MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 111 



Function: According to Sirena (1871), this muscle and the an- 

 coneus are synergists in keeping the olecranon in place against the 

 pull of the dorso-epitrochlearis. 



DEEP SERIES 



M. flexor digitorum profundus (fig. 32): The muscle is entirely 

 covered by those of the superficial series and lies directly over the 

 forearm bones and the interosseous membrane. The flexor profundus 

 is formed by two heads, a caput ulnare and a caput radiale which 

 remain separate until they form a common tendinous structure below 

 the flexor retinaculum. (1) Caput radiale originates by fleshy fibers 

 from an extensive area of the radius bound proximally by the tuber- 

 ositas radii, distally by the attachment of m. pronator quadratus, 

 laterally by that of the round pronator, and medially by the line 

 along which the interosseous membrane is fixed to the radius (fig. 27). 

 This head receives proximaUy a small fleshy bundle from the ulnar 

 epicondyle and many fascicles from the tendinous sheet formed on 

 the deep aspect of the flexor superficiahs, caput superficiale. All its 

 fibers end on a tendon which develops high up and forms the medial 

 margin of the beUy. It enters the flexor retinaculum on the radial side 

 of the tendon of the ulnar head. Here they are both joined into a 

 single tendinous structure, the radial fibers seeming to be contributed 

 to digits I and II. (2) Caput ulnare has fleshy origin from the ulnar 

 shaft over an area limited medially by the attachment to the bone of 

 the medial intermuscular septum and the flexor carpi ulnaris, lateraUy 

 by that of the interosseous membrane, and proximally by thebrachi- 

 alis insertion, although a good many of the flexor fibers ascend over 

 the medial surface of the olecranon to the proximity of the upper end 

 of this process. Distally the area of origin stops where the fibers of 

 pronator quadratus are inserted on the ulna (fig. 27). The fibers of 

 this head insert on a long tendon which develops along its radial 

 margin and later on, in the carpal canal, it joins the tendon of the 

 radial head. The fascicles of the ulnar head seem to go to digits III, 

 IV, and V. From the common central structure five tendons are given 

 off, one for every finger. That for the first is smaller and attached to 

 the base of the distal poUical phalanx. The other four run within the 

 fibrous flexor vaginal sheaths beneath those of the flexor superficiahs, 

 which they perforate to end on the palmar base of the last respective 

 phalanx. Individual vincula brevia unite them to the second phalanges. 



The information about this flexor in Alouatta jusca is obscured by 

 the attempt of Sirena (1871) to name the parts of this muscle with 

 terms used in human anatomy. Nevertheless, the pattern he describes 

 corresponds to that of my red howlers. 



Nerve supply: The radial head is innervated by branches of the 

 median nerve, the ulnar by the ulnar nerve. 



