ARTIFICIAL RHYTHM.
If the umbrella of Aurelia aurita has been paralyzed by the
removal of its lithocysts, and if it is then subjected to faradaic
stimulation of minimal intensity, the response which it gives is not
tetanic, but rhythmic. The rate of this artificial rhythm varies
in different specimens, but the limits of variation are always
within those which are observed by the natural rhythm of different
specimens. The artificial rhythm is not in every case strictly
regular; but by carefully adjusting the strength of the current, and
by shifting the electrodes from one part of the tissue to another
until the most appropriate part is ascertained, the artificial rhythm
admits in most cases of being rendered tolerably regular, and in many
cases as strictly regular as is the natural rhythm of the animal. To
show this, I append a tracing of the artificial rhythm (Fig. 25),
which may be taken as a fair sample of the most perfect regularity
that can be obtained by minimal faradaic stimulation.[24]
[24] This and all the subsequent tracings I obtained by the
method already described.
[Illustration: Fig. 25.]
[Illustration: Fig. 26.]
This artificial rhythm may be obtained with a portion of irritable
tissue of any size, and whether a large or small piece of the tissue
employed be included between the electrodes.
As the fact of this wonderfully rhythmic response to faradaic
irritation was quite unexpected by me, and as it seemed to be a fact
of great significance, I was led to investigate it in as many of its
bearings as time permitted. First, I tried the effect on the rhythm
of progressively intensifying the strength of the faradaic current. I
found that with each increment of the current the rate of the rhythm
was increased, and this up to the point at which the rhythm began to
pass into tetanus due to summation of the successive contractions.
But between the slowest rhythm obtainable by minimal stimulation and
the most rapid rhythm obtainable before the appearance of tetanus,
there were numerous degrees of rate to be observed. I here append
another tracing, to show the effect on the rate of the rhythm of
alterations in the strength of the current (Fig. 26).
It will also be observed from this tracing that, in consequence of
the current having been strengthened slightly beyond the limit within
which strictly rhythmic response was attainable, the curves in the
middle part of the tracing, where the current was strengthened, are
slightly irregular. This irregularity is, of course, due to the first
appearance of tumultuous tetanus. If the faradaic stimulation had in
this case been progressively made still stronger, the irregularity
would have become still more pronounced up to a certain point, when
it would gradually have begun to pass into more persistent tetanus.
But as in this case, instead of strengthening the current still
further, I again weakened it to its original intensity, the rhythm
immediately returned to its original rate and regularity.
Such being the facts, the question arises as to their interpretation.
At first I was naturally inclined to suppose that the artificial
rhythm was due to a periodic variation in the strength of the
stimulus, caused by some slight breach of contact between the
terminals and the tissue on each contraction of the latter. This
supposition, of course, would divest the phenomena in question of
all physiological meaning, and I therefore took pains in the first
instance to exclude it. This I did in two ways: first, by observing
that in many cases (and especially in Cyanæa capillata) the rate of
the rhythm is so slow that the contractions do not follow one another
till a considerable interval of total relaxation has intervened; and
second, by placing the terminals close together, so as to include
only a small piece of tissue between, and then firmly pinning the
tissue all round the electrodes to a piece of wood placed beneath the
Medusæ. In this way the small portion of tissue which served as the
seat of stimulation was itself prevented from moving, and therefore
the rhythmic motions which the rest of the Medusa presented cannot
have been due to any variations in the quality of the contact between
the electrodes and this stationary seat of stimulation.
Any such merely mechanical source of fallacy being thus, I think,
excluded, we are compelled to regard the facts of artificial rhythm
as of a purely physiological kind. The question, therefore, as to
the explanation of these facts becomes one of the highest interest,
and the hypothesis which I have framed to answer it is as follows.
Every time the tissue contracts it must as a consequence suffer a
certain amount of exhaustion, and therefore must become slightly
less sensitive to stimulation than it was before. After a time,
however, the exhaustion will pass away, and the original degree of
sensitiveness will thereupon return. Now, the intensity of faradaic
stimulation which is alone capable of producing rhythmic response,
is either minimal or but slightly more than minimal in relation
to the sensitiveness of the tissue when fresh; consequently, when
this sensitiveness is somewhat lowered by temporary exhaustion, the
intensity of the stimulation becomes somewhat less than minimal
in relation to this lower degree of sensitiveness. The tissue,
therefore, fails to perceive the presence of the stimulus, and
consequently fails to respond. But so soon as the exhaustion is
completely recovered from, so soon will the tissue again perceive
the presence of the stimulus; it will therefore again respond, again
become temporarily exhausted, again fail to perceive the presence
of the stimulus, and again become temporarily quiescent. Now it is
obvious that if this process occurs once, it may occur an indefinite
number of times; and as the conditions of nutrition, as well as those
of stimulation, remain constant, it is manifest that the responses
may thus become periodic.
In order to test the truth of this hypothesis, I made the following
experiments. Having first noted the rate of the rhythm under faradaic
stimulation of minimal intensity, without shifting the electrodes
or altering the intensity of the current, I discarded the faradaic
stimulation, and substituted for it single induction shocks thrown
in with a key. I found, as I had hoped, that the _maximum_ number
of these single shocks which I could thus throw in in a given time
_so as to procure a response to every shock_, corresponded with the
number of contractions which the tissue had previously given during
a similar interval of time when under the influence of the faradaic
current of similar intensity. To make this quite clear, I shall
describe the whole course of one such experiment. The deganglionated
tissue under the influence of minimal faradaic stimulation manifested
a perfectly regular rhythm of thirty contractions per minute, or one
contraction in every two seconds. While the position of the platinum
electrodes and the intensity of the current remained unchanged,
single induction shocks were now administered with a key at any
intervals which might be desired. It was found that if these single
induction stimuli were administered at regular intervals of two
seconds or more, the tissue responded to every stimulus; while if
the stimuli were thrown in more rapidly than this, the tissue did
not respond to every stimulus, but only to those that were separated
from one another by an interval of at least two seconds' duration.
