Case reports on the pectoralis quartus and the pectoralis intermedius muscles
Sawada, M.; Ishibashi, Y.; Suzuki, T.; Chiba, S.
Kaibogaku Zasshi. Journal of Anatomy 66(2): 99-105
1991
ISSN/ISBN: 0022-7722 PMID: 1950433 Document Number: 371492
Each one case of the pectoralis quartus and the pectoralis intermedius muscles was found on the left thoracic wall in a 73 year old male and on the right thoracic wall in a 57 year old female respectively. The pectoralis quartus was a thin muscle of triangular shape, the base of which was the origin arising from the left thoracic wall at the level of the 6th rib between the pectoralis major and the latissimus dorsi, being separated from both margins of the muscles. The aberrant muscle ascended left-upwards about 10 cm to insert to the inner surface of the pectoralis major near its lower margin. The muscle was innervated by the most caudal pectoral nerve, passing around the lower margin of the pectoralis minor. The pectoralis quartus is extremely rare in man, and only two cases were reported by Bluntschli (1906) and Frey (1921). From the comparative anatomical point of view, the pectoralis quartus muscle was supposed to be a remnant of the ventral part of the subcutaneous trunci muscle in lower mammals, differing from the ordinary muscular arch of the axilla which was believed to derive from the dorso-cranial part of the muscle. The pectoralis intermedius was located in the deep layer of the right pectoralis major, lying about 2 cm below the lower margin of the pectoralis minor. It arose in the 4th and 5th ribs and extended right-upwards, running almost parallel with the pectoralis minor. The origin of the pectoralis minor shifted cranially to the 2nd and 3rd ribs. The clavicular part of the pectoralis major was normal, while the sternocostal part was supplied exclusively by the pectoral nerves passing cranial to the pectoralis minor. The sternalis muscle was found superficially to the latter. The abdominal part of the pectoralis major seemed to adhere to the inferior margin of the aberrant muscle. The most noticeable findings was that the reflection of the lower margin of the sternocostal part (the so-called Pectoralis-tasche) was lacking, and then it looked as if the reflecting part was separated from the parent muscle to be independent as the pectoralis intermedius. Three branches of the pectoral nerves supplied the aberrant muscle, passing cranial to, piercing through and passing caudal to the pectoralis minor respectively. As the result of these findings, the pectoralis intermedius was considered to be a muscle being separated from the sternocostal part of the pectoralis major to form an independent muscle, instead of being united with the rest of the part to form the reflecting part of the pectoralis major.