EXPERIMENTS IN SECTION OF NAKED-EYED MEDUSÆ.
_Distribution of Nerves in Sarsia._
My experiments have shown that the nervous system in the naked-eyed
Medusæ is more highly organized, or integrated, than it is in the
covered-eyed Medusæ; for whereas in the latter I obtained no evidence
of the gathering together of nerve-fibres into definite bundles or
trunks (the plexus being evenly distributed over the entire surface
of the neuro-muscular sheet lining the umbrella), in the former I
found abundant evidence of this advance in organization. And as the
experiments in this connection serve to substantiate the histological
researches of Professors Haeckel, Schultz, Eimer, and Hertwig, in
as far as the distribution of the main nerve-trunks is concerned, I
shall here detail at some length the character and results of these
experiments in the case of Sarsia.
The occurrence of reflex action in Sarsia is of a very marked and
unmistakable character. I may begin by stating that when any part
of the internal surface of the bell is irritated, the manubrium
responds; but as there is no evidence of ganglia occurring in the
manubrium, this cannot properly be regarded as a case of reflex
action. But now the converse of the above statement is likewise true,
viz. that when any part of the manubrium is irritated, the bell
responds; and it is in this that the unequivocal evidence of reflex
action consists, for while the sympathy of the manubrium with the
bell is not in the least impaired by removing the marginal ganglia of
the latter, the sympathy of the bell with the manubrium is by this
operation entirely destroyed.
We have thus very excellent demonstration of the occurrence of reflex
action in the Medusæ. Further experiments show that the reflex action
occurs, not between the marginal ganglia and every part of the
manubrium, but only between the marginal ganglia and the point of the
bell from which the manubrium is suspended--it being only the pull
which is exerted upon this point when the manubrium contracts that
acts as a stimulus to the marginal ganglia. But the high degree of
sensitiveness shown by the marginal ganglia to the smallest amount
of traction of this kind is quite as remarkable as their lack of
sensitiveness to disturbances going on in the manubrium.
Turning now to the physiological evidence of the distribution of
nerves in Sarsia, when one of the four tentacles is very gently
irritated, it alone contracts. If the irritation be slightly
stronger, all the four tentacles, and likewise the manubrium,
contract. If one of the four tentacles be irritated still more
strongly, the bell responds with one or more locomotor contractions.
If in the latter case the stimulus be not too strong, or, better
still, if the specimen operated on be in a non-vigorous or in a
partly anæsthesiated state, it may be observed that a short interval
elapses between the response of the tentacles and that of the bell.
Lastly, the manubrium is much more sensitive to a stimulus applied
to a tentacle, or to one of the marginal bodies, than it is to a
stimulus applied at any other part of the nectocalyx.
These facts clearly point to the inference that nervous connections
unite the tentacles with one another and also with the manubrium;
or, perhaps more precisely, that each marginal body acts as a
co-ordinating centre between nerves proceeding from it in four
directions, viz. to the attached tentacle, to the margin on either
side, and to the manubrium. This, it will be observed, is the
distribution which Haeckel describes as occurring in Geryonia, and
Schultz as occurring in Sarsia. It is, further, the distribution to
which my explorations by stimulus would certainly point. But, in
order to test the matter still more thoroughly, I tried the effects
of section in destroying the physiological relations which I have
just described. These effects, in the case of the tentacles, were
sufficiently precise. A minute radial cut (only just long enough to
sever the tissues of the extreme margin) introduced between each pair
of adjacent marginal bodies completely destroyed the physiological
connection between the tentacles. If only three marginal cuts were
introduced, the sympathy between those two adjacent tentacles
between which no cut was made continued unimpaired, while the
sympathy between them and the other tentacles was destroyed.
The nervous connections between the tentacles and the manubrium
are of a more general character than those described between the
tentacles themselves; that is to say, severing the main radial
nerve-trunks produces no appreciable effect upon the sympathy between
the tentacles and the manubrium.