Thus, for instance, if the shocks were thrown in at the rate of one
a second, the tissue only, but always, responded to every alternate
shock. And similarly, as just stated, if any number of shocks were
thrown in, the tissue only responded once in every two seconds. Now,
as this rate of response precisely coincided with the rate of rhythm
previously shown by the same tissue under the influence of faradaic
stimulation of the same intensity, the experiment tended to verify
the hypothesis which it was designed to test.
[Illustration: Fig. 27.]
I may give one other experiment having the same object and tendency.
Employing single induction shocks of slightly more than minimal
intensity, and throwing them in at twice the rate that was required
to produce a strong response to every shock, I found that midway
between every two strong responses there was a weak response. In
other words, a stimulus of uniform intensity gives rise alternately
to a strong and to a weak contraction, as shown in the appended
tracing (Fig. 27). It will be observed that in this tracing each
large curve represents the whole time occupied by the strong
contraction, the latter beginning at the highest point of the
curve on the left-hand side in each case. The effect of the weak
contraction is that of momentarily interrupting the even sweep of
diastole after the strong contraction, and therefore the result on
the tracing is a slight depression in the otherwise even curve of
ascent. Lest any doubt should arise from the smallness of the curves
representing the weak contractions that the former are in some
way accidental, I may draw attention to the fact that the period
of latent stimulation is the same in the case of all the curves.
To render this apparent, I have placed crosses below the smaller
curves, which show in each case the exact point where the depressing
effect of these smaller curves on the ascending sweeps of the larger
curves first become apparent--_i.e._ the point at which the feeble
contraction begins. Now, what I wish to be gathered from the whole
tracing is this. If the strength of the induction shocks had been
much greater than it was, _all_ the contractions would have become
strong contractions, and tetanus would have been the result. But,
as the strength of the induction shocks was only slightly more than
minimal, the exhaustion consequent on every strong contraction so
far diminished the irritability of the tissue that when, during
the process of relaxation, another shock _of the same intensity_
was thrown in, the stimulus was only strong enough, in relation
to the diminished irritability of the partly recovered tissue, to
cause a feeble contraction. And these facts tend still further to
substantiate the hypothesis whereby I have sought to explain the
phenomena of artificial rhythm.
Now, I think that the strictly rhythmic action of the paralyzed
swimming-bell of Aurelia in answer to constant stimulation is a
fact of the highest significance; for here we have a tissue wholly,
or almost wholly, deprived of its centres of spontaneity, yet
pulsating as rhythmically in answer to artificial stimulation as
it previously did in answer to ganglionic stimulation.[25] Does
not this tend to show that for the production of the natural rhythm
the presence of the ganglionic element is non-essential; that if we
merely suppose the function of this element to be that of supplying a
constant stimulus of a low intensity, without in addition supposing
the presence of any special resistance-mechanism to regulate the
discharges, the periodic sequence of systole and diastole would
assuredly result; and, therefore, that the rhythmical character
of the natural swimming motions is dependent, not on the peculiar
relations of the ganglionic, but on the primary qualities of the
contractile tissue? Or, if we do not go so far as this (and, as I may
parenthetically observe, I am not myself inclined to go so far), must
we not at least conclude that the natural rhythm of these tissues is
not _exclusively_ due to any mechanism whereby the discharges of the
ganglia are interrupted at regular intervals; but that whether these
discharges are supposed to be interrupted or continuous, the natural
rhythm is probably in a large measure due to the same cause as the
artificial rhythm, viz. in accordance with our previous hypothesis,
to the alternate exhaustion and recovery of the excitable tissues?
This much, at least, must be allowed even by the most cautious of
critics, viz. that if, as current views respecting the theory of
rhythm would suppose, it is exclusively the ganglionic element which
in the unmutilated Aurelia causes the rhythm of the swimming motions
by intermittent stimulation, surely it becomes a most unexpected
and unaccountable fact, that after the removal of this element the
contractile tissues should still persist in their display of rhythm
under the influence of constant stimulation. At any rate no one, I
think, will dispute that the facts which I have adduced justify us in
reconsidering the whole theory of rhythm as due to ganglia.
[25] It will not be forgotten that there are a multitude of
ganglion-cells distributed throughout the contractile tissues
of the Medusæ; but forasmuch as these are comparatively rarely
instrumental in originating stimulation, I think it is probable
that artificial stimulation acts directly on the contractile
tissues, and not through the medium of these scattered cells.
