EXPERIMENTS IN STIMULATION.
_Mechanical, Chemical, and Thermal Stimulation._
So far as my observations extend, I find that all Medusæ, after
removal of their locomotor centres, invariably respond to every
kind of stimulation. To take the case of Sarsia as a type, nothing
can possibly be more definite than is the single sharp contraction
of the mutilated nectocalyx in response to every nip with the
forceps. The contraction is precisely similar to the ordinary ones
that are performed by the unmutilated animal; so that by repeating
the stimulus a number of times, the nectocalyx, with its centres
of spontaneity removed, may be made to progress by a succession of
contractions round and round the vessel in which it is contained,
just as a frog, with its cerebral hemispheres removed, may be made to
hop along the table in response to a succession of stimulations.[8]
[8] In the case of the covered-eyed Medusæ, however, the
paralyzed umbrella sometimes responds to a single stimulation
with two, and more rarely with three contractions, which are
separated from one another by an interval of the same duration as
the normal diastole of the unmutilated animal.
Different species of Medusæ exhibit different degrees of irritability
in responding to stimuli; but in all the cases I have met with
the degree of irritability is remarkably high. Thus, I have seen
responsive contractions of the whole umbrella follow upon the
exceedingly slight stimulus caused by a single drop of sea-water
let fall upon the irritable surface from the height of one inch. As
regards chemical stimulation, 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.
It is somewhat difficult, in the case of paralyzed swimming organs,
to prove the occurrence of a contraction in response to thermal
stimulation, from the fact that while these tissues are not nearly
so sensitive to this mode of excitation as might be anticipated,
they are, as just observed, extraordinarily sensitive to mechanical
excitation. It therefore becomes difficult to administer the
appropriate thermal stimulus without at the same time causing a
sufficient mechanical disturbance to render it doubtful to which
of the stimuli the response is due. This may be done, however, by
allowing a few drops of heated sea-water to run over the excitable
surface while it is exposed to the air. In this and in other ways I
have satisfied myself that the paralyzed tissues of swimming organs
respond to sudden elevations of temperature.
_Luminous Stimulation._
It is interesting to note that, in the case of some of the naked-eyed
Medusæ, the action of light as a stimulus is most marked and
unfailing. In the case of Sarsia, for instance, a flash of light let
fall upon a living specimen almost invariably causes it to respond
with one or more contractions. If the animal is vigorous and swimming
freely in water, the effect of a momentary flash thrown upon it
during one of the natural pauses is immediately to originate a bout
of swimming. But if the animal is non-vigorous, or if it be removed
from the water and spread flat upon an object-glass, it usually gives
only one contraction in response to every flash. There can thus be
no doubt that a sudden transition from darkness to light acts upon
Sarsia as a stimulus, and this even though the transition be but of
momentary duration. The question therefore arises as to whether the
stimulus consists in the presence of light, or in the occurrence
of the sudden transition from darkness to light and from light to
darkness. To answer this question, I tried the converse experiment
of placing a vigorous specimen in sunlight, waiting till the middle
of one of the quiescent stages in the swimming motions had come
on, and then suddenly darkening. In no case, however, under these
circumstances, did I obtain any response; so that I cannot doubt it
is the light per se, and not the sudden nature of the transition
from darkness to light, which in the former experiment acted as
the stimulus. Indeed, the effect of the converse experiment just
described is rather that of inhibiting contractions; for, if the
sunlight be suddenly shut off during the occurrence of a swimming
bout, it frequently happens that the quiescent stage immediately
sets in. Again, in a general way, it is observable that Sarsia are
more active in the light than they are in the dark, the comparative
duration of the quiescent stages being less in the former than in
the latter case. Light thus appears to act towards these animals as
a constant stimulus. Lastly, it may be stated that when the marginal
bodies of Sarsia are removed, the swimming-bell, although still able
to contract spontaneously, no longer responds to luminous stimulation
of any kind or degree. But if only one body be left _in situ_, or if
the severed margin alone be experimented upon, the same unfailing
response may be obtained to luminous stimulation as that which is
obtained from the entire animal.