The nervous connections between the whole excitable surface of the
nectocalyx and the manubrium are likewise of this general character,
so that, whether or not the radial nerve-trunks are divided, the
manubrium will respond to irritation applied anywhere over the
internal surface of the nectocalyx. The manubrium, however, shows
itself more sensitive to stimuli applied at some parts of this
surface than it is to stimuli applied at other parts, although in
different specimens there is no constancy as to the position occupied
by these excitable tracts.
_Distribution of Nerves in Tiaropsis Indicans._
[Illustration: Fig. 22.]
We have seen that in Sarsia reflex action obtains between the
manubrium and the nectocalyx; we shall now see that in Tiaropsis
indicans something resembling reflex action obtains between the
nectocalyx and the manubrium. The last-named species is a new one,
which I have described elsewhere, and I have called it "indicans"
from a highly interesting and important peculiarity of function
which is manifested by its manubrium. The Medusa in question measures
about one and a half inches in diameter, and is provided with a
manubrium of unusual proportional size, its length being about
five-eighths of an inch, and its thickness being also considerable.
Now, if any part of the nectocalyx be irritated, the following
series of phenomena takes place. Shortly after the application of
the stimulus, the large manubrium suddenly contracts--the appearance
presented being that of an exceedingly rapid crouching movement.
The crouching attitude in which this movement terminates continues
for one or two seconds, after which the organ begins gradually to
resume its former dimensions. Concurrently with these movements on
the part of the manubrium, the portion of the nectocalyx which has
been stimulated bends inwards as far as it is able. The manubrium
now begins to deflect itself towards the bent-in portion of the
nectocalyx; and this deflection continuing with a somewhat rapid
motion, the extremity of the manubrium is eventually brought, with
unerring precision, to meet the in-bent portion of the nectocalyx.
I here introduce a drawing of more than life-size to render a
better idea of this _pointing_ action by the manubrium to a seat
of irritation located in the bell. It must further be stated that
in the unmutilated animal such action is quite invariable, the
tapered extremity of the manubrium never failing to be placed on the
exact spot in the nectocalyx where the stimulation is being, or had
previously been, applied. Moreover, if the experimenter irritates
one point of the nectocalyx, with a needle or a fine pair of forceps
for instance, and while the manubrium is applied to that point he
irritates another point, then the manubrium will leave the first
point and move over to the second. In this way the manubrium may be
made to indicate successively any number of points of irritation;
and it is interesting to observe that when, after such a series
of irritations, the animal is left to itself, the manubrium will
subsequently continue for a considerable time to visit first one and
then another of the points which have been irritated. In such cases
it usually dwells longest and most frequently on those points which
have been irritated most severely.
I think the object of these movements is probably that of stinging
the offending body by means of the urticating cells with which the
extremity of the manubrium is armed. But, be the object what it
may, the fact of these movements occurring is a highly important
one in connection with our study of the distribution of nerves in
Medusæ, and the first point to be made out with regard to these
movements is clearly as to whether or not they are truly of a reflex
character. Accordingly, I first tried cutting off the margin, and
then irritating the muscular tissue of the bell; the movements in
question were performed exactly as before. I was thus led to think
it probable that the reflex centres of which I was in search might
be seated in the manubrium. Accordingly, I cut off the manubrium,
and tried stimulating its own substance directly. I found, however,
that no matter how small a portion of this organ I used, and no
matter from what part of the organ I cut it, this portion would do
its best to bend over to the side which I irritated. Similarly,
no matter how short a stump of the manubrium I left in connection
with the nectocalyx, on irritating any part of the latter, the
stump of the manubrium would deflect itself towards that part of
the bell, although, of course, from its short length it was unable
to reach it. Hence there can be no doubt that every portion of the
manubrium--down, at least, to the size which is compatible with
conducting these experiments--is independently endowed with the
capacity of very precisely localizing a point of irritation which is
seated either in its own substance or in that of the bell.
