We present a rare case of costohumeralis muscle, an accessory slip originating from the sixth rib near costochondral junction and running along the lower border of the pectoralis major muscle, inserting onto the medial epicondyle of the humerus on the right side. The occurrence of this costohumeralis muscle also showed a coincidental loss of the classical twisting of the tendinous insertion of the pectoralis major muscle, which proves that this muscle is variable in its development and attachment. Clinical considerations of such a variation include ulnar nerve entrapment and functionally limited abduction of humerus.
