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Innervation of the panniculus carnosus in the armadillo Dasypus novemcinctus.

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Department of Anatomy, Johns Hoplcins Uniz;ersity, Baltimore, Yaryland
There has been considerable recent attention directed at the
subject of peripheral nerve-muscle relationship. Probably the
concensus of opinion favors the thesis that the Furbringer
theory, as originally set forth, is not rigidly applicable. The
relationship of a nerve arid its muscle although usually conservative in major respects is not always inflexible. Both
variation and evolution in these, as in other anatomical details,
can and do occur. There are grounds for believing, too, that
afferent pathways are more labile than efferent, and that
routes are more variable peripherally than proximally.
I n Dasypus novemcinctus it was fouiid that the innervation
of the panniculus carnosus was obscured by a diffuse and
intricate net-work of nerves on its medial surface. In addition
to the anterior thoracic nerve (from the pectoral group), this
net-work was contributed to by the spinal nerves. These were
of two types analogous to the anterior rami and the lateral
rami of the intercostal nerves in man. The former, instead
of emerging from the intercostal space close to the sternum,
branched from the intercostal nerve more laterally, thus extending directly to the inferomedial aspect of the panniculus.
The latter instead of passing through the intercostal space to
emerge on the lateral aspect of the thorax, emerged dorsally
close to the spinal column and passed laterally and ventrally
beneath the trapezius to the dorso-medial aspect of the
From this complex mass of nerve fibers, from the three
different sources, lying in a dense fatty connective tissue,
nerves were found to enter the panniculus. Thus it was considered that there was a strong possibility of a double motor
innervation of the paiiniculus, nerve fibers of the anterior
thoracic (pectoral) field having been rerouted over intercostal
nerves in correlation with the extreme specialization of the
panniculus in this animal. To determine the nature of these
nerves, they were electrically stimulated and traced as far as
possible by dissection,
Zeiger in his work cin the panniculus in the Edentata concluded, after careful dissection of Dasypus novemcincta,
Euphractus villosa and other forms, that the innervation of
this muscle was by the anterior thoracic nerve alone.
Experimed N o . 1
Distal stimulation of branches of the anterior thoracic, and
the lateral and anterior brannches of the iiztercostal nerves.
The animal, a 10 lb. male, was anaesthetized intraperitoneally
with 1.5 cc. of nembutal. A longitudinal incision was made
ventrally along the inner edge of the shell from forearm to
thigh, uncovering the complex network of nerves on the medial
aspect of the panniculus carnosus. This plexus, formed by the
anterior branches of the intercostal nerves from T, to T, or 8 ,
by branches of the anterior thoracic (pectoral) nerve, and by
lateral branches of the intercostal nerves, lies in a thick, almost
viscous, fatty connective tissue, from which the nerves were
separated largely by blunt dissection. I n this manner a fairly
large area of the paiiniculus was exposed.
The nerves were stimulated with a unipolar electrode from
a transformer using alternating house current. Voltages
from 0.1 to 0.5 volts were used. The indifferent electrode was
buried in the pectoral muscles of the opposite side. The experiment lasted between 4 and 5 hours at the end of which
time the animal appeared to be in good condition.
Resztlts. It was found that comparatively few of the nerves
isolated and stimulated elicited any response. This was of
Fig. 1 Diagrammatic representation of the panniculus and its innervation i n
Dasypus iiovemcinctus.
A. Shell intact. B. 8hell removed. C. Panniculus removed. D. Inner aspect
of shell showing nerve net-work on medial surface of panniculus (modified from
Zeiger). E. Section through thoracic region showing nerve relationships. F.
Brachial plexus. Anterior thoracic nerve to panniculus black, t o pectoral muscle
stippled. Dorsal roots cut.
Abbreviations: Br. Lat. N. 1ntercos.-Ramus
lateralis nervi intercostah
(lateral branch of intercostal nerve)-&.
Ant. N. Inter.-Ramus anterior nervi
intercostah (anterior branch of intercoatal nerve).
two kinds; first, a generalized twitching or contraction of
abdominal and thoracic musculature ; second, a localized response of the panniculus. After cutting the nerve, tlie former
occurred following stimulation of the central stump only, while
tlie latter followed stimulation of the peripheral stump only.
With each nerve the type of response was constant if any
action at all was found. However, in some cases response was
absent at first but appeared later, or vice-versa.
Stimulation of the branches of the anterior thoracic
(pectoral) nerve, both close to the brachial plexus and more
distally, gave either 110 response or contractions of the
Stimulation of the anterior branches of the intercostal
nerves which were separated with difficulty from the plexus
of nerves to the panniculus (each of which was not absolutely
identified) in no case elicited contractions of the panniculus
but in some cases resulted in twitching of the thoracic
Stimulation of the lateral branches of the intercostal nerves
in the thoracic region gave results similar to those on stimulation of the anterior branches, although the thoracic musculature contractions wcrc more constant. Likewise, stimulation
of the spinal nerves in the abdominal region gave contractions
of the abdominal musculature alone.
Summary aad conclusions. No motor response and therefore no intercostal innervation of the panniculus was demonstrated by stimulation of branches of the intercostal nerves.
