NATURAL RHYTHM.
It will be convenient here to introduce all the observations that
I have been able to make with regard to the natural rhythm of
the Medusæ. As Dr. Eimer has also made some observations in this
connection, before proceeding with the fresh points having relation
to this subject, I shall consider those to which he alludes.
In Aurelia aurita, as Dr. Eimer noticed, the rate of the rhythm has a
tendency to bear an inverse proportion to the size of the individual.
Size, however, is far from being the only factor in determining the
differences between the rate of the rhythm of different specimens,
the individual variations in this respect being very great even among
specimens of the same size. What the other factors in question may
be, however, I am unable to suggest.
Dr. Eimer also affirms that the duration of the natural pauses,
which in Aurelia habitually alternate with bouts of swimming, bears
a direct proportion to the number and strength of the contractions
that occurred in the previous bout of swimming. I observed that
Sarsia are much better adapted than Aureliæ for determining whether
any such precise relation obtains; for, in the first place, the
strength of the contraction is more uniform, and, in the next place,
the alternation of pauses with bouts of swimming is of a more
decided character in Sarsia than in healthy specimens of Aureliæ. I
further observed that in Sarsia no such precise relation did obtain,
although in a very general way it is true, as might be expected, that
unusually prolonged bouts of swimming were sometimes followed by
pauses of unusual duration. As all the observations are very much the
same, I shall only quote two of them:--
--------------------------+---------------------------------
_Sarsia._ | _Sarsia_ (another specimen).
-------------+------------+--------------+------------------
Number of | Seconds of | Number of | Seconds of
pulsations. | rest. | pulsations. | rest.
-------------+------------+--------------+------------------
54 | 90 | 40 | 60
20 | 15 | 29 | 90
9 | 92 | 32 | 132
51 | 40 | 33 | 92
38 | 60 | 18 | 59
1 | 43 | 8 | 63
63 | 45 | 15 | 35
1 | 14 | 2 | 85
60 | 15 | 11 | 63
6 | 50 | 30 | 33
38 | 50 | 17 | 81
22 | 32 | 19 | 67
25 | 12 | 3 | 65
56 | 55 | 19 | 36
65 | 20 | 41 | 123
42 | 15 | 80 | 23
35 | 40 | 61 | 150
76 | 43 | 45 | 145
| | 40 | 120
| | 10 | 97
| | 14 | 35
-------------+------------+--------------+------------------
These observations may be taken as samples of others which it would
be unnecessary to quote, as it will be seen from the above that there
is no precise relation between the number of the pulsations and
the duration of the pauses. Nevertheless, that there is a general
relation may be seen from some cases in which unusually prolonged
pauses occur. The following instance will serve to show this:--
_Sarsia_ (another specimen).
Number of pulsations. | Seconds of rest.
38 | 30
22 | 35
49 | 40
30 | 45
46 | 20
2 | 15
24 | 380
112 | 20
45 | 185
894 | 30
6 | 45
4 | 140
2 | 185
30 | 210
200 | 60
In this case, the relation between the long pause of 380 seconds
and the subsequent prolonged swimming bout of 112 pulsations is
obvious; also, as the latter was then followed by a short pause of
twenty seconds and another comparatively short bout of forty-five
pulsations, the refreshing influence of the previous 380 seconds rest
may be supposed to have been not quite neutralized by the exhausting
effect of the foregoing 112 pulsations. At any rate, looking to
the general nature of the previous proportions (viz. in their sum
185/211), it is certain that 380/112 leaves a large preponderance in
favour of nutrition, which preponderance is not much modified by
adding the next succeeding proportion, thus, (380 + 20)/(112 + 45) =
400/157. Consequently, the organism may fairly be supposed to have
entered upon the next prolonged period of rest (viz. 185 seconds)
with a large balance of reserve power; so that when to this large
balance there was added the further accumulation due to the further
rest of 185 seconds, we are not surprised to find the next succeeding
swimming bout comprising the enormous number of 894 pulsations.
But this great expenditure of energy seems to have been somewhat
in excess of the energy previously accumulated by the prolonged
rest, for this unusual expenditure seems next to have entailed an
unusually prolonged period of exhaustion. At any rate, it is plainly
observable that the next succeeding proportions are greatly in
favour of repose; for it is not until 360 seconds have elapsed, with
only twelve pulsations in the interval, that energy enough has been
accumulated to cause a moderate bout of thirty pulsations. But next
another long and sustained pause of 240 seconds supervenes, and, the
animal being now fully refreshed with a large surplus of accumulated
energy, the next succeeding swimming bout comprises two hundred
pulsations. Lastly, there succeeded sixty seconds of rest, and here
the observation terminated.[22]
[22] If the reader takes the trouble to ascertain the average
proportion between the number of pulsations and the seconds of
rest in the first observations as far down as the first long
pause, viz. as above stated, 185/211, and if he then balances
the succeeding income and expenditure of energy of all the rest
of the observations, he will find the net result to accord very
precisely with the proportion he previously obtained. But, as
already stated, any such precision as this is certainly the
exception rather than the rule.