The fact last mentioned indicates that the marginal bodies are organs
of special sense, adapted to respond to luminous stimulation; or,
in more simple words, that they perform the functions of sight. Now
it has long been thought more or less probable that these marginal
bodies are rudimentary or incipient "eyes," but hitherto the
supposition has not been tested by experiment, and was therefore of
no more value than a guess.[9] The guess in this instance, however,
happens to have been correct, as the results of the following
experiments will show.
[9] As Professor Haeckel observes in his monograph already
alluded to, "Die Deutung der Sinnesorgane niederer Thiere gehört
ohne Zweifel zu den schwierigsten Objecten der vergleichenden
Physiologie und ist der grössten Unsicherheit unterworfen. Wir
sind gewohnt, die von den Wirbelthieren gewonnenen Anschauungen
ohne Weiteres auch auf die wirbellosen Thiere der verschiedenen
Kinese zu übertragen und bei diesen analoge Sinnesempfindungen
anzunehmen als wir selbst besitzen ... Noch weniger freilich als
die von den meisten Autoren angenommene Deutung der Randbläschen
unserer Medusen als Gehörorgane kann die von Agassiz und Fritz
Müller vertretene Ansicht befriedigen, dass dieselben Augen
seien.... Alle diese Verhältnisse sind mit der Deutung der
Concretion als 'Linse' und des sie umschliessenden Sinnesganglion
als 'Sehnerv' durchaus unvereinbar."
It may not be unnecessary to say that, although the simple
experiment above described effectually proves that the marginal
bodies have a visual function to subserve, we are not for this
reason justified in concluding that these are so far specialized
as organs of sight as to be precluded from ministering to any
other sense.
Having put two or three hundred Sarsia into a large bell-jar, I
completely shut out the daylight from the room in which the jar
was placed. By means of a dark lantern and a concentrating lens,
I then cast a beam of light through the water in which the Sarsia
were swimming. The effect upon the latter was most decided. From all
parts of the bell-jar they crowded into the path of the beam, and
were most numerous at that side of the jar which was nearest to the
light. Indeed, close against the glass they formed an almost solid
mass, which followed the light wherever it was moved. The individuals
composing this mass dashed themselves against the glass nearest
the light with a vigour and determination closely resembling the
behaviour of moths under similar circumstances. There can thus be no
doubt about Sarsia possessing a visual sense.
The method of ascertaining whether this sense is lodged in the
marginal bodies was, of course, extremely simple. Choosing a dozen of
the most vigorous specimens, I removed all the marginal bodies from
nine, and placed these, together with the three unmutilated ones, in
another bell-jar. After a few minutes the mutilated animals recovered
from their nervous shock, and began to swim about with tolerable
vigour. I now darkened the room, and threw the concentrated beam of
light into the water as before. The difference in the behaviour of
the mutilated and of the unmutilated specimens was very marked. The
three individuals which still had their marginal bodies sought the
light as before, while the nine without their marginal bodies swam
hither and thither, without paying it any regard.
A further question, however, still remained to be determined. The
pigment spot of the marginal body in Medusæ is, as L. Agassiz
observed, placed _in front of_ the presumably nervous tissue, and
for this reason he naturally enough suggested that if the marginal
body has a visual function to perform, the probability is that the
rays by which the organ is affected are the heat-rays lying beyond
the range of the visible spectrum. Accordingly I brought a heated
iron, just ceasing to be red, close against the large bell-jar which
contained the numerous specimens of Sarsia; but not one of the latter
approached the heated metal.
