MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 79 



Ateles (Schiik, 1913b), and that from C 5 in Brachyteles (Hill, 1962). 

 With regard to the number of digitations which form the pars caii- 

 daUs, it is interesting to note that the six sUps of the howler align 

 this genus \\'ith the quadrupedal group rather than with the semi- 

 brachiators of Ashton and Oxnard (1963). Unfortunately, there are 

 no data that would allow us to compare, even if only in a qualitative 

 manner, the differential degree of development of pars cranialis 

 versus its caudal counterpart in the genera we are considering. Such 

 information might permit us to evaluate the relative functional 

 importance of each of them. Alouatta, with a not too powerful caudal 

 portion in relation to the cranial, appears in position intermediate 

 between the semibrachiators and quadrupeds of Ashton and Oxnard 

 (1963). 



M. latissimus dorsi: This is a broad, flattened muscular sheet 

 covering the dorsolateral aspect of the thorax and extending from 

 the vertebral column and lower ribs to the humerus. Of roughly 

 triangular shape, its base corresponds to the dorsal midhne and 

 caudal part of the thorax, the apex to the axilla. The upper border 

 is nearly horizontal and the lateral obUquely directed toward the 

 insertion. The origin is double: (1) Strong tendinous fibers arise from 

 the spinous processes of T 7 down to L 1 or 2. They form a continuous 

 aponeurotic lamina intimately joined to the thoracolumbar fascia 

 (fig. 12). (2) Fleshy digitations begin in ribs 8-12 in one 14-rib male, 

 7-11 in a 13-rib male, 9-12 in the other 14-rib male, and 8-11 in the 

 13-rib female. On reaching the floor of the axilla the muscular elements 

 form a strong flat tendon and are tightly bound to m. teres major by 

 connective tissue. The highest fibers of the latissimus pass along the 

 dorsal border of the tendon, those from the lower ribs continue on 

 its ventral edge. Insertion is at the bottom of the broadened sulcus 

 intertubercularis. This ending does not impress a fossa on the bone, 

 but as in the case of Ateles (Napier and Davis, 1959; Schon, 1965) 

 and also in Cehus (Schon, 1964b), the groove for the muscular ending 

 is broad and its caudal Umit ill-defined. 



Sirena (1871) found the muscle arising by four fleshy digitations 

 from ribs 9-12 in all his three specimens. He describes the spinal 

 origin as reaching T 6 by means of an aponeurotic lamina fused Avith 

 the thoracolumbar fascia. The cranial origin in Alouatta palliata is T 

 7 (Campbell, 1937) and its highest costal shp comes from rib 7 or 8. 



Nerve supply: The thoracodorsal nerve, arising from the posterior 

 cord of the brachial plexus (fig. 26). 



Function: The muscle retracts the forearm and rotates it medially. 

 Its main function, nevertheless, is that of propeUing the body (Ashton 

 and Oxnard, 1963). The not too strong development of the latissimus 

 in the howler indicates that it does not play a very active part in 

 propeUing the body from above. 