As I have already said, I am not inclined to deny that there is
probably some truth in the current theory of rhythm as due to
ganglia; I merely wish to point out distinctly that this theory is
inadequate, and that in order to cover all the facts it will require
to be supplemented by the theory which I now propose. The current
theory of rhythm as due to ganglia attributes the whole of the effect
to the ganglionic element, and thus fails to meet the fact of a
rhythm which is artificially produced after the ganglionic element
has been removed. It also fails to meet a number of other facts of
the first importance; for it is beyond all doubt that rhythmic action
of the strictest kind occurs in an innumerable multitude of cases
where it is quite impossible to suppose anything resembling ganglia
to be present. Not to mention such cases as the Snail's heart, where
the most careful scrutiny has failed to detect the least vestige
of ganglia, but to descend at once to the lowest forms of animal
and vegetable life, rhythmic action may here be said to be the rule
rather than the exception. The beautifully regular motions observable
in some Algæ, Diatomaceæ, and Ocillatoriæ, in countless numbers of
Infusoria, Antherozoids, and Spermatozoa, in ciliary action, and even
in the petioles of Hedysarum gyrano, are all instances (to which many
others might be added) of rhythmical action where the presence of
ganglia is out of the question. Again, in a general way, is it not
just as we recede from these primitive forms of contractile tissue
that we find rhythmic action to become less usual? And, if this is
so, may it not be that those contractile tissues which in the higher
animals manifest rhythmic action are the contractile tissues which
have longest retained their primitive endowment of rhythmicality?
To my mind it seems hard to decide in what respect the beating of
a Snail's heart differs from that of the pulsatile vesicles of the
Infusoria; and I do not think it would be much easier to decide in
what essential respect it differs from the beating of the Mammalian
heart. The mere fact that the presence of ganglia can be proved in
the one case and not in the other, seems to me scarcely to justify
the conclusion that the rhythm is in the one case wholly dependent,
and in the other as wholly independent, of the ganglia. At any rate,
this fact, if it is a fact, is not of so self-evident a character
as to recommend to us the current theory of ganglionic action on _à
priori_ grounds.
Coming, then, to experimental tests, we have already seen that in the
deganglionated swimming organ of Aurelia aurita, rhythmic response is
yielded to constant faradaic stimulation of low intensity. The next
question, therefore, which presents itself in relation to our subject
is as to whether other modes of constant stimulation elicit a similar
response. Now, in a general way, I may say that such is the case,
although I have chosen faradaic stimulation for special mention,
because, in the first place, its effect in producing rhythmic
action is the most certain and precise; and, in the next place, the
effects of administering instantaneous shocks at given intervals
admit of being compared with the effects of constant faradaic
stimulation better than with any other kind of constant stimulation.
Nevertheless, as just stated, other modes of constant stimulation
certainly have a more or less marked effect in producing rhythmic
response. The constant current, during the whole time of its passage,
frequently has this effect in the case of the paralyzed nectocalyx
of Sarsia; and dilute spirit, or other irritant, when dropped on the
paralyzed swimming organ of Aurelia aurita, often gives rise to a
whole series of rhythmical pulsations, the systoles and diastoles
following one another at about the same rate as is observable in the
normal swimming motions of the unmutilated animal.
From this it will be seen that, both in the case of mechanical and
of chemical stimulation, the same tendency to the production of
rhythmic response on the part of the paralyzed tissues of Aurelia
may be observed as in the case of electrical stimulation. The
principal differences consist in the rhythm being much less sustained
in the former than in the latter case. But, by experimenting on
other species of Medusæ, I have been able to obtain, in response to
mechanical and chemical stimulation, artificial rhythm of a much more
sustained character than that which, under such modes of stimulation,
occurs in Aurelia. I have no explanation to offer why it is that some
species, or some tissues, present so much more readiness to manifest
sustained rhythm under certain modes of stimulation, and less
readiness to manifest it under other modes, than do other species or
tissues. Probably these differences depend on some peculiarities in
the irritability of the tissues which it is hopeless to ascertain;
but, in any case, the facts remain, that while Aurelia, Cyanæa, and
the covered-eyed Medusæ generally are the best species for obtaining
artificial rhythm under the influence of faradaic stimulation, some
of the naked-eyed Medusæ are the best species for obtaining it
under the influence of the constant current, and also under that
of mechanical and chemical stimulation. I have already spoken of
this effect of the constant current in the case of Sarsia; I shall
now proceed to describe the effects of mechanical and chemical
stimulation on the same species.
It is but rarely that artificial rhythm can be produced in the
paralyzed nectocalyx of Sarsia by means of mechanical stimulation,
but in the case of the manubrium, a very decided, peculiar, and
persistent rhythm admits of being produced by this means. In this
particular species, the manubrium never exhibits any spontaneous
motion after the ganglia of the nectocalyx have been removed. But if
it be nipped with the forceps, or otherwise irritated, it contracts
strongly and suddenly; it then very slowly and gradually relaxes
until it has regained its original length. After a considerable
interval, and without the application of any additional stimulus,
it gives another single, sudden, though slight contraction, to be
again followed by gradual relaxation and a prolonged interval of
repose, which is followed in turn by another contraction, and so on.
These sudden and well-marked contractions occur at intervals of many
seconds, and show a decided tendency to rhythmic periodicity, though
the rhythm is not always perfectly exact. This intensely slow rhythm,
as the result of injury, may continue for a long time, particularly
if the injury has been of a severe character. There can be no doubt,
therefore, that the mechanical (or other) injury in this case acts
as a source of constant irritation; so that here again we have
evidence of rhythmic action independent of ganglia, and caused by the
alternate exhaustion and recovery of contractile tissues.[26]
[26] We may pretty safely conclude that ganglia are altogether
absent in the manubrium of Sarsia, not only because Schultz
has failed to detect them in this organ microscopically, but
also because of the complete absence of spontaneity which it
manifests. I may here mention that this case of the manubrium of
Sarsia is precisely analogous to another which I have observed
in a widely different tissue, namely, the tongue of the frog.
