MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 69 



in the thyrohyoid space. The air that had been drawn into these 

 membranous structures will then be blown back into the air passage. 



M. geniohyoideus: This strong and paired muscle has origin by 

 short tendinous fascicles from the basis mandibulae at each side of 

 the midline (fig. 5). The attachment is frequently marked by corre- 

 sponding bony flanges. The fibers are dorsally directed toward the 

 laterolingual surface of the bulla hyoidea where they insert over an 

 extensive area (fig. 7). Fatty masses separate the muscle from the 

 overlying genioglossus and the anterior part of the mylohyoideus 

 covers it in the submandibular triangle. The descriptions of this 

 muscle by Lampert (1926), Sandifort (1834), Sirena (1871), and 

 Schon (1964a) do not differ from the present one. 



Nerve supply : The XII cranial. 



Function: It pulls on the bulla thus allowing for the opening of 

 the thyrohyoid canal. This role was discussed in a previous commu- 

 nication (Schon, 1964a). 



Comparative anatomy: Campbell (1937) reported the absence of an 

 omohyoid in Lagothrix, Ateles, and Alouatta, but since his study was 

 not particularly concerned with the neck, I am not able to determine 

 if that muscle is replaced by a costothyroideus in any of these two 

 genera besides the howler. Lampert (1926) could not differentiate an 

 omohyoideus at the point of insertion in the spider monkey, but 

 indicates its presence in the wooUy monkey. The fact that Lamport's 

 observations were done mainly in neck organs and not always in 

 complete specimens should be considered in evaluating his results. 

 Hill (1962) says that an omohyoid is absent in Brachyteles and does 

 not indicate the presence in this animal of a muscle that could be 

 identified with the costothyroideus. M. omohyoideus was found in 

 Cebus (Campbell, 1937). Lampert (1926) does not describe the muscle 

 in his specimen of this genus, but mentions the relations of m. ster- 

 nothyroideus with it. I therefore assume that the capuchin monkey 

 has such a muscle. The most relevant difference between Alouatta and 

 the other prehensile-tailed ceboids in relation to aU these muscles is 

 one of size. They attain a prominent development in the howler 

 monkey parallel to the increase in proportions of the hyolaryngeal 

 apparatus. Otherwise, no major divergences can be pointed out except 

 to say that the superficial fibers of the sternothyroideus do not pass 

 up to continue with those of the thyrohyoideus in Lagothrix (Lampert, 

 1926). 



M. rectus abdominis: A long, ventrally compressed, paired muscle 

 lying at each side of the midline between the thorax and the pubis. 

 It arises by thin but resistant musculotendinous digitations from the 

 ventral surfaces of the highest costal cartilages (1-6 in one 13-rib 

 male, one 14-rib male, one female; 2-6 in the other 14-rib male where 