We have here, then, a curious fact, and one which it will be well to
bear in mind during our subsequent endeavours to frame some sort of a
conception regarding the nature of these primitive nervous tissues.
The localizing function, which is so very efficiently performed
by the manubrium of this Medusa, and which if anything resembling
it occurred in the higher animals would certainly have definite
ganglionic centres for its structural co-relative, is here shared
equally by every part of the exceedingly tenuous contractile tissue
that forms the outer surface of the organ. I am not aware that such
a diffusion of ganglionic function has as yet been actually proved
to occur in the animal kingdom, but I can scarcely doubt that future
investigation will show such a state of things to be of common
occurrence among the lower members of that kingdom.[16]
[16] The only case I know which rests on direct observation,
and which is at all parallel to the one above described, is the
case of the tentacles of Drosera. Mr. Darwin found, when he cut
off the apical gland of one of these tentacles, together with
a small portion of the apex, that the tentacle thus mutilated
would no longer respond to stimuli applied directly to itself.
Thus far the case differs from that of the manubrium of Tiaropsis
indicans, and, in respect of localization of co-ordinating
function, resembles that of ganglionic action. But Mr. Darwin
also found that such a "headless tentacle" continued to be
influenced by stimuli applied to the glands of neighbouring
tentacles--the headless one in that case bending over in whatever
direction it was needful for it to bend, in order to approach the
seat of stimulation. This shows that the analogue of ganglionic
function must here be situated in at least more than one part
of a tentacle; and I think it is not improbable that, if trials
were expressly made, this function would be found to be diffused
throughout the whole tentacle.
I shall now proceed to consider the nature of the nervous connections
between the nectocalyx and manubrium of this Medusa.
Bearing in mind that in an unmutilated Tiaropsis indicans the
manubrium invariably localizes with the utmost precision any minute
point of irritation situated in the bell, the significance of the
following facts is unmistakable, viz. that when a cut is introduced
between the base of the manubrium and the point of irritation in the
bell, the localizing power of the former, as regards that point in
the latter, is wholly destroyed. For instance, if such a cut as that
represented at _a_ (see Fig. 22) be made in the nectocalyx of this
Medusa, the manubrium will no longer be able to localize the seat of
a stimulus applied below that cut, as, for instance, at _b_. Now,
having tried this experiment a number of times, and having always
obtained the same result, I conclude that the nervous connections
between the nectocalyx and the manubrium, which render possible the
localizing action of the latter, are connections the functions of
which are intensely specialized, and the distribution of which is
radial.
So far, then, we have highly satisfactory evidence of tissue-tracts
performing the function of afferent nerves. But another point
of interest here arises. Although, in the experiment just
described, the manubrium is no longer able to _localize_ the seat
of stimulation in the bell, it nevertheless continues able to
perceive, so to speak, that stimulation is being applied in the bell
_somewhere_; for every time any portion of tissue below the cut a
is irritated, the manubrium actively dodges about from one part of
the bell to another, applying its extremity now to this place and
now to that one, as if seeking in vain for the offending body. If
the stimulation is persistent, the manubrium will every now and
then pause for a few seconds, as if trying to decide from which
direction the stimulation is proceeding, and will then suddenly move
over and apply its extremity, perhaps to the point that is opposite
to the one which it is endeavouring to find. It will then suddenly
leave this point and try another, and then another, and another,
and so on, as long as the stimulation is continued. Moreover, it is
important to observe that there are _gradations_ between the ability
of the manubrium to localize correctly and its inability to localize
at all, these gradations being determined by the circumferential
distance from the end of the cut and the point of stimulation. For
instance, in Fig. 22, suppose a cut A B, quarter of an inch long,
to be made pretty close to the margin and concentric with it, then
a stimulus applied at the point _c_, just below the middle point of
A B, would have the effect of making the manubrium move about to
various parts of the bell, without being able in the least degree
to localize the seat of irritation. But if the stimulus be applied
at _d_, the manubrium will probably be so far able to localize the
seat of irritation as to confine its movements, in its search for the
offending body, to perhaps the _quadrant_ of the bell in which the
stimulation is being applied. If the stimulation be now supplied at
_e_, the localization on the part of the manubrium will be still more
accurate; and if applied at _f_ (that is, _almost_ beneath one end of
the cut A B), the manubrium may succeed in localizing quite correctly.
