Okajimas Folia Anat. Jpn., 67(6): 449-456, March, 1991 Innervation of the Sternalis Muscle Accompanied by Congenital Partial Absence of the Pectoralis Major 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 nerve, 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 nerves 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 intercostal nerves 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 rectus sheath (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 pectoralis minor 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. subclavius 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.