MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 85 



Function: A stabilizer of the scapulohumeral joint, it rotates the 

 arm laterally. Its small size testifies to the relative unimportance of 

 the muscle. 



M. supraspinatus (fig. 25): This muscle is as large as the supra- 

 spinous fossa from which it arises. Peripheral fibers originating from 

 the vertebral and cranial borders of the scapula are tendinous. They 

 soon join the fleshy bundles arising from the rest of the excavation 

 (fig. 24). The muscle receives an additional contribution of fibers 

 whose origin is on the infraspinous fossa at the root of the spine. All 

 these elements converge to form a robust and central tendon within 

 the muscle as this passes below the acromion. It crosses over the capsule 

 of the shoulder joint to which it is bound by fibrous strands. The 

 tendon inserts on the upper facet of the greater humeral tuberosity 

 (fig. 27) . A large subacromial bursa separates the supraspinatus from 

 the acromion, coracoid, coracoacromial ligament, and distal part of 

 the clavicle. As it traverses the joint, the tendon of the muscle covers 

 the posterior border of the coracohumeral ligament. The supraspinatus 

 is not different in the other howlers. 



Nerve supply: The suprascapular nerve (fig. 26). 



Function: It is believed that this muscle, which in man initiates 

 the abduction of the arm, actually begins its protraction in the howler 

 because of the angle at which the humerus is set on the scapula and 

 the position of this last bone with respect to the thorax. This results 

 in the eventual elevation of the arm. Otherwise, the supraspinatus 

 stabilizes the joint. 



M. infraspinatus (fig. 25): The infraspinatus is also noticeable 

 for its strong development. It exceeds the supraspinatus in size for 

 about one-quarter and, like that muscle, it is also of triangular shape 

 in correspondence with the infraspinous fossa from which it originates. 

 The surface of this spacious area is not entirely occupied by the 

 muscle, a part near the axillary border being reserved for the two 

 mm. teres. The fleshy fibers of the infraspinatus arise from the rest 

 of the fossa including the under surface of the spine (fig. 24), and 

 also from part of the infraspinal fascia. Tendinous fascicles are 

 superficial and peripherally distributed as they come from the lower 

 lip of the posterior border of the spine, the vertebral margin, and 

 the proximities of the lower angle. The infraspinatus is a multi- 

 pennate muscle formed by about five closely arranged bundles, 

 all of which are directed toward the shoulder. Three of them are 

 central with respect to the fossa, while the other two follow its axUlary 

 margin and the root of the spine. These two meet superficial to the 

 other three near the neck of the scapula. At this level a central and 

 powerful tendon is formed which crosses the back of the scapulo- 

 humeral capsule, adheres to it, and is inserted on the intermediate 