These facts may also be well brought out by another mode of section,
viz. by cutting round a greater or less extent of the marginal
tissue, leaving one end of the resulting slip free, and the other
end attached _in situ_. If this form of section be practised on
Tiaropsis indicans, as represented at _g_ _k_ in the figure, it may
also be observed that irritation of a distant point in the marginal
strip, such as _g_ or _h_, causes the manubrium to move in various
directions, without any special reference to that part of the bell
which the irritated point of the marginal strip would occupy if _in
situ_. But if the stimulation be applied only one or two millims.
from the point of attachment of the marginal strip, as at _i_, the
manubrium will confine its localizing motions to perhaps the proper
quadrant of the bell; and if the stimulus be applied still nearer
to the attachment of the severed strip, as at _j_, the localizing
motions of the manubrium may become quite accurate.
Again, with regard to _radial_ distance, if the cut A B in the figure
were situated higher up in the bell, as at A´ B´, and the arc, _c_,
_d_, _e_, _f_, of the margin irritated as before, the manubrium
would be able to localize better than if, as before, the radial
distance between A B and _c_, _d_, _e_, _f_ were less. The greater
this radial distance, the better would be the localizing power of
the manubrium; so that, for instance, if the cut A´ B´ were situated
nearly at the base of the manubrium, the latter organ might be able
to localize correctly a stimulus applied, not only as before at _f_,
but also at _e_ or _d_. In such comparative experiments, however, it
is to be understood that the higher up in the bell a cut is placed,
the shorter it must be; for a fair comparison requires that the
two ends of the cut shall always touch the same two radii of the
nectocalyx. Still, if the cut is only a very short one (say one or
two millims. long), this consideration need not practically be taken
into account; for such a cut, if situated just above the margin, as
represented at _a_, will have the effect of destroying the localizing
power of the manubrium as regards the corresponding arc of the
margin; but if situated high up in the bell, even though its length
be still the same, it will not have this effect.
From all this, then, we have seen that the connections which render
possible the _accurate_ localizing functions of the manubrium are
almost, though not quite, exclusively radial. We have also seen that
between accurate localization and mere random movements on the part
of the manubrium there are numerous gradations, the degree of decline
from one to the other depending on the topographical relations
between the point of stimulation and the end of the section (the
section being of the form represented by A B in the figure). These
relations, as we have seen, are the more favourable to correct
localization: (_a_) the greater the radial distance between the point
of stimulation and the end of the section; and (_b_) the less the
circumferential distance between the point of the stimulation and
the radius let fall from the end of the section. But we have seen
that the limits as regards severity of section within which these
gradations of localizing ability occur, are exceedingly restricted--a
cut of only a few millims. in length, even though situated at the
greatest radial distance possible, being sufficient to destroy all
localizing power of the manubrium as regards the middle point of the
corresponding arc of the margin, and a stimulus applied only a few
millims. from the attached end of a severed marginal strip entirely
failing to cause localizing action of the manubrium. Lastly, we have
seen that even after all localizing action of the manubrium has been
completely destroyed by section of the kinds described, this organ
nevertheless continues actively, though ineffectually, to search for
the seat of irritation.
[Illustration: Fig. 23.]
