Okajimas Folia Anat. Jpn., 67(6): 449-456, March, 1991

Innervation of the Sternalis Muscle Accompanied by Congenital Partial Absence of the Muscle

By

Masahiko Y. KIDA and Hiroyuki KUDOH

Department of Anatomy, Sapporo Medical College, Sapporo 060, Japan. Department of Anatomy, St. Marianna University School of Medicine, Kawasaki 213, Japan.

-Received for Publication, October 29, 1990-

Key words: Sternalis muscle, Pectoralis major's portial absence, Innervation, Pectoral , Intercostal nerve.

Summary: In one case accompanied by congenital partial absence of the pectoralis major muscle the sternalis muscle was examined to confirm its innervation by means of analysis of intramuscular nerve distribution. It was proved that the sternalis muscle was supplied only by the pectoral even in the case of sternalis in direct contact with the proper thoracic wall. These findings as well as the results of Ura (1937) and Morita (1944) favor the interpretation presented by Eisler (1901), in which the sternalis muscle was described as being supplied only by the pectoral nerves. However, the problem of double innervation of the sternalis requires continued discussion because the relationships between the pectoral nerves and the branches of the or extramural nerves (Yamada & Mannen, 1985; Kodama et al., 1986) have not yet been resolved. The precise genesis of the sternalis muscle should be also examined though it has already been proved to be derived from the pectoralis muscle group including the subcutaneous trunci muscle.

The sternalis muscle is one of the most famous minor influence upon, that the problem of exactly what variant muscles in the human muscular system. kind of nerves innervate the sternalis muscle has not Statistical investigations of this variation were widely yet been completely resolved. carried out in many races. The variation has in the past The authors encountered a rare case of the sternalis, also attracted much morphological interest in its accompanied by the partial absence of the pectoralis phylogenic origin and innervation. According to the major muscle and directly in contact with the proper description of Le Double (1897), Eisler (1901) and Ruge thoracic wall through the cleavage resulting from the (1905), the sternalis muscle was reported, on the basis partial absence of the pectoralis major muscle. of its morphology and/or innervation, to be closely Therefore, the authors examined the above-mentioned related to such muscles as the rectus abdominis, case to confirm the innervation of the sternalis muscle. sternocleidomastoid, platysma, pectoralis major, subcutaneous trunci, etc. From the morphological view point, the sort of innervation of such muscles must Materials and Methods correspond to that of the sternalis muscle. With regard to innervation of the sternalis, however, the participa- The sternalis muscle was observed on each side of tion of only two kinds of nerves, namely the intercostal a 45-year-old female cadaver in the dissecting class of nerves and the anterior thoracic nerves (the pectoral St. Marianna University School of Medicine, Kawasaki, nerves), was already demonstrated in those days 1987. The sternalis muscle of the left side was accom- (Cunningham, 1888; Le Double, 1897). panied by congenital partial absence of the pectoralis After the contributions made by the studies of Ura major muscle, while the muscle of the right side was (1937 & 1938) and Morita (1944), all the cases of a typical sternalis. The left sternalis muscle was carefully sternalis muscle are now commonly recognized to be detached from the proper thoracic wall marking every supplied by pectoral nerves and to be derived from the structure that appeared to be a branch from the inter- pectoral muscle group. However, the participation of costal nerves supplying the sternalis muscle. Then the the intercostal nerves in the innervation has also been sternalis muscle was removed en bloc, together with the insisted upon by many authors (e.g., Dwight, 1888; Le pectoral muscles, the axillar arch, the latissimus dorsi Double, 1890; Bardeleben, 1905; Fick, 1918; Taniguchi muscle and the brachial plexus. The sternalis muscle I ochiliara, 1932; Fukuvama, 1940; Kitamura et al., and its nerves of supply were examined by means of 1985). This fact gives the impression, though having analysis of intramuscular nerve distribution and nerve 450 M.Y. Kida and H. Kudoh