Here, too, the presence of ganglion-cells has never been observed
microscopically, though specially sought for by Dr. Klein and
others. Yet, under the influence of mechanical and other modes
of stimulation, I find that I am able to make the excised organ
pulsate as rhythmically as a heart.
With regard to artificial rhythm caused by chemical stimuli, by far
the most conspicuous instance that I have observed is that of the
paralyzed nectocalyx of Sarsia. This consists in a highly peculiar
motion of a flurried, shivering character, which is manifested by
this organ when its marginal ganglia have been removed and it is
exposed to the influence of faintly acidulated water. Now, when read
in the light of the foregoing facts, there can be no doubt that
the present one falls into its place very satisfactorily: it is an
additional and very valuable instance of the display of artificial
rhythm under the influence of a constant stimulus of low intensity;
for the shivering motions of the mutilated nectocalyx under these
circumstances are most unmistakably of a rhythmic nature. Viewed from
a little distance, indeed, these motions are not distinguishable
from the natural swimming motions of the unmutilated animal,
except that, not being of quite such a powerful character, they
are not so effective for locomotion. Viewed more closely, however,
it may frequently be seen that the whole bell does not contract
simultaneously, but that, as it were, clouds of contraction pass now
over one part and now over another. Still, whether the contractions
are partial or universal, they are more or less rhythmical. As this
was the only case that had ever been observed of rhythm as due to a
constant chemical stimulus, I studied it with much care, and shall
now give a full description of what appears to me an important body
of physiological facts.
Ten to twenty drops of acetic acid having been added to one thousand
cubic centimetres of sea-water, and the paralyzed bell of Sarsia
having been placed in the mixture, an interval of about half a minute
will elapse before any movement begins. Sooner or later, however, the
artificial rhythm is sure to be induced, and it will then continue
for a variable time--occasionally as long as an hour, and generally
for a considerable number of minutes. After it ceases it may often
be made to recommence, either by adding a few more drops of acid to
the sea-water, or by supplying an additional stimulus to the bell
by nipping it with the forceps. Eventually, however, all movement
ceases, owing to the destruction of irritability by the action of the
acid. By this time the whole inner surface of the bell has become
strongly opalescent, owing to the destructive influence of the acid
on the epithelial cells which overspread the irritable tissues.
The latter fact is worth mentioning, because in no case does the
artificial rhythm set in until this opalescence has begun to show
itself; and as this opalescence is but an optical expression of the
damage which the epithelial coat is undergoing, the explanation of
the time which elapses after the first immersion of the bell in the
acidulated water and the commencement of the artificial rhythm no
doubt is, that during this time the acid has not obtained sufficient
access to the excitable tissues to serve as an adequate stimulus.
During the soaking stage of the experiment--_i.e._ before the
artificial rhythm begins--the excitability of the tissues may be
observed progressively and abnormally to increase; for soon after the
soaking stage begins, in response to a single nip with the forceps
the bell may give two or three locomotor contractions, instead of
a single one, as is _invariably_ the case with a paralyzed bell of
Sarsia in normal water. Later on during the soaking stage, four
or five successive contractions may be yielded in response to a
single mechanical stimulus, and shortly after this a whole bout of
rhythmic contractions may be started by the same means. Indeed, in
some cases the artificial rhythm in acidulated water requires such
a single additional stimulus for its inauguration, the shivering
movements failing to begin spontaneously, but beginning immediately
upon the application of the additional stimulus. Similarly, after
the shivering movements have ceased, a fresh bout may very often
be started by again giving the motionless nectocalyx a single
stimulation. The interpretation of these facts would seem to be that
the general irritability of the excitable tissues is exalted by the
universal and constant stimulus supplied by the acid to an extent
that is just bordering on that which gives rise to rhythmic movement,
so that when the violent contraction is given in response to the
mechanical stimulus, the disturbance serves to start the rhythmic
movement.
If a paralyzed nectocalyx, while manifesting its artificial
rhythm in acidulated sea-water, be suddenly transferred to normal
sea-water, the movements do not cease immediately, but continue for
a considerable time. This fact can easily be explained by the very
probable, and indeed almost necessary, supposition that it takes some
time after the transference to the normal sea-water for the acid to
be washed out from contact with the excitable tissues. Sooner or
later, however, as we should expect, in the normal sea-water the
rhythmic movements entirely cease, and the bell becomes quiescent,
with a normal irritability as regards single stimuli. If it be now
again transferred to the acidulated water, after a short interval
the rhythmic movements will again commence, and so on during several
repetitions of this experiment, until the irritability of the tissues
has finally become destroyed by the influence of the acid.
Other chemical irritants which I have tried produce substantially
similar effects on the paralyzed bell of Sarsia. I shall, therefore,
only wait to describe the influence of one of these irritants, the
action of which in some respects differs from that of acids, and
which I have found to be one of the most unfailing in its power
to produce the rhythmic movements in question. This irritant is
glycerine, and in order to produce its full effect it requires to be
added to the sea-water in about the proportion of five per cent. The
manifestation of artificial rhythm in solutions of this kind is quite
unfailing. It begins after an exposure of from fifteen to thirty
seconds, and continues for a variable number of seconds. It generally
begins with powerful contractions, of a less shivering character than
those which are produced by acids, and therefore still more closely
resembling the normal swimming motions of the unmutilated animal.
