'^IMIA SATMU'S. TUF ORANG-OUTANG 



493 



A lorthrain index dI 83 |)c'i" cent alln's the orang-outaiifi w itli the firoup of 

 aiiiinals reeogmzed as ha\ iiiii \vell-ad\ aneed maiuial thllerentiation, as is the 

 ease with all ol the primates. 



Surface Appeakaxce oi- riii I^kmn i\ ihe Oha\(,-Ol iang 



FISSURES AND LOBES 



In pattern the brain ot tin- orang-Dutang is riehly <:;> reneephalic, its 

 lateral, basal and nusial surlaees presentniu many eonxohitions, tlie dis|)()si- 

 tion 1)1 w Imh almost exaetiv emneides with that nl the human brain. The 

 hemisplu'ie in its oeeipital portion I'ompletely eoxers the eerel)c-lhim. I he 

 superior l()n;j;itudinal lissiirt- at tlu' oeeijjital pole ol the hemis|)lu'ri' tends to 

 diverge slightly in order to aeeommodate t lu' ele\ation eaiisi'd in the superior 

 vermis of the eerebellum. This di\ergenee at the eaudal e\tremit\ ol the 

 superior longitudinal lissure is most pronounced in the lower and inti'i- 

 mediatt' primati-s. It gradualK beeomes less conspicuous in the higher nu-ni- 

 bers ol the [jrimate group. 



The lissiiral [xittern of the- lateral eon\e\it\ of the cerebral hemispiiere is 

 grouped about three mam lissures, namel\ , i 1 ) the lissure ol S\ l\ lus, (2) 

 the lissure of I^oiando and 1 3) the semilunar or simian hssurt-. I liese fissures 

 determine a distinctlx human t\ pe ol lobat ion m the luiiiis|jhcres. Flic lissure 

 of Rolando, starting about midwa\ between tht- Irontal and oeeipital poles, 

 at or lu-ar the supt'rior longitudinal lissiiri', extends oblicniel\ downward and 

 forward toward the S\ K ian lissure. it presents two gemillexions, both less 

 promiiu'iit than in tlu' human brain. |-5\' means ol this Rolandie lissure, the 

 Irontal lobe is st'parated from tlu' parietal lobe. Ihe S\l\ian li.ssure, some- 

 what shorter than in man, presents its usual di\ isions, and up(»n the latt'ral 

 surface extends oblic|ut-l\ backward and upward to be surrouncktl b\ the 

 marginal conxolution at its caudal e\tremit\. It constitutes a boundary 

 Ix'twi'en the temporal lobe and the parietal lobe. The sulcus simiarum is 



