MUSCULAR SYSTEM OF THE RED HOWLING MONKEY H 



area of the mastoid region (figs. 3, 4, 5), which is described in relation 

 to the sternocleidomastoid. The elongated belly is found posterior to 

 m. stylohyoideus and lig. stylomandibulare. Its girth reduces gradually 

 as the muscular bundles end on the lateral surface of the intermediate 

 tendon. This pierces the stylohyoideus deep to the mandibular angle. 

 There are no aponeurotic structures connecting the tendon to the 

 hyoid bone. The fibers of the anterior belly are a continuation of 

 those of the tendon and form a laterally compressed, rather large and 

 fan-like belly whose fascicles are attached to the inner surface of the 

 mandibular ramus and corpus (fig. 2) . They mark a shallow but broad 

 triangular fossa in the bone which is limited behind by the insertion 

 of the pterygoideus medialis, above by the mylohoid line, and below 

 by the basis mandibulae. The anterior belly is located between the 

 expanded hyoid body lined here by m. mylohyoideus and the mandible. 

 Sirena (1871) points out that the anterior belly ends on the inner 

 mandibular surface at the junction of the angle with the ramus and 

 without reaching the symphysis. Sandifort (1834) describes a similar 

 attachment in three specimens of Mycetes (=Alouatta) senicvlus. 

 Leche (1912) indicates that this insertion does not reach forward 

 beyond the level of the second molar and corresponds behind with the 

 line of the coronoid process. Starck (1933) noted that this ending of 

 the digastricus in a young red howler extends further back and 

 coincides with the lower border of the medial pterygoid. Van den 

 Broek (1920) also says that the digastric area reaches below that of 

 the pterygoid in another young specimen that he examined. The 

 exception to these claims for a peculiar location of the digastric 

 insertion is Bijvoet (1908), who asserts that in the howling monkey 

 this attachment occurs from the level of the masseter to the mental 

 symphysis. It would thus be no different from the pattern in other 

 monkeys (see Leche, 1912). 



Nerve supply: A small branch of the postauricular division of the 

 facial nerve reaches the posterior belly. The anterior is innervated by 

 the mylohoid nerve. 



Function : Depressor and retractor of the mandible. Tension of the 

 intermediate tendon should help the stylohyoideus in bringing about 

 more effectively the tilting backward of the hyoid bulla as proposed 

 by Schon (1964a). 



M. mylohyoideus: This is a broad quadrilateral muscular layer 

 joining the inner surface of the corpus mandibulae to the hyoid 

 buUa. The development of the latter has crowded the muscle into a 

 narrow space between the two bones. Origin is by a fine aponeurotic 

 lamina on the mylohoid line (fig. 2). Rostral fibers arise as close to 

 the mandibular torus (see Scott, 1963, for definition of this term) as 

 0.5 cm., hinder ones come from very near the mandibular foramen 