Sometimes, however, the first manifestation of the artificial rhythm
is in the form of a few gentle rhythmic contractions, to be followed
by a few seconds of quiescence, and then by the commencement of the
sustained bout of strong contractions. In either case, when the
bout of strong contractions sets in, the rate of the rhythm becomes
progressively and rapidly increased, until it runs up into incipient
tetanus. The rate of the rhythm still quickening, the tetanus rapidly
becomes more and more pronounced, till at last the bell becomes
quiescent in tonic spasm.[27]
[27] Sometimes, however, the order of events is slightly
different, the advent of the spasm being more sudden, and
followed by a mitigation of its severity, the bell then
exhibiting what is more usually the first phase of the series,
namely, the occurrence of the locomotor-like contractions.
Occasionally, also, rhythmical shivering contractions may be seen
superimposed on the general tonic contraction, either in a part
or over the whole of the contractile tissues.
If the bell is still left in the glycerine solution nothing further
happens; the tissues die in this condition of strong systole. But if
the bell be transferred to normal sea-water immediately after, or,
still better, slightly before the tonic spasm has become complete, an
interesting series of phenomena is presented. The spasm persists for
a long time after the transference without undergoing any change,
the length of this time depending on the stage in the severity
and the spasm at which the transference is made. After this time
is passed, the spasm becomes less pronounced than it was at the
moment of transference, and a reversion takes place to the rhythmic
contractions. The spasm may next pass off entirely, leaving only
the rhythmic contractions behind. Eventually these too fade away
into quiescence, but it is remarkable that they leave behind them
a much more persistent exaltation of irritability than is the case
with acid. For in the case of glycerine, the paralyzed bell which
has been exposed to the influence of the irritant and afterwards
become quiescent in normal sea-water, will often continue for hours
to respond to single stimuli with a bout of rhythmic contractions.
This effect of glycerine in producing an extreme condition of exalted
irritability is also rendered apparent in another way; for if,
during the soaking stage of the experiment--_i.e._ before the first
of the rhythmic contractions has occurred--the bell be nipped with
the forceps, the effect may be that of so precipitating events that
the whole of the rhythmic stages are omitted, and the previously
quiescent bell enters at once into a state of rigid tonic spasm. This
effect is particularly liable to occur after prolonged soaking in
weak solutions of glycerine.
As in the case of stimulation by acid, so in that of stimulation by
glycerine, the artificial rhythm never begins in any strength of
solution until the epithelial surface has become opalescent to a
considerable degree.
In the case of stimulation by glycerine, the behaviour of the
manubrium is more unequivocal than it is in the case of stimulation
by acid. I have therefore reserved till now my description of the
behaviour of this organ under the influence of constant chemical
stimulation. This behaviour is of a very marked though simple
character. The manubrium is always the first to respond to the
stimulation, its retraction preceding the first movements of the
bell by an interval of several seconds, so that by the time the bell
begins its rhythmic response the manubrium is usually retracted to
its utmost. The initial response of the manubrium is also rhythmic,
and the rhythm which it manifests--especially if the glycerine
solution be not over-strong--is of the same slow character which
has already been described as manifested by this organ when under
the influence of mechanical stimulation. The rhythm, however, is
decidedly quicker in the former than in the latter case.
Lastly, with regard to the marginal ganglia, it is to be observed
that in the case of all the chemical irritants I have tried,
if unmutilated specimens of Sarsia be immersed in a sea-water
solution of the irritant which is of a sufficient strength to evoke
artificial rhythm in paralyzed specimens, the spontaneity of the
ganglia is destroyed in a few seconds after the immersion of the
animals, _i.e._ in a shorter time than is required for the first
appearance of artificial rhythm. Consequently, whether the specimens
experimented upon be entire or paralyzed by removal of their margins,
the phenomena of artificial rhythm under the influence of chemical
stimulation are the same. But although the spontaneity of the ganglia
disappears before the artificial rhythm sets in, such is not the case
with the reflex activity of the ganglia; for on nipping a tentacle of
the quiescent bell before the artificial rhythm has set in, the bell
will give a single normal response to the stimulation.
Hence, in historical order, on dropping an unmutilated specimen of
Sarsia into a solution of glycerine of the strength named, the usual
succession of events to be observed is an follows. First, increased
activity of the normal swimming motions, to be quickly followed by
a rapid and progressive decrease of such activity, till in about
fifteen seconds after the immersion total quiescence supervenes. Four
or five seconds later the manubrium begins to retract by rhythmical
twitches, the rate of this rhythm rapidly increasing until it ends
in tonic contraction. When the manubrium has just become fully
retracted--or very often a little earlier--the bell suddenly begins
its forcible and well-pronounced rhythmic contractions, which rapidly
increase in their rate of rhythm until they coalesce into a vigorous
and persistent spasm. If the animal be now restored to normal
sea-water, spontaneity will return in a feeble manner; but there is
always afterwards a great tendency displayed by the bell to exhibit
shivering spasms instead of normal swimming movements in response
to natural or ganglionic stimulation. And, as already observed, this
peculiarity of the excitable tissues is also well marked in the case
of the artificial stimulation of deganglionated specimens under
otherwise similar conditions.
