A Statistical Study of the Intermediate Intercostal Muscles

The internal intercostal muscles were divided in the outer layer, the intermediate intercostal muscles, and the inner layer, the (intrinsic) internal intercostal muscles, by the intercostal nerves passed between both layers (P. Eisler 1912). The intermediate intercostal muscles are present in each intercostal space in all cases except in one case which has 13 ribs, but the internal intercostal muscles are absent in the 1st upper intercostal spaces constantly, and are also absent in the upper and the lower intercostal spaces in some cases. The intermediate intercostal muscles in the intercostal spaces extend from the angle of ribs to the sternum. At the situation where the cutaneous lateral branches of the intercostal nerves pierce the lateral wall of the thorax, the internal intercostal muscles in each intercostal space disappear and the intercostal muscles are alone in the intercostal spaces. Therefore, the inside of anterior thorax was occupied not by the internal intercostal muscles, by the intermediate intercostal muscles.