MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 17 



other specimen the muscle is a thin fascicle with origin on the anterior 

 part of the helix. Descending in the subcutaneous preauricular fascia 

 it ends in front of the ear cartilage. Schreiber (1928) found three dif- 

 ferent arrangements for this muscle, but none of them was similar to 

 either of those I have described. 



M. zygomatico-orhital: This muscular mass is an extensive and 

 well-developed sheet lying over the zygomatic, supraorbital, infra- 

 orbital, and orbital regions with little differentiation of its two parts 

 (fig. 6) : The fibers of pars zygomatica arise from the outer surface of 

 the anterior two- thirds of the jugal arch and run forward as a broad 

 band across the malar bone and the large zygomatico-orbital foramen 

 to reach the infraorbital region. They expand fanwise toward the 

 upper lip and nose where they end (1) by intermixing mth those of 

 the orbicularis oris, and (2) under cover of the orally descending fibers 

 of the nasolabialis. This part and the platysma colli et faciei form a 

 single lamina as described before. The bundles of pars orbitalis have 

 origin on the medial palpebral ligament and run laterally forming 

 arcades within both the upper and lower eyelids. Deep fascicles attach 

 to the respective tarsal plates, superficial ones end on the skin. The 

 longest reach the lateral palpebral commissure. Two zones can be 

 recognized in pars orbitalis: one occupies only the lids, the other is 

 thicker and formed by the more peripheral bundles. 



M. depressor supercilii could not be demonstrated. 



M. corrugator supercilii: It is composed of a few fibers and lies 

 in the supraorbital region between the pars orbitalis of the zygomatico- 

 orbital plate and the frontalis (fig. 6). Origin is on the glabella and 

 its fascicles spread laterally along the supercihary arch to end by 

 intermixing with those of the pars orbitalis and in the skin of the 

 region. 



M. nasolabialis: It forms a muscular sheet extending from as 

 high as the interorbital space, over the root of the nose, to the in- 

 fraorbital and oral regions (fig. 6). Origin is from the skin of the 

 glabella and insertion takes place by intercrossing with m. orbicularis 

 oris. The muscles of each side are continuous without interruption 

 over the midline. Schreiber (1928) calls it m. levator labii alaeque 

 nasi and describes some of its fibers as passing upward covering m. 

 frontalis (his m. orbitotemporo-auricularis) . They perhaps correspond 

 to m. corrugator supercilii. 



M. maxillo-nasolabialis: It appears as a well differentiated but 

 rather weak slip extended from the maxilla to the upper lip across the 

 infraorbital region where it lies under the zygomatico-orbital mass 

 (fig. 1). Origin of its fleshy fibers is from the alveolar border of the 

 maxilla above the premolars and the attachment of the buccinator 

 (pars maxillaris). They pass over the infraorbital vessels, the nerve 



