102 U.S. NATIONAL MUSEUM BULLETIN 273 



sively narrower aponeurotic band. Insertion is on the ventral part 

 of the lesser tuberosity (fig. 27). 



Nerve supply: A branch from the ansa pectoralis. 



M. subclavius: A strong and elongated muscle found between the 

 scapula and the lower surface of the clavicle at one end, and the 

 first rib at the other. It has an approximately oval cross section 

 which is larger near the shoulder than at the thorax. The muscle 

 arises by fleshy fibers from the upper surface and distal end of the 

 first rib where some of them come also from the corresponding costo- 

 chondral joint. Its belly is directed laterally, cranially and a little 

 backward, crossing the costoclavicular space to reach the region of 

 the coracoclavicular joint. The subclavian bundles are inserted on 

 the posteromedial surface of the trapezoid ligament (the conoid is 

 absent) and adjacent inferior surface of the clavicle. A sharp edge is 

 usually formed in the posterior border of the bone at the attachment 

 of the muscle (fig. 24) . A large insertion occurs also on the base of the 

 coracoid process which is flattened and expanded for the reception 

 of the subclavian bundles. Finally, some of its fibers reach the supra- 

 scapular ligament and the lateral border of the notch (fig. 24). The 

 muscle is surrounded by the clavipectoral fascia. The coracoid in- 

 sertion is also indicated by Ashton and Oxnard (1963), Campbell 

 (1937), andSirena (1871). 



Nerve supply: A branch from the brachial plexus (see fig. 26). 



Function of the extrinsic muscles. — The pectoral mass acts 

 in medially rotating the proximal segment of the forelimb (pars 

 sternoclavicularis) . It is also important in puUing back the segment 

 as in retracting the arm (pars sternocostalis) . The subclavius is a 

 stabilizer of the pectoral girdle. 



Comparative anatomy of the extrinsic series. — -The clavicular 

 origin of m. pectoralis major occurs also in Lagothrix (Ashton and 

 Oxnard, 1963; Campbell, 1937; Hill, 1962; Robertson, 1944), in 

 Brachyteles (HiU, 1962) where it is said to be reduced to a small 

 slip, and in Ateles (Ashton and Oxnard, 1963; Hill, 1962). Campbell 

 (1937) negates its presence in the spider monkey and Hill (1962) 

 comments that it is often thin and aponeurotic in this animal. The 

 howling monkey conforms with the semibrachiators of Ashton and 

 Oxnard (1963) in (1) the more prominent development of the sterno- 

 clavicular part of the muscle and (2) the extent of the muscular 

 origin on the thorax. In quadrupeds the pars sternocostalis is the 

 stronger and reaches the eighth rib in 78.5% of their cases (Ashton 

 and Oxnard, 1963). 



There are again no differences among Ateles, Brachyteles, Lago- 

 thrix, and the howling monkey in the other two pectoral muscles 

 and the subclavius. Cehus, on the other hand, lacks a pars clavicularis 



