MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 45 



(1963) has already pointed out that if we define the spinal regions on 

 the basis of the type of articulations, the number of lumbar vertebrae 

 in Alouatta is on the average eight with a range of seven to nine in 21 

 adult specimens. The opposition to lateral rotation found in the tradi- 

 tionally defined lumbar column is thereby extended cranially to in- 

 clude the low rib-bearing segments of the spine. In apparent agreement 

 with these osteological details is the presence of conspicuous bundles 

 of longissimus arising from the first three or four lumbar vertebrae 

 (in Erikson's sense). Such fascicles would in all likeUhood contribute 

 favorably to dorsiflexion movements, but restrict those of lateral 

 rotation. Unfortunately, there is no available information about 

 Ateles, Brachyteles, Cebus, or Lagothrix dealing in similar details with 

 the epaxial musculature. Due to this fact my interpretation has to be 

 taken with some reservation until more material either substantiates 

 or negates it. 



The presence of a m. semispinalis capitis fused with bundles coming 

 from the spines of C 6 to T 4 which I regard as m. spinalis capitis, 

 could perhaps be another adaptation to the role of extending the head 

 and neck against the flexing influences of the hyolaryngeal organs. 

 De Pina (1930) reports similar spinal fibers from Tl to T5 in the left 

 side of a female Lagothrix humboldtii whose right side, however, did 

 not have these origins. Again, absence of comparative information 

 about other cebids demands caution in accepting this interpretation 

 for the fusion of the two muscles. 



The situation of a combined longissimus cervicis et capitis, as 

 described by Sirena (1871) and myself in the howler, could also be an 

 adaptation to the problems of balancing the skull and the large voice - 

 producing structures upon the vertebral column. Once more, the lack 

 of related information about other cebids requires caution respecting 

 this opinion. 



The remarkable development of the spinous process of the axis is 

 paralleled by the peculiar arrangement of the cervical interspinales. 

 Von Eggeling (1922) noted the proportion of the axial spine which, 

 according to Schultz (1961), corresponds to 214 percent of the mid- 

 sagittal diameter of the body in that vertebra. Such a value is the 

 highest among platyrrhines and is surpassed only by Pongo (266%) 

 within the order Primates (Schultz, 1961). These characters are prob- 

 ably another adaptation to the same problem. 



Comparative anatomy of the epaxial musculature. — As it has 

 been said in the previous paragraphs, the knowledge about the epaxial 

 muscles in Ateles, Brachyteles, Cebus, and Lagothrix is practically 

 zero. With respect to m. splenius, the only differences between the 

 howler and these genera seem to be (1) the number of vertebral 

 segments taken up by its origin, and (2) the variable presence of a 