The last-mentioned fact shows that after excitational continuity
of a higher order has been destroyed, excitational continuity of a
lower order nevertheless persists; or, to state the case in other
words, the fact in question shows that after severance of the almost
exclusively radial connections between the bell and the manubrium, by
which the perfect or unimpaired localizing function of the latter
is rendered possible, other connections between these organs remain
which are not in any wise radial. I therefore next tested the degree
in which these non-radial connections might be cut without causing
destruction of that excitational continuity of a lower order which
it is their function to maintain. It will here suffice to record one
mode of section which has yielded definite results. A glance at the
accompanying illustration (Fig. 23) will show the manner in which
the Medusa is prepared. The margin having been removed (in order to
prevent possible conduction by the marginal nerve-fibres), a single
deep radial cut (_a_ _a_) is first made, and then a circumferential
cut (_a_, _b_, _c_) is carried nearly all the way round the base of
the manubrium. In this way the nectocalyx, deprived of its margin,
is converted into a continuous band of tissue, one of the ends of
which supports the manubrium. Now it is obvious that this mode of
section must be very trying to nervous connections of any kind
subsisting between the bell and the manubrium. Nevertheless, in many
cases, irritating any part of the band _a_ _l_ has the effect of
causing the manubrium to perform the active random motions previously
described. In such cases, however, it is observable that the further
away from the manubrium the stimulus is applied, the less active
is the response of this organ. In very many instances, indeed, the
manubrium altogether fails to respond to stimuli applied at more than
a certain distance from itself. For example, referring to Fig. 23,
the manubrium might actively respond to irritation of any point in
the division _d_, _e_, _f_, _g_, while to irritation of any point
in the division _f_, _g_, _h_, _i_ its responses would be weaker,
and to irritation of any point in _h_, _i_, _j_, _k_, they would be
very uncertain or altogether absent. Hence in this form of section
we have reached about the limit of tolerance of which the non-radial
connections between the bell and manubrium are capable.
Another interesting fact brought out by this form of section is, that
the radial tubes are tracts of comparatively high irritability as
regards the manubrium; for the certainty and vigour with which the
manubrium responds to a stimulus applied at one of the severed radial
tubes, _f_, _g_, or _h_, _i_, or _j_, _k_, contrast strongly with the
uncertainty and feebleness with which it often responds to stimuli
applied between any of these tubes. Indeed, it frequently happens
that a specimen which will not respond at all to a stimulus applied
between two radial tubes, will respond at once to a stimulus applied
much further from the manubrium, but in the course of the radial tube
_f_ _k_.
And this leads us to another point of interest. In such a form of
section, when any part of the mutilated nectocalyx is irritated, the
manubrium shows a very marked tendency to touch some point in the
tissue-mass _a_ _a_ _d_ _e_ (Fig. 23) by which it still remains in
connection with the bell, and through which, therefore, the stimulus
must pass in order to reach the manubrium. And it is observable that
this tendency is particularly well marked if the section has been
planned as represented in Fig. 23, _i.e._ in such a way as to leave
the tissue-tract _a_ _a_ _d_ _e_ pervaded by a nutrient-tube _d_ _e_,
this tube being thus left intact. When this is done, the manubrium
most usually points to the uninjured nutrient-tube _d_ _e_ every time
any part of the tissue-band _a_ _l_ is irritated.
Let us now very briefly consider the inferences to which these
results would seem to point. The fact that the localizing power of
the manubrium is completely destroyed as regards all parts of the
bell lying beyond an incision in the latter, conclusively proves,
as already stated, that all parts of the bell are pervaded by
radial lines of differentiated tissue, which have at least for one
of their functions the conveying of impressions to the manubrium.
