might
not so readily be expected that strychnine, for instance, should
block contraction-waves; yet it does so, even in doses so small as
only just to taste bitter. Nay, even fresh water completely blocks
contraction-waves after the strip has been exposed to its influence
for about half an hour, and exerts a permanently slowing effect after
the tissue is restored to sea-water. These facts show the extreme
sensitiveness of the neuro-muscular tissues of the Medusæ to any
change in the character of their surrounding medium, a sensitiveness
which we shall again have occasion to comment upon when treating of
the effects of poisons.
In conclusion, I may mention an interesting fact which is probably
connected with the summation of stimuli before explained. When a
contractile strip is allowed to rest for a minute or more, and
when a wave is then made to traverse it, careful observation will
show that the passage of the first wave is slower than that of its
successor, provided the latter follows the former after not too great
an interval of time. The difference, however, is exceedingly slight,
so that to render it apparent at all the longest possible strips
must be used, and even then the experimenter may fail to detect the
difference, unless he has been accustomed to signalling, by which
method all these observations on rate have to be made.
_Stimulus-waves._
The rate of transmission of tentacular waves is only one-half that of
contraction-waves, viz. nine inches a second. This fact appeared to
me very remarkable in view of the consideration that the tentacular
wave is the optical expression of a stimulus-wave, and that there can
be no conceivable use in a stimulus-wave being able to pass through
contractile tissue independently of a contraction-wave, unless the
former is able to travel more rapidly than the latter; for the only
conceivable use of the stimulus-wave is to establish physiological
harmony between different parts of the organism, and if this wave
cannot travel more rapidly than a contraction-wave which starts from
the same point, it would clearly fail to perform this function.
In view of this anomaly, I was led to think that if the rate of the
stimulus-wave is dependent in a large degree on the strength of the
stimulus that starts it, the slow rate of nine inches a second might
be more than doubled, if, instead of using a stimulus so gentle as
not to start a contraction-wave, I used a stimulus sufficiently
strong to do this. Accordingly I chose a specimen of Aurelia wherein
the occurrence of tentacular waves was very conspicuous, and found,
as I had hoped, that every time I stimulated too gently to start a
contraction-wave, the tentacular wave travelled only at the rate
of nine inches a second; whereas, if I stimulated with greater
intensity, I could always observe the tentacular wave coursing an
inch or two in front of the contraction-wave.
It is remarkable, however, that in this, as in all the other
specimens of Aurelia which I experimented upon, the reflex response
of the manubrium was equally long, whatever strength of stimulus
I applied to the umbrella; or, at any rate, the time was only
slightly less when a contraction-wave had passed than when only a
tentacular wave had done so. The loss of time, however, appears to
take place in the manubrium itself, where the rate of response is
astonishingly slow. Thus, if one lobe be irritated, it is usually
from four to eight seconds before the other lobes respond. But the
time required for such sympathetic response may be even more variable
than this--the limits I have observed being as great as from three
to ten seconds. In all cases, however, the response, when it does
occur, is sudden, as if the distant lobe had then for the first
time received the stimulus. Moreover, one lobe--usually one of those
adjacent to the lobe directly irritated--responds before the other
two, and then a variable time afterwards the latter also respond.
This time is, in most cases, comparatively short, the usual limits
being from a quarter of a second to two seconds. How much of these
enormous intervals is occupied by the period of ganglionic latency,
and how much by that of transmission, it is impossible to say; but I
have determined that the rate of transmission from the end of a lobe
of the manubrium to a lithocyst (deducting a second for the double
period of latent stimulation) is the same as the rate of a tentacular
wave, viz. nine inches a second. The presumption, therefore, is that
the immense lapse of time required for reflex response on the part of
the manubrium is required by the lobular ganglia, or whatever element
it is that here performs the ganglionic function.