One further experiment may here be mentioned. Having split open
the paralyzed bell of Sarsia along the whole of one side from base
to apex of the cone, I suspended the now sheet-like mass of tissue
by one corner in the air, leaving the rest of the sheet to hang
vertically downwards. By means of a rack-work support I now lowered
the sheet of tissue, till one portion of it dipped into a beaker
filled with a solution of glycerine of appropriate strength. After
allowing this portion to soak in the solution of glycerine until it
became slightly opalescent, I dropped the entire mutilated bell, or
sheet of tissue, into another beaker containing sea-water. If the
exposure to the glycerine solution had been of sufficient duration, I
invariably found that in the normal sea-water the rhythmic movements
were performed by the whole tissue-mass quite as efficiently as was
the case in my other experiments, where the whole tissue-mass, and
not merely a portion, had been submitted to the influence of the
irritant. But on now suddenly snipping off the opalescent portion
of the tissue-mass, _i.e._ the portion which had been previously
alone submitted to the influence of the irritant, all movement in
the remainder of the tissue-mass instantly ceased. This experiment
I performed repeatedly, sometimes exposing a large and sometimes a
small portion of the tissue to the influence of the irritant. As I
invariably obtained the same result, there can be no doubt that in
the case of chemical stimulation the artificial rhythm depends for
its manifestation on the presence of a constant stimulus, and is not
merely some kind of obscure fluttering motion which, having been
started by a stimulus, is afterwards kept up independently of any
stimulus.
Such being the case, I naturally expected that if I were to supply
a constant stimulus of a thermal kind, I should also obtain the
phenomena of artificial rhythm. In this, however, my expectations
have not been realized. With no species of Medusa on of artificial
rhythm by immersing the paralyzed animals in heated water. I can only
explain this fact by supposing that the stimulus which is supplied
by the heated medium is of too uniform a character over the whole
extent of the excitable tissues; it would seem that in order to
produce artificial rhythm there must be a differential intensity of
stimulation in different parts of the responding tissue, for no doubt
even the excitatory influence of acidulated water is not of nearly so
uniform an intensity over the whole of the tissue-area as is that of
heated water.
In now quitting the subject of artificial rhythm as it is manifested
by the paralyzed bell of Sarsia, it is desirable again to observe
that sustained artificial rhythm cannot be produced by means of
chemical irritation in the case of any one of the species of
covered-eyed Medusæ that I have met with. In order to evoke any
response at all, stronger solutions of the irritants require to be
employed in the case of the covered than in that of the naked-eyed
Medusæ, and when the responses do occur they are not of so suggestive
a character as those which I thought it worth while so fully to
describe. Nevertheless, even in the covered-eyed Medusæ well marked,
though comparatively brief, displays of artificial rhythm may often
be observed as the result of constant chemical stimulation. Thus,
for instance, in the case of Aurelia, if the paralyzed umbrella be
immersed in a solution of glycerine (ten to twenty per cent.), a few
rhythmic pulsations of normal rate are usually given; but shortly
after these pulsations occur, the tissue begins to go into a tetanus,
which progressively and rapidly becomes more and more pronounced
until it ends in violent tonic spasm. So that the history of events
really resembles that of Sarsia under similar circumstances, except
that the stage of artificial rhythm which inaugurates the spasm is of
a character comparatively less pronounced.
Thus far, then, I have detailed all the facts which I have been able
to collect with reference to the phenomena of artificial rhythm, as
produced by different kinds of constant stimulation. It will not be
forgotten that the interest attaching to these facts arises from the
bearing which they have on the theory of natural rhythm. My belief is
that hitherto the theory of rhythm as due to ganglia has attributed
far too much importance to the ganglionic as distinguished from
the contractile tissues, and I have founded this belief principally
on the facts which have now been stated, and which certainly prove
at least this much: that after the removal of the centres of
spontaneity, the contractile tissues of the Medusæ display a marked
and persistent tendency to break into rhythmic action whenever they
are supplied with a constant stimulus of feeble intensity. Without
waiting again to indicate how this fact tends to suggest that the
natural rhythm of the unmutilated organisms is probably in large
part due to that alternate process of exhaustion and restoration of
excitability on the part of the contractile tissues, whereby alone
the phenomena of artificial rhythm can be explained,[28] I shall go
on to describe some further experiments which were designed to test
the question whether the influences which affect the character of the
natural rhythm likewise, and in the same manner, affect the character
of the artificial rhythm. I took the trouble to perform these
experiments, because I felt that if they should result in answering
this question in the affirmative, they would tend still further to
substantiate the view I am endeavouring to uphold, viz. that the
natural rhythm may be a function of the contractile as distinguished
from the ganglionic tissue. Of the modifying causes in question, the
first that I tried was temperature.
[28] It is of importance to point out the fact that some of my
previously stated experiments appear conclusively to prove that
the natural stimulation which is supplied by the marginal ganglia
of the Medusæ resembles all the modes of artificial stimulation
which are competent to produce artificial rhythm in one important
particular; the _intensity_ of the stimulation which the marginal
ganglia supply is shown by these experiments to be about the same
as that which is required to produce artificial rhythm in the
case of artificial stimulation. In proof of this point, I may
allude particularly to the observations which are detailed on pp.
131-136.
Having already treated of the effects of temperature on the natural
rhythm, it will now be sufficient to say that we have seen these
effects to be similar to those which temperature exerts on the rhythm
of ganglionic tissues in general. Now, I find that temperature
exerts precisely the same influence on the artificial rhythm of
deganglionated tissue as it does on the natural rhythm of the
unmutilated animal. To economize space, I shall only quote one of my
observations in a table which explains itself. I also append tracings
of another observation, to render the difference in the rate of the
artificial rhythm more apparent to the eye (Fig. 28).
