i63
Muscle Receptor Organs in the Abdomen of Homarus
vulgaris and Palinurus vulgaris
By J. S. ALEXANDROWICZ
{From the Laboratory of the Marine Biological Association,
Plymouth)
With three plates
SUMMARY
I . In the abdomen and thorax of some groups of Crustacea (Stomatopoda, Decapoda Macrura, and Anomura) ganglion cells have been found with ramifications into
special muscle-fibres. It is assumed that these are organs for response to stimuli resulting from muscular activity and therefore the name 'muscle receptor organs' has
been adopted for them. Each muscle receptor unit consists of (a) a thin muscle, (6)
one ganglion cell connected with this muscle by means of numerous dendritic processes, and (c) various nerves supplying the muscle and entering into connexion with
ganglion cells.
This paper describes the results of a study of these organs in the abdomen of Homarus
vulgaris and Palinurus vulgaris.
2. In each of the six abdominal segments of these animals there are two muscle
receptor units on each side lying close to one another at the level of the superficial
dorsal muscles. Their muscle components are quite distinct from the neighbouring
muscles and preserve their individuality throughout their whole course and at their
attachments. Moreover, the two muscles of the same side exhibit differences in their
length, their attachments, and even their histological structure. Each muscle in about
the middle of its length has a region made up of connective tissue fibres which may
be regarded as an intercalated tendon.
3. Situated near to and in connexion with each of these muscle units is one large
nerve-cell; there are, therefore, four such cells in each segment and a total of twentyfour in the abdomen. The cells are multipolar in shape with a variable number of short
dendritic processes abundantly ramifying in the intercalated tendinous region of the
muscle. The long processes, the axons, join the dorsal branch of the nerve supplying
the extensor muscles and run in it towards the ganglionic cord.
4. In preparations made from embryonic lobsters it has been possible to establish
that these axons bifurcate after entering the ganglionic cord, and the resulting branches
run in opposite directions. Associating with similar fibres from other segments they
form a tract situated in the nerve-cord near to its median line and running through all
the ganglia of the abdominal and thoracic segments.
5. It has been found that in addition to the ganglion cells, at least three kinds of
nerves take part in the innervation of the muscle receptors. They have been described
under the names of: (a) motor nerves, (b) thick accessory nerve, and (c) thin accessory
nerve.
6. Special means for protecting the muscle receptor organs are present. The nervecells are encapsuled and encircled by several layers of thin membranous tissue. The
muscles are surrounded by connective tissue fibres and a special arrangement of these
fibres supports the muscles in position.
7. As regards the function of these organs, the hypothesis is put forward that they
might come into action during vigorous movements of the abdomen in the escape
[Quarterly Journal of Microscopical Science, Vol. 92, part 2, June 1951.]
164
Alexandrowicz—Muscle
Receptor Organs in the
reaction of the animal. If this be so, they may perhaps convey inhibitory impulses to
the elements causing the rapid contractions of the flexor muscles. As these contractions
are governed by the giant fibre system it might be expected that the neurons of the
receptor organs enter into relation with some elements of that system.
CONTENTS
PAGE
INTRODUCTION
METHODS
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RESULTS
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M u s c l e E l e m e n t s of t h e R e c e p t o ^ O r g a n s
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T o p o g r a p h y of t h e M u s c l e s of t h e R e c e p t o r O r g a n s .
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Structure of t h e M u s c l e s of the Receptor O r g a n s
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Nerve-cells
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Nerve-fibres
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Motor Nerves
Thick Accessory Nerve
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T h i n Accessory Nerve
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E l e m e n t s of M u s c l e Receptors in t h e E m b r y o n i c Lobster
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192
DISCUSSION
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194
REFERENCES
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EXPLANATION OF PLATES
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INTRODUCTION
I
N the course of investigations made on the innervation of the heart of
Squilla mantis many years ago, I became aware that some muscle-fibres in
the dorsal part of the body of this animal have a peculiar nerve-supply: there
are ganglion cells that ramify in the muscle with numerous short branches.
Later on, during a visit to Naples in 1939, I examined various species of
crustaceans on this point and was able to establish the presence of similar
cells on the dorsal side of the abdominal segments in the stomatopods and all
decapod Macrura and Anomura examined. There were four cells in each
segment, two on each side. It became evident that these cells and the musclebundle with which they are connected constitute an unknown neuromuscular organ endowed most probably with some receptor function. More
detailed observations on the structure of these elements were then made in
Squilla mantis and Pagurus striatus, but unfortunately during the war years
all my preparations and notes were lost. It only became possible to return
to these interrupted investigations again in 1949, at the Laboratory of the
Marine Biological Association in Plymouth. I would like to express my most
cordial thanks to the Director of the Laboratory, Mr. F. S. Russell, F.R.S.,
for having given me the opportunity of working there and for the great
interest he has taken in my work.
Owing to the rarity of stomatopods off Plymouth the investigations have
been continued with Homarus vulgaris, which is available in, sufficient
numbers. Some data have also been obtained from Palinurus vulgafis. In both
Abdomen of Homarus vulgaris and Palinurus vulgaris
165
species the organs in question could be found in the six abdominal segments
and, moreover, in certain thoracic muscles. The present paper is concerned
with observations on the abdominal segments only. Those relating to the
thoracic muscles will be dealt with in a further contribution.
METHODS
The results described below were obtained by means of methylene blue
staining. In both the lobster and the rock-lobster the nerves in the organs
under examination stain on the whole very satisfactorily, so that no preparation is ever a complete failure, though, of course, all are not of equal value.
The staining was achieved either by immersing the tissues in the methylene
blue solution (10-15 drops of 0-5 per cent, solution in distilled water to 100 c.c.
of sea-water), or by injection of the dye into the living animal (1 volume of
methylene blue, 0-5 per cent, aq., with 2-5 volumes of sodium chloride,
1-5 per cent, aq.), or by combining the two processes, i.e. staining the tissue in
the solution as above, after the animal had been previously injected. Dissection
of the specimens was performed 1-4 hours after injection. The last method
was often used in order to correct the staining when it was unsatisfactory
after injection.
Fixation. The preparations were fixed with solutions of ammonium molybdate. The following formulae were used:
1.
Ammonium molybdate, 8per cent.
P l a t i n u m c h l o r i d e ,i p e r c e n t .
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Osmium tetroxide, 2per cent.
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g oc.c.
3c . c .
3 drops
2. In one of my previous papers in which this formula was recommended
(Alexandrowicz, 1932), the addition of cane sugar (saccharose) to the ammonium molybdate solution was suggested, so as to make it isotonic for marine
invertebrates. In the present series of experiments it was found that the shape
of the ganglion cells in the receptor organs was preserved better when the
quantity of saccharose amounted to about half that needed to make the solution isotonic:
A m m o n i u m molybdate 10 p e rcent.
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Saccharose
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100 c.c.
17g m .
To this platinum chloride and osmium tetroxide may also be added.
3. The mixture with glycerine and hydrochloric acid, as recommended
by E. C. Cole (1934), was also tried and it was found that for my purpose it
was advisable to make the mixture with less glycerine and acid. This modified
formula was as follows:
A m m o n i u m molybdate
Glycerine
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Dist. water
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H C 1 , s p . g r . 1-124
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2 5 c.c.
7 5 c.c.
15 d r o p s
The solution of ammonium molybdate to which saccharose or glycerine
has been added becomes blue when exposed to light. It was noticed in the
i()(»
Alexandrowicz—Muscle
Receptor Organs in the
earlier experiments that this colour disappeared when the bottle was put in a
shaded place, and also after addition of platinum chloride and osmium
tetroxide. In the course of the recent work it was found that a similar solution,
probably owing to some different properties of the sample of ammonium
molybdate used, became very dark blue and that it did not even turn colourless if kept in brown bottles. What is worse, the tissues of the preparations
may retain a greyish-blue coloration.
With each of the above formulae satisfactory fixation may be obtained.
The preparations were left in the fixing fluid for 6-18 hours, washed out for
about the same time in distilled water, then dehydrated in absolute alcohol,
transferred to xylene and mounted in xylene dammar.
Although the staining and fixing of these organs proved comparatively
easy, there were difficulties that had to be overcome. Chief among these was
to ensure the access of the dye to the nervous elements during staining.
Methylene blue has a low penetrating capacity so that a small fold or some
overlapping parts of the tissue may hinder the staining of the nerves. It is
therefore very important to get the nerves exposed on the surface of wellstretched preparations. To do this I use the method already recommended
(Alexandrowicz, 1932); i.e. the portion removed from the animal is pinned to
paraffin-wax plates with hedgehog spines. When the preparations have been
thus fixed in a fully stretched state, all manipulations on them are much
easier. In my investigation it was necessary to remove as much of the surface
connective tissue and blood-vessels as possible. This is a very delicate operation and needs to be done when, after some minutes, the staining has advanced
so far that the different tissues are becoming differentiated. Sometimes pieces
of muscle have to be pushed aside and fixed to the plate or even removed.
Unfortunately, operations in which the muscles are squashed or cut often
have an unfavourable influence on the staining of the adjoining nerves. The
preparations are left attached to the paraffin plates during staining and fixation ; it is also better to keep them for some time on these plates while washing
in water, since after treatment with ammonium molybdate some tissues have
a tendency to shrink and others to swell. After detaching them it is advisable
to remove all superfluous parts. The muscle layer can be thinned by removal
with scissors from the underside. Before transferring into alcohol the preparations should be put on the slide and as much water as possible sucked away
with blotting-paper. If some regions are still too thick, they can be reduced
further when in alcohol or xylene by scraping or by cutting out parts of them.