From these observations, therefore, I conclude that in Sarsia
the faculty of appreciating luminous rays is present, and that
this faculty is lodged exclusively in the marginal bodies; while
from observations conducted on the covered-eyed Medusæ, I have
come to the same conclusion respecting them. But although I have
tested many species of naked-eyed Medusæ besides Sarsia, I have
obtained indications of response to luminous stimulation only
in the case of one other. This is a species which I have called
Tiaropsis polydiademata, and the response which it gives to luminous
stimulation is even more marked and decided than that which is given
by Sarsia; for a sudden exposure to sunlight causes this animal to
go into a kind of tonic spasm, the whole of the nectocalyx being
drawn together in a manner resembling cramp. Now, in one remarkable
particular this response to luminous stimulation on the part of
Tiaropsis polydiademata differs from that given by Sarsia tubulosa;
and the difference consists in the fact that, while with Sarsia the
period of latency (_i.e._ the time between the fall of the stimulus
and the occurrence of the response) is, so far as the eye can judge,
as instantaneous in the case of response to luminous stimulation as
it is in the case of response to any other kind of stimulation, such
is far from being true with Tiaropsis polydiademata. The period of
latency in the last-named species is, so far as the eye can judge,
quite as instantaneous as it is in the case of Sarsia, when the
stimulus employed is other than luminous; but in response to light,
the characteristic spasm does not take place till slightly more than
a second has elapsed after the first occurrence of the stimulus.
As this extraordinary difference in the latent period exhibited by
the same animal towards different kinds of stimuli appeared to me
a matter of considerable interest, I was led to reflect upon the
probable cause of the difference. It occurred to me that the only
respect in which luminous stimulation of the Medusæ differed from
all the other modes of stimulation I had employed consisted in
this--that, as proved by my previous experiments on Sarsia, which I
repeated on Tiaropsis, luminous stimulation directly affected the
ganglionic tissues. Now, as in Tiaropsis polydiademata luminous
stimulation differed from all the other modes of stimulation in
giving rise to an immensely longer period of latency, I seemed
here to have an index of the difference between the rapidity of
the response to stimuli by the contractile and by the ganglionic
tissues respectively. The next question, therefore, which presented
itself was as to whether the enormous length of time occupied by
the process of stimulation in the ganglia was due to any necessity
on the part of the latter to accumulate the stimulating influence
prior to originating a discharge, or to an immensely lengthened
period of latent stimulation manifested by the ganglia under the
influence of light.[10] This is an interesting question, because if
such a lengthened period of latent stimulation occurs in this case,
it would stand in curious antithesis to the very short period of
latent stimulation manifested by the contractile tissues of the same
animal under other modes of irritation. To test these alternative
hypotheses, I employed the very simple method of first allowing a
continuous flood of light to fall suddenly on the Medusa, and then
noting the time at which the responsive spasm first began. This
time, as already stated, was slightly more than one second. I next
allowed the animal to remain for a few minutes in the dark to recover
shock, and, lastly, proceeded to throw in single flashes of light
of measured duration. I found that unless the flash of light was of
slightly more than one second in its duration, no response was given;
that is to say, the minimal duration of a flash required to produce
a responsive spasm was just the same as the time during which a
continuous flood of light required to operate in order to produce a
similar spasm. From this, therefore, I conclude that the enormously
long period of latent excitation in response to luminous stimuli was
not, properly speaking, a period of latent excitation at all; but
that it represented the time during which a certain summation of
stimulating influence was taking place in the ganglia, which required
somewhat more than a second to accumulate, and which then caused
the ganglia to originate an abnormally powerful discharge. So that
in the action of light upon the ganglionic matter of this Medusa we
have some analogy to its action on certain chemical compounds in
this respect, that, just as in the case of those compounds which
light is able to split up, a more or less lengthened exposure to its
influence is necessary in order to admit of the summating influence
of its vibrations on the molecules, so in the ease of this ganglionic
material, the decomposition which is effected in it by light, and
which terminates in an explosion of nervous energy, can only be
effected by a prolonged exposure of the unstable material to the
summating influence of the luminous vibrations. Probably, therefore,
we have here the most rudimentary type of a visual organ that is
possible; for it is evident that if the ganglionic matter were a
very little more stable than it is, it would altogether fail to be
thrown down by the luminous vibrations, or would occupy so long a
time in the process that the visual sense would be of no use to its
possessor. How great is the contrast between the excitability of
such a sense-organ and that of a fully evolved eye, which is able
to effect the needful molecular changes in response to a flash as
instantaneous as that of lightning.
