INNERVATION O F THE PANNICULUS CARNOSUS I N THE ARAIADILLO, DASYPUS NOVEMCINCTUS SAMUEL S. MILES Department of Anatomy, Johns Hoplcins Uniz;ersity, Baltimore, Yaryland ONE FIGURE 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 panniculus. 273 TIIE ANATOMICAL RECORD, VOL. JOVEXBER, 1940 78, NO. 3 274 SAMUEL S. MILES 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). 275 276 SAMUEL S. MILES 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 panniculus. 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 musculature. 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 experiment. 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 IKNERTATION O F PANNICULUS CARNOSUS 277 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 multiplicity. 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. 278 SAMUEL S. MILES 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 preparations. 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. INNERVATIOX O F PANNICULUS CARNOSUS 279 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 determined. 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. LITERATURE CITED 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 D novemcinetus. Morph. Jahrb., Bd. 63, 8. 260. .