The fact in question also proves that the particular effect which
is produced on the manubrium by stimulating any one of these lines
cannot be so produced by stimulating any of the other lines. But
although these tracts of differentiated tissue thus far resemble
afferent nerves in their function, we soon see that in one important
particular they differ widely from such nerves; for we have seen
that, after they have been divided, stimulation of their peripheral
parts still continues to be transmitted to their central parts,
as shown by the non-localizing movements of the manubrium. Of
course this transmission cannot take place through the divided
tissue-tracts themselves; and hence the only hypothesis we can
frame to account for the fact of its occurrence is that which would
suppose these tissue-tracts, or afferent lines, to be capable of
vicarious action. Such vicarious action would probably be effected
by means of intercommunicating fibres, the directions of which would
probably be various. In this way we arrive at the hypothesis of the
whole contractile sheet being pervaded by an intimate plexus of
functionally differentiated tissue, the constituent elements of which
are capable of a vicarious action in a high degree.
Now we know from histological observation that there is a plexus of
nerve-fibres pervading the whole expanse of the contractile sheet,
and therefore we may conclude that this is the tissue through which
the effects are produced. But, if so, we must further conclude that
the fibres of this nerve-plexus are capable of vicarious action in
the high degree which I have explained.
And this hypothesis, besides being recommended by the consideration
that it is the only one available, is confirmed by the fact that
the stimuli which it supposes to escape from a severed phalanx of
nerve-fibres, and then to reach the manubrium after being diffused
through many or all of the other radial lines (such stimuli thus
converging from many directions), are responded to when they reach
the manubrium, not by any decided localizing action on the part of
the latter, but, as the hypothesis would lead us to expect, by the
tentative and apparently random motions which are actually observed.
Moreover, we must not neglect to notice that these tentative or
random movements resemble in every way the localizing movements,
save only in their want of precision. Again, this hypothesis is
rendered more probable by the occurrence of those _gradations_ in
the localizing power of the manubrium which we have seen to be
so well marked under certain conditions. The occurrence of such
gradations under the conditions I have named is what the theory
would lead us to expect, because the closer beneath a section that
a stimulus is applied, the greater must be the immediate lateral
spread of the stimulus through the plexus before it reaches the
manubrium. Similarly, the further the circumferential distance from
the nearest end of such a section that the stimulus is applied, the
greater will be its lateral spread before reaching the manubrium.
Lastly, the present hypothesis would further lead us to anticipate
the fact that when Tiaropsis indicans is prepared as represented in
Fig. 23, the manubrium refers a stimulus applied anywhere in the
mutilated nectocalyx to the band of tissue by which it is still left
in connection with that organ; for it is evident that, according to
the hypothesis, the radial fibres occupying such a band are the only
ones whose irritation the manubrium is able to perceive, and hence it
is to be expected that it should tend to refer to these particular
fibres a source of irritation occurring anywhere in the mutilated
bell.
It is not quite so easy to understand why, in the last-mentioned
experiment, the manubrium should tend to refer a seat of irritation
to the unsevered nutrient tube, or nerve-trunk, rather than to the
unsevered nerves in the general nerve-plexus on either side of
that nerve-trunk; for if this nerve-trunk at all resembles in its
functions the nerve-trunks of higher animals, the afferent elements
collected in it ought to communicate to the manubrium the impression
of having had their _distal_ terminations irritated, and therefore
the fact of a number of such elements being collected into a single
trunk ought not to cause the manubrium to refer a distant seat of
irritation to that trunk rather than to any of the parts from which
the plexus-elements may emanate. Concerning this difficulty, however,
I may observe that we seem to have in it one of those cases in which
it would be very unsafe to argue, with any confidence, from the
highly integrated nervous systems with which we are best acquainted,
to the primitive nervous systems with which we are now concerned. And
although it would occupy too much space to enter into a discussion
of this subject, I may further observe that I think it is not at
all improbable that the manubrium of Tiaropsis indicans should, in
the absence of more definite information, refer a distant seat of
injury to that tract of collected afferent elements through which it
actually receives the strongest stimulation.