[10] The period of latent stimulation merely means the time
after the occurrence of an excitation during which a series of
physiological processes are taking place, which terminate in
a contraction; so that, whether the excitation is of a strong
or of a weak intensity, the period of latent stimulation is
not much affected. The above question, therefore, was simply
this--Does the prolonged delay on the part of these ganglia, in
responding to light, represent the time during which the series
of physiological processes are taking place in response to an
adequate stimulus, or does it represent the time during which
light requires to act before it becomes an adequate stimulus?
With regard to luminous stimulation, it is only necessary further to
observe that responses were given equally well to direct sunlight,
diffused daylight, and to light reflected from a mirror inclined at
the polarizing angle. It must also be stated that responses are given
to any of the luminous rays of the spectrum when these are employed
separately; but that neither the non-luminous rays beyond the red,
nor those beyond the violet, appear to exert the smallest degree of
stimulating influence.
_Electrical Stimulation._
All the excitable parts of all the Medusæ which I have examined are
highly sensitive to electrical stimulation, both of the constant and
of the induced current.
Exploration with needle-point terminals and induction shocks of
graduated strength showed that certain parts or tracts of the
nectocalyx are more sensitive than others. The most sensitive
parts are those which correspond with the distribution of the main
nerve-trunks, _i.e._ round the margin of the nectocalyx and along
the course of the radial tubes. The external or convex surface of a
nectocalyx or umbrella is totally insensitive to stimulation, and
the same statement applies to the whole thickness of the gelatinous
substance to which the neuro-muscular sheet is attached.
In all other respects the excitable tissues of the Medusæ in their
behaviour towards electrical stimulation conform to the rules which
are followed by excitable tissues of other animals. Thus, closure
of the constant current acts as a stronger stimulus than does
opening of the same, while the reverse is true of the induction
shock; and exhaustion supervenes under the influence of prolonged
excitation. Moreover, I have obtained evidence of that polarization
of nerve-tissues under the influence of the constant current,
which is known to physiologists by the term "electrotonus;" but
it would be somewhat tedious to detail the evidence on this head
which I have already published elsewhere.[11] Tetanus produced by
faradaic electricity is not of the nature of an apparently single
and prolonged contraction, but that of a number of contractions
rapidly succeeding one another, as in the case of the heart under
similar excitation. This at least applies to Sarsia. In the case of
Aurelia, tolerably strong faradization does cause a more or less
well-pronounced tetanus. The continuity of the spasm is, indeed,
often interrupted by momentary and partial relaxations. These
interruptions are the more frequent the weaker the current; so that,
at a certain strength of the latter, the tetanus is of a wild and
tumultuous nature; but with strong currents the spasm is tolerably
uniform. That in all cases the tetanus is due to summation of
contractions may be very prettily shown by the following experiment.
An Aurelia is cut into a spiral strip, and all its lithocysts are
removed; single induction-shocks are then thrown in with a key at
one end of the strip--every shock, of course, giving rise to a
contraction wave. If these shocks are thrown in at a somewhat fast
rate, two contraction waves may be made at the same time to course
along the spiral strip, one behind the other; but if the shocks are
thrown in at a still faster rate, so as to diminish the distance
between any two successive waves, a point soon arrives at which
every wave mounts upon its predecessor; and if several waves be thus
made to coalesce, the whole strip becomes thrown into a state of
persistent contraction.
[11] See "Croonian Lectures," 1875. _Philosophical Transactions_,
vol. 166, part I. pp. 284-6.