_Exhaustion._
In various modes of section of Aurelia I have several times observed
a fact that is worth recording. It sometimes happens that when the
connecting isthmus between two almost severed areas of excitable
tissue is very narrow, the passage of contraction-waves across the
isthmus depends upon the freshness, or freedom from exhaustion,
of the tissue which constitutes the isthmus. That is to say, on
faradizing one of the two tissue-areas which the isthmus serves
to connect, the resulting contraction-waves will at first pass
freely across the isthmus; but after a time it may happen in some
preparations that every now and then a contraction-wave fails to
pass across the isthmus. When this is the case, if the stimulation
is still continued, a greater and greater proportion of waves fail
to pass across the isthmus, until perhaps only one in every five or
six becomes propagated from the one area to the other. If single
induction-shocks be then substituted for the faradaic stimulation,
it may be found that by leaving an interval of four or five seconds
between the successive shocks, every wave that is started in the
one area will be propagated across the isthmus to the other area.
But if the interval between the successive shocks be reduced to
two or three seconds, every now and then a wave will fail to pass
across the isthmus; and if the interval be still further reduced to
one second, or half a second, comparatively few of the waves will
pass across. Now, however, if the tissue be allowed five minutes'
rest from stimulation, and the single shocks be thrown in at one
second's intervals, all the first six or ten waves will pass across
the isthmus, after which they begin to become blocked as before. It
may be observed also that when the waves are thus blocked, owing to
exhaustion of the connecting isthmus, they may again be made to force
a passage by increasing the intensity of the stimulation, and so
giving rise to stronger waves having a greater power of penetration.
Thus, on re-enforcing the electrical stimulus with the simultaneous
application of a drop of spirit, the resulting waves of contraction
are almost sure to pass across the isthmus, even though this has been
exhausted in the manner just described.
_Ganglia appearing to assert their Influence at a Distance from their
own Seat._
Another fact, which I have several times noticed during my sections
of Aurelia, also deserves to be recorded. I have observed it under
several modes of section, but it will be only necessary to describe
one particular case.
In the Aurelia of a portion of which the accompanying woodcut (p.
102) is a representation, seven of the lithocysts were removed,
while the remaining one was almost entirely isolated from the
general contractile tissue by the incisions _aa_, _bb_, _cc_. The
lithocyst continued to animate the tissue-area _xxxx_, and through
the connecting passage _y_ the contraction-waves spread over the
remainder of the sub-umbrella tissue _zzzz_. So far, of course, the
facts were normal; but very frequently it was observed that the
contraction-waves did not start from the lithocyst, or from the area
_xxxx_, but from the point o in the area _zz_. After this origination
of the contraction-waves from the point _o_ had been observed a great
number of times, I removed the lithocyst. The effect was not only
to prevent the further origination of contraction-waves in the area
_xxxx_, but also to prevent their further origination from the point
_o_, the entire umbrella thus becoming paralyzed. Hence, before the
removal of the lithocyst, the contraction-waves which originated at
the point _o_, no less than those which originated at the lithocyst
itself, must in some way or other have been due to the ganglionic
influence emanating from the lithocyst and asserting itself at the
distant point _o_.
[Illustration: Fig. 21.]
This property, which lithocysts sometimes present, of asserting their
ganglionic influence at a distance from their own locality, can only,
I think, be explained by supposing that at the point where under
these circumstances the contractions originate, there are situated
some scattered ganglionic cells of considerable functional power,
but yet not of power enough to originate contraction-waves unless
re-enforced by some stimulating influence, which reaches them from
the lithocyst through the nervous plexus.
_Regeneration of Tissues._
The only facts which remain to be stated in the present chapter
have reference to the astonishing rapidity with which the excitable
tissues of the Medusæ regenerate themselves after injury. In this
connection I have mainly experimented on Aurelia aurita, and shall,
therefore, confine my remarks to this one species.
If with a sharp scalpel an incision be made through the tenuous
contractile sheet of the sub-umbrella of Aurelia, in a marvellously
short time the injury is repaired. Thus, for instance, if such an
incision be carried across the whole diameter of the sub-umbrella,
so as entirely to divide the excitable tissues into two parts while
the gelatinous tissues are left intact, the result of course is that
physiological continuity is destroyed between the one half of the
animal and the other, while the form of the whole animal remains
unchanged--the much greater thickness of the uninjured gelatinous
tissues serving to preserve the shape of the umbrella. But although
the contractile sheet which lines the umbrella is thus completely
severed throughout its whole diameter, it again reunites, or heals
up, in from four to eight hours after the operation.