96o EVOLUTIONAL MODIFICATIONS 



transverse groove, icirntilied l)\ Elliot Smith and utIuTs as the sulcus occip- 

 italis transversus, which Rctzius nianitanis is the honiologuc ot the sulcus 

 simiarum ("Affenspalte"). Lateral to this trans\erse occipital fissure is an 

 equally faint groove indicatnig the position ot the sulcus occipitalis lateralis. 

 \\ ith reference to the sulcus centralis, or fissure ol Rolando, it nia\' be said 

 that in the brain of the Ccbidae, particularix in the brain ol niycetes, this 

 fissure has nuicli that is reminiscent of the hssura cruciata ol the lower 

 mammals. There is, ho\\e\er, an im]iortant difference. Its angle of inclina- 

 tion with reference to the superior longitudinal fissure is now considerably less 

 than ()0 degrees. In mycetes, therefore, as in most of the new-world monkeys, 

 the lissural fundaments of the primate brain are clearly discermble. Here- 

 alter, in the further steps of evolutional de\clopment, it is simply a matter ol 

 increasing complexity, particularly of expansion in the frontal and occipital 

 lobes, which brings into existence all of the lissural characters of the highest 

 primates. 



In the lower di\ ision of the old-world monkexs, here described as the 

 intermediate ])rimates, the simian characters in the lissural j)attcrn ol the 

 brain a])j)ear in their ultimate piominciice. The pattern is now delinitely 

 trifissural with a fissure of SyK lus, a fissure of Rolando and a simian fissure 

 constituting easily rec(jgnizetl boundaiN lines between the lour great lobes 

 appearing on the lateral coii\c'\it\- of the lu'inisphcrc. The hssure ol Sybius 

 has departed from its jjrimitixe wrtical direction and is tending to approach 

 closer to the horizontal, characteristic of it in the higher species. The fissure 

 of Rolando also has reduced the angle ol its inclmat mn in reference to the 

 superior longitudinal lissure and now runs a courst- of sonu'thing o\'er "j 

 degrees with the median plane. 1 lu' sulcus simiarum extends outward Irom 

 the superior longitudinal fissure at an angle of ()o degri'cs and reaches as far 

 lateral as the latt'ral occipital lissure. In the baboon, \lacacus rhesus and 

 in the gibbon, these lissural relations are all essentially similar. In none ol 



