Abstract
Two cases of M. supracostalis anterior were observed on anatomical dissection of male corpses. It is believed that this kind of anomaly is an atavistic and conservative variation, or a “homologon” of M. obliquus externa thoraco-abdominalis which is common with lower mammals. Appearance of this abnormal muscle is very rare and seldom with mankinds.
We named the muscles “M. intercostalis extima”, for the muscles run between ribs, are located on the proper intercostal muscles, and the nerves supplied to both muscles are considered to be the same origin.
The features of the abnormal muscles are as follows:
1) The muscles are found on both sides of the anterior breast wall symmetrically.
2) The muscles are covered with M. pectoralis minor and lie vertically between the origins of M. pectoralis minor and of M. serratus anterior.
3) The origin is lower margin of the first rib and 5 cm. externally from Articulatio sternocostalis I. The muscles individually run over the 2nd or even the 3rd rib, inserting themselves to upper margin of the 3rd or the 4th rib.
4) The muscles are thin and board-like or bandage-like and are mixed with considerably large amount of tendon fibers.
5) The origin of the nerves supplied to the muscles has been unknown and we, in the first case, could trace back a fine branch nearly to the region of the intercostal nerves.
