Right sternalis muscle

Myotome portion of the somites give rise to most of the skeletal muscles of the body. Muscles of the extremities develop from somatic mesoderm. Splanchnic mesoderm gives rise to cardiac muscle and most of the smooth muscles; whereas mesoderm of the pharyngeal arches gives rise to some of the head and neck muscles. The sternalis muscle is a derivative of superficial part of the rectus abdominis muscle [1]. Sternalis is a flat, ribbonshaped muscle that begins from the lower part of the ribs, rectus sheath and then courses upward, finally inserting into the upper part of the sternum and ribs or the sternocleidomastoid muscle [2]. Various authors have classified sternalis muscle under 4 main categories regarding the structure it has been derived: (a) from pectoralis major, (b) from rectus abdominis, (c) from sternomastoid and (d) from the panniculus carnosus [3]. It is necessary to record and discuss unusual anatomical variants with the use of advanced diagnostic and therapeutic tools as these variants could present a challenge to the radiologist or surgeon in establishing a diagnosis [4].