It may here be stated that after the sixty seconds of rest
above recorded, the animal began another swimming bout. It was
then immediately bisected, and the subsequent observations are
detailed in the next footnote.
_Effects of Segmentation on the Rhythm._
We have next to consider Dr. Eimer's observations concerning the
effects on the rhythm of Aurelia which result on cutting the animal
into segments; and here, again, I much regret to say that I cannot
wholly agree with this author. He says he found evidence of a very
remarkable fact, viz. that by first counting the natural rhythm of an
unmutilated Aurelia, and then dividing the animal into two halves,
one of these halves into two quarters, and one of these quarters into
two eighths; the sum of the contractions performed by these four
segments in a given time was equal to the number which had previously
been performed in a similar time by the unmutilated animal. And
not only so, but the number of contractions which each segment
contributed to this sum was a number that stood in direct proportion
to the size of the segment; so that the half contracted half as many
times, the quarter a quarter as many times, and the eighth parts
one-eighth part the number of times that the unmutilated Aurelia had
previously contracted in a period of equal duration. I am glad to
observe that Dr. Eimer does not regard this rule otherwise than as
liable to frequent exception; for, as already observed, I cannot say
that my experiments have tended to confirm it. I am only able to say
that there is general tendency for the smaller segments of an Aurelia
divided in this way to contract less frequently than the larger
segments.
It would be tedious and unnecessary to quote any observations in
this connection; but as these observations brought out very clearly
a fact which I had previously suspected, I may detail one experiment
to illustrate this point. The fact in question is, that the _potency
of the lithocysts_ in any given segment of a divided Aurelia has
more to do with the frequency of its pulsations than has the size
of the segment. As previously mentioned, one or more lithocysts may
often be observed to be permanently prepotent over the others; and
I may here observe that the segmentation experiments just described
have shown the converse to be true, viz. that one or more lithocysts
are often permanently feebler than the others. Well, if a specimen
of Aurelia exhibiting decided prepotency in one or more of its
lithocysts be watched for a considerable length of time, so as to be
sure that the prepotency is not of a merely temporary character, and
if the animal be then divided into segments in such a way that the
prepotent lithocysts shall occupy the smaller segments, it may be
observed, provided time be left for the tissues to recover, that the
segments containing the prepotent lithocysts, notwithstanding their
smaller size, contract more frequently than do the larger segments.
Conversely, if the larger segments happen to contain feeble
lithocysts, their contractions will be but few. I have, indeed, seen
cases in which the lithocysts appeared to be quite functionless, so
far as the origination of stimuli was concerned.
The following observations were made on a healthy specimen of
Aurelia having all its lithocysts in good condition, but prepotency
being well marked in the case of one of them, and also, though in
a lesser degree, in the case of another. I divided the animal so
as to leave one of these two prepotent lithocysts in each of the
eighth-part segments, and the next most powerful lithocysts in the
quadrant segment. In the following description, I shall call the
two eighth-part segments A and B, the former letter designating
the segment containing the most powerful lithocyst. The Aurelia
before being divided manifested for several hours a very regular and
sustained rhythm of thirty-two per minute. After its division, the
various segments contracted at the following rates, in one-minute
intervals:--
-----------+------------+------------+--------------+--------------
Time after | | | |
operation.|Segment 1/2.|Segment 1/4.|Segment 1/8 A.|Segment 1/8 B.
-----------+------------+------------+--------------+--------------
1/2 hour. | 20 | 25 | 27 | 15
1 " | 20 | 25 | 27 | 15
2 hours.| 29 | 25 | 27 | 16
4 " | 19 | 16 | 27 | 12
-----------+------------+------------+--------------+--------------
Next morning, the water which contained the segments was somewhat
foul, and this, as is always the case, gave rise to abnormally long
pauses. This effect was much more marked in the case of some of the
segments than in that of others. I therefore observed the segments
over five-minute intervals, instead of one-minute intervals as on the
previous day. The following is a sample of several observations, all
yielding the same general result.
----------------------+------------+---------------+---------------
Segment 1/2. |Segment 1/4.|Segment 1/8 A. |Segment 1/8 B.
-----------+----------+------------+---------------+---------------
Number of |Seconds of| |Continued |Rhythm
pulsations.| rest. | |persistently to|tolerably
-----------+----------+ |contract with a|perfect at 78
12 | 120 |No motion |nearly perfect |in the 5
3 | 10 |during the |rhythm of |minutes; but
2 | 20 |hour of |78 in the 5 |this was
44 | 130 |observation.|minutes during |occasionally
12 | 20 | |the hour |interrupted by
-----------+----------+ |of observation.|long pauses of
73 | 5 min. | | |4 or 5 minutes'
| | | |duration.