In this way sustained tetanus, or single contraction waves, or any
intermediate phase, may be instantly produced at pleasure. In such
experiments, moreover, it is interesting to observe that, no matter
how long the strip be, whatever disturbances are set up at one end
are faithfully transmitted to the other. For instance, if an Aurelia
be cut into the longest possible strip with a remnant of the disk
left attached at one end, as represented in Fig. 11 (p. 70), then
all the peculiar time relations between successive contractions
which are intentionally caused by the experimenter at one end of the
strip, are afterwards accurately reproduced at the other end of the
strip by the remainder of the disk. Now, as this fact is observable
however complex these time relations may be, and however rapidly
the successive stimuli are thrown in, I think it is a point of some
interest that these complicated relations among rapidly succeeding
stimuli do not become blended during their passage along the thirty
or forty inches of contractile tissue. The fact, of course, shows
that the rate of transmission is so identical in the case of all
the stimuli originated, that the sum of the effects of any series
of stimuli is delivered at the distal end of the strip, with all
its constituent parts as distinct from one another as they were at
starting from the proximal end of the strip.
_Period of Latency, and Summation of Stimuli._
I shall now give an account of my experiments in the period of
latency and the summation of stimuli. To do this, I must first
describe the method which I adopted in order to obtain a graphic
record of the movements which were given in response to the stimuli
supplied. As Aurelia aurita is the only species on which I have
experimented in this connection, my remarks under this heading will
be confined to it alone.
The method by which I determined the latent period in the case of
this species was as follows. A basin containing the Medusa was
filled to its brim with sea-water, and placed close beside a smoked
cylinder, which, while it lay in a horizontal position, could be
rotated at a known rate. The Aurelia[12] was placed with its concave
aspect uppermost, and an inch or two below the surface of the water.
The animal was held firmly in this position by means of a pair of
compasses thrust through it and forced into a piece of wood, which
was fastened to the bottom of the basin. The legs of the compasses
were provided with india-rubber sliders, so that by placing these
under the Medusa, the latter might be kept at any elevation in the
water which might be desired. The manubrium and lithocysts were now
removed, and also a segment of the umbrella. A light straw was then
forced through the gelatinous substance of the umbrella in a radial
direction, and close to the gap caused by the missing segment. The
other, or free, end of this straw was firmly joined to a capillary
glass rod, which was suitably bent to avoid contact with the rim of
the basin, and also to write on the smoked cylinder. If the straw
was not itself sufficient to support the weight of the capillary
rod, a small cross-piece of cork might easily be tied to it, so as
to add to the flotation power. A part of the excitable tissue was
now raised above the surface of the water by means of a disk of cork
placed beneath it, and on the part of the tissue thus raised there
were placed a pair of platinum electrodes. These electrodes proceeded
from an electro-magnetic apparatus, which was arranged in such a
way that every time the current in it was opened or closed, it gave
an induction shock and moved a lever at the same instant of time.
This lever was therefore placed upon the cylinder immediately above
the capillary glass-writer which proceeded from the Medusa, care
being taken to place the two writers in the same line, parallel to
the axis of the cylinder. Such being the arrangement, the cylinder
was rotated, and thus two parallel lines were marked upon it by the
two writers. If the current was now closed, an induction shock was
thrown into the tissue at the same instant that the electro-magnet
writer recorded the fact, by altering its position on the cylinder.
Again, as soon as the paralyzed Medusa responded to the induction
shock, the radii of the vacant segment were drawn apart, and in
this way a curve was obtained by the other writer on the rotating
cylinder. Now, by afterwards dropping a perpendicular line from
the point at which the electro-magnet writer changed its position,
to the parallel line made by the other writer, and then measuring
the distance between the point of contact and the point on the
last-mentioned line on which the curve began, the period of latent
stimulation was determined. A glance at Figs. 3 and 4 (p. 55)
will render this description clear to any one who is not already
acquainted with the method, when it is stated that the upper line is
a record of the movements of the electro-magnet writer, and the lower
line that of the movements of the other writer. It will be observed
that the point _a_ in the upper line marks the point at which the
induction shock was thrown in; so that by first producing the
perpendicular till it meets the lower line at _b_, and then measuring
the distance between the point _b_ and the point _c_, at which the
curve in the lower line first begins, the latent period (_b c_) is
determined--the time occupied by the rotation of the cylinder from
_b_ to _c_ being known.
