MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 51 



Nerve supply: Small twigs from the corresponding intercostal 

 nerv^es. 



Function: The main function of the inferior serratus is accepted 

 to be that of fixing the ribs on which it is inserted. It may also depress 

 these costal segments. By the absence of muscular fibers in the area 

 of the superior serratus the role of elevating the upper ribs in res- 

 piration is probably taken over by the scalenes. 



Comparative anatomy: The separation between the two posterior 

 serrati appears to be less complete in Cebus where the last digitation 

 of the upper and the first of the inferior overlap by ending both on 

 rib 7 (3 individuals) or 6 (one) (Ogushi, 1920). In Lagothrix these 

 insertions are on the sixth and seventh ribs with the sixth intercostal 

 space intervening (Ogushi, 1920). In the single howler studied by 

 Ogushi (1920) the two muscles were separated by the sixth rib and its 

 t^^■o adjacent intercostal spaces. The two posterior serrati in all of 

 my male specimens were always separated by two ribs and the inter- 

 vening spaces, the female resembling the woolly monkey. Ogushi 

 (1920) found the widest separation in Ateles where the serratus 

 posterior superior inserted on ribs 2 to 4, and the inferior on ribs 10 

 to 14. 



Mm. levatores costarum: Strong triangular muscles are found 

 between the dorsal end of every external intercostal muscle and the 

 corresponding costotransverse ligament. They are aU covered by the 

 longissimus system. Every levator arises by its apex from the trans- 

 verse process of a cranially located vertebra and its fibers fan out 

 caudally to be inserted on the cranial border of the next lower rib 

 just lateral to the tubercle. The tip of every thoracic transverse 

 process presents on a lateral view, (1) a cranially directed tuberosity 

 (metapophyseal tubercle), (2) another directed caudally (anapophy- 

 seal tubercle) and (3) a ventrally projecting small eminence for the 

 levator costae. These three landmarks, particularly the first two, 

 become increasingly separated one from the other in lower thoracic 

 levels. The third tends to remain closer to the anapophysis. M. levator 

 costae primus occupies the space between the end of the sixth cervical 

 transverse process and the first rib, where it is inserted from the 

 capitulum to the attachment of the scalenus medius accesorius 

 (fig. 16). The last member of the series ends on the last rib. 



Mm. intercostales externi: A typical intercostalis externus extends 

 between the levator costae of the corresponding space dorsally and the 

 l)roximity of the sternal end of the ribs ventrally. It arises from the 

 lower border of the cranially placed rib and its fibers pass caudally and 

 forward to be inserted on the upper border of the rib below. The 

 muscle is thicker dorsally and is continued between the costal cartilages 

 by the thin membrana intercostalis externa. 



