84 U.S. NATIONAL MUSEUM BULLETIN 2 73 



M. deltoideus (fig. 25) : This triangular muscle is formed by thick 

 bundles which arise from the clavicle, acromion, and scapular spine 

 and converge upon the humerus. Origin of pars clavicularis is by 

 fleshy fibers from the lateral third of the collar bone below the attach- 

 ment of the trapezius (fig. 24). Pars acromialis arises by muscular 

 fascicles from the lateral edge of the acromion (fig. 24) Tendinous 

 fibers forming a short aponeurotic pars spinalis arise from the lower 

 lip of the posterior spinal (fig. 24) border and the infraspinal fascia. 

 Pars spinalis was separated by a narrow cleft from the other two in 

 both sides of one male. Acromial fibers are inserted on the proximal 

 third of the humeral shaft between the deltopectoral crest and the 

 lateral border of the bone (fig. 27). Its superficial fascicles become 

 tendinous and attach more distally than the deep ones. Those from 

 the clavicular and spinal parts converge upon the same area, ending 

 on the bone and upon the tendinous elements of pars acromialis. 

 Many deep fibers from the clavicle cross the deltopectoral interval 

 and are fixed to the tendon of the pectoralis major. The area corre- 

 sponding to the deltoid tuberosity is wide across the anterolateral 

 surface of the humerus and extends proximally to the lower part of 

 the greater tubercle. 



Sirena (1871) explains how both m. trapezius and the deltoideus 

 are interconnected through their attachment to the infraspinous 

 aponeurosis. No difference can be noted in the same muscle studied 

 by Campbell (1937) in Alouatta palliata and Ashton and Oxnard 

 (1963) in Alouatta sp. 



Nerve supply: A branch of the inferior division of the axillary 

 nerve which seems to comprise fibers from C V and VI (fig. 26) . 



Function: Elevates the arm. 



M. teres minor: This muscle is small when compared with those 

 surrounding it. The teres minor is narrow but long and lies between 

 the infraspinatus and teres major (fig. 25). Origin is by fleshy fibers 

 from the dorsal lip of the axillary border where they occupy a narrow 

 strip of bone between the origins of the two said muscles, but fall 

 short of reaching either the angle or the neck of the scapula (fig. 24) . 

 Fibers are also contributed to the muscle from its investing sheath. 

 All these fascicles have a bipennate arrangement and form a strong 

 and round tendon after crossing superficial to the long head of the 

 triceps. More laterally, the muscle adheres to the back of the scapu- 

 lohumeral capsule and ends on the lower impression of the greater 

 tubercle (fig. 27) . 



As in the case of the deltoideus, m. teres minor in the other two 

 species of howler appears to be the same. 



Nerve supply: A branch of the axillary nerve is given off in the 

 quadrilateral space and reaches the muscle. 