_Staurophora Laciniata._
This is a Medusa about the size of a small saucer which responds to
stimulation of its marginal ganglia, or radial nerve-trunks, by a
peculiar spasmodic movement. This consists in a sudden and violent
contraction of the entire muscle-sheet, the effect of which is
to draw together all the gelatinous walls of the nectocalyx in a
far more powerful manner than occurs during ordinary swimming. In
consequence of this spasmodic action being so strong, the nectocalyx
undergoes a change in form of a very marked and distinctive
character. The corners of the four radial tubes, being occupied by
comparatively resisting tissue, are not so much affected by the
spasm as are other parts of the bell; and they therefore constitute
a sort of framework upon which the rest of the bell contracts, the
whole bell thus assuming the form of an almost perfect square, with
each side presenting a slight concavity inwards. These spasmodic
movements, however, are quite unmistakable when they occur even in
a very minute portion of detached tissue; for, however large or
small the portion may be, when in a spasm it folds upon itself with
the characteristically strong and persistent contraction. I say
_persistent_ contraction, because a spasmodic contraction, besides
being of unusual strength, is also of unusual duration; that is to
say, while an ordinary systolic movement only lasts a short time,
a spasm lasts from six to ten seconds or more, and this whether it
occurs in a large or in a small piece of tissue. Again, the diastolic
movements differ very much in the case of an ordinary locomotor
contraction and in that of a spasm; for while in the former case the
process of relaxation is rapid even to suddenness, in the latter it
is exceedingly prolonged and gradual, occupying some four or five
seconds in its execution, and, from its slow but continuous nature,
presenting a graceful appearance. Lastly, the difference between the
two kinds of contraction is shown by the fact that, while a spasm is
gradually passing off the ordinary rhythmical contractions may often
be seen to be superimposed on it--both kinds of contraction being
thus present in the same tissue at the same time.
Now the point with which we shall be especially concerned is, that
it is only stimulation of _certain parts_ of the organism which has
the effect of throwing it into a spasm. These parts are the margin
(including the tentacles) and the courses of the four radial tubes
(including the manubrium, which in this species is spread over the
radial tubes). This limitation, however, is not invariable; for I
have often seen individuals of this species respond with a spasm
to irritation of the general contractile tissue. Nevertheless,
such response to such stimulation in the case of this species is
exceptional--the usual response to muscular irritation being an
ordinary locomotor contraction, which forms a marked contrast to the
tonic spasm that _invariably_ ensues upon stimulation of the margin,
and _almost_ invariably upon the stimulation of a radial tube.
The first question I undertook to answer was the amount of section
which the excitable tissues of Staurophora laciniata would endure
without losing their power of conducting the spasmodic contraction
from one of their parts to another. This was a very interesting
question to settle, because Staurophora laciniata, like all the other
species of discophorus naked-eyed Medusæ, differs from Aurelia, etc.,
in that the ordinary contraction-waves are very easily blocked by
section. It therefore became interesting to ascertain whether or not
the wave of spasm admitted of being blocked as easily. First, then,
as regards the margin. If this be all cut off in a continuous strip,
with the exception of one end left attached _in situ_, irritation
of any part of the almost severed strip will cause a responsive
spasm of the bell, so soon as the wave of stimulation has time to
reach the latter. I next continued this form of section into the
contractile tissues themselves, carrying the incision round and round
the bell in the form of a spiral, as represented in the case of
Aurelia by Fig. 11, page 70. In this way I converted the whole Medusa
into a ribbon-shaped piece of tissue;[17] and on now stimulating
the marginal tissue at one end of the ribbon, a portion of the
latter would go into a spasm. The object of this experiment was to
ascertain how far into the ribbon-shaped tissue the wave of spasm
would penetrate. As I had expected, different specimens manifested
considerable differences in this respect, but in all cases the degree
of penetration was astonishingly great. For it was the exception to
find cases in which the wave of spasm failed to penetrate from end
to end of a spiral strip caused by a section that had been carried
twice round the nectocalyx; and this is very astonishing when we
remember that the ordinary contraction-waves, whether originated by
stimulation of the contractile tissues or arising spontaneously from
the point of attachment of the marginal strip, usually failed to
penetrate further than a quarter of the way round. Moreover, these
waves of spasm will continue to penetrate such a spiral strip even
after the latter has been submitted to a system of interdigitating
cuts of a very severe description.