Intercostal innervation was not absolutely disprovcn by this
In all probability the lateral and anterior branches of the
intercostal nerves in this area carry sensory fibers, and response of the abdominal and thoracic musculature to stimulation of the central stump of the cut nerves is of reflex nature.
PossiFZe experimcntd errors. The nerves stimulated in this
plexus were in many cases very fine, their ramifications
complex and numerous, and their origins obscured by the
density of the surrounding fat and connective tissue. These
anatomical factors may have given rise to drying of fine fibers,
injury by stretcliing on blunt dissection and actual breaking
of many of the finer strands. Isolation of all fibers, much less
their stimulation, was an obvious impossibility owing to their
The above possible errors diminish the value of negative
results. The positive results were perhaps influenced by other
factors. Unipolar stimulation may allow spread of stimulus,
but the specific muscular reactions and their constancy would
seem to eliminate this factor, as would the results following
cutting of the nerves. At one time in the experiment the indifferent electrode was placed in the panniculus on the stimulated side and no differences in response were noted.
The positive results may also lose some value because of
the possibility of common pathways of intercostal and anterior
thoracic branches. Since the reactions seemed to fall consistently into the pattern described above, such haphazard
fasciculation of dissimilar fibers would appear unlikely.
Experiment N o . 2
Proximal stimdation of the intercostal nerzies. A large
female was anaesthetized with nembutal, and the panniculus
uncovered as in the previous experiment. The anterior
thoracic (pectoral) nerve was isolated and stimulated with 0.1
volt, with resultaiit contraction of the panniculus.
The thorax was then opened by a mid-line incision from
xiphoid to clavicles and the second to fifth intercostal nerves
quickly isolated and stimulated on one side. After each stimulation, the anterior thoracic nerve of the same side was also
stimulated. Both unipolar and bipolar electrodes were used.
With the cessation of breathing following the opening of the
thoracic cavity, the heart action quickly stopped. Nevertheless, throughout the experiment, the panniculus was effectively
stimulated by way of the anterior thoracic nerve, and reacted
well for over one-half hour. The intercostal nerves were
stimulated within 5 to 10 minutes after death.
Results. I n no instance did the paniiiculus contract 011
stimulation of the intercostal nerves, T, to T,, although
voltages as high a s 0.5 volts were used.
Co~clusion. Intercostal nerve branches do not carry motor
fibers to the panniculus carnosus.
Possible experimental errors. The intercostal branches to
the plexus of nerves beneath the panniculus may have been
injured on exposing the muscle. This would seem unlikely,
from the limited extent of the dissection.
It also may be significant that the consistency of response
diminishes as the anterior thoracic nerve is more proximally
stimulated. Thus, since the intercostals were stimulated f a r
from the muscle, a n unknown factor may have appeared. This
conceivably may depend upon the fibers in question being too
deeply buried in other surrounding nerve fibers or in the connective tissue sheath. The strong voltages used probably
eliminated any such factor.
The fact that most of the experiment was carried out after
the death of the animal I believe to be of 110 importance, since
the panniculus responded well to anterior thoracic stimulation
one-half hour after death, and since the anconeus responded
to stimulation of both its radial and ulnar innervation about
45 minutes after death.
StaiNimg of nerve f i b c r s to the panniczdtis. An attempt was
made to stain the nerves to the panniculus in order to follow
their courses to the nerve endings on the muscle fibers. Two
methods were used, by Sililer’s solution, as outlined in Byrnes’
paper, and by the methylene blue technique. The latter method
was discarded because oxidation of the stain to the colored
condition could not be accomplished in the necessarily thick
I n the former, Ehrlich’s acid hematoxylin was used. Adequate staining was not obtainable in my preparations, since
the extensiveness of the nerve network demanded the removal
of large thick portions of the muscle. The material used had
unfortunately been fixed for some weeks, being removed from
a n animal that had been injected with stock embalming fluid.
Nevertheless, it was possible to demonstrate that intimate
anastomoses (as Zeiger stated), occur between the anterior
thoracic (pectoral) fibers and those of the anterior and lateral
branches of the intercostal nerves within the body of the panniculus. Many of these branches were so extremely fine that
it seemed very unlikely that they passed through the muscle
to the overlying shell, and it was thus concluded that there are
nerve endings of spinal (non-brachial plexus) origin in the
panniculus. The nature of these endings could not be
Summary. No motor innervation of the panniculns could
be demonstrated other than that of the anterior thoracic
(pectoral) nerve. Some fibers from spinal nerves apparently
do end in this muscle and are therefore probably of sensory
nature. The majority of the spinal nerves in question pass
through the muscle to the overlying shell.
BYRNES,C. M. 1926 A contribution to the pathology of paralysis agitans.
Arch. Neurol. & Psych., vol. 15, pp. 407-443.
ZEIQEE, K. 1927 Beitrage zur Kenntnis der Hmtmuskulatur der Saugetiere.
11. Die Hantmuskulatur am Rumpf des Kugelgiirteltieres (Tolypeutes)
Morph. Jahrb.. Bd. 58, 8. 64.
-1929 Beitrage zur Kenntnis der Hautmuskulatur der Saugetiere.
111. Die Hautmuskeln am Rumpf. ~ O Dasgpus
novemcinetus. Morph.
Jahrb., Bd. 63, 8. 260.
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dasypus, carnosus, armadillo, panniculus, innervation, novemcincta
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