fiber analysis. biceps and the anterior border of the latissimus dorsi. In the sternocostal part of the pectoralis major, small Findings amount of muscular fibers immediately abov e the cleavage was attached independently of the other main The sternalis muscle observed on the left side of this part of the pectoralis major. Namely, the fibers wound cadaver extended between the sternal angle and the from before backwards, first below and then behind upper end of the anterior layer of the (Fig. the inferior border of the pectoralis major at the axillar 1). Although the whole of the sternalis muscle, including fossa, to insert into the short head of the biceps after its origin and insertion, was convex to the lateral side, joining the tendon of the axillar arch. its fleshy part, the muscle belly, ran almost straight. The sternalis muscle of this case was innervated by The muscle belly was 7 cm in maximum length and 4 three twigs from the two branches of the loop (ansa cm in maximum width. pectoralis) which was formed with the lateral and The pectoralis major muscle was partially absent at medial pectoral nerves (Figs. 2 & 3 and Plt. 1). Of the the middle sternocostal portion. This absence resulted two branches, the cranial one sent a twig to the in a large cleavage, which ran in the direction of the and then extended over the superior muscle fibers of the pectoralis major at the same level border of the pectoralis minor muscle to distribute to of the upper part of the muscle belly of the sternalis. the pectoralis major and sternalis muscles, and the The nerves of supply to the sternalis muscle running caudal one penetrated the pectoralis minor after having medially on the surface of the pectoralis minor muscle sent branches to the same muscle, so that the two were directly observable through the cleavage of the branches lay between the pectoralis major and minor pectoralis major muscle (Fig. 1). muscles, where they were rearranged into three twigs. In this cadaver there was also a muscular axillar arch Among the three, the cranial twig directly reached the (Achselbogen) extending between the short head of the sternalis muscle through the wide cleavage of the

Fig. 1. Anterior view of the sternalis muscles and the pectoralis major muscles of this cadaver. The anterior cutaneous branches of the intercostal nerves appeared at intervals along the medial border of thesternalis muscle (ST) on each side. On the right side, the two twigs (indicated by white arrow heads) from the pectoral nerves emerged from the pectoralis major and then ran towards the sternalis on the surface of the pectoralis major muscle. There was no variation except for the sternalis in the right pectoralis major. On the left side, the sternalis muscle, accompanied by the partial absence of the pectoralis major and the a\ illar arch (Achselhogen) was seen. The latter is not shown in this figure. The nerves of supply to the stemili, (indicated by vkhite arrow heads) and the pectoralis minor rutkcic (marked by a %%biteaqeri,k) were partiall^ ei 111,1101 the cleavage of the pectoralis major muscle. Their description, are gisen in the lest. PA, h( :111,1 I arc .ahbre,iations of the abdominal part , clavicular part and creriloco,,tal part. of the pectoralis ira Jr i ii rI I(', rpcc,ti,ely. Innervation of the Seeman,' Muscle 451

apparent overlapping distribution, the prime area of distribution of each twig was recognizable. Namely, there was a tendency for the more lateral and deeper part of the muscle belly to receive the nerve fibers from the lower segments in contrast with the innervation of the more medial and superficial part being supplied by the higher segments (Fig. 5). In the left pectoralis major muscle of this case the abdominal portion could not be clearly distinguished though its appearance and the distribution pattern of its nerves were examined.

Discussion

According to the description of Cunningham (1888), the sternalis muscle was first described at the beginning of the 17th century and Hallett (1848) reported its nerve