In all these manipulations the muscle receptor organs, as a rule, remained in
connexion with the neighbouring dorsal muscles and with the soft chitinous
parts to which they were attached. It is, however, possible to isolate them.
This can be done before staining, but the final results are much better if it is
done when the preparations are in water after fixation or even when they have
been transferred to xylene.
It is not particularly difficult, when the preparations are in water, to cut out
the receptor organs, viz. the muscle-bundles with their nerve elements, the
Abdomen of Homarus vulgaris and Palinurus vulgaris
167
latter in connexion with the main dorsal nerve-trunk. The whole should be
put on the cover-glass, stretched properly, passed into alcohol, &c. When the
separation has to be made after the pieces have been brought into xylene, the
isolation of the required parts can be done by teasing off small pieces of
the adjoining muscles. Caution is needed as the muscles and nerves are brittle
and there is always some risk of damaging the preparations at the important
spot, but after some practice not many of them become lost in that way.
These processes for reducing the thickness of the preparations or separating
some parts of them are applicable also to those which have been mounted for
a long time: after keeping them some hours in xylene the cover-glass can be
removed, the desired improvements effected, and the preparations mounted
again.
Some hints on dissecting the animal in order to expose the muscle receptor
organs are given later (p. 174) as they can be better understood after a detailed
account of the situation of these organs has been given.
RESULTS
The symbol MRO will be used in this paper as an abbreviation for muscle
receptor organ.
In Homarus (Text-fig. 1) MRO appears on the dorsal surface of each of the
six abdominal segments as a very thin muscle-bundle, approximately equal in
length to the superficial dorsal muscles and situated at the level of these
muscles. Two largish ganglion cells lying quite near to this muscle-bundle
send short branches to it while their axons, starting from the opposite poles,
join the nerve-trunk running transversely on the superficial dorsal muscles
and supplying these muscles; the same trunk carries also several nerve-fibres
destined for the MRO. We have therefore to consider three components of
this organ, viz. (a) muscle elements, (b) nerve-cells, and (c) nerve-fibres.
Muscle Elements of the Receptor Organs
Topography of the Muscle Receptor Organs
The situation of the muscle components of MRO can be best understood in
relation to the superficial dorsal muscles. The latter were described in
Homarus by Milne Edwards (1834) and mentioned by W. Schmidt (1915),
who, in his monograph of the muscle system of the common crayfish, gives
some particulars regarding the lobster. Daniel (1930) investigated the muscle
system of various crustaceans and among them that of the lobster, but from
his diagrammatic figures one can hardly get an idea of what these muscles look
like when seen from the dorsal side. The only illustration representing them
which could be found was that of Milne Edwards, but it is not quite exact.
Some remarks about the general topography of this region may therefore not
be superfluous.
168
Akxandrowicz—Muscle
Receptor Organs in the
In Homarus vulgaris, to which the following description refers, the superficial dorsal muscles form in the 2nd to 5th abdominal segments two distinct
portions; both originate at the forward edge of one segment and run forward
as thin band-like muscle units; they have their attachments to the integument
TEXT-FIG, I. Homarus vulgaris. A. Muscle receptor organs in the 2nd abdominal segment
(left side). The receptor organs have been a little displaced in dissection from the edge of the
lateral superficial dorsal muscle for the sake of clarity. A certain amount of connective tissue
has been left surrounding the organs, B. Muscle receptor organs in the ist abdominal segment
(right side).
of the next segment not far from its anterior edge. As is shown in Text-fig. 2A
(representing the 2nd segment with the adjoining parts of the ist and the
3rd), the fibres of the median portion run parallel to the longitudinal axis
while those of the lateral portion take an oblique course. In the resulting gap
parts of the deeper thicker layer of the dorsal muscles lying immediately
beneath the superficial ones become exposed to view. These deeper muscles
also come into view between the median borders of the superficial ones. There
are also in this region tiny muscle units attached to the interarticular membrane (Text-fig. 2A).
Abdomen of Homanis vulgaris and Palinurus vulgaris
169
f^^-STz
••:-
\
- » •
D
TEXT FIG. 2. Homanis vulgaris. Diagrams showing the situation of the muscle receptor
organs in the 2nd abdominal segment from (A) the dorsal and (B) the ventral side. c. Sections
through the same segment in the plane of the receptor organs; the muscle-fibres are diagrammatically represented as if they were lying with their insertions in the same plane. In Fig. c the
abdomen is in a bent position, in D it is straightened. •
170
Alexandrowicz—Muscle
Receptor Organs in the
The general arrangement of the muscles is the same in the segments 2-5.
There are only certain differences in the anterior attachments of the lateral
portion, which in each succeeding segment is situated a little farther back
than in the one next in front of it. Consequently the oblique portion becomes
gradually shorter. In the 4th and especially the 5th segment the gap between
the two portions appears comparatively large.
In the 1st segment the median portion takes an oblique course, too, and
thus the fibres of the whole superficial muscle run approximately parallel with
each other. The gap between them is closed and the two portions may appear
as one muscle, although by more careful examination they can be distinguished
as two units.
In the 6th segment the superficial dorsal muscles are not divided into two
portions and are greatly reduced both in width and length. They begin at the
forward edge of the telson and run for a short distance, occupying only about
one-third of the length of the segment.
The situation of the muscle-bundle of MRO may be roughly defined as
being between the two portions of the superficial muscles but more or less
close to the lateral portion. On microscopic examination it may be seen that
this thin muscle-bundle actually consists of two muscle units of about 150/A in
diameter; though closely associated in the middle part of their course, they
preserve their individuality throughout. They are, moreover, not of the same
length and they have different points of attachment, and they also show a
difference in their histological structure. For the convenience of the description that muscle which has the more superficial attachments will be called
RM 1 and the other RM 2. The following description refers to the 2nd segment which is represented in Text-fig. 2.
The muscle RM 1, the shorter of the two, has its posterior insertion at the
point where the fibres of the two superficial dorsal muscles meet at an acute
angle and it may therefore be most easily recognized at this spot. In its forward
course it runs at first along the edge of the lateral superficial muscle but soon
departs from it and comes to lie in the triangular space between the two portions of the superficial muscles and on the bundles of the deeper dorsal
muscles. Bending slightly inwards and dorsalwards it usually divides into a
few bundles of fibrils which are attached to the integument at the point
between the two superficial muscles and a good deal behind their anterior
insertions.
To understand the course of the muscle RM 2 Text-fig. 2A should be
compared with 2B, in which the same segments are depicted as seen from the
ventral side with all the muscles removed except the superficial ones, so that
the whole muscle RM 2 is exposed to view; in Text-fig. 2A those parts of
RM 2 that are hidden from view are drawn in dotted lines.
The anterior attachment of RM 2 lies a little to the side of and much in
front of that of RM i, approximately at the level of the insertion of the
median bundles of the lateral superficial muscle. In its backward course RM 2
passes beneath RM 1 and runs alongside it in such a position that a good deal
Abdomen of Homarus vulgaris and Palinurns vulgaris
171
of RM 2 or even the whole of it may be seen on the median side of RM 1.
Held together by connective tissue the fibres run for a distance in close
association, but they separate again as RM 2 passes underneath the lateral
superficial muscle, crosses its fibres obliquely near to their insertion (Text-figs.
1 A, 2B) and runs so far backwards that its attachment takes place in the vicinity
of the superficial muscles (median portion) of the next following segment.
It ought to be noted, as this feature may have some importance for the
function of these muscles, that RM 2 looks in the preparations as if it were
more loose, so that even in its middle part the fibres of RM 2 may be often
distinguished from those of RM 1 by their undulating course.
The length of RM 2 in the 2nd and also in the 3rd segment is not much less
than twice that of RM 1. In the preparation which served as a basis for the
drawing of Text-fig. 2 the respective lengths were 19 and 10 mm. As to their
calibre, RM 2 appears a little stouter in that region where both muscles run
together. Moreover, each appears as if it were not. of uniform thickness
throughout its whole length, RM 1 looking thinner anteriorly and RM 2
thinner posteriorly. However, the evaluations of their calibre as seen during
dissection are not exact, as variations are also caused by deformations of the
cylindrical fibres, which are flattened in various directions by the neighbouring organs.
As the muscle RM 1 approaches its anterior attachment it frequently
splits into several tiny bundles of fibrils which spread out fanwise. The
posterior end of this muscle and both ends of RM 2 only show an enlargement
due to a slight divergence of the fibrils.
The course of the muscles RM drawn to greater magnification is shown in
Text-fig. 3A. We see that they must cross each other twice, since in the middle
part RM 2 always lies nearer the median line than RM 1. It is to be emphasized
that at both crossings RM 2 always passes ventrally to RM 1.
In front of and behind the crossing RM 2 runs at a deeper level. These
points are made clear in the diagrammatic figures 2c and D; these represent
a dorso-ventral section of the same segments as 2A, made in the plane of
the muscles RM of the 2nd segment. In Text-fig. 2c the abdomen is supposed
to be in almost maximal flexion and consequently the muscles are stretched;
their longitudinal dimensions correspond in this state to those represented in
A. and B, which were drawn from preparations that had been stretched and
flattened, with, therefore, the muscles fixed in the stretched position. In the
living animal, when the abdomen becomes straightened, the points of attachment of the muscles RM assume the position shown in 2D. Where they are
linked together in their middle parts, they do not deviate from a straight
course. Elsewhere they are bent; this applies especially to the posterior part
of RM 2, which has to turn round the edge of the following segment and,
moreover, is separated at this place from RM 1 by the thickness of the lateral
superficial muscle (not represented in this figure).