[12] It may here be stated that in all the experiments on
stimulation subsequently to be detailed, there is no difference
to be observed between the behaviour of an entire swimming organ
deprived of its ganglia, and that of a portion of any size which
may be separated from it.
_Summation of Stimuli._--In this way I have been able to ascertain
the period of latent stimulation in Aurelia aurita with accuracy.
It must be stated at the outset, however, that the period is subject
to great variations under certain varying conditions, so that we can
only arrive at a just estimation of it by understanding the nature
of the modifying causes. To take the simplest cause first, suppose
that the paralyzed Aurelia has been left quiet for several minutes in
sea-water at forty-five degrees, and that it is then stimulated by
means of a single induction shock; the responsive contraction will
be comparatively feeble with a very long period of latency, viz.
five-eighths of a second. If another shock of the same intensity be
thrown in as soon as the tissue has relaxed, a somewhat stronger
contraction, with a somewhat shorter latent period, will be given.
If the process is again repeated, the response will be still more
powerful, with a still shorter period of latency; and so on, perhaps,
for eight or ten stages, when the maximum force of contraction of
which the tissue is capable will have been attained, while the period
of latency will have been reduced to its minimum. This period is
three-eighths of a second, or, in some cases, slightly less.
Now, we have here a remarkable series of phenomena, and as it is
a series which never fails to occur under the conditions named,
I append tracings to give a better idea of the very marked and
striking character of the results. The first tracing (Fig. 2) is a
record of the successive increments of the responses to successive
induction shocks of the same intensity, thrown in at three seconds'
intervals--the cylinder being stationary during each response, and
rotated a short distance with the hand during each interval of repose.
[Illustration: Fig. 2.]
The second tracing (Figs. 3 and 4) is a record of the difference
between the lengths of the latent period, and also between the
strengths of the contraction, in the case (_a_) of the first of
such a series of responses (Fig. 3), and (_b_) of the last of such
a series (Fig. 4). From these tracings it will be manifest, without
further comment, how surprising is the effect of a series of stimuli;
first, in _arousing_ the tissue, as it were, to increased _activity_,
and, second, in developing a state of _expectancy_.
In accordance with the now customary terminology, I shall call such
a series of responses as are given in Fig. 2 a "staircase." Such a
staircase has a greater number of steps in it if caused by a weak
current (compare Figs. 2 and 5); and if the strength of the current
be suddenly increased after the maximum level of a staircase has
been reached by using a feeble current, this level admits of being
slightly raised (see Fig. 5). Moreover, I find that a stimulus,
which at the bottom of a staircase is of less than minimal intensity,
is able, at the top of a staircase, to give rise to a contraction
of very nearly maximum intensity. That is to say, by employing
an induction stimulus of slightly less than minimal intensity in
relation to the original irritability of the tissue, no response is
given to the first two or three shocks of a series; but at the third
or fourth shock a slight response is given, and from that point
onward the staircase is built up as usual. This was the case in the
experiment of which Fig. 2 is a record, no response having been given
to the first two shocks.
[Illustration: Fig. 3.]
[Illustration: Fig. 4.]
[Illustration: Fig. 5.]
With regard to this interesting staircase action, two questions
naturally present themselves. In the first place, we are anxious
to know whether the arousing effect which is so conspicuous in
a staircase series is due to the occurrence of the previous
_stimulations_, or to that of previous _contractions_; and, in the
next place, we should like to know whether, during the _natural_
rhythm of the tissue, each contraction exerts a beneficial influence
on its successor, analogous to that which occurs in the case of
contractions which are due to _artificial_ stimuli. To answer the
first of these questions, therefore, I built up a staircase in the
ordinary way, and then suddenly transferred the electrodes to the
opposite side of the umbrella from that on which they rested while
constructing the staircase. On now throwing in another shock at this
part of the contractile tissue so remote from the part previously
stimulated, the response was a maximum response. Similarly, if the
electrodes were transferred in the way just described, not after the
maximum effect had been attained, but at any point during the process
of constructing a staircase, the response given to the next shock was
of an intensity to make it rank as the next step in the staircase.
