88 U.S. NATIONAL MUSEUM BULLETIN 2 73 



and the deep surface of the tendon formed by the other two heads. 



The muscle is not different in the howlers studied by other authors 

 (Campbell, '.937; Sirena, 1871; Ashton and Oxnard, 1963). 



Nerve supply: The long head receives a high branch of the 

 radial nerve. In one male, the lower branch of the axillary nerve 

 sends an additional twig to it. The medial head is innervated by 

 several branches of the ulnar collateral nerve; the lateral by the 

 radial, as this spirals around the humeral shaft. 



Function: Its strong development indicates the importance of 

 the muscle as extensor of the antebrachium. In addition, the broad 

 scapular attachment allows the triceps to retract the arm at the 

 scapulohumeral joint. 



M. dorso-epitrochlearis: This is a broad but fiat muscular band 

 found superficially on the dorsomedial aspect of the arm immediately 

 beneath the skin and brachial fascia. It arises by fleshy fibers from the 

 lower border and adjacent parts of the anterior and posterior aspects 

 of the latissimus tendon, as this travels through the floor of the 

 axilla. The dorso-epitrochlear fibers run distally, covering the triceps 

 and narrowing down to about one-half of the original width of the 

 muscle as they approach the elbow. The fleshy fascicles now become 

 continuous with the fibrous tracts of the brachial fascia. In this 

 manner the dorso-epitrochlearis gains insertion to the medial epi- 

 condyle, its ridge, and the olecranon. There is some grouping of 

 fibers in the part of the fascia leading from the distal end of the 

 muscle to the dorsum and medial aspect of the olecranon, but this 

 was not always a very obvious detail. 



Sirena (1871) describes a similar muscle, but only the olecranal 

 ending. Ashton and Oxnard (1963), on the other hand, assign the 

 msertion to the medial epicondyle. Campbell (1937) adds that the 

 muscle has its origin extended to the axiUary border of the scapula 

 by means of a condensation of fibrous tissue within the substance of 

 the latissimus. 



Nerve supply: The first branch given off the radial nerve, before 

 entering the radial canal, is for the dorso-epitrochlearis. 



Function: It has a combined action of extensor of the forearm 

 through its fascial continuation to the olecranon, and adductor of 

 the arm. 



Comparative anatomy of the upper arm group. — The triceps 

 brachii appears to be the same in the other genera. Alouatta also shares 

 with them the supplementary anchorage of the long head to the spine 

 of the scapula by means of the infraspinal fascia (Campbell, 1937; 

 Hill, 1962). The distal attachment of m. dorso-epitrochlearis is var- 

 iously described by those who have studied the muscle. In Ateles it 

 is reported to end on the intermuscular septum and epicondyle by 



