Unilateral presentation of three muscle variants in the pectoral region

A rare case of three muscle anomalies in the pectoral region was discovered during routine dissection of an 85-year-old female cadaver. The muscle anomalies include the congenital partial absence of the pectoralis major muscle, a sternalis muscle, and a pectoralis quartus muscle. All three variants presented on the right side. The pectoralis major muscle demonstrated a normal clavicular head but lacked an abdominal part and had a sternocostal head that attached only to the manubrium. The sternalis arose from the manubrium and appeared to share a common tendon with the contralateral sternocleidomastoid muscle. The pectoralis quartus arose from the costal cartilages of ribs six and seven and inserted onto the fascia of the coracobrachialis muscle deep to the pectoralis major muscle. The sternalis muscle was innervated by intercostal nerves and the pectoralis quartus was innervated by both the medial pectoral and intercostal nerves. The documentation of pectoral muscle variants is not only important for the anatomical record but has clinical implications for surgical procedures in the axillary region and the interpretation of CT and MRI scans.