MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 113 



of the muscle for this genus, but the central part is said to come 

 only from the raphe. The description by Senft (1907) of the flexor 

 profundus is incomplete. He does not mention the conspicuous pars 

 ulnaris and points out that m. flexor poUicis longus is absent. The 

 formation of a common tendon by the different parts of the muscle, 

 in the carpal canal, is described by Hill (1962) and Senft (1907), 

 but Straus (1949) shows one case of Ateles where the fusion is in- 

 complete, and I found the same situation in my spider monkey. 

 From either the completely or partly conjoined structure, tendons 

 are sent off to digits II to V. Those for the first two on the preaxial 

 side of the hand derive their fibers from the combined centroradial 

 head and are stronger. The postaxial tendons come from the ulnar 

 belly (Schon, 1965). A triple-headed deep flexor occurs also in Brachy- 

 teles (Hill, 1962), where a common tendon is not mentioned in the 

 carpal canal. The situation in Lagothrix (Robertson, 1944) is similar 

 to that of the howler \vdth radial, ulnar, and humeral parts forming 

 a common tendon distributed to all five digits. Senft (1907) does 

 not mention the antebrachial double origin of the flexor profundus 

 in Cehus, and the muscle is said to arise apparently only from the 

 medial epicondyle. Straus (1949) illustrates an arrangement in the 

 capuchin monkey similar to what Grand (1967), Sirena (1871), 

 and this uTiter have found in the howler, and Robertson (1944) 

 in Lagothrix. The pronator quadratus of all these animals seems to 

 be the same. An m. epitrochleo-anconeus is present also in Brachy- 

 teles (Hill, 1962) and Lagothrix (Robertson, 1944). 



Hand Group 



SUPERFICIAL SERIES, SUPERFICIAL LAYER 



M. abductor pollicis hrevis (figs. 32, 34) : The more superficial 

 and lateral of the thenar muscles is flat and not very strong. Origin 

 is from the lateral border of the palmar aponeurosis, skin and flexor 

 retinaculum, the navicular, and the radial carpal sesamoid. The 

 muscle runs distally, covering part of the flexor pollicis brevis to 

 its insertion on the radial half of the ventral base of the first pollical 

 phalanx. This ending is fleshy and was found to be fused vAih. that 

 of m. flexor pollicis brevis on one side of one male and on both of 

 another. The terminal part of the muscle glides over the radial ses- 

 amoid of the metacarpophalangeal joint in digit I. Sirena (1871) 

 spoke of this muscle as being just as in man. That this is not true 

 can be seen from the previous description. The abductor polHcis 

 brevis of the howler is underdeveloped when compared to that of 

 man. Moreover, it is not formed by two parts, receives no contri- 

 bution from the long abductor, and has no insertion on the extensor 