Hence, shifting the position of the electrodes in no wise modifies
the peculiar effect we are considering; and this fact conclusively
proves that the effect is a general one, pervading the whole mass of
the contractile tissue, and not confined to the locality which is
the immediate seat of stimulation. Nevertheless, this fact does not
tend to prove that the staircase effect depends on the process of
contraction as distinguished from the process of stimulation, because
the wave of the former process must always precede that of the
latter. But, on the other hand, in this connection it is of the first
importance to remember the fact already stated, viz. that a current
which at the beginning of a series of stimulations is of slightly
less than minimal intensity presently becomes minimal, and eventually
of much more than minimal intensity--a staircase being thus built up
of which the first observable step (or contraction) only occurs in
response to the second, third, or even fourth shock of the series.
This fact conclusively proves that the staircase effect, at any
rate at its commencement, depends on the process of stimulation as
distinguished from that of contraction; for it is obvious that the
latter process cannot play any part in thus constructing what we may
term the invisible steps of a staircase.
To answer the second of the above questions, I placed an Aurelia with
its concave surface uppermost, and removed seven of its lithocysts;
I then observed the spontaneous discharge of the remaining one, and
found it to be conspicuous enough that, after the occurrence of one
of the natural pauses (if this were of sufficient duration), the
first contraction was feeble, the next stronger, the next still
stronger, and so on, till the maximum was attained. This natural
staircase action admits of being very prettily shown in another
way. If a tolerably large Aurelia is cut into a spiral strip of
small width and great length, and if all the lithocysts are removed
except one at one end of the strip, it may be observed that,
after the occurrence of a natural pause, the first discharge only
penetrates perhaps about a quarter of the length of the strip, the
next discharge penetrates a little further, the next further, and so
on, till finally the contraction waves pass from end to end. On now
removing the ganglion, waiting a few minutes, and then stimulating
with successive induction shocks, the same progressive penetration is
observable as that which previously took place with the ganglionic
stimulation. Lastly, the identity of natural and artificial
staircase action may be placed beyond all doubt by an experiment in
which the effects of induction shocks and of ganglionic discharges
are combined. To accomplish this, all the lithocysts save one are
removed, and a staircase is then built up in the ordinary way by
successive induction shocks. It will now occasionally happen that the
ganglion originates a discharge during the process of constructing
the staircase, which is being built up by the artificial stimuli;
when this happens the resulting contraction takes its proper rank
in the series, and this at whatever point the natural contraction
happens to come in.
Thus, then, to summarize and conclude these observations, we have
seen that if a single stimulation, whether of a natural or artificial
kind, is supplied to the excitable tissues of a jelly-fish, a short
period, called the period of latency, will elapse, and then the
jelly-fish will give a single weak contraction. If, as soon as the
tissue has relaxed, the stimulation is again repeated, the period of
latency will be somewhat shorter, and will be followed by a somewhat
stronger contraction. Similarly, if the stimulation is repeated a
third time, the period of latency will be still shorter, and the
ensuing contraction still stronger. And so on up to nine or ten
times, when the period of latency will be reduced to its _minimum_,
while the force of the contraction will be raised to its _maximum_;
so that in the jelly-fish, the effect of a series of excitations
supplied at short intervals from one another is that of both arousing
the tissue into a state of increased _activity_, and also of
producing in it a state of greater _expectancy_. We have, moreover,
seen that this effect depends upon the repetition of the process of
stimulation, and not upon that of the process of contraction.