[17] It may be stated that while conducting this mode of section
of Staurophora laciniata, the animal responds to each cut of the
contractile tissues with a locomotor contraction (or it may not
respond at all); but each time the section crosses one of the
radial tubes, the whole bell in front of the section, and the
whole strip behind it, immediately go into a spasm.
Now, we have here to deal with a class of facts which physiologists
will recognize as of a perfectly novel character. Why it should be
that the very tenuous tracts of tissue which I have named should
have the property of responding even to a feeble stimulus by issuing
an impulse of a kind which throws the contractile tissues into a
spasm; why it should be that a spasm, when so originated, should
manifest a power of penetration to which the normal contractions of
the tissues in which it occurs bear so small a proportion; why it is
that the contractile tissues should be so deficient in the power of
originating a spasm, even in response to the strongest stimulation
applied to themselves;--these and other questions at once suggest
themselves as questions of interest. At present, however, I am wholly
unable to answer them; though we may, I think, fairly assume that it
is the ganglionic element in the margin, and probably also in the
radial tubes, which responds to direct stimulation by discharging
a peculiar impulse, which has the remarkable effect in question.
For the sake of rendering the matter quite clear, let us employ a
somewhat far-fetched but convenient metaphor. We may compare the
general contractile tissues of this Medusa to a mass of gun-cotton,
which responds to ignition (direct stimulation) by burning with
a quiet flame, but to detonation (marginal stimulation) with an
explosion. In the tissue, as in the cotton, every fibre appears to
be endowed with the capacity of liberating energy in either of two
very different ways; and whenever one part of the mass is made, by
the appropriate stimulus, to liberate its energy in one of these
two ways, all other parts of the mass do the same, and this no
matter how far through the mass the liberating process may have to
extend. Now, employing this metaphor, what we find is that, while the
contractile fibres resemble the cotton fibres in the respects just
mentioned, the ganglion cells resemble detonators, when themselves
directly stimulated. In other words, the ganglion-cells of this
Medusa are able to originate two very different kinds of impulse,
according as they liberate their energy spontaneously or in answer to
direct stimulation, and the muscular tissues respond with a totally
different kind of contraction in the two cases. Possibly, indeed,
direct stimulation of the ganglia is followed by a spasm of the
muscular tissue only because a greater amount of ordinary ganglion
influence is thus liberated than in the case of a merely spontaneous
discharge. If this were the explanation, however, I should not
expect so great a contrast as there is between the facility with
which a spasm may be caused by stimulation of the margin and of the
contractile tissue respectively. The slightest nip of the margin of
Staurophora laciniata, for instance, is sufficient to cause a spasm,
whereas even crushing the contractile tissues with a large pair of
dissecting-forceps will probably fail to cause anything other than an
ordinary contraction. Nevertheless, pricking the margin with a fine
needle usually has the effect of causing only a locomotor contraction.
In conclusion, I may state that anæsthetics have the effect of
blocking the spasmodic wave in any portion of tissue that is
submitted to their influence. It is always observable, however, that
this effect is not produced till after spontaneity has been fully
suspended, and even muscular irritability destroyed as regards direct
stimulation. Up to this stage the certainty and vigour of the spasm
consequent on marginal irritation are not perceptibly impaired;
but soon after this stage the intensity of the spasm begins to
become less, and later still it assumes a _local_ character. It is
important, also, to notice that at this stage the effect of marginal
stimulation is very often that of producing a _general locomotor_
contraction, and sometimes a series of two or three such. During
recovery in normal sea-water all these phases occur in reverse order.