In Text-fig. 2c and D allowance must be made for the fact that the insertions of both muscles are shown in the same plane, although this does not
172
A
Alexandrowicz—Muscle
W
Receptor Organs in the
B
D
TEXT-FIG. 3. Homarus vulgaris. A. Muscle elements RM 1 and RM 2 of the receptor organs
in the. 2nd abdominal segment. B. Same of the 5th segment. C. One of the types of the insertions of the muscle-fibres in the 5th segment. D. Diagram of the dorsal view of the abdomen
showing the positions of the receptor organs and of the superficial dorsal muscles by transparency.
Abdomen of Homarus vulgaris and Palinurus vulgaris
173
agree with their topography as represented in Text-fig. 2A and B. On the
other hand, it must be pointed out that the position of these elements as
observed on stretched and flattened preparations does not reproduce exactly
their three-dimensional relations, in which the convexity of the abdomen also
plays its part.
The arrangement of the muscles RM in the first five segments is basically
the same; there are, however, some minor differences. In the 1st segment
they lie between the two portions of the superficial muscles, which, as has
been said above, run parallel to each other (Text-fig, IB). Both muscles RM
show a greater independence of each other so that in their middle parts they
may be found less closely linked than in other segments. Their anterior insertions lie nearer to each other than in the 2nd segment and thus their
difference in length is not so pronounced.
In the 3rd segment the arrangement is almost identical with that in the 2nd,
but it has been noticed that in many cases the muscles RM run a little more
closely to the lateral superficial muscle.
In the 4th segment the muscles RM are shorter than in the foregoing. This
relates particularly to RM 2 which in its caudal part at first takes a course
similar to that in the anterior segments, i.e. under the lateral superficial
muscle; but after crossing the line of its attachment it bends sideways and
soon ends near this line.
In the 5th segment the insertions of RM 2 are not far from those of RM 1
and thus the difference in the lengths of the muscles becomes still smaller.
Their shape and relative dimensions are shown in Text-fig. 3B, which is
drawn to the same scale as 3A. The muscles are represented as uncrossed in
their anterior part, as this seems to be frequently the case.
In this connexion it may be mentioned that variations in the course of the
muscles RM are not uncommon. These occur rather often in their anterior
parts, especially in the 4th and 5th segments: they can be found attached
farther forward, or more centrally or laterally; in some cases, as shown in
Text-fig. 3 c, RM 1 divides into two branches, attached one on each side of
RM 2. In their middle parts both muscles may be found nearer to or farther
from the lateral superficial muscle or even lying on the muscle itself. These
differences may not infrequently be due to displacement produced artificially
during the dissection, but this cannot be the cause for all observed cases. For
instance, in one preparation RM 2 running on the dorsal side of the lateral
muscle was found piercing this muscle and then crossing it obliquely in the
median direction till it reached the point of its usual anterior attachment.
This case is worth mentioning because it makes evident the completely
independent character of this muscle unit, which, though running through
the fibres of another muscle, retains its own and different course.
In the 6th segment the muscles RM run along the median edges of the
superficial muscles and therefore are situated very near to the median line of
the body. They are parallel with each other, RM 1 being more superficial and
partly overlying RM 2. Owing to certain difficulties in preparation of this
174
Akxandrowicz—Muscle
Receptor Organs in the
segment the muscles are easily torn away or displaced, but as far as I have
been able to ascertain they are of equal or nearly equal length, and their
insertions are near to those of the superficial muscle.
The question of the presence or absence of MRO in the telson could not be
solved, since I was unable to overcome the difficulty of making preparations
without injuring muscles to such an extent that the tiny fibres might have
been damaged and overlooked. In freshly moulted specimens one might
perhaps be more successful either in finding these elements or else in proving
their absence.
The position of MRO in all segments of the straightened abdomen in
relation to the external outlines of the body is shown in Text-fig. 3D.
In Palinurus the topography of the dorsal muscles and of MRO is a little
different, and the recognition of MRO is somewhat more difficult since the
bundles of the superficial dorsal muscles do not divide into two distinct portions and they thus do not leave the space in which the MRO are easily to be
found in Homarus. Moreover, in Palinurus the MRO are partly covered by the
bundles of the dorsal muscles and if they have to be exposed to view some of
these bundles must be pulled aside. Only in the 6th segment do they lie more
superficially in Palinurus than in Homarus.
The muscles RM do not exhibit much difference in length in Palinurus and
consequently the points of attachments of RM 1 and RM 2 lie nearer to each
other than in Homarus. On the other hand, they show more marked difference in their diameter, RM 2 being in the middle part of its course much
thicker than RM 1.
The animals can be anaesthetized by adding alcohol to the water in which they
are living. As soon as they become insensitive the abdomen should be detached
from the thorax and its dorsal part cut away with strong scissors. The two lines of
incision should run on the outer sides of the hinge-like articulations of the segments;
later, these hinges must be severed by additional transverse incisions.
The MRO can be exposed from either the dorsal or ventral sides. In the former
method, the greater part of each tergum should be removed, leaving its anterior
portion with the attachments of the muscles. To do this it is necessary first to cut
through the interarticular membrane along its anterior insertion with fine scissors
or a sharp scalpel, and great care must be taken not to push the blade too far in. If,
after this membrane has been cut, the posterior edge of the tergum is lifted, several
strands of tissue are to be seen stretching between the dorsal muscles and the integument. The most conspicuous of these strands are two tiny muscles situated near
the median line (Text-fig. 2A). Their posterior attachments are on the interarticular
membrane and the anterior ones at about the middle of the tergum. These muscles,
which were known to Daniel (1930), may be called muscles of the interarticular
membrane since their function is obviously to pull this membrane forwards so that
it does not get pinched between the terga during the movements of the abdominal
segments.
The other strands consist of connective tissue with blood-vessels and some
nerve-fibres, except for one of them which is the anterior insertion of the
muscle RM 1. If, therefore, one decides to remove the greater part of the tergum
Abdomen of Homarus vulgaris and Palinurus vulgaris
175
and before doing so cuts all these strands through, the anterior portion of RM 1
becomes severed.
After the dorsal surfaces of the muscles have been exposed in this way, all the
superfluous parts of the integument and of the muscles should be cut away, and the
preparations pinned on to the paraffin plates and treated as described above.
In Homarus the 2nd and 3rd segments are the most suitable for studying the
MRO. In the 4th, and especially in the 5th, the anterior attachments, not only of
RM 1 but also of RM 2, often become injured. Moreover, the more convex shape of
the dorsal wall of the abdomen and the arrangement of the dorsal muscles make the
location of MRO more difficult. In the 6th segment the short muscles RM are so
situated that, even after careful dissection of the interarticular membrane and
exposure of the superficial dorsal muscles, they are often damaged.
The ease with which the terga can be dissected depends on the consistency of the
integument. If the calcified layer is very hard and cracks when cut, the operation is
difficult. Animals that have just moulted are by far the most suitable, but these are
only available on rare occasions. Animals that are just about ready to moult are also
quite suitable; in these the calcified layer can easily be separated from the new
integument, which can be cut with safety. The worst animals for dissection are those
at an early stage of the moulting-cycle, when the softer layer comes away easily but
is so incoherent that it disintegrates when gripped by the forceps; the small fragments adhering to the surface of the muscles have to be removed bit by bit.
To expose the MRO from the ventral side it is necessary, after dissecting off the
dorsal portion of the abdomen, to remove all the muscles there except the median
and lateral parts of the superficial dorsal muscles, which, it may be recalled, have a
shape of very thin bands. The MRO may either be seen in the triangular space
between these muscles (Text-fig. 2B) or may be overlain by the median bundles of
the lateral muscle. In the latter case these bundles have to be pulled aside and fixed
in that position, or part of them has to be cautiously removed. Here again the 2nd
and 3rd segments prove to be the most suitable for making the preparations.
Both methods of dissection have their advantages and disadvantages. Approaching
from the dorsal side one gets the nerve elements lying superficially and the common
nerve-trunk exposed in a longer part of its course. On the other hand, the anterior
attachments of RM 1 are as a rule cut off. When the organs are exposed from the
ventral side, the topography of MRO and of the supporting connective tissue is
better preserved, but only a small part of the common nerve-trunk becomes stained.
Moreover, if the calcified chitin layer does not come away, the observation of the
staining process is made difficult.
For Palinurus the same methods of dissection may be used, but as already mentioned, the search for the MRO is somewhat more difficult owing to the arrangement
of the muscles.
Structure of the Muscles of the Receptor Organs
Each RM is composed of bundles of myofibrils enclosed in delicate sheaths
of connective tissue, the whole surrounded by a layer of much stronger connective tissue fibres. The myofibril bundles do not form such individual and
separate fibres as do typical elements of the striated muscles, since they may
branch. Therefore, in cross-sections of RM they appear as areas varying in
number and size. Their nuclei are placed centrally or at the periphery.
176
Akxandrowicz—Muscle
A
Receptor Organs in the
B
connective
tissue
TEXT-FIG. 4. Homarus vulgaris. A. Semi-diagrammatic view of a longitudinal section of the
muscles RM 1 and RM 2 showing the regions of the intercalated connective tissue. B. Semidiagrammatic view of the anterior portions of the muscles RM 1 and RM 2 with the connective
tissue fibres; the fibres lying on the muscles which would obscure the latter are omitted in the
drawing.