--------------+----------+------------+---------------+----------------
Average rate | |Continuous |Interrupted
14-3/5 |No motion. |rhythm at the |rhythm at the
per minute. | |rate of 15-3/5 |rate of 15-3/5
| |per minute. |per minute.
----------------------+------------+---------------+----------------
I now transferred all the segments to fresh sea-water, with the
following results:--
Rhythm during first quarter of an hour immediately after
transference, in five-minute intervals.
----------+-------------+--------------+--------------+-------------
Time. | Segment | Segment |Segment |Segment
| 1/2. | 1/4. 1/8 A. 1/8 B.
----------+-------------+--------------+--------------+-------------
First 5 | | | |
minutes. |139 | 0 |83 |20
|(irregular). | (regular). (irregular).
Second | | | |
5 minutes.| 0 | 0 |68 " | 75
| | (regular).
Third | | | |
5 minutes.|100 |39 |70 " | 69 "
|(regular). | |(irregular). |
----------+-------------+--------------+--------------+-------------
Rhythm two hours after transference (five-minute intervals).
--------------+---------------+----------------+----------------
Segment 1/2. | Segment 1/4. | Segment 1/8 A. | Segment 1/8 B.
--------------+---------------+----------------+----------------
82 (regular). | 77 (regular). | 70 (regular). | 62 (regular).
--------------+---------------+----------------+----------------
Rhythm next day (five-minute intervals).
-------------+--------------+----------------+---------------
Segment 1/2. | Segment 1/4. | Segment 1/8 A. | Segment 1/8 B.
-------------+--------------+----------------+---------------
68 | 55 | 17 | Dead.
-------------+--------------+----------------+---------------
Next day all the segments were dead except the largest one, in which
a single lithocyst still continued to discharge at the rate of
twenty-four in five minutes.
Now, with regard to these tables, it is to be observed that during
the first day the prepotent lithocyst in the eighth-part segment
A maintained an undoubted supremacy over all the others, and that
the same is true of the comparatively potent lithocysts in the
quadrant. (This is not the case with segment B; probably the degree
of prepotency of the lithocyst in this case was not sufficient to
counteract the antagonistic influence of the small size of the
segment.) But next day the supremacy of the small segment A was not
so marked; for although its rhythm was more regular in the stale
water than was that of the largest segment, its actual number of
contractions in a given time was just about equal to that of the
largest segment. Again, after transference to fresh sea-water,
the balance began to fall on the side of the larger segments;
for even the quadrant, which in the stale water had ceased its
motions altogether, now held a middle position between that of the
half-segment and the prepotent eighth-part segment. On the next day,
again, the balance fell decidedly in favour of the larger segments,
and the weaker eighth-part segment died. Lastly, next day all the
smaller segments were dead.
Hence the principal facts to be gathered from these observations are,
that as time goes on the rhythm of all the segments progressively
decreases, and that the decrease is more marked in the case of the
smaller than in that of the larger segments. This lesser endurance
of the smaller segments also finds its expression in their earlier
death. Now as these smaller segments started with a greater
proportional amount of ganglionic power than the larger segments,
their lesser amount of endurance can only, I think, be explained by
supposing that the process of starvation proceeds at a rate inversely
proportional to the size of the segment, a supposition which is
rendered probable if we reflect that the smaller the segment the
greater is the proportional area of severed nutrient tubes.[23] And
in this connection it is interesting to observe that, although the
endurance of the smaller segments was less than that of the larger
as regards the deprivation of nutriment, it was greater than that
of the larger segments as regards the deprivation of oxygen. This
is shown by the greater regularity of the rhythm manifested by the
smaller than by the larger segments in the stale water, and the fact
is presumably to be accounted for by the consideration that the
ganglia in the smaller segments were more potent than those in the
larger.
[23] It may be thought that the greater area of general
tissue-mass in the larger segments than in the smaller, and
not the lesser proportional area of tube-section, is the cause
of the larger segments living longer than the smaller ones. I
am led, however, to reject this hypothesis, because in Sarsia,
where segmentation entails a comparatively small amount of
tube-section, there is no constant rule as to the larger segments
showing more endurance than the smaller ones--the converse
case, in fact, being of nearly as frequent occurrence. I can
only account for this fact by supposing that the endurance of
the segments of Sarsia is determined by the degree in which the
three or four minute open tube-ends become accidentally blocked.
