MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 169 



The stylopharyngeus and other muscles of the pharynx have also 

 undergone a considerable increase in size (fig. 8). The variabiUty 

 manifested in the glossopharyngeal part of the superior constrictor of 

 the pharynx is comparable to that of the triangularis. The importance 

 of the middle constrictor, pars membranacea, \\ith relation to the 

 emptying of the air sacs, and of the inferior constrictor in a similar 

 role, and in the control of the vocal cords, has been discussed when 

 deaHng with these muscles. 



Of the muscles operating on the hyoid bone (fig. 3), some belong to 

 the facial group, as m. stylohyoideus; others to the lingual, as m. 

 genioglossus, m. hyoglossus, and m. styloglossus; the rest to the 

 ventral group of hypaxial muscles, as m. sternohyoideus and m. 

 thyrohyoideus. They all present a remarkable increase in size and in 

 addition the styloglossus ^vas absent in two out of five specimens I 

 examined (this report; Schon, 1964a). That the styloglossus does not 

 substitute functionally for the glossopharyngeal part of the superior 

 constrictor is evident from the fact that when the first is lacking the 

 second is not found either. The importance of the stylohyoideus was 

 dealt vnih previously (Schon, 1964a). 



The muscular differences summarized so far between the howler 

 and other prehensile-tailed cebids are either directly related to the 

 production and modification of the voice of the howler or depend on 

 the overall growth process affecting its hyolaryngeal parts and 

 extending into the surrounding area. 



Adaptations in the musculature of the trunk. — Some of the 

 modifications encountered in these muscles of the howler, particularly 

 in the epaxial group, I beUeve to be related to the need for more 

 powerful extensors of the neck. The scarcity of comparative informa- 

 tion calls for caution in accepting the following conclusions. The 

 subdivisions of the epaxial muscles in Alouatta are only moderately 

 differentiated. The arrangement of its erector spinae (fig. 13) can be 

 compared to that observed by Howell and Straus (1933) in the rhesus 

 monkey. It suggests a rather rigid body in the sense that its vertebral 

 column and associated muscles do not allow great freedom for lateral 

 rotation over this axis. I am unfortunately in no position at the 

 }3resent time to say whether this pattern is more typical of a quadruped 

 than of a brachiator monkey of the New World. The dorsal muscles 

 in the neck have details which are perhaps related to their role as 

 extensors. A combined longissimus capitis et cervicis (fig. 14) and a 

 spinalis et semispinalis capitis (fig. 15) might be adaptations to this 

 function in the nuchal region where the need for strong extensors is 

 evident. One cannot discount the very Ukely explanation that the 

 combined beUies of these othermse separate muscles is merely a 

 generalized i)rimate character. The peculiar arrangement of the 



