MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 65 



foramina, (2) the body of all the caudal vertebrae, and (3) the corre- 

 sponding hemal arches or sesamoids. It is also serially arranged. One 

 given segment forms a powerful long tendon which approaches the 

 midline to be inserted on the sesamoids or tubercles of the sixth or 

 seventh vertebrae away from its origin. 



The first three caudal muscles are essentially the same in the 

 southern howler where Su-ena (1871) calls them pubococcygeus, 

 ileococcygeus, and ischiococcygeus externus. The differences relate to 

 their attachments, which he describes as being on hemal arches 2 

 and 3 for the first, 2, 3, and 4 for the second, and the transverse 

 (costal) processes of Cd 2, 3, 4, and 5 for the last. The medial and 

 lateral flexors of the tail in the two species of howler are likewise 

 similar. 



Nerve supply of the caudal group.^ — The pubo- and iliocaudalis 

 are innervated by a common branch from the ventral ramus of S 

 III (fig. 35). The nerve to the ischiocaudalis is formed by contribu- 

 tions from Cd I and S I (fig. 35). The medial and lateral flexors of 

 the tail are innervated by ventral rami of the caudal nerves. 



Function of the caudal group. — Flexors and abductors of the 

 tail. 



Comparative anatomy of the caudal group. — Brachyteles (Hill, 

 1962) appears to have an arrangement similar to that of the two 

 species of howlers. I suspect that the same is true also for the remain- 

 ing prehensile-tailed monkeys. 



Ventral Group 



M. stemohyoideus: This paired muscle lies at each side of midline 

 enclosed within the more superficial lamina of the fascia of the infra- 

 hyoid muscles. It extends from the thorax to the hyoid bone (fig. 3) 

 and originates from (1) the deep aspect of the first two costal cartilages 

 over their sternal half, (2) the corresponding chondrosternal joints, 

 and (3) adjacent sternal pieces. The fleshy bundles ascend into the 

 neck medial to each costothyroideus with which they cover the 

 sternothyroideus. The stemohyoideus attains its maximum width in 

 front of the swollen thjo-oid cartilage past which, in front of the 

 thyrohyoid membrane, it narrows down a little but not so much as 

 at the origin. Insertion is along the lower subcircular margin of the 

 apertura buUae hyoideae. The attachment at each side of the midline 

 extends from the midpoint to the very much reduced hypohyal. A 

 triangular space whose base corresponds to the incisura sternalis 

 separates the stemohyoideus of each side below the prominence of 

 the thyroid cartilage. The two muscles come together along the 

 midline in front of the thyrohyoid space and appear to form here a 

 single sheet. Lampert (1926) and Sirena (1871) noted also the strong 