Serial sections show that the amount of muscle tissue at various levels of the
muscles RM is not the same and that these differences are much greater than
one would expect from the appearance of these muscles when seen during
dissection. It is obvious that many of the fasciculi of myofibrils terminate in
the connective tissue sheath before reaching the end of the muscle. This is
particularly noticeable in the anterior portions of RM 1, near the insertions.
Abdomen of Homarus vulgaris and Palinurus vulgaris
177
Both muscles RM exhibit a peculiar feature in having the muscle-fibres
replaced in one particular region by connective tissue (Text-fig. 4A). This
region is situated at about the middle of the length of each RM and, as we
TEXT-FIG. 5. Homarus vulgaris. Cross-sections through the middle of the connective tissue
region of (A) RM I and (B) RM 2 passing through nerve-cell 1 and nerve-cell 2 respectively;
in (A) RM 2 and in (B) RM 1 consists entirely of muscle elements, c. Section passing between
the two nerve-cells; muscle elements in both RM reduced to few bundles. D. Section near the
end of the connective tissue region of RM 2.
shall see later, it is also the area of distribution of the dendrites of the nervecells. In sections stained with Mallory (the best results were obtained after
fixing in Zenker's fluid) the fibres in this region reveal their collagenous
nature by staining in blue, and they supplant the muscle elements in the whole
thickness of each of the RM (Text-fig. 5). They are in communication with
178
Ahxandrovncz—Muscle
Receptor Organs in the
the surrounding connective tissue fibres but mainly run longitudinally,
vidently having the function of intercalated tendons. The muscles coming
from each side terminate in them somewhat unevenly, so that serial sections
show a gradual reduction in the number of myofibrils. A central portion is
left containing connective tissue fibres, among which can also be distinguished
some thicker processes of the nerve-cells. Only a few myofibrils are to be
found passing through this connective tissue bridge. The nuclei, which occur
here in fair numbers, seem to be of two kinds, some belonging to the connective tissue and others to the sheaths of the nerve elements.
The total lengths of these intercalated tendons, obtained by counting the
cross-sections between the first endings of the myofibrils to be seen on each
side, in a preparation from the 3rd segment of a lobster 17 cm. long, were 520 [M
in RM 1 and 250/* in RM 2. These will be lower figures than those in the
living animal owing to the shrinking of the preparations during fixation and
embedding. As has been mentioned, the muscles RM 1 and RM 2 show
certain differences in their histological structure. In RM 1 the fibrils are
thicker and more regularly arranged, i.e. their alinement is more strictly
parallel and the lines of cross-striations are often seen passing at the same
level through several fibrils. In RM 2 the fibrils are thinner, their alinement
is less regular and their cross-striations, if not independent for each fibril,
run as single lines across fewer elements than in RM 1. Moreover, in RM 2
this cross-striation is much finer than in RM 1.
The possibility that this unexpected feature might have been accidental
and have resulted from different states of the fibres caused by sectioning and
fixation can be discounted, because the same differences could be observed
on a great many preparations fixed and stained in various ways. It is, besides,
so distinct that it can hardly be put down to anything else than specialized
histological structure. What is more, none of the muscles RM appears to be
histologically identical with the neighbouring ordinary muscles. These particulars may well be seen on the photomicrographs (Text-fig. 6) made from the
same preparation of the same segment and under the same magnification
(oil-imm. | inch), which show portions of the dorsal superficial muscle,
muscle RM 1, and muscle RM 2. It is evident that, when compared with the
dorsal muscle, RM 1 exhibits similar differences to those distinguishing
RM 2 from RM 1, i.e. less regular arrangement of the fibrils and a finer
cross-striation. On measuring the spacing of the latter in the same segment
it has been found that for every 10 sarcomeres of the dorsal muscles there are
approximately 14 of the muscle RM 1 and 23 of RM 2.
The differences in the structure of RM 1 and RM 2 may also be ascertained
in cross-sections which show that the myofibrils in RM 2 are finer and more
densely grouped together than in RM 1.
The muscles RM are held in position by the connective tissue, which
stretches over the neighbouring muscles as a fine membrane strengthened by
fibres of varying calibre and which supports the nerves and blood-vessels.
From the arrangement of the elements of this tissue in relation to the muscles
Abdomen of Homarus vulgaris and Palinurus vulgaris
179
RM it may be inferred that these tiny muscle entities, of such unusual dimensions, are given particular protection: the fibres of connective tissue, becoming
here markedly stronger and more numerous, surround the muscle RM in the
form of a continuous sheath. They run chiefly in the longitudinal direction,
but they also send branches to the sides (Text-fig. 4B). The latter are obviously
superficial
dorsal muscle
RM1
RMZ
TEXT-FIG. 6. Homarus vulgaris. Photomicrographs showing the different histological
structures of the superficial dorsal muscles, muscle RM i, and muscle RM 2. All photographs
have been made from the same preparation and with the same magnification (oil-imm. 1 ,'7 in.).
responsible for maintaining the muscles RM in their courses, which, as we
have seen, deviate from the straight line. A few connective tissue fibres of
particular strength accompany the muscles RM throughout their whole
length and become attached in the vicinity of their ends, giving off also
additional branches. As RM 1 approaches its anterior end and undergoes
subdivision, the connective tissue fibres follow the bundles of myofibrils more
or less regularly but spread over a wider area.
The particular strength of these few accompanying fibres was revealed in
preparations in which, after fixation in ammonium molybdate mixture, the
MRO were excised and put into distilled water. It could be noticed that after
180
Akxandrowicz—Muscle
Receptor Organs in the
some time the muscles were becoming shorter and taking on an undulating
shape. This proved to be due to shrinkage of the connective tissue fibres
which had to be cut in order to get the muscles stretched again.
The Mallory preparations show that the majority of fibres ensheathing the
muscles RM are collagenous in nature. However, there are among the bluestained elements some other fine fibres revealing a different character by
staining orange-red.
Nerve-cells
The nerve-cells of the MRO are situated at the point immediately in front of
the crossing of the muscles RM by the main nerve running transversely on the
surface of the dorsal muscles. This nerve, which always crosses the muscles
RM on their dorsal side, will for descriptive purposes be called the 'common
nerve-trunk', as it carries nerves for the dorsal muscles, for the muscles
of the receptor organ, and, besides, is joined by the axons of the latter's
nerve-cells.
In relation to the length of the median portion of the superficial muscles,
the nerve-cells in different segments must be looked for as follows: in segment
i approximately midway along this muscle, in segments 2-5 at the front of the
posterior third, and in segment 6 again nearer to the midpoint of the superficial muscles; but as the latter are very short the cells become situated very
near to the telson.
The cell connected with the muscle RM 1, which will be called (nerve-)cell
1, always lies in front of the second, which, as it belongs to the muscle RM 2,
will be called (nerve-)cell 2. Cell 1 is always, and cell 2 most frequently,
situated in front of the common nerve-trunk. In some cases cell 2 may be
found directly beneath this nerve or even, as is usually the case in the 1st
segment, behind it (Text-fig, IA, B). This arrangement makes it possible to
distinguish at first sight whether the preparation is from the left or from the
right side and which are its anterior and posterior ends.
The distance between the two cells is not equal in all segments, even in the
same specimen. As a rule they are farther apart in segment 1 and very close to
each other in segment 6. In the remaining segment the spacing is variable but
less than in the 1st segment.
The cells have one long process, the axon, joining the common trunk and
running in it towards the ganglionic cord, and a variable number of dendrites,
short, richly ramifying processes, penetrating one of the muscles RM.
In Homarus the dendrites of cell 1 (Text-fig. 7B; PL 1, fig. 2), are more
numerous than in cell 2 and sometimes up to ten of them could be counted,
but their real number may be even greater, as those lying more deeply do not
stain well and some, particularly the thinner ones, may remain invisible.
The majority of the dendrites originate from the side of the cell nearest the
muscle, but they can arise from any point on the cell or even occasionally
from the axon, or rather from that part of the cell which from the morphological point of view would be called the axon (PL 1, fig. 4, PL 2, fig. 7).
Abdomen of Homarus vulgaris and Palinurus vulgaris
*
A
181
D
0__500JX
TEXT-FIG. 7. Homarus vulgaris. Photomicrographs showing various features of the nervecells of the receptor organs. A. Cell 2 with two dendrites. B. Cell 1 with many dendrites, cell 2
with one thick and four thin dendrites. c. Cells with distinct capsules and surrounding connective tissue. D. Cells whose dendrites, together with the branches of the accessory nerves,
form a deeply stained neuropile-like network; its shape and dimensions are different in each
of the muscle units; the capsules of the cells are clearly distinguishable, E. Muscle receptor
organs in the 6th segment; note the shape of the axons, which increase in size and attain a
large diameter at some distance from the cells.
Cell 2 is frequently situated at a little greater distance from its muscle and
its dendrites are consequently elongated in this direction (Text-fig. 7 A, B, C).
It should be noted that when crossing the muscle RM 1 the processes of cell 2
always pass on its dorsal side. Their number is reduced, so that often one
finds only three, two, or even one stout process; in the latter case the cell
might appear to be bipolar. However, even in such cells thin additional
182
Alexandrowicz—Muscle
Receptor Organs in the
processes arising at different levels may be often observed (Text-fig. 7B). In
*aany preparations the elongation of the cell 2 becomes artificially exaggerated
during the attempts to fix the muscles and nerves in the stretched position.