Fig. 2. Diagram of the left brachial plexus of this cadaver. of supply to be from the intercostal nerves for the first All the abbreviations of this diagram are as follows, time. In the 19th and the first half of the 20th centuries, A: the axillary artery, AB: nerve to the axillar arch the sternalis muscle was widely studied. At the (Achselbogen), AM: the medial antebrachial beginning, several sorts of muscles were postulated to cutaneous nerve, AX: the axillary nerve, BM: the be the genesis of the sternalis. Thereafter, only two medial brachial cutaneous nerve, CB: nerve to the coracobrachialis muscle, EX: the extramural branch, muscles, that is, the pectoralis major muscle and the M: the median nerve, MC: the musculocutaneous subcutaneous trunci muscle, survived to be commonly nerve, R: the radial nerve, SBC: nerve to the recognized as the genesis of the sternalis muscle. , SBS: the subscapular nerve, SS: On the other hand, the intercostal and/or pectoral the suprascapular nerve, TD: the thoracodorsal nerves were reported as the nerve supply to the sternalis nerve and U: the ulnar nerve. The mark "?" indicates a branch of unknown distribution. muscle. However, the participation of the intercostal nerves in the innervation of the sternalis muscle would pectoralis major, while the middle and caudal twigs not agree morphologically with the innervation of its penetrated the pectoralis major muscle to innervate the genesis, the pectoralis major or the subcutaneous trunci, sternalis muscle. Nerve fiber analysis indicated that the because both of these are supplied by the pectoral cranial twig (A in Fig. 3) mainly consisted of the 7th nerves. and 8th cervical nerves and the caudal twig (C in Fig. Eisler (1901) studied innervation of the sternalis 3) of the 8th cervical and first thoracic nerves. The muscle in detail on the basis of analysis of intramuscular middle twig (B in Fig. 3) received nerve fibers from the nerve distribution. Since the sternalis was innervated cranial (A) and the caudal (C) twigs. Therefore, these specifically by the anterior thoracic nerves, he concluded three twigs slightly but definitely showed a metameric that the sternalis muscle was derived from the pectoralis gradient from A to C. The three twigs entered into the major muscle, and that the direction of the muscle belly of the sternalis muscle also in that order from fibers changed from transverse to longitudinal in the superomedial to inferolateral. process of the formation of the sternalis muscle. Binocular microscopic observations clarified that Namely, the superior side of the belly of the sternalis every marked structures between the sternalis and the at its original condition usually becomes the medial side thoracic wall, which initially appeared to be branches of the belly after completing the rotation. supplying the sternalis muscle from the intercostal Although he also accepted that the sternalis was nerves, were not nerves but connective tissue or blood innervated by the pectoral nerves, Ruge (1905) vessels. The results of examination by means of analysis supported on the basis of his comparative myological of intramuscular nerve distribution clarified that the studies the postulation of Turner (1867) that the nerve fibers of the three twigs were distributed to almost sternalis muscle was derived from the subcutaneous all muscle fibers of the sternalis (Figs. 4 & 5). Therefore, trunci muscle (the panniculus carnosus) which is also the intercostal nerves had no relation to the innerva- supplied by the pectoral nerve. tion of the sternalis muscle of this case. The results of Thereafter, three important reports concerning the the tmetlysisalso showed the metameric distribution of pectoral and sternal muscles were published in the flue(' twigs. Although those twigs communicated Japanese, two by Ura (1937, 1938) and the other by with each other in the muscular tissue and showed Morita (1944) on the basis of extensi\ e examinations 452 M.Y. Kida and H. Kin:loh

Fig. 3. Schematic diagram based on the nerve fiber analysis to show the gradient of the metameric distribution of the left pectoral nerves of this cadaver on the deep surfaces of the pectoralis major and sternalis muscles. Dots express the points where twigs enter into the pectoralis major muscle, and asterisks into the pectoralis minor. Circles stand for the points where twigs penetrate into or out of the pectoralis minor, while only in the distal part of the twigs "B" and "C" do the circles mean the points where the twigs come into and out of the pectoralis major muscle. The communication between the pectoral ansa and the Medianus Schlinge (the point of confluence of the medial and lateral cords) is omitted.

in many kinds of mammals. Ura (1937) clarified that the pectoral nerves were separable into three groups and that the caudalmost group, the pectoralis caudalis, innervated the subcutaneous trunci muscle including the pectoralis abdominalis muscle. Subsequently, based on the results of his observation Ura (1938) offered his hypothesis on the sternalis muscle that the genesis of the muscle was not homogeneous, because there were cases of the sternalis receiving the pectoralis caudalis. Namely, the subcutaneous trunci muscle participates in the genesis in individual cases. However, Morita (1944) reaffirmed that the sternalis muscle was innervated by the pectoralis intermedius. This means that Eisler's theory was more reliable than that of Ruge, because the sternalis was not supplied by the pectoralis caudalis. In spite of reliable reports of nerve supply by the pectoral nerves to the sternalis, there still remains the problem of the participation of intercostal nerves in the innervation of the sternalis muscle. Many reports