Now, effects very similar to these have been found to occur in the
case of the excitable plants by Dr. Burdon-Sanderson; in the case of
the frog's heart by Dr. Bowditch; and in the case of reflex action
of the spinal cord by Dr. Stirling. Indeed, the only difference in
this respect between these four tissues, so widely separated from one
another in the biological scale, consists in the _time_ which may be
allowed to elapse between the occurrence of the successive stimuli,
in order to produce this so-called summating effect of one stimulus
upon its successor: the _memory_, so to speak, of the heart-tissue
for the occurrence of a former stimulus being longer than the memory
of the jelly-fish tissue; while the memory of the latter is longer
than that of the plant tissue. And I may here add that even in our
own organization we may often observe the action of this principle of
the summation of stimuli. For instance, we can tolerate for a time
the irritation caused by a crumb in the larynx, but very rapidly
the sense of irritation accumulates to a point at which it becomes
impossible to avoid coughing. And similarly with tickling generally,
the convulsive reflex movements to which it gives rise become more
and more incontrollable the longer the stimulation is continued,
until they reach a _maximum_ point, where, in persons susceptible
to this kind of stimulation, the muscular action passes completely
beyond the power of the will. Lastly, I may further observe, what I
do not think has ever been observed before, that even in the domain
of psychology the action of this principle admits of being clearly
traced. We find it, for instance, in the rhythmical waves of emotion
characteristic of grief, and at the other extreme we find it in the
case of the ludicrous. We can endure for a short time, without giving
any visible response, the psychological stimulation which is supplied
by a comical spectacle; but if the latter continues sufficiently
long in a sufficiently ludicrous manner, our appropriate emotion
rapidly runs up to a point at which it becomes uncontrollable, and
we burst into an explosion of ill-timed laughter. But in this case
of psychological tickling, as in the previous case of physiological
tickling, some persons are much more susceptible than others.
Nevertheless, there can be no doubt that from the excitable tissues
of a plant, through those of a jelly-fish and a frog, up even to the
most complex of our psychological processes, we have in this recently
discovered principle of the summation of stimuli a very remarkable
uniformity of occurrence.
_Effects of Temperature on Excitability._
I shall now conclude this chapter with a brief statement of the
effects of temperature on the excitability of the Medusæ; and before
stating my results, I may observe that in all my experiments in this
connection I changed the temperature of the Medusæ by drawing off
the water in which they floated with a siphon, while at the same
time I substituted water of a different temperature from that which
I thus abstracted. In this way, without modifying any of the other
conditions to which the animals were exposed, I was able to observe
the effects of changing the temperature alone.
With regard to the effect of temperature on the latent period of
stimulation, the following table, setting forth the results of one
among several experiments, explains itself.
Period of latent stimulation of the deganglionated tissues of Aurelia
aurita as affected by temperature:--
-------------------------------+-------------------------------
Temperature of water (Fahr.). | Period of latent stimulation.
-------------------------------+-------------------------------
70° | 1/5 second
50° | 1/3 second
35° | 2/5 second
20° | 1/2 second
-------------------------------+-------------------------------
In the case of each observation, several shocks were administered
before the latent period was taken, in order to decrease this period
to its _minimum_ by the staircase action. When this is not done, the
latent period at 20° may be as long as 1-1/5 seconds; but soon after
this irritability disappears.
The extraordinary sluggishness of the latent period at very low
temperatures is fully equalled by the no less extraordinary
sluggishness of the contraction.
In order to render apparent the degree in which both these effects
are produced, I here append a pair of tracings which were procured
from the same piece of tissue when exposed to the different
temperatures named. (N.B.--The seconds are wrongly marked in Fig. 7;
they ought to be the same as in Fig. 6.)
[Illustration: Fig. 6.]
[Illustration: Fig. 7.]
I may as well state here that in water at all temperatures, within
the limits where responses to stimuli are given at all, the staircase
action admits of being equally well produced; but in order to
procure the _maximum_ effect for any given temperature, the rate at
which the successive stimuli are thrown in must be quicker in warm
than in cold water.