The ganglion cells are conspicuously large, being more than 100/x across:
when stained they can thus be seen even with the naked eye. Exact estimation
of their size is hardly possible owing to their variable shape, and it is difficult
to ascertain the true size-relations between the two cells in the same segment;
if there is any difference in their volume it cannot be great. In segment 1,
cell 1 often looks as it were larger than cell 2, and, conversely, in segment 6 the
latter seems to be the larger. All estimations are made even more uncertain by
the fact that the cells exhibit the interesting peculiarity of being encapsuled.
The capsules may be seen quite distinctly on many preparations when the
cells become fixed in such a position that a space is left between the capsule
and the periphery of the cell-body. The capsule consists of a thin membrane
in which no structure could be seen in the methylene-blue preparations;
it includes the roots of the dendrites and of the axon and is continuous with
the sheath of the latter; in other words, it is formed by the prolongation of
the neurilemma of the axon expanding in a more rigid membrane around the
nerve-cell (Text-fig. 7c, D; PI. 1, figs. 4, 5). The capsule, which has obviously
a protective function and is probably filled with some fluid, is in its turn encased in the connective tissue, which, as already stated, spreads over the
muscles, nerves, and blood-vessels. Around the ganglion cells it becomes
arranged in concentric layers in which numerous pale nuclei may be noticeable. In sections it can be seen that the nuclei are more numerous in the layer
close to the cell and these may belong to the capsule.
In the majority of preparations, when the process of staining has been prolonged in order to show as many nerve elements as possible, the cells appear
dark blue and do not show any details of their structure. It is only when
staining is interrupted, or when the cells for some reason remain pale, that a
smallish nucleus and granulation of the cytoplasm may be observed (PI. 1,
fig- 5).
The axons of both cells increase their diameter greatly at some distance
from the cells and become nearly as large as the thickest motor fibres in the
common nerve-trunk. In the 6th segment the axons are even the thickest of all
nerve-fibres running over the surface of the muscles (Text-fig. 7E).
Cell 2 is surrounded by very fine nerve-fibres which form a kind of basketwork on the surface of the capsule and also penetrate into it. Although this
feature can be but rarely noticed it seems most probable that all these fibres,
or at least the majority of them, enter into close relation with the cell itself.
These fine fibres are given off, as will be shown later, by one of the nerves
supplying the MRO. Some of them, before reaching the nerve-cell, approach
its axon and run close to it (PI. 1, fig. 6).
The dendrites of each ganglion cell, some of very stout calibre, penetrate
into one of the RM and give off a number of short, tortuous, abundantly
ramifying branches which finally break up into a mass of fine fibres (PI. 1,
Abdomen of Homarus vulgaris and Palinurus vulgaris
183
fig. 3); such very fine fibres arise also directly from the stout trunks of the
dendrites. They all make their way among the fibres of the connective tissue
intercalated in the muscles RM, and the areas through which they are distributed evidently correspond exactly to the region occupied by this connective
tissue. Accordingly, this area is always longer in RM 1 than in RM 2 and the
difference is so constant that by this feature alone each of the two muscles RM
may be recognized, if the ramifications of the dendrites are stained. It is true
that these areas, if measured in methylene-blue preparations, prove to be
longer than when evaluated from cross-sections, and the question might be
asked whether they do not extend on to the muscle tissue as well. However,
the shape and limits of these areas, which are particularly well shown for RM 2
(Text-fig. 7D; PI. 1, figs. 1, 3), give no support for such an assumption; and
the difference of the dimensions may be easily explained by the fact that in the
two methods of preparation the muscles become deformed in opposite ways:
those cut in sections are shortened by shrinking, while those stained in
methylene blue and mounted as a whole become overstretched.
• The preparations do not reveal the intimate relations of the nerve elements
with the connective fibres, but it is obvious that contact is made over as large
a surface as possible. The nerve-supply at this spot, which as we shall see
later is reinforced by other nerve elements, is so dense that sometimes in the
more deeply stained preparations all the entanglements of the terminal fibres,
forming here a neuropile-like network, appear a solid mass (Text-fig. 7D).
In Palinurus vulgaris cell 2 seems to be a little larger than cell 1 and this
may be in agreement with the larger calibre of the muscle RM 2. The shape
of the cells is also variable, but similar in any one segment, so that they cannot
be distinguished by their outlines alone (PI. 2, fig. 2). Further, the disposition
of the dendrites of the cell 2 may be often seen to be such that one dendrite (or
perhaps more) passes on the other side of, i.e. ventrally to, the muscle RM 1.
This feature, if it is constant, might have something to do with the shape of
the cell, which, with its dendrites astride of RM 1, cannot become elongated
towards their terminal ramifications.
The fine fibres forming a basket-work round cell 2 appear iri Palinurus to be
much more numerous and of finer calibre. It may be that they are in reality
more numerous in this species, but it is also possible that this apparent
difference is due to their unequal staining properties.
There are certain peculiarities in the outlines of the nerve-cells, the disposition of the dendrites, and in their mode of branching, which enables one to
distinguish the Palinurus preparations from those of Homarus; but they are all
of minor importance. The capsules and the connective tissue round the cells
are constructed in a similar manner.
Nerve-fibres
The nerves supplying the muscle receptors are of various thicknesses and
appearance, and there can be no objection to the assumption that they must be
of different kinds with different functions. It is, however, much more difficult
184
Alexandrowicz—Muscle
Receptor Organs in the
common nerve trunk
TEXT-FIG. 8. Homarus vulgaris. A. Diagram showing the motor nerves of the muscle
receptors. The main motor fibres run in the common nerve-trunk which crosses the muscles
RM; additional motor fibres can be seen on the left side of the figure branching from a nerve
supplying the neighbouring dorsal muscles. B. Enlarged view of the additional motor fibres
to show the common innervation of the dorsal muscles and of the receptor organs, c. Motor
nerves and their finer branches in the muscles; between the two main motor fibres are to be
seen several fibres, presumably elements of multiple motor innervation; a, fibre arising from
a nerve running to the median portion of the superficial dorsal muscles. D. Thick and thin
accessory nerves.
Abdomen of Homarus vulgaris and Palinurus vulgaris
185
to obtain positive evidence as to how many kinds there actually are, and only
after long deliberation have I come to the conclusion that at least three
categories should be distinguished. For descriptive purpose they will be
called (a) motor nerves, (b) thick accessory nerve, and (c) thin accessory nerve.
Motor Nerves
There are several fibres which, on all the evidence, may be regarded as
carrying motor impulses to both muscles of MRO. Two of them, the main
ones, run in the common nerve-trunk; one, generally of thinner calibre,
supplies RM 1, while the thicker is destined for RM 2. On approaching these
muscles each nerve bifurcates and its branches run along the muscles in
opposite directions giving off abundant ramifications ending amidst the
myoflbrils (Text-fig. 8A, B, C). The bifurcation of the main motor fibres
may take place in the common trunk at varying distances from the muscles
themselves (Text-figs. 8A, 9). In the first case the primary branches and their
ramifications may take various courses: either they run in the common trunk
or join at first nerves arising from this trunk and supplying the dorsal muscles.
Having travelled with these nerves for some distance they turn finally towards
their destination (Text-fig. 9.B). Such a course of the motor fibres is particularly often noticeable in the innervation of the muscle RM 1.
It should be remarked that the main motor fibres of MRO stain rather
rarely while they are running in the common trunk and one cannot therefore
expect to get a picture as described above in every preparation that is quite
satisfactory in other respects.
The primary branches of the motor fibres may be followed for a great
distance along the muscles in those preparations in which they alone stain and
in which their courses are not obscured by mixing with other nerve elements.
In such cases it is surprising to find a fibre of quite stout calibre near to the
insertion of the muscle, the more so since the distal parts of the latter may get
a supply from the additional motor branches given off by the nerves of adjoining dorsal muscles (Text-fig. 8A, B). These additional motor nerves are not
often met with as they may be easily overlooked or destroyed during the dissection and only one or two may be rather rarely seen. Their real number is
difficult to ascertain but it cannot be much greater. This additional motor
innervation perhaps varies in different segments and these variations might
account for the observation that the main motor fibres in the posterior segments seem to have a comparatively thicker calibre and are more frequently
stained.
In Palinurus the main motor fibre running to RM 2 is of particularly thick
calibre (PI. 2, fig. 8).
The question arises: has this motor innervation of MRO, if considered to
be a part of the motor innervation of the dorsal muscles, the same multiple
character, i.e. are there at least two nerve units reaching the muscle ? In fact,
in so far as the additional fibres are concerned it may be distinctly seen that
they include branches of the same fibres which supply the dorsal muscles,
186
AUxandrowicz—Muscle
Receptor Organs in the
TEXT-FIG. 9. Homarus vulgaris. Photomicrographs showing motor nerves {mot) of the
receptor organs. In A the main motor fibre of RM 2 is seen bifurcating on this muscle; in B
two branches of the motor fibres run for a short distance alongside a branch of the common
nerve-trunk (cf. Text-fig. 8A) ; in c main motor fibres are accompanied by several fibres; as
many as 8 fibres, some of which are out of focus, enter into the composition of this bundle.
and, as Text-fig. 8B shows, two such fibres can be seen running side by side
and these are presumably the elements of double innervation. As regards the
main motor nerves the matter is more complicated: they often appear accompanied by several finer fibres (Text-figs. 8c, 9c) among which there are
most probably some that establish multiple motor innervation, but as we
Abdomen of Homarus vulgaris and Palinurus vulgaris
187
shall see below, there may also be branches of other nerves supplying the
MRO.