Temperature of water | Number of contractions
(Fahr.). | per minute.
25° | 24
45° | 40
75° | 60
During the whole progress of such experiments the faradaic
stimulation was, of course, kept of uniform intensity; so that the
progressive acceleration is undoubtedly due to the increase of
temperature alone. With each increment of temperature the rate of
the artificial rhythm increases suddenly, just as it does in the
case of the natural rhythm. Moreover, there seems to be a sort of
rough correspondence between the amount of influence that any given
degree of temperature exerts on the rate of the natural and of the
artificial rhythm respectively. Further, it will be remembered that
in warm water the natural rhythm, besides being quicker, is not so
regular as it is in cold water; thus also it is with the artificial
rhythm. Again, water below 20° or above 85° suspends the natural
rhythm, _i.e._ stops the contractions; and the artificial rhythm
is suspended at about the same degrees. Lastly, just as there are
considerable individual variations in the extent to which the natural
rhythm is affected by temperature, so the artificial rhythm is in
some cases more influenced by this cause than in others, though in
all cases it further resembles the natural rhythm in showing some
considerable degree of modification under such influence.
[Illustration: Fig. 28.]
On the whole, then, it would be impossible to imagine two cases more
completely parallel than are these of the effects of temperature on
natural and on artificial rhythm respectively; and as it must be
considered in the last degree improbable that all these coincidences
are accidental, I conclude that the effects of temperature on the
natural rhythm of Medusæ (and so, in all probability, on the natural
rhythm of other ganglio-muscular tissues) are for the most part
exerted, not on the ganglionic, but on the contractile element.
In order to test the effects of gases on the artificial rhythm, I
took a severed quadrant of Aurelia, and floated it in sea-water,
with its muscular surface just above the level of the water. Over
the tissue I lowered an inverted beaker filled with the gas the
effects of which I desired to ascertain, and by progressively forcing
the rim of the beaker into the water I could submit the tissue to
various pressures of the atmosphere of the gas I was using. By an
appropriate arrangement the electrodes passed into the interior of
the beaker, and could then be manipulated from the outside, so as to
be properly adjusted on the tissue. In this way I was able to observe
that different gases exerted a marked influence on the rate of the
artificial rhythm.
The following table gives the ratios in the case of one experiment:--
Rate of artificial rhythm, | |
in air. | In oxygen. | In carbonic acid.
| |
36 per minute. | 50 per minute. | 25 per minute.
It may here be observed that to produce these results, both carbonic
acid and oxygen must be considerably diluted with air, for otherwise
they have the effect of instantaneously inhibiting all response,
even to the strongest stimulation. When this is the case, however,
irritability returns very soon after the tissue is again exposed
to air or to ordinary sea-water. But I desire it to be understood
that the results of my experiments on the influence of oxygen,
both on the natural and on the artificial rhythm, have proved
singularly equivocal; so that as far as this gas is concerned further
observations are required before the above results can be accepted as
certain.
I have still one other observation of a very interesting character
to describe, which is closely connected with the current views
respecting ganglionic action, and may therefore be more conveniently
considered here than in any other part of this treatise. I have
already stated that in no case is the manubrium of a Medusa affected
as to its movements by removal of the periphery of the swimming-bell;
but in the case of Sarsia a very interesting change occurs in the
manubrium soon after the nectocalyx has been paralyzed by excision
of its margin. Unlike the manubriums of most of the other Medusæ,
this organ, in the case of Sarsia, is very highly retractile. In
fresh and lively specimens the appendage in question is carried in
its retracted state; but when the animals become less vigorous--from
the warmth or impurity of the water in which they are confined, or
from any other cause--their manubriums usually become relaxed. The
relaxation may show itself in various degrees in different specimens
subjected to the same conditions, but in no case is the degree of
relaxation so remarkable as that which may be caused by removing
the periphery of the nectocalyx. For the purpose of showing this
effect, it does not signify in what condition as to vigour, etc., the
specimen chosen happens to be in; for whether the manubrium prior to
the operation be contracted or partially relaxed, within half an hour
after the operation it is sure to become lengthened to a considerable
extent.
In order to show the surprising degree to which this relaxation may
proceed, I insert a sketch of a specimen both before and after the
operation. The sketches are of life size, and drawn to accurate
measurement (Figs. 29 and 30).[29]
[29] I may here mention that the fact of the manubrium of Sarsia
undergoing this extreme elongation after the removal of the
marginal ganglia, serves to render the artificial rhythm of the
organ under the influence of injury, as previously described, all
the more conspicuous.
[Illustration: Fig. 29.]
[Illustration: Fig. 30.]
With regard to this remarkable effect on the manubrium of removing
the margin of the nectocalyx, it is now to be observed that in it
we appear to have very unexceptionable evidence of such a relation
subsisting between the ganglia of the nectocalyx and the muscular
fibres of the manubrium as elsewhere gives rise to what is known as
muscular tonus. This interpretation of the facts cannot, I think, be
disputed; and it fully explains why, in the unmutilated animal, the
degree of elongation on the part of the manubrium usually exhibits
an inverse proportion to the degree of locomotor activity displayed
by the bell. I may here state that I have also observed indications
of muscular tonus in some of the other Medusæ, but for the sake of
brevity I shall now restrict myself to the consideration of this one
case.