This supposition is the only one I can think of to account for
the astonishing contrasts as to endurance that are presented
by different segments of the same individual, and, I may add,
of different individuals when deprived of their margins and
afterwards submitted to the same conditions. For instance, a
number of equally vigorous specimens had their margins removed,
and were then suspended in a glass cage attached to a buoy in
the sea. Four days afterwards some of the specimens were putrid,
while others were as fresh as they were when first operated on.
Again, as an instance of the experiments in segmentation of
Sarsia, I may quote an experiment in which a score of specimens
were divided in all sorts of ways, such as leaving the manubrium
attached, to one half, or three marginal bodies in one portion
and the remaining marginal body in the other portion, etc.
Yet, although it was very exceptional to find the two portions
presenting an equal degree of endurance, no uniform results
pointing to the cause of the variations could be obtained. In
most cases, however, the energy, as distinguished from the
endurance of the larger segments, was conspicuously greater than
that of the smaller. (But it is curious that in many cases the
effects of _shock_ appeared to be more marked in the larger than
in the smaller segments--the latter, for some time after the
operation, contracting much more frequently than the former.) To
show both these effects, one experiment may be quoted. A specimen
of Sarsia was divided into two parts, of which one was a quadrant.
Immediately after the operation the results were as follows:--
----------------------++----------------------
Portion 1/4. || Portion 3/4.
------------+---------++------------+---------
Number of | Minutes || Number of | Minutes
pulsations | of rest || pulsations | of rest
------------+---------++------------+---------
20 | 0 || 0 | 5
4 | 4 || 10 | 2
15 | 5 || 46 | 1
6 | 3 || 23 | 2
------------+---------++ 49 | 1
45 | 12 || 900 | 1
| || 117 | 1
------------+---------++------------+---------
|| 1145 | 13
----------------------++------------+---------
To show the difference between the _endurance_ of two halves of
a bisected specimen of Sarsia, I may quote one experiment which
was performed on the same specimen as the one mentioned in the
text to show the general relationship between the duration of the
pauses and that of swimming bouts. (See last footnote.)
Immediately after bisection.
----------------------++----------------------
1/2 A. || 1/2 B.
------------+---------++------------+---------
Number of | Seconds || Number of | Seconds
pulsations | of rest || pulsations | of rest
------------+---------++------------+---------
56 | 10 || 82 | 180
150 | 150 || 51 | 20
68 | 335 || 14 | 60
130 | 30 || 13 | 50
46 | 45 || 46 | 45
2 | 10 || 38 | 65
99 | 66 || 18 | 45
103 | 360 || 23 | 60
12 | 4 || 35 | 130
------------+---------++ 105 | 70
Pauses now become || |
longer, and swimming|| |
bouts shorter. || |
----------------------++------------+---------
Twenty-four hours after the operation.
----------------------++----------------------
1/2 A. || 1/2 B.
------------+---------++------------+---------
Number of | Seconds || Number of | Seconds
pulsations | of rest || pulsations | or rest
------------+---------++------------+---------
2 | 363 || 50 | 20
12 | 362 || 81 | 25
4 | 666 || 37 | 101
25 | 300 || 2400 | 60
------------+---------++------------+---------
But although in the case of Sarsia the leaser endurance of
the smaller segment than of the larger cannot be regarded as
a general rule, it may be so regarded, as already stated, in
the case of Aurelia. The following experiment exemplifies this
particular rule even more prettily than does the one quoted in
the text, from the fact that the segments survived the operation
for a greater number of days.
An Aurelia having a regular and well-sustained rhythm of twenty
per minute was divided as already described in the text. In
five-minute intervals on successive days the average rates of the
four segments were as follows:--
Four hours after the operation.
-----------+-----------+-----------+-------------
Seg. 1/2. | Seg. 1/4. | Seg. 1/8. | Seg. 1/8 A.
-----------+-----------+-----------+-------------
100 | 100 | 85 | 90
-----------+-----------+-----------+-------------
Next day.
-----------+-----------+-----------+-------------
88 | 90 | 64 | 58
-----------+-----------+-----------+-------------
Next Day.
-----------+-----------+-----------+-------------
86 | 82 | 62 | 57
-----------+-----------+-----------+-------------
Next Day.
-----------+-----------+-----------+-------------
59 | 45 | 24 | 20
-----------+-----------+-----------+-------------
Next day.
-----------+-----------+-----------+-------------
50 | 49 | 20 | 10
-----------+-----------+-----------+-------------
Next day.
-----------+-----------+-----------+-------------
43 | 33 | 18 | 4
-----------+-----------+-----------+-------------
Next Day.
-----------+-----------+-----------+-------------
39 | 32 | 19 | Dead.
-----------+-----------+-----------+-------------
Next day.
-----------+-----------+-----------+-------------
Seg. 1/2. | Seg. 1/4. | Seg. 1/8. | Seg. 1/8 A.