Thick Accessory Nerve
Both accessory nerves, each represented by one fibre, run in the same
common trunk as the main motor fibres, but in their paths they show a closer
association to the axons of the ganglion cells. Thus, for instance, if the common trunk lies at a certain distance from the cells, or even when it is pulled
aside during the dissection, it may be observed that the accessory nerves
remain with the axons while the motor fibres prove to be connected more
firmly with the rest of the common trunk.
In Homarus the thick accessory nerve appears in many preparations as the
most conspicuous nerve element reaching the MRO. There' may be some
doubt whether it is actually thicker than the motor fibres, but, as the latter
remain most frequently colourless or are incompletely stained, the accessory
nerve, which stains readily, dominates the whole picture (Text-fig, IOA). AS
it approaches the MRO the thick accessory nerve gives off several branches,
some of which, although of approximately the same calibre, are of different
lengths. The short ones pass near the ganglion cells and break up into tufts
of very fine fibres in the same area of RM where the ramifications of the
dendrites of the nerve-cells end (Text-fig. 8D). AS, during the staining process,
the thick accessory nerves often appear before any of the other elements of
MRO and before the ganglion cells can be distinguished, one can see these
areas as characteristic spots looking as though very fine blue powder had been
sprayed on to the muscles. The appearance of such spots in preparations may
be very helpful in identifying the position of the muscles RM and of the
nerve-cells, which are still invisible. When the ramifications of the dendrites
also become stained, both elements are entangled in the neuropile-like network already mentioned above.
Although both ganglion cells are abundantly supplied by the branches of
the thick accessory nerve, it often looks as though cell 2 were the better
supplied. In many preparations, when cell 1 is approached by one branch of
the accessory nerve, cell 2 has two of them running in a characteristic way
past either side (Text-figs. 9.B, IOA; PI. 1, fig. 4); it seems also as if the density
of the branches connecting with cell 2 is greater than of those of cell 1.
The long branches of the thick accessory nerve run in both directions
along the muscles RM. They may be seen associating with the motor fibres
and take the same tortuous paths, but while each main motor fibre in its
course along the muscle gives off its branches to one of RM, the thick accessory nerve on its way supplies both of them.
Thin Accessory Nerve
The thin accessory nerve can be best distinguished at that point where
the thick one bifurcates. It repeats more or less closely the mode of this
branching so that the fibres of both nerves run side by side and their different
188
AUxandrowicz—Muscle
Receptor Organs in the
characteristics may be easily noticed (Text-fig, IOB; PI. i, figs. 2, 4, PI. 2,
figs. 7, 9). Running to the ramifications of the dendrites, the fibres of the thin
accessory nerve also enter into the neuropile-like network.
TEXT-FIG. 10. Homarus vulgaris. A. Thick accessory nerve with branches to the dendrites
of the nerve-cells and to the muscles. B. Thick and thin accessory nerves.
The thin accessory nerve also gives rise to branches which apparently do
not end in this network and which perhaps take part in the nerve-supply of
the muscles RM, but, owing to their small calibre and the complex innervation of RM, they could not be traced with sufficient certainty.
If the thin accessory nerve be followed backwards from the point at which
it first branches, it can be seen that in the common trunk it preserves its individuality and that it can be traced as a separate fibre as far as the other nerves,
that is, for a distance of about 5 mm.
Abdomen of Homams vulgaris and Palinurus vulgaris
189
The particular features of the nerves described above seem to leave little
doubt that they belong to different systems of nerve elements. A less certain
answer can be given to the question whether these three categories include all
the kinds of nerves supplying the MRO. As a matter of fact, still more fibres
can be seen running in the common trunk which take part in this innervation:
some approach the ganglion cells, others run to the muscles, and very often
one cannot be certain among which of the above categories they should be
reckoned and whether perhaps some more should not be added to them.
Pictures such as those of accessory nerves accompanied by additional fibres
undergoing similar divisions (PI. 2, fig. 9) especially cause one to think that
there may be other nerve units associated with the two accessory nerves
described above; and it remains doubtful whether the fibres running close to
the axon of cell 2 and surrounding the cell itself derive from the accessory
nerves or from these accompanying fibres. On the other hand, there are indications that so great a number of fibres might result from the division and
subdivision of the motor and the two accessory nerves having already taken
place in the common nerve-trunk far from the muscle receptors. As such
branches show a tendency to associate with those belonging to other categories, each of the nerves may be accompanied by thin fibres, the origin of
which can but rarely be traced with certainty. One must therefore be cautious
in attributing to every fibre seen in the common trunk the character of a
special unit. For the same reason the question of the motor nerves already
touched on above cannot be satisfactorily answered, since even if one sees the
main motor fibres associated with other fibres there remains a doubt as to the
true origin of the latter.
In Palinurus (PL 2, fig. 8) the accessory nerves show a similar arrangement;
the main differences already pointed out are that the thick accessory nerve
does not attain the calibre of the motor nerves and that the thin fibres surrounding cell 2 appear to be more numerous.
With all the elements of the MRO combined as in Text-fig. 11 and which
also may be seen in PL 2, fig. 7, we may gain an idea how complex its structure
is if we imagine that all the nerves, which are shown in Text-fig. 11 in their
outlines only, branch abundantly in the muscles and that to them must be
added all the fibres of uncertain origin which are represented in Text-fig. 12.
The muscles RM are supplied over their whole length by (a) the motor
fibres, which as in other muscles are probably composed of various elements,
(b) the thick accessory nerve, and perhaps by (c) the thin accessory nerve. The
innervation of that region with which the nerve-cells are connected is very
complicated: here on a small area are agglomerated the terminations of (a)
the dendrites of the nerve-cells, (b) the thick accessory nerve, (c) the thin
accessory nerve, and (d) fibres of dubious origin coming from the common
nerve-trunk. The motor fibres are also here, presumably supplying the scarce
myofibrils passing over this region.
The tracing of individual nerve-fibres on to the muscles, when they run in
one bundle, is liable to many errors. Even in such seemingly clear preparations
fvona
re
p
been
1
s
I
1.
TEXT-FIG. \Z. Diagram showing other nerve-fibres, as well as those in Text-fig, n which are drawn with dotted
lines, o, fibres running parallel to the accessory nerves, cf. PI. 2, fig. 9; b, branches of the accessory nerves springing at
different distances from the receptor organs and taking various courses; c,fibresaccompanying the main motor fibres; d,
fibres accompanying the main motor fibres but deviating towards the nerve-cells, seen also on Text-fig, IOA and PI. 1,
fig. 1; e, branch springing fr<im a nerve running to the median portion of the superficial dorsal muscle.
192
Alexandrowicz—Muscle Receptor Or gam in the
as represented in PI. 2, fig. 8, one soon loses the trail of the zigzagging
fibres and in such places as are represented in PI. 2, fig. 12, tracing becomes
hopeless. On the mutual relations of those nerve elements of various origin in
their terminal branches nothing can be said, except that, judging from the
course of the thicker nerves, it may be assumed that the muscles RM through
all their length are provided with various kinds of nerve-fibres.
The branches arising from the thicker nerve-bundles send in turn fibres
which for the most part take a longitudinal course in the muscles (Text-fig.
8c, PI. 2, figs. 10,11). It must be emphasized that the nerve-fibres penetrate
the muscles at all depths throughout their thickness and therefore only portions of them can be seen in the photomicrographs, since the others are out
of focus. It is interesting to note that the general appearance of the distribution of the branches of the finer nerves is not the same in both muscle units:
those in RM 1 are coarser while those in RM 2 are more dense (PI. 2, figs. 10,
11). This is in accordance with what has already been said about the structure
of the two muscles.
No terminations of special structure could be noticed and in the best
preparations the finest nerve-fibrils still noticeable are to be seen running
alongside the bundles of myofibrils until their blue colour becomes gradually
fainter and finally disappears. One gets the impression that their true terminations are beyond the capabilities of the staining method and very probably
beyond the limits of resolution of the ordinary microscope.
Elements of Muscle Receptors in the Embryonic Lobster
It was not possible to trace the axons of the cells of the MRO back to the
central nervous system nor could any indications as to how they might end be
found in the published accounts of crustacean nervous systems. Retzius (1890),
in his well-known work, described fibres in Astacus fluviatilis which enter
the 2nd root of the abdominal ganglia and give off ramifying branches ending
in the 'Punktsubstanz*. The axons of our cells might be among these fibres,
but his general definition referring to them as 'feine Fasern' could hardly be
applied to these elements, which are remarkable for their large diameter. It is
true that Retzius saw some thickerfibrestoo, butjudging by their general appearance he considered them to belong to some cells situated in the ganglion itself.
The works of E. J. Allen (1894, 1896) on the embryonic lobster also did not
offer the desired clue. Yet it seemed worth while to investigate again the same
organism with which Allen was so successful in discovering many details of
fundamental importance for our knowledge of the central nervous system of
decapods. As will be seen later, the assumption that Dr. Allen's work did not
contain any observations of the elements of MRO proved not to be an exact
one; for he had, in fact, seen these cells but classified them among ordinary
sensory elements.
The lobster embryos available were all in the same Pre-zoea stage. I am
indebted to Dr. Marie V. Lebour for her great help in examining the material
and determining the stage of development.
