MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 21 



the large irregular fossa for the jugular and carotid foramen medially. 

 It is traversed from behind forward by the horizontal ramus of the 

 postmastoid crest (see description of the mastoid zone with sterno- 

 cleidomastoid muscle and fig. 9) as this passes orally just lateral to the 

 vascular openings. This extension of the crest forms the lateral and 

 well defined margin of the carotideo-jugular fossa and is quite sharp in 

 relation to the arterial foramen where it gives rise to the stylomandib- 

 ular ligament. Midway between the venous and arterial apertures a 

 laterally directed and irregular ridge branches off toward the exit of 

 n. facialis. It corresponds in young specimens to the junction of the 

 anulus tympanicus with the mastoid zone of pars petrosa. Its develop- 

 ment is variable in adults where we have seen it often replaced by 

 just a short knob. It gives rise to the styloid muscles and together 

 with the part of the crest around and in front of the carotid opening 

 corresponds to the petrous crest of Zukerman, Ashton, and Pearson 

 (1962). 



M. stylohyoideus: A long, well-developed muscle which connects 

 the anteroinferior aspect of the petrous bone to the hypohyale (Schon, 

 1964a; Sirena, 1871). Origin is from the lateral half of the petrous 

 crest where short tendinous fibers form a flat and broad band. Its 

 continuing fleshy belly is penetrated from behind and below by the 

 intermediate tendon of the digastric upon which some of its fibers 

 are directly inserted. The rest of the muscle attaches to the hypohyale 

 as a fine tendon. In both sides of the female the medial bundles are 

 continuous with those of the mylohyoideus. 



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

 facial nerve. 



Function : The jDossible importance of this muscle in relation to the 

 howling mechanism was previously reported (Schon, 1964a). It should 

 suffice here to say that the contraction of the stylohyoideus would 

 tilt the hyoid bulla back and upward. By this movement the hy- 

 olaryngeal canal can be closed, as it is then pressed between the 

 epiglottis and the receding hyoid. 



Glossopharyngeal Group 



M. stylopharyngeus: This strong and well differentiated muscle 

 arises by fleshy fibers from the lateral half of the petrous crest (see 

 description of this bony landmark in relation Avith the styloid region) 

 between the origin of m. stylohyoideus and the carotid foramen 

 (fig. 5) . Its round belly lies against the upper pharyngeal constrictor 

 and the pharyngobasilar membrane, both of which are lined by the 

 buccopharyngeal fascia. The stylopharyngeus runs doAvnward and 

 medially to enter the pharynx through the middle pharyngeal hiatus 

 together with cranial nerve IX. Its fibers spread out now as an in- 