-----------+-----------+-----------+-------------
33 | 7 | Dead. | 0
-----------+-----------+-----------+-------------
Next day.
-----------+-----------+-----------+-------------
28 | Dead. | 0 | 0
-----------+-----------+-----------+-------------
Next day, the temperature unfortunately rose sufficiently to
cause the death of the single surviving segment, which otherwise
would probably have lived for one or two days longer.
With regard, therefore, to the original point under consideration,
I conclude that, although the size of the segments is doubtless one
factor in determining the relative frequency of contraction, there
are at least two other factors quite as important, viz. the relative
potency of the lithocysts, and the length of time that elapses
between performing the operation and observing the rhythm. Hence it
is that in my experience I have found but very few examples of Dr.
Eimer's rule.
_Effects of Other Forms of Mutilation on the Rhythm._
The next point I have to dwell upon is one of some interest. If
the manubrium of Aurelia, or of any other covered-eyed Medusa, be
suddenly cut off at its base, the swimming motions of the umbrella
immediately become accelerated. This acceleration, however, only
lasts for a few minutes, when it gradually begins to decline,
the rate of the rhythm becoming slower and slower, until finally
it comes to rest at a rate considerably less than was previously
manifested by the unmutilated animal. If a circular piece be now cut
out from the centre of the umbrella, the rhythm of the latter again
becomes temporarily quickened; but, as before, gradual slowing next
supervenes. This slowing, however, proceeds further than in the last
case, so that the rate at which the rhythm next becomes stationary
is even less than before. If, now, another circular ring be cut from
the central part of the umbrella--_i.e._ if the previously open ring
into which this organ had been reduced by the former operation be
somewhat narrowed from within--the same effects on the rhythm are
again observable; and so on with every repetition of the operation,
the rate of the rhythm always being quickened in the first instance,
but then gradually slowing down to a point somewhat below the rate
it manifested before the previous operation. It will here suffice to
quote one experiment among many I have made in this connection:--
An Aurelia manifested a regular and sustained rhythm of 26
Immediately after removal of manubrium, rhythm rose to 36
Rate then gradually fell for a quarter of an hour, and became
stationary at 20
Circular incision just including ovaries caused rhythm to
rise to 26
After gradual fall during quarter of an hour, rhythm became
stationary at 17
Another circular incision carried round midway between the
former one and the margin caused rhythm to rise to 24
Rate again gradually declined, and in a quarter of an hour was 12
Another circular incision was carried round as close to the
margin as was compatible with leaving the physiological
continuity of all the lithocysts intact. Rhythm rose to 14
Within a few minutes it fell to 6
Excepting the cases where the effects of shock are apparent, some
such series of phenomena as those just recorded are always sure to
ensue when a covered-eyed Medusa is mutilated in the way described,
and this kind of mutilation, besides producing such marked effects
on the _rate_ of the rhythm, also produces an effect in impairing
the _regularity_ of the rhythm. In some specimens the latter effect
is more marked than it is in others. The following series of
observations will serve to give a good idea of this effect:--
An Aurelia manifested a regular and sustained rhythm of 36.
Immediately after the removal of the manubrium, the rate of rhythm
in successive minutes was as follows: 40, 39, 37, 35, 32, 30, 29,
26, 24, 18, 14 (40 seconds' pause), 16, 15, 14, 15, 16 (40 seconds'
pause), 22, 20, 19, 15, 16, 17, 14, 13, 13, 15, 16, 16, 17, 18,
14, 12, 13, 11, 12, 9, 15, 16, 14, 12, 9, etc., the rhythm now
continuing very irregular. An hour after the operation, the following
were the number of contractions given in one-minute intervals, the
observations being taken at intervals of ten minutes: 15, 15, 12, 22,
14, etc.
In this experiment, therefore, as soon as the acceleration and
slowing-stages had been passed, viz. about a quarter of an hour after
the operation, a great disturbance was observable in regularity of
the rhythm; for before the removal of the manubrium, the Medusa had
been swimming for hours with perfect regularity.
Before concluding my description of these experiments, it may
perhaps be as well to mention one other, which was designed to meet
a possible objection to the inferences which, as I shall immediately
argue, these experiments seem to sustain. It occurred to me as a
remote possibility that the slowing and irregularity of the rhythm,
which are observable about a quarter of an hour after the operations
described, might be due to the deprivation of adequate nourishment
suffered by the ganglia, in consequence of the escape of nutrient
matter from the cut ends of the nutrient tubes. Accordingly, instead
of cutting off the manubrium, I tried the effect of momentarily
immersing it in hot water, and found that the subsequent disturbances
of the rhythm were precisely similar to those which result from
removal of the manubrium.