Abdomen of Homarus vulgaris and Palinurus vulgaris
193
The embryos, after removal from the egg-membranes, were stained,
fixed, and mounted in the usual way so that the observations could be made on
permanent preparations.
If the staining is successful, and this is not always so since the methylene
blue finds its way with difficulty under the integument, there appear on the
dorsal side of each abdominal segment one or two bipolar cells each with a
short distal process and a long one running towards the abdominal nerve-cord.
The short process may vary in appearance: its end may be pointed or swollen,
but when most of it is revealed by staining, it appears to be attached to a tiny
strand of tissue lying obliquely to the main axis of the body at the median edge
of the lateral superficial muscle. This may be seen in PI. 3, fig. 15, and (better
stained) in PL 3, fig. 16; but in the latter it is partly overlain by the cell and
therefore the process of the cell appears fore-shortened.
There can be little doubt that these are the elements of the muscle receptor
organs at that stage of development when the nerve-cell has not yet acquired
its multipolar shape but is already connected with its muscle by means of a
single process. In some instances even the branching of this process at its
point of junction with the muscle-fibre could be noticed. The coloration of
the muscle itself is probably due to the staining of its particularly abundant
nerve-fibres, which could not, however, be discriminated because their staining was either too diffuse or in the form of small dots.
For the most part only one cell on each side of the segment becomes visible,
but in favourable preparations the second appears also, though less distinctly
stained. The second muscle could not be distinguished, and the process of the
cell was observed terminating in a rounded diffusely stained swelling.
The long processes of the cells run towards the nerve-cord close under the
integument (PL 3, fig. 13). After entering the ganglia they pass near to the
median line and each divides into two branches which run in opposite directions and can be followed through several ganglia. Although in the majority of
preparations only one fibre is stained (PL 3, fig. 17) the presence of two fibres
entering the same root could be established (PL 3, fig. 18). Associating with
similar fibres from other segments they form in the nerve-cord a tract which
can be followed through all abdominal and thoracic segments, but its
definite connexions with other elements of the central nervous system could
not be made clear. In relation to the giant fibres, one pair of which, most
probably the median ones, often stain quite well, this tract is situated more
ventrally in the abdomen and at the same distance from the median line or a
little nearer to it than the giant fibres (PL 3, figs. 17, 18); in the thorax it lies
on the inside of all the fibres which usually stain with methylene blue (PL 3,
fig. 19).
The fibres of which this tract is composed appear in varying numbers.
Sometimes only one or two are visible, at other times a whole bundle of them
become stained, but in the latter case the counting is uncertain. Assuming, as
seems very likely, that each axon sends its bifurcating branches through all
ganglia rostral and caudal to its own segment, then the total number would
194
Alexandrovdcz—Muscle
Receptor Organs in the
amount to 16 throughout, corresponding to the 12 MRO cells in the abdomen
plus the 4 in the thorax.
It is interesting to note that Allen saw these elements and figured them
(1894, PL 36, figs. 10, 11, 12; 1896, PL 4, figs, i, 3). He observed the cells,
describing them as sensory cells, followed the course of their processes, and
was able to trace an individual fibre in the nerve-cord for a long distance, in
some cases as far as through seven ganglia. As for the distal process he noticed
that it may be 'flattened out' and pictured such a cell in his fig. 10 (1894).
Having no reason for suspecting that they might not be ordinary sensory cells
he did not attach special importance to this peculiarity, which on closer
scrutiny has revealed details proving the true nature of these elements. The
other point on which Allen's data can be supplemented by the present observations is the establishment of the occurrence of two cells and of two fibres
entering each side of the ganglion. This is in agreement with the presence of
two muscle receptor units in the adult animal.
Observation of the elements of MRO in embryonic lobster is facilitated by
their property of appearing during the earlier stage of the staining process.
When other nerve elements take up the dye the picture becomes confused,
since the motor and ordinary sensory fibres pass through the same root of the
ganglion; moreover, in the vicinity of the cells of the MRO some bodies
appear which look like smaller cells and which are perhaps the common
sensory cells. In such cases the discrimination of the different elements is very
difficult and the tracing of individual fibres in the common trunk quite
impossible.
To sum up, it may be stated that all the evidence leads to a belief that the
cells on the dorsal side of the abdomen in the lobster embryo already seen by
Allen are in fact the nerve-cells of the muscle receptor organs. Objections
could be raised against the statements concerning the course of their axons on
the grounds that, because the same root is composed of different elements,
some other kind of fibres might be mistaken for these axons. It is true that
the following of individual fibres round the convexity of the abdomen is
difficult, but it seems improbable that in well-stained preparations the central
processes arising unmistakably from these cells would on every occasion
become confused with some other element before reaching the ganglionic
cord.
DISCUSSION
Consideration of the nature of the peculiar organs described above raises
some interesting questions. The first is whether they are present exclusively
in the dorsal part of the body or whether they occur elsewhere. This will
only cease to be open to question if their presence should be proved in other
parts of the body. Negative statements have hardly any conclusive value
since, if similar elements should be hidden somewhere deeper among the
muscles, they might only be found by chance. At any rate, no such organs
have yet been seen and at present all one can say is that muscle receptors of
Abdomen of Homarus vulgaris and Palinurus vulgaris
195
this kind are probably restricted to the dorsal region of the abdomen and of
the thorax.
As to the problem of their function it must be emphasized that the present
investigation is only aimed at the elucidation of their morphological structure.
However, special attention has been paid to those points which should be
taken into account when future researches on their role are made by appropriate methods. Meanwhile, it seems worth while to consider what conclusions
might be derived from these morphological data. One of these conclusions
has already been expressed in the designation adopted—muscle receptor
organs—since it seems evident that, if a special muscle unit has connexions
with that kind of ganglion cell which sends its axon towards the central
nervous system, the whole must fulfil some receptor function.
It should now be noted that this ganglion cell shows two peculiar features,
namely, the ramifications of its short arborizations in a particularly modified
region of the muscle, and the very stout calibre of its axon. There is only one
kind of cells as yet known in crustaceans in which there is a certain similarity
to these elements, viz. the ganglion cells in the heart. Their dendrites have
similar short processes ending in the muscles (Alexandrowicz, 1932, though
a special change in the structure of the muscles was not described) and their
axons are of very large calibre; but as the latter give rise to branches innervating the whole heart their size is understandable. If, however, the nerve-cells
of MRO acted only as elements conveying the impulses to the central nervous
system, the dimensions of their long processes compared with those of other
receptor elements of Crustacea would be quite unusual; this suggests that the
higher conduction-velocity of these fibres must have some particular functional
importance.
Granting that the adopted designation of muscle receptors has been justified, the next question is at which instant of the muscle movement do they
come into action. The assumption that these organs are present only in the
abdomen and in the posterior part of the thorax leads to the idea that they
have something to do with muscular movements peculiar to this part of the
body, and this suggests the activity responsible for the vigorous flexions of
the 'tail' by means of which the animal shoots backwards through the water.
Of the two possible states in which the muscle receptor may dispatch impulses,
it may be conjectured that the stimulation of the nerve-cells takes place
during the stretching of the muscles RM, i.e. when the abdomen is bent by
the action of its ventral muscles. At that instant the function of the stimulated
nerve-cell of MRO might be to transmit inhibiting impulses in order to bring
about relaxation of the mighty flexor muscles. If this be so, the nerve-cells of
the MRO might be expected to have connexions with the system of giant
fibres, which are responsible for the contraction of the abdominal muscles in
the escape reaction of these animals (Johnson, 1926; Wiersma, 1938, 1947).
Johnson (1924) and Holmes (1942) have given interesting details about these
giant fibres, but knowledge of their connexions has yet to be completed.
Other hypotheses about the function of these MRO could be put forward,
196
Alexandrowicz—Muscle
Receptor Organs in the
Hut if we hold to the one suggested above we must try to answer the question
whether both units of the MRO have the same function or a different one.
The posing of this problem is justifiable if only because the units are always
duplicated on each side. Some differences in the structure of the cells, such as
the basket-works around cell 2, are not conclusive, since their absence in
cell 1 might be considered as not sufficiently proved; for it is known that sometimes certain nerve elements do not stain readily at certain places. There are,
however, other features such as the different insertions of the two RM and the
striking difference of their cross-striation which must lead to the conclusion
that they cannot be physiologically identical. On the other hand, they have
too much in common in their innervation to be regarded as endowed with
totally different functions. It seems therefore conceivable that they each have
a similar function but that they do not act at the same time during the
flexion of the tail. If so, it would seem more likely that nerve-cell 1 becomes
excited first, and cell 2 afterwards, when the abdomen becomes more, or
perhaps maximally, bent. This supposition seems to agree with what may be
deduced from the courses and differences of the insertions of the muscles, and
is supported by the observation that RM 1 appears to be the slacker of the
two.
According to this conjecture RM 1 would reach the threshold for stimulation of its nerve-cell earlier, and it must be assumed that this instant does not
coincide with the maximal capacity for extension of the muscle because otherwise the latter would be overstretched by the further flexion of the tail.
These speculations have been based on hypothetical assumptions and it is
obvious that by substituting others one might arrive at different conclusions
with several variants. The one that has been chosen fits with the available
data, although not quite satisfactorily; for instance, similar organs are
present in the hermit crab, in which the abdomen has not to perform such
movements as in the Macrura.