.4. Diagram showing the intramuscular nerve distributionof thiscase. The nervesof A, Band C correspond to those indicated in Fig. 3. Circles expressthe pointswhere nervesenter the muscular I issue. MS: stands for the musclespindle and VP: the Pacinian lamellatedcorpuscle, respectively. Innervation of the Sternalis Muscle 453

Fig. 5. Diagram showing the relation of the intramuscular nerve distribution to the innervation units of muscle fibers of the sternalis in this case. This relationship is superimposed on one transverse section of the muscle. Each compartment means a unit of muscle fibers separated by the perimysium. The horizontal stripes stand for the distribution of the A nerve, the vertical ones of the C nerve and dots stand for the distribution of the B nerve. Some compartments received double- or triple-innervation of the three nerves. : Distribution of the A nerve : Distribution of the B nerve Distribution of the C nerve supporting the concept of the participation of the inter- sternalis, however, seems to have been resolved mainly costal nerves have been published. Certainly , the by studies of the pectoral nerves (Eisler, 1901; Ura, possible participation of the intercostal nerves cannot 1937; Morita, 1944). Namely, it is highly reliable that be totally denied. the sternalis muscle is innervated only by the pectoral In this case the sternalis muscle was innervated by nerves. In these days, therefore, problems concerning the pectoralis intermedius and the nerves of supply also the innervation of the sternalis should be discussed in showed a metameric gradient. Even in this case of relation to understanding the stratified structure of the sternalis muscle which was in direct contact with the human body. proper thoracic wall, the authors could not find any In the problem of the double innervation of the nerves of supply from the intercostal nerves. These sternalis muscle, the existence of communication findings also support Eisler's theory . between the pectoral nerve and the anterior branch of Recently, Kitamura et al. (1985) studied a case of the intercostal nerve (Cunningham, 1888) requires large sternalis accompanied by congenital partial further examination. In addition, when the sternalis absence of the pectoralis major muscle and reported muscle is accompanied by the partial absence of the its double innervation by the intercostal and pectoral pectoralis major, the secondary and partial participa- nerves. However, they did not demonstrate findings tion of the extramural branches (Yamada & Mannen, clearly showing the double innervation . This is the same 1985; Kodama et al., 1986) in the innervation of the weakpoint of many other reports supporting the sternalis muscle should also be examined. participation of the intercostal nerves. Moreover, there remains the problem of the genesis The problem of the participation of the intercostal of the sternalis, namely the problem of whether the nerves in the innervation of the sternalis muscle is very sternalis muscle is always supplied by the pectoralis important for phylogenetic understanding of the human intermedius, the innervation of which surely proves that muscular system. The sternalis muscle is thought to the sternalis muscle does not originate from the sub- belong to the rectus group if it is supplied by the inter- cutaneous trunci muscle. The location of the sternalis costal nerves, though the rectus group is commonly muscle shows much variety. When the sternalis is accepted to be restricted to the abdomen in mammals. located laterocaudally on the surface of the pectoralis Therefore, the innervation of the sternalis muscle must major, the participation of the pectoralis caudalis in the be analyzed in detail at the level of intramuscular nerve innervation of the sternalis should be examined distribution. preferably because the abdominal part of the pectoralis The classic problem of mere innervation of the major is as a rule understood to be a rudiment of the 454 M.Y. Kida and H. Kudoh

