We examined 94 axillary regions of 47 Japanese adults and found the muscular arch of the axilla (Maa) in five sides of three cadavers as well as the tendinous arch of the axilla (Taa) in two sides of two cadavers. The results are summarized as follows: 1) The frequency of Maa was 6.4% of the total bodies and 5.3% of the sides in this series. 2) In the left side of a 57-year-old male (No. 427), Maa was attached to the surface of the coracobrachialis muscle after fusing with the dorsal surface of the inserting tendon of the pectoralis quartus muscle. Both muscles were supplied by the caudal pectoral nerve (Npc) from the medial pectoral nerve. Moreover, in this same specimen, the sternalis muscle was recognized on the ventral surface of the pectoralis major muscle. In the left side of a 93-year-old female (No. 386), the cranial part of the muscular arch of the axilla (Cpa) was extended to the coracoid process by a tendon and attached to the abdominal part of the pectoralis major by two muscle bundles supplied by independent branches from Npc. One muscle bundle was attached to the lower margin of the abdominal part of the pectoralis major on the same plane, and the other bundle was located on the dorsal surface of the abdominal part. In a 74-year-old female (No. 411), the well-developed lateral part of the muscular arch of the axilla (Lpa) was attached to the inferior side of the tendinous arch. According to Ruge (1914) and Kasai et al. (1977), this arch was in the transition of the muscle bundle of Cpa to the arch. In the right side of the same specimen, only the thoracodorsal nerve (Ntd) was distributed into Lpa, whereas in the left side, only Npc supplied branches to Lpa. 3) The axillary arch was classified into 8 types based on the form and the supplying nerve of Cpa and Lpa. Cpa consisting of the muscle bundle is Type I, and Cpa consisting of the tendinous arch is Type II. We proposed that only Type II-A, with Cpa as tendinous arch and no Lpa, be designated as Taa (found in two cases), and the others as Maa. The following types were found in this study: Type I-A, consisting of only Cpa supplied by Npc (two cases); Type I-D, consisting of Cpa supplied by Npc and Lpa supplied by Ntd (one case); Type II-B, consisting of the tendinous arch and Lpa supplied by Npc (one case); Type II-D, consisting of the tendinous arch and Lpa supplied by Ntd (one case). 4) From the above findings, it can be suggested that Maa of varying shapes have been formed by a portion of the latissimus dorsi muscle supplied by Ntd, together with the pectoralis subcutaneous muscle, consisting of the pectoralis abdominalis, humeroabdominalis, humerodorsalis and ventrolateralis muscles supplied by Npc. The latter three muscles were proposed by Ura (1937) as the panniculus carnosus muscle, which was well developed in some lower mammalian orders. However, early investigators suggested that Maa was derived from the panniculus. Maa might have occurred as a rudimentary phylogenetic remainder in an early human embryonic stage.