The bilateral sternalis muscles (M. sternalis) observed during an anatomical class

In the present report, we describe the finding concerning sternalis muscles, a rare muscle with an appearance rate of 6%, in the chest of an 81-year-old man (cause of death : senility) during an anatomical class at the Asahi University, School of Dentistry in 2022. The length of the right sternalis muscle was 17.4cm and its maximum width was 4.12cm, originating from the anterior lobe of the rectus abdominis muscle sheath and then inserting in the origin tendon of sternocleidomastoid muscle’s sternal part, the origin tendon of the left pectoralis major muscle’s clavicular part, and the periosteum of the front of the manubrium of sternum. The lateral pectoral nerve and the anterior cutaneous branch of the intercostal nerve also entered from the posterior aspect. The length of the left sternalis muscle was 13.25cm and its maximum width was 4.6cm. It consisted of two muscle bundles, inner and outer, with the inner bundle originating from the anterior lobe of the left rectus abdominis muscle sheath and the outer bundle originating from the pectoralis major muscle’s fascia under the nipple, which merged and then stopped at the fascia of the anterior surface of the pectoralis major muscle. From the vicinity of the insertion of the pectoralis major muscle, the muscular axillary arch originated and stopped at the lateral border of the latissimus dorsi muscle. The lateral pectoralis nerve was observed as the nerve entering the posterior aspect of the sternalis muscle. The bilateral sternalis muscles seen in this report were well developed and compared to previous studies, may be the largest ones observed in Japanese. Based on the location of the origin and stop of the muscles, his sternalis muscles clearly showed a very rare form. As for the origin of this muscle, the morphology of the muscle and its innervating nerve found in this report suggests that it may be a part of the pectoralis major muscle or a remnant of the cutaneous muscle of trunk in other mammals.