pectoralis abdominalis, a part of the subcutaneous Okajimas Folia Anat. Japon., 19: 69-92, 1940. trunci muscle being supplied by the pectoralis caudalis 7) Hallett: Muscular system of the human body. Edimb . Med. (Ura, 1938). J., 69: 12, 1848. (cited from 2) 8) Kitamura, S., Yoshioka, T., Kaneda, M., Nlatsuoka , K., Chen, K. & Sakai, A.: A case of the congenital partial defect of the pectoralis major accompanied by the sternalis with enormous Acknowledgement size. Acta Anat. Japon., 60: 728-732, 1985. (In Japanee with English summary) The authors sincerely express their thanks to Prof. 9) Kodama, K., Kawai, K., Okamoto, K., Yamada, M. & Iwataro Morimoto, St. Marianna University School of Koizumi, M.: The extramural nerve and its system in the Gorilla. Med. Biol., 113: 371-375, 1986. (In Japanese) Medicine, for the permission to use the material. They 10) Le Double, A. F.: Sur trente-trois muscles presternaux. Mem. are greatly indebted to Prof. Yukio Dodo, Sapporo l'Acad. med. Paris, 1890. (cited from 11 and 14) Medical College, Prof. Tatsuo Kasai, Hirosaki 11) Le Double, A. F.: Traite des variations du systeme musculaire University School of Medicine and Prof. Masaharu de l'homme. Tome I, 275-286, 1897. 12) Morita, M.: Observations of M. sternalis and Mm. pectorales Horiguchi, Iwate Medical University School of in mammals and a morphological interpretation of the essence Medicine, for the useful advice. They are also very of M. sternalis. Acta Anat. Japon., 22: 357-366, 1944. (In grateful to Assoc. Prof. J.P. Barron of St. Marianna Japanese) University School of Medicine, for grammatical review 13) Ruge, G.: Zusammenhang des M. sternalis mit der Pars of the manuscript. abdominalis des M. pectoralis major und mittels dieser mit dem Achselbogen. Morphol. Jahrb., 33: 348-373, 1905. 14) Taniguchi, T. & Tochihara, J.: Studies of the sternalis muscle in Japanese (No. 2), with special reference to fetus and References anencepalon of Japanese. Acta Anat. Japon., 7: 1232-1249, 1932. (in Japanese) 1) Bardeleben,v. K.: Muskelnund Muskelmechanik.Ergebn. 15) Turner, W.: On the musculus sternalis. J. Anat. Physiol., 1: Anat. Entwickelungsgesch.,9: 66-67, 1899. 246-253, 1867. 2) Cunningham,D. J.: The musculussternalis. J. Anat. Physiol., 16) Ura, R.: Uber die allgemeine Differenzierung der 22: 391-407,1888. oberflachlichen Brustmuskeln mit besonderer Behicksichtigung 3) Dwight, T.: Notes on muscular abnormalities. J. Anat. der Hautrumpfmuskeln der Saiigetiere. Mitteil. der Physiol., 22: 96-102, 1888. Medizinisch. Gesellschaft zu Tokyo, Bd 51: 216-288, 339-390, 4) Eisler, P.: Der Musculus sternalis, seine Ursache und 1937. (in Japanese with German summary) Entstehung,nebst Bemerkungen Uber die Beziehungenzwischen 17) Ura, R.: A personal interpretation of the essence of the sternalis Nery und Muskel.Zeitschr. Morphol. Anthropol., 3: 21-92, muscle. Acta Anat. Japon., 7: 64-65, 1938. (in Japanese) 1901. 18) Yamada, M. & Mannen, H.: Manual of human dissection. 1st 5) Fick, R.: Zur Frage der Nervenversorgungdes M. sternalis. ed., 34-56, 464-465, 569-571, Nankohdo, Tokyo, 1985. (in Anat. Anz., 50: 406-410,1917-1918. Japanese) 6) Fukuyama,U.: Der Musculus sternalis bei den Nordchinesen. Innervation of the Sternalis Muscle 455

Plate I

Explanation of Figure

Plate I

Plt. 1: Photograph of the deep surfaces of the left pectoralis major and sternalis muscles to show their nerve supply after the analysis of nerve fibers. The twigs of the pectoral nerves show the gradient of the metameric distribution . AB: stands for the axillar arch (Achselbogen), LD: for the , PECT: for the pectoralis major muscle and ST: for the sternalis muscle, respectively. C5, C6, C7, C8 and T1 indicate the segments of the spinal nerves.