To my mind, then, it is an interesting fact that ganglionic tissue,
where it can first be shown to occur in the animal kingdom, has for
one of its functions the maintenance of muscular tonus; but it is
not on this account that I now wish to draw prominent attention to
the fact before us. Physiologists are almost unanimous in regarding
muscular tonus as a kind of gentle tetanus due to a persistent
ganglionic stimulation, and against this opinion it seems impossible
to urge any valid objection. But, in accordance with the accepted
theory of ganglionic action, physiologists further suppose that the
only reason why some muscles are thrown into a state of tonus by
ganglionic stimulation, while other muscles are thrown into a state
of rhythmic action by the same means, is because the resistance to
the passage of the stimulation from the ganglion to the muscle is
less in the former than in the latter case. Here, be it remembered,
we are in the domain of pure speculation: there is no experimental
evidence to show that such a state of differential resistance as
the theory requires actually obtains. Hence we are quite at liberty
to suppose any other kind of difference to obtain, either to the
exclusion of this one or in company with it. Such a supposition I
now wish to suggest, and it is this--that all rhythmical action
being regarded as due (at any rate in large part) to the alternate
exhaustion and restoration of excitability on the part of contractile
tissues, the reason why continuous ganglionic stimulation produces
incipient tetanus in the case of some muscles and rhythmic action,
in the case of others, is either wholly or partly because the
irritability of the muscles in relation to the intensity of the
stimulation is greater in the former than in the latter case. If
this supposition as to differential irritability be granted, my
experiments on paralyzed Aurelia prove that tetanus would result
in the one case and rhythmic action in the other. For it will be
remembered that in these experiments, if the continuous faradaic
stimulation were of somewhat more than minimal intensity, tetanus was
the result; while if such stimulation were but of minimal intensity,
the result was rhythmic action. Now, that in the particular case
of Sarsia the irritability of the tonically contracting manubrium
is higher than that of the rhythmically contracting bell is a
matter, not of supposition, but of observable fact; for not only is
the manubrium more irritable than the bell in response to direct
stimulation of its own substance, but it is generally more so even
when the stimuli are applied anywhere over the excitable tissues
of the bell. And from this it is evident that the phenomena of
muscular tonus, as they occur in Sarsia, tend more in favour of the
exhaustion than of the resistance theory.[30]
[30] The evidence, however, is not altogether exclusive of the
resistance theory, for it is quite possible that in addition to
the high irritability of the manubrium there may be conductile
lines of low resistance connecting this organ with the marginal
ganglia. I entertain this supposition because, as explained in my
Royal Society papers, I see reason to believe that the natural
swimming movements of Sarsia are probably in part due to an
intermittent discharge of the ganglia. I think, therefore, that
in this particular case the ganglia supply a tolerably constant
stimulation to the manubrium, while it is only at intervals
that their energy overflows into the bell, and that the higher
degree at irritability on the part of the manubrium ensures the
tonic response of this organ at a small cost of nervous energy.
How far the rhythm of the nectocalyx is to be attributed to the
resistance mechanism of the ganglia, and how far to the alternate
exhaustion and recovery of the contractile tissues, I think it is
impossible to determine, seeing that it is impossible exactly to
imitate the natural ganglionic stimulation by artificial means.
But it is, I think, of importance to have ascertained at least
this much, that in Sarsia the tonus of one organ and the rhythm
of another, which apparently both received their stimulation from
the same ganglia, must at any rate in part be attributed to a
differential irritability of these organs, as distinguished from
their differential stimulation.
I will now sum up this rather lengthy discussion. The two theories
of ganglionic action may be stated antithetically thus: in both
theories the accumulation of energy by ganglia is supposed to be
a continuous process; but while the resistance theory supposes
the rhythm to be exclusively due to an intermittent and periodic
discharge of this accumulated energy on the part of the ganglionic
tissues, the exhaustion theory supposes that the rhythm is largely
due to a periodic process of exhaustion and recovery on the part
of the responding tissues. Now, I submit that my experiments have
proved the former of these two theories inadequate to explain all the
phenomena of rhythm as it occurs in the Medusæ; for these experiments
have shown that even after the removal of the only ganglia which
serve as centres of natural stimulation, the excitable tissues still
continue to manifest a very perfect rhythm under the influence of any
mode of artificial stimulation (except heat), which is of a constant
character and of an intensity sufficiently low not to produce
tetanus. And as I have proved that the rhythm thus artificially
produced is almost certainly due to the alternate process of
exhaustion and recovery which I have explained, there can scarcely be
any doubt that in the natural rhythm this process plays an important
part, particularly as we find that temperature and gases exert the
same influences on the one rhythm as they do on the other. Again, as
an additional reason for recognizing the part which the contractile
tissues probably play in the production of rhythm, I have pointed
to the fact that in the great majority of cases in which rhythmic
action occurs the presence of ganglia cannot be suspected. For it is
among the lower forms of life, where ganglia are certainly absent,
and where the functions of stimulation and contraction appear to be
blended and diffused, that rhythmic action is of the most frequent
occurrence; and it in obvious with how much greater difficulty the
resistance theory is here beset than is the one I now propose.
Granted a diffused power of stimulation with a diffused power of
response, and I see no essential difference between the rhythmic
motions of the simplest organism and those of a deganglionated Medusa
in acidulated water. Lastly, the facts relating to the tonus of
the manubrium in Sarsia furnish very striking, and I think almost
conclusive, proof of the theory which I have advanced.