Now, to draw any inferences from such meagre facts as the above would
be hazardous, unless we recognize that in so doing our inferences are
not trustworthy. But, with this recognition, I think there will be no
harm in briefly stating the deductions to which the facts, such as
they are, would seem to point.
Physiologists are undecided as to the extent in which many apparently
automatic actions may not really be actions of a reflex kind. Given
any ganglio-muscular tissue which is rhythmically contracting, how
are we to know whether the action of the ganglia is truly automatic,
or sustained from time to time by stimuli proceeding from other parts
of the organism? In most cases experiments cannot be conducted with
reference to this question, but in the case of the Medusæ they may be
so, and it was with the view of throwing light on this question that
the experiments just described were made. Now in these experiments
the fact is sufficiently obvious that mutilations of any part of the
organism modify the rhythm of the marginal ganglia most profoundly.
That this modification does not proceed from shock, would seem to be
indicated by the facts that the first effect of the mutilation is to
_quicken_ the rhythm; that there is a sort of general proportion to
be observed between the amount of tissue abstracted and the degree of
slowing of the rhythm produced; and that the slowing effects continue
for so long a time. All these facts seem to show that we have here
something other than mere shock to deal with.
A strong suspicion, therefore, arises that the cause of the slowing
of the rhythm which results from removing the manubrium, or a
part of the general contractile tissue of the bell, consists in
the destruction of some influence of an afferent character which
had previously emanated from the parts of the organism which have
been removed, and that the normal rhythm before the operation was
partly due to a continuous reception, on the part of the ganglia,
of this afferent or stimulating influence. In support of this
view are the facts that the first effect of such an operation as
we are considering is greatly to accelerate the rhythm, and that
this acceleration then gradually declines through a period of
about a quarter of an hour. These facts tend to support this view,
because, if it is correct, they are what we might anticipate. If the
manubrium, for instance, while _in situ_ is continually supplying
a gentle stimulus to the marginal ganglia, when it is suddenly cut
off, the nerve-tracts through which this stimulating influence had
previously been conveyed must be cut through; and as it is well
known how irritable nerve-fibres are at their points of section,
it is to be expected that the irritation caused by cutting these
nerve-tracts, and probably also by the action of the sea-water on
their cut extremities, would cause them to stimulate the ganglia
more powerfully than they did before their mutilation. And here I
may state that on several occasions, with vigorous specimens, I have
observed a sudden removal of the manubrium to be followed, not merely
with a quickening of the rhythm on the part of the bell, but with a
violent and long-sustained spasm.
Again, as regards the other fact before us, it is obvious that as
soon as the cut extremities of the nerves begin to die down, and
so gradually to lose their irritability, the effect on the rhythm
would be just what we observe it to be, viz. a gradual slowing till
the rate falls considerably below that which was exhibited by the
unmutilated animal. And even the _irregularity_ which is at this
stage so frequently observable is, I think, what we should expect
to find if this view as to the essentially reflex character of the
natural rhythm is the true one.
If this view is the true one, the question next arises as to the
nature of the process which goes on in the excitable tissues, and
which afterwards acts as a stimulus on the ganglionic tissues. This
question, however, I am quite unable to answer. Whether the process
is one of oxygenation, of chemical changes exerted by the sea-water,
or a process of any other kind, further experiments may be able to
show; but meanwhile I have no suggestion to offer.
_Effects of lessening the Amount of Tissue adhering to a Single
Ganglion._
The above experiments led me to try the effects of cutting out a
single lithocyst of Aurelia, and, after the rhythm of the detached
segment had become regular, progressively paring down the contractile
tissues around the ganglion. I found that this process had no very
marked effect on the rhythm, until the paring reached within an inch
or two of the ganglion: then, however, the effect began to show
itself, and with every successive paring it became more marked.
This effect consisted in slowing the rate of the rhythm, but more
especially in giving rise to prolonged pauses: indeed, if only a very
little contractile tissue was left adhering to the ganglion, the
pauses often became immensely prolonged, so that one might almost
suppose the ganglion to have entirely ceased discharging. But if a
stimulus of any kind were then applied, the rhythmic discharges at
once recommenced. These generally continued for some little time at
a slower rate than that which they had manifested before they were
affected by the paring down of the contractile tissue.
_Effects of Temperature on the Rhythm._
The effects of temperature on the rhythm of Medusæ are very decided.
For instance, a specimen of Sarsia which in successive minutes gave
the following number of pulsations, 16, 26, 0, 0, 26, gave sixty
pulsations during the next minute, while a spirit-lamp was held
under the water in which the Medusa was swimming. If hot water be
added to that in which Sarsia are contained until the whole is
about milk-warm, their swimming motions become frantic. If the same
experiment be performed after the margins of the Sarsia have been
removed, the paralyzed bells remain quite passive, while the severed
margins exhibit the frantic motions just alluded to.
