Innervation of the sternalis muscle accompanied by congenital partial absence of the pectoralis major muscle.

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.