MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 99 



Function: The extensor of the index and pollex extends the first 

 digit at both the metacarpophalangeal and interphalangeal joints, but 

 only the metacarpophalangeal articulation of the second. M. extensor 

 tertii proprius produces a similar effect upon the third digit at the 

 metacarpophalangeal level and by its ending on the ulnar side ex- 

 pansion of the extensor digitorum tendon, it abducts this finger. This 

 attachment of the extensor of the third digit and that of the smaller 

 tendon of m. extensor digiti minimi to the same side on the fourth and 

 third finger is, perhaps, one of the anatomical bases for the zygodacty- 

 lous grasp in Alouatta. Conceivably, the other might be the control of the 

 pollex and index by one single muscle, the extensor poUicis et indicis 

 longus. 



Comparative anatomy: The division of the deep digital extensor 

 of the forehmb into a first part for the index and pollex and a second 

 for the other digits appears to be found normally only in platyrrhines 

 and Tarsius (Straus, 1941). My observations in Alouatta senicvlus 

 and those of Sirena (1871) in Alouatta jusca sustain this generalization, 

 but the situation is not so clear in Ateles, Brachyteles, Lagothrix, and 

 Cebus. The description of jthis muscle, or its parts, is not always done 

 with the same approach and the results of the wording are sometimes 

 confusing. The arrangement described for Lagothrix by Robertson 

 (1944) appears to resemble that of the howler in the existence of two 

 separate tendons for digits I and II coming from a single muscular 

 belly, but it seems as if, in this genus, contributions are given also to 

 III and IV before the subdivisions to the pollex and index had formed. 

 In Brachyteles (Hill, 1962) the deep extensor is said to be represented 

 by two bellies which, after origin from the radius, form fine tendons 

 destined to join the dorsal expansions of the index. The pattern 

 reported by Senft (1907) in Ateles is that of a deep extensor dividing 

 into three parts, one joining the extensor aponeurosis to III, the other 

 two blending ^ith that to II. What he describes as m. extensor polUcis 

 longus in Cebus mth origin from the middle third of the ulna and inter- 

 osseous membrane divides into tendons, one for the first digit, the other 

 for the second and third. Straus (1941) reports a separate extensor 

 poUicis longus in one Cebus. It seems as if these ceboids are no exception 

 to the great variability which the second part of the deep extensor has 

 been shown to have in all primates (Straus, 1941). 



Ventral (Flexor) Musculature 

 Shoulder Girdle Group 



EXTRINSIC SERIES 



M. pectoralis major: This triangular muscle narrows from its 

 extensive origin along part of the clavicle and the midline of the 