As to the reasons for the differences in structure of the two muscles RM
and especially their unequal cross-striation, the question appears to be
obscure. That the cross-striations of muscle-fibres may be more or less
widely spaced is a known fact, but it seems that no satisfactory explanation of
this has yet been given. It has been suggested that the finer cross-striation
occurs in muscles contracting more quickly, but as Heidenhain (1911) has
pointed out, the muscles of insects have a much coarser striation than those of
the turtle. Thus, a general law cannot be established. This does not exclude
the possibility that in the same animal muscles of different structure differ in
the speed of contraction, and this is the view adopted by Brenner (1939). It
should be mentioned that in our preparations from the dorsal part of the
abdomen of the lobster, differences in cross-striation could also be observed
between the units of the ordinary muscles.
As regards the problem of the function of the various nerves supplying the
MRO, it must be emphasized that the function of all of them, except those
here described as motor fibres, is very doubtful. It is true that the assumption
Abdomen of Homarus vulgaris and Palinurus vulgaris
197
that they form part of the system of motor nerves supplying the ordinary
muscles might meet some objections, in view of the fact that the thickest
'main' fibres put into this category run in the common trunk, apparently as
individual units. On the other hand, it can be stated that in some muscle
receptor organs of the thorax the 'main' motor fibres are missing and the
whole of the motor innervation is given off by the nerves of the neighbouring
muscles.
When we consider the elements described as accessory nerves it must be
pointed out that their outstanding peculiarity lies in the connexions of the
same fibre with both the nerve-cells and the muscles. These connexions could
be established, at any rate for the thick accessory nerve, beyond any doubt.
There is hardly any indication in the morphology of these nerves that would
afford a clue as to their function. Since they are of two kinds one might suppose that one exerts a sensitizing, and the other a suppressing, action on the
excitability of MRO and that in that way they regulate the functioning of the
whole so as to convey the impulses to the central nervous system only during
a certain type of muscular activity. It is natural, when analysing the structure
of organs that might act as muscle receptors, to draw analogies with those
organs which are known to have such a function, i.e. the muscle spindles of
vertebrates. If compared with the latter the MRO in crustaceans show a
remarkable individuality of their muscle components, which, as has been
emphasized, are individual separate muscle units, whereas the muscle spindles
represent only modified muscle-fibres lying within the bundles of the
ordinary muscle-fibres. A common feature may be seen in the modification of a certain region of the muscle elements. In the crustaceans, as
has been shown, there is an interruption of the muscle elements by connective tissue. In the muscle spindles the equatorial region is devoid of crossstriated myofibrils, being transformed into what is termed by Barker (1948)
a 'nuclear bag', a 'great mass of smaller spherical nuclei which at one point
completely fills and often distends the body of the muscle fibre'.
In the innervation of the MRO in crustaceans the close association of the
ganglion cells with the muscles appears at first sight as the particularly
striking feature; yet this is in accord with the peripheral situation of all kinds
of receptor cells in crustaceans as well as in other invertebrates. The actual
differences in the schemes representing the afferent neurons of receptor
organs in vertebrates and crustaceans would therefore be the greater length
of the distal processes of the cells in the former. A more important difference
is the occurrence in the muscle spindles of various kinds of sensory endings,
a view adopted by Barker in his valuable recently published work (1948).
If other nerve elements be considered, it may be stated that in both cases
there are several systems of fibres. One of them has obviously a motor function.
As to the others, even in the muscle spindles, although they have been the
subject of many investigations, there are still points requiring elucidation;
and in crustaceans the whole problem awaits a physiological approach. In the
meantime, as regards the hypothesis that one of the accessory nerves might
198
Alexandrowicz—Muscle
Receptor Organs in the
sensitize and the other suppress the excitability of MRO, it may be remarked
^ ^ t the former would have a similar function to the 'small fibres' in the
muccle spindles, which, as Leksell (1945) has shown, may 'facilitate the discharge from the afferents'.
Finally, in such a comparison it should be observed that the functions of
these organs in vertebrates and crustaceans are most likely far from being
identical. In vertebrates they serve to control the delicately graded adjustments of the muscles, whereas in crustaceans they enter in action, if our
hypothesis holds good, only during particularly violent contractions of the
muscles.
REFERENCES
ALEXANDROWICZ, J. S., 1932. Quart. J. micr. Sci., 75, 181.
ALLEN, E. J., 1894. Ibid., 36, 461.
1896. Ibid., 39, 33.
BARKER, D., 1948. Ibid., 89, 143.
BRENNER, H., 1939. Z. f. Zellforsch. u. mikr. Anat., 29, cited from H0NCKE, P., 1947. Acta
Physiol. Scand., 15.
COLE, E. J., 1934. Stain Tech., 9, 89.
DANIEL, R. J., 1930. Rep. Lancashire Sea Fish. Lab.
HEIDENHAIN, M., 1911. Plasma und Zelle, fol. 2, p. 627.
HOLMES, W., 1942. Phil. Trans. Roy. Soc. B, 231, 293.
JOHNSON, G. E., 1924. J. comp. Neurol., 36, 323.
1926. Ibid., 42, 19.
LEKSELL, L., 1945. Acta Physiol. Scand., 10, Suppl.
M I L N E EDWARDS, H., 1834. Histoire naturelle des CrustacSs.
RETZIUS, G., 1890. Biol. Unters., N.F. 1.
SCHMIDT, W., 1915. Z. wiss. Zool., 113, 165.
WIERSMA, C. A. G., 1938. Proc. Soc. Exp. Biol. and Med., 38, 661.
1947. J. Neurophysiol., 10, 23.
EXPLANATION OF PLATES
All photomicrographs have been made from preparations stained with methylene blue,
fixed in ammonium molybdate and mounted in xylene-dammar.
PLATE I . Elements of the muscle receptor organs in Homarus vulgaris.
FIG. I . Nerve-cells of the 2nd abdominal segment (right side).
FIG. 2. Nerve-cell 1 of the 1st abdominal segment with the dendrites penetrating the connective tissue region of the muscle RM 1. Thick and thin accessory fibres can be seen running
close together; the third fibre running near to the accessory nerves is the axon of the cell 2.
FIG. 3. Ramifications of the dendrites of the nerve-cell 2.
FIG. 4. Nerve-cell 1 in a capsule which encloses the roots of the dendrites and of the axon.
The small bodies in the vicinity of the cell are the nuclei of the connective tissue. On the
sides of the faintly stained cell 2 are seen branches of the two accessory nerves.
FIG. 5. Nerve-cells enclosed in the capsules and surrounded by sheets of connective
tissue.
FIG. 6. Nerve-cell 2 with fine fibres running close to the axon and forming a basket-work
round the cell.
PLATE 2. All photomicrographs exceptingfig.8 are made from Homarus vulgaris.
FIG. .7. Muscle receptor organs of the 5th abdominal segment (left side). Cell 1 appears
much larger than cell 2 because of the exceptional dimensions of its capsule, which is deeply
stained. One of the dendrites arises from the axon. mot, main motor fibres; one of the fibres
overlies the other along part of its course and in the photograph the two appear to form a
Abdomen of Homarus vulgaris and Palinurus vulgaris
199
common trunk; ace, the two accessory nerves. Note the branch of the thick accessory nerve on
the right side taking its course alongside the muscles.
FIG. 8. Palinurus vulgaris. 2nd abdominal segment (left side), mot, main motor nerve of the
muscle RM 2; ace, accessory nerves. Note also fine fibres approaching cell 2 which will form
a basket-work around this cell; the latter, owing to the dark staining of the cell, cannot be seen
in the photograph. On the right side of the picture fibres of various origins and calibres run
to the muscle-fibres. In this preparation the abnormal position of the axon of cell i was
caused artificially during dissection.
FIG. 9 shows finer fibres accompanying the accessory nerves.
Fics. io and n . Distribution of the nerve-fibres in the muscles RM I and RM z. The
finerfibresare running alongside the myofibrils at all levels, in the muscles and therefore only
some of them are in focus. The small dots seen in the figures are nerve-fibres, which when
stained with methylene blue generally assume a beaded appearance and after prolonged
staining break up into chains of small particles. Note finer aspect of these elements in the
muscle RM z.
FIG. 12. Nerve-bundle on the muscles RM showing that it is composed of several fibres.
PLATE 3. Embryos of Homarus vulgaris in the Pre-zoea stage.
FIG. 13. Part of the abdomen seen from the ventral side. The fibres reaching the ganglionic
cord from both sides are the axons of the nerve-cells of the muscle receptor organs. The cells
lying on the dorsal side may be seen but they are out of focus.
FIGS.1 4, 15, 16. Nerve-cells of the muscle receptors from the abdomen. In fig. 16 the
muscle-bundle of the receptor organ is well stained but is partly overlain by the cell.
FIG. 17. Axon of the cell of the receptor organ entering the ganglion of the nerve-cord and
bifurcating in it. On the opposite side the course of the same element is obscured by a giant
fibre stained on that side only.
FIG. 18. Two fibres of the receptor organs entering the abdominal ganglion.
FIG. 19. Parts of the thorax and abdomen from the dorsal side showing the tract of the
muscle receptor organs.
Quart. Journ. Micr. Sci., Third Series, Vol. 92
J. S. AI.EXANDROWICZ—PLATE I
Quart. Journ. Micr. Sri., Third Series, Vol. 92
J. S. ALEXANDROWICZ—PLATE II
Quart. Journ. Micr. Sci., Third Series, Vol. 92
J. S. A L E X A N D R O W I C Z — P L A T E III