MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 71 



digitations are connected by ligamentous fibers. The diaphragm was 

 not described by Sirena (1871). 



Nerve supply: Right and left phrenic nerves. Twigs were seen 

 reaching the muscle from the subcostal nerves. 



Function: Respiration. 



Comparative anatomy: The diaphragm of Ateles is regarded by 

 van den Broek (1908) as possessing a central tendon larger than in 

 any other platyrrhine and shaped very much like that of man. 

 Winckler (1926) studied the crura in several mammals and found the 

 right crus of the spider monkey larger than the left, arising by two 

 fascicles from the ventral aspect of the sixteenth and eighteenth 

 thoracolumbar vertebrae. The two crura met in front of the eighteenth 

 body where they were firmly anchored and fused with the fibers of 

 the anterior longitudinal ligament. Winckler (1926) does not give the 

 number of rib-bearing vertebrae in his specimen. If we take 15 as the 

 most representative value for Ateles (see Schultz and Straus, 1945), 

 the origin of the crura in this genus could be assigned to the second 

 and fourth lumbar vertebrae and, therefore, regarded as not too 

 different from the situation in the howler. The right crus in Brachyteles 

 (Hill, 1962) is also the larger, arising as far as L 3, whereas the left 

 comes from L 1 and 2. Hill (1962) describes a xiphosternal origin in 

 this animal. 



Muscles of the Upper Extremity 



Dorsal (Extensor) Musculature 

 Shoulder Girdle Group 



EXTRINSIC SERIES 



M. atlantoscapularis anterior (fig. 16): This muscle extends be- 

 tween the atlas and the shoulder girdle. Together with m. trapezius it 

 forms the dorsal border of the narrow posterior cervical triangle. 

 Tendinous fibers arise from the transverse process of C 1, lateral to 

 and partially overlapping the origin of the scalenus medius (rein- 

 forcing bundle). These fascicles change shortly into a muscular belly 

 which spreads out toward the shoulder and crosses the distal half of 

 the craniolateral margin of the trapezius. Insertion is on the lateral 

 half of the clavicle, the acromioclavicular capsule, and the upper 

 surface of the acromion, always between the attachments of m. 

 deltoideus and the trapezius (fig. 24). 



The atlantoscapularis anterior of the red southern howler cor- 

 responds to that of my description. Sirena (1871), nevertheless, adds 

 that there is an aponeurotic expansion between the muscle and the 