In the case of Aurelia aurita, the characteristic effects of
temperature on rhythm may be better studied than in that of Sarsia,
from the fact that the natural motions are more rhythmical and
sustained in the former than in the latter genus. I have, therefore,
in this connection made more observations on Aurelia than on Sarsia.
The following may be taken as a typical experiment.
A small and active specimen of Aurelia contracted with the greatest
regularity 33 times per minute in water kept at 34°; but on
transference to water kept at 49°, the contractions always became
irregular, in respect (_a_) of not having a perfectly constant
rhythm, and (_b_) of exhibiting frequent pauses, which was never the
case in colder water. The rate of rhythm in the warmer water varied
from 37 to 49; and as in these observations no allowance was made
for the occurrence of the pauses, the actual rate of rhythm during
the swimming motions was about 60 per minute. The following are some
sample observations in the case of this specimen:--
----------------------------+---------------------+----------------
Temperature of water (Fahr.)|Number of pulsations.|Seconds of rest.
----------------------------+---------------------+----------------
40° | 41 | 5
" | 49 | 4
Transferred to 34° | 33 | 0
" " | 33 | 0
" " | 33 | 0
" " | 33 | 0
Transferred to 49° | 45 | 4
" " | 39 | 10
" " | 37 | 15
Transferred to 34° | 20 | 0
" " | 30 | 0
" " | 33 | 0
" " | 33 | 0
" " | 33 | 0
" " | 33 | 0
----------------------------+---------------------+----------------
This rate continued quite regularly for a quarter of an hour, when
the observation terminated.
It might naturally be supposed that when the alterations of
temperature between 34° and 49° produce such marked effects on the
rhythm, still greater alterations would be attended with still
greater effects. Such, however, is not the case. Water at 70° or 80°,
for instance, has the effect of permanently _diminishing_ the rate of
the rhythm, after having temporarily raised it for a few seconds. The
following experiment will serve to convey a just estimation of these
facts.
An Aurelia whose rhythm in water at 40° was very regular at
eighteen per minute, was suddenly transferred to water at 80°. In
the immediately succeeding minutes the rhythm was 22, 20, 14. The
latter rate continued for nearly half an hour, when the observation
terminated.
The effect of very warm water, therefore, is to slow the rhythm, as
well, I may add, as to enfeeble the vigour of the contractions. The
case of Medusæ thus differs, in the former respect, from that of the
heart; and I think the reason of the difference is to be found in
the following considerations. Even slight elevations of temperature
are quickly fatal to the Medusæ, so it becomes presumable that
considerable elevations act very destructively on the neuro-muscular
tissues of those animals. This destructive effect of high
temperatures may, therefore, very probably counteract the stimulating
effect which such temperatures would otherwise exert on the natural
rhythm, and hence a point would somewhere be reached at which the
destructive effect would so far overcome the stimulating effect as
to slow the rhythm. That this is probably the true, as it certainly
is the only explanation to be rendered, will, I think, be conceded
when I further state that if an Aurelia be left for some little time
in water at 80°, and then again transferred to water at 30° or 40°,
its original rate of rhythm at the latter temperature does not again
return, but the rhythm remains permanently slowed. And, in favour
of the explanation just offered, it may be further pointed out that
the first effect of sudden immersion in heated water is to _quicken_
the rhythm, it not being for a few seconds, or for even a minute or
two after the immersion, that the rhythm becomes slowed. Lastly, the
slowing takes place gradually; and this is what we should expect if,
as is probable, the destructive effect takes somewhat more time to
become fully developed than does the stimulating effect.
Before leaving the subject of temperature in relation to rhythm, I
must say a few words on the effects of cold. The following may be
regarded as typical experiments.
An Aurelia presenting a regular rhythm of twenty per minute in water
at 45° was placed in water at 19°. Soon after the transference
the rhythm began to slow, and the strength of the contractions
to diminish. Both these phenomena rapidly became more and more
pronounced, till the rhythm fell to ten per minute (still quite
regular), and the contractions ceased to penetrate the muscular
tissue further than an inch or so from the marginal ganglia. Shortly
after this stage pauses became frequent, but mechanical or other
irritation always originated a fresh swimming bout. Next, only one
very feeble contraction was given at long and irregular intervals,
a contraction so feeble that it was restricted to the immediate
vicinity of the lithocyst in which it originated. Soon after this
stage irritability towards all kinds of stimuli entirely ceased,
including even strong spirit dropped on the under surface of the
animal when taken momentarily out of the water. All these stages thus
described were passed through rapidly, the whole series occupying
rather less than five minutes. On now leaving the specimen for ten
minutes and then restoring it to its original water at 45°, all
the above-mentioned stages were passed through in reverse order.
