100 U.S. NATIONAL MUSEUM BULLETIN 273 



thorax towards its insertion on the upper part of the humeral di- 

 aphysis (fig. 31). Sternoclavicular and sternocostalis parts can be 

 recognized but not separated. The first arises by thick, fleshy bundles 

 from the ventral aspect of the medial clavicular half, the capsule 

 of the sternoclavicular joint, the sternal process, and the remainder 

 of the manubrium. Pars sternocostalis has a muscular origin from 

 the lateral half of the sternum, adjacent sternocostal joints and 

 cartilages down to the sixth in all the males, but only to the fourth 

 in the female. The caudal part of the pectoralis major in front of 

 the sixth cartilage actually comes from the rectus sheath. The sterno- 

 clavicular fibers run horizontally outward, while the sternocostal 

 follow an increasingly oblique and ascending direction toward the 

 shoulder. The insertion is on the upper two-thirds of the crest of 

 the greater tubercle (fig. 27) by a double layered, strong, and flat 

 tendon. Its two laminae are joined along their caudal margins and 

 formed as follows: (1) the superficial by the sternoclavicular fibers, 

 (2) the deep by those with sternocostal origin. The distal portion 

 of the pectoralis major is partially covered near its insertion by 

 the pars clavicularis of m. deltoideus, the two muscles being inti- 

 mately connected by fibrous tissue with the cephalic vein running 

 along the del topee toral interval. This same part of the pectoral lies 

 over the two heads of the biceps, m. coracobrachialis, and the latis- 

 simus tendon. More medially, the pectoralis major contributes to 

 form the ventral axillary wall and, over the thorax, it covers the 

 other two pectorals. 



Sirena (1871) considers this muscle in Alouatta jusca to be in 

 general like that of man but contrasting in (1) the lack of separation 

 between pars clavicularis and the sternocostalis, and (2) the midline 

 intercrossing in front of the sternum. A clavicular origin occurs also 

 in Alouatta palliata (Campbell, 1937). Hill (1962) states that the 

 clavicular origin takes place on the medial sixth of that bone and 

 that the sternocostal origin covers the first six ribs. 



Nerve supply: The lateral pectoral nerve from C VI and VII 

 (fig. 26). 



M. pectoralis minor (fig. 31): This is an elongated and flat muscle 

 found close to the midline of the rib cage deep to the pectoralis 

 major. Its origin lies between the sternum and the highest costal 

 digitations of m. rectus abdominis. It begins by fleshy fibers from 

 the ventral aspect of the third, fourth, and fifth costal cartilages, 

 but from the first to the third in the female. The fascicles form broad 

 digitations that reach the muscle on its deep surface. Pectoralis 

 minor ascends craniolaterally in front of the first five ribs, passes 

 across the subclavius muscle and the clavicle to reach the shoulder 

 as a strong and flat tendon. The insertion is on the ventromedial 