The first faint marginal contraction was confined to the immediate
vicinity of the prepotent lithocyst, and all subsequent contractions
continued to be so for the next three minutes. Rhythm very slow.
Contractions now began to penetrate round the margin, and in eight
minutes from the restoration had gone all the way round, the rate
of their rhythm meanwhile increasing. In two minutes more all the
umbrella was contracting at the rate of fifteen per minute.
In another specimen, subjected to the same conditions, the rate of
recovery was even more rapid, occupying only two minutes altogether;
but in every case the process of recovery is a gradual one, and
differs only in the time it occupies in passing through the various
stages.
_Effects of Freezing Medusæ._
In conclusion, I will describe some rather interesting experiments
that consisted in freezing some specimens of Aurelia into a solid
block of ice. Of course, as sea-water had to be employed, the cold
required was very considerable; but I succeeded in turning out the
Medusæ encased on all sides in a continuous block of sea-water.
By now immersing this block in warm water, I was able to release
the contained specimens, which then presented a very extraordinary
appearance. The thick and massive gelatinous bell of a Medusa is,
as every one knows, chiefly composed of sea-water, which everywhere
enters very intimately into the structure of the tissue. Now,
all this sea-water was, of course, frozen _in situ_, so that the
animals were everywhere and in all directions pierced through by an
innumerable multitude of ice-crystals, which formed a very beautiful
meshwork, pervading the whole substance of their transparent tissues.
These experiments were made in order to ascertain whether the Medusæ,
after having been thus completely frozen, would survive on being
again thawed out, and, if so, whether the freezing process would
exert any permanent influence on the rate of their rhythm. Now in
all the cases the Medusæ, after having been thawed out, presented a
ragged appearance, which was due to the disintegrating effect exerted
by the ice-crystals while forming in the tissues; yet notwithstanding
this mechanical injury superimposed on the physiological effects of
such extreme cold, all the Medusæ recovered on being restored to
sea-water of the normal temperature. The time occupied by the process
of recovery varied in different individuals from a few minutes to
half an hour or more, and it was observable that those specimens
which recovered soonest had the rate of their rhythm least affected
by the freezing. In no case, however, that I observed did the rate
of the rhythm after the freezing return fully to that which had been
manifested before the freezing.
_Effects of Certain Gases on the Rhythm._
_Oxygen._--I will now conclude my remarks on rhythm by very briefly
describing the effects of certain gases. Oxygen forced under
pressure into sea-water containing Sarsia has the effect of greatly
accelerating the rate of their rhythm. The following observation on a
single specimen will serve to render this apparent.
Number of pulsations given by Sarsia in successive five-minute
intervals.
In ordinary sea-water 472, 527, 470
In oxygenated sea-water 800
In ordinary sea-water 268, 350, 430
It will be seen from this observation that the acceleration of the
rhythm due to the oxygenation was most marked; indeed, the pulsations
followed one another so rapidly that it was no easy matter to count
them. It must also be stated that while the animal was under the
influence of oxygen, the duration of the natural pauses between the
swimming bouts was greatly curtailed, the swimming motions, in fact,
being almost quite continuous throughout the five minutes that the
Medusa was exposed to such influence. Lastly, it will be observed
from the above table that the unnatural amount of activity displayed
by the organism while in the oxygenated water entailed on it a
considerable degree of exhaustion, as shown by the fact that even a
quarter of an hour after its restoration to normal water its original
degree of energy had not quite returned.
_Carbonic acid._--As might be expected, this gas has the opposite
effects to those of oxygen. It is therefore needless to say more
about this agent, except that if administered in large doses it
destroys both spontaneity and irritability. Nevertheless, if its
action is not allowed to last too long, the Medusæ will fully recover
on being again restored to normal sea-water.
_Nitrous oxide._--This gas at first accelerates the motions of
Sarsia, but eventually retards them. I omitted, however, to push the
experiment to the stage of complete anæsthesia, which would doubtless
have supervened had the pressure of the gas been sufficiently great.
_Deficient aëration._--It may now be stated that the Medusæ are
exceedingly sensitive to such slight carbonization of the water in
which they are contained as results from their being confined in a
limited body of it for a few hours. The rhythm becomes slowed and
the contractions feeble, while the pauses between the swimming bouts
become more frequent and prolonged. If the water is not changed,
all these symptoms become more marked, and, in addition, the rhythm
becomes very irregular. Eventually the swimming motions entirely
cease; but almost immediately after the animals are restored to
normal sea-water, they recover themselves completely, the rate and
regularity of their rhythm being then quite natural. The suddenness
with which this return to the normal state of things is effected
cannot but strike the observer as very remarkable, and I may mention
that it takes place with equal suddenness at whatever stage in the
above-described process of asphyxiation the transference to normal
sea-water is accomplished.