PARIETAL REGION IN THE PRIMATE BRAIN 293 



Difficulties of such a character must always be interesting and 

 it is the hope of throwing some light upon those of the parietal 

 region of the brain that this work is undertaken. 



Comparative anatomy rather than embryology must form 

 the groundwork upon which to build for the reason that the struc- 

 tures in question are relatively new, in a phylogenetic sense. The 

 only near approach to the complex conditions obtaining in man 

 is found in the highest of the Simiidae, but even between these 

 latter forms and man the differences and difficulties in interpre- 

 tation are greater than between the highest and lowest of the Old 

 World Apes. There is present in man an extensive cortical area 

 which is small in the Lemurs, New World and lowest Old World 

 forms but which gradually attains a sufficient development so that 

 in the higher Simiidae cortical conditions are reached which can 

 be compared with those in man. This area of cortex, in the lower 

 types, is situated in front of the sulcus lunatus, or rather the bottom • 

 of the sulcus, and extends, above, as far forward as the ante- 

 rior limb of the interparietal sulcus but below it does not encroach 

 to any extent upon the temporal region, only reaching as far for- 

 ward as the posterior end of the fissura Sylvii. From this strip 

 of cortex with its few and simple foldings as it occurs in the Ce- 

 bidae and Cercopithecidae, there is developed the major portion 

 of the entire lateral aspect of the hemisphere behind the post- 

 central sulcus. In fact the entu'e hemisphere, excepting the fron- 

 tal and the anterior portion of the temporal regions, has been 

 profoundly influenced by the expansion of the area just men- 

 tioned, and it will be our task in the following pages to. endeavor 

 to trace the course of this great development. 



In many of the Prosimiae, in Tarsus and the lowest American 

 forms (the Hapalidae, and even some of the Cebidae such as Cal- 

 lithrix), the only sulci present on the lateral aspect of the hemi- 

 sphere are the Sylvian and some shallow and inconstantly placed 

 furrows which may be considered as interparietal and superior 

 temporal. The lemurs and marmosets are small animals and 

 no great extent of cortex has to be accommodated on the surface 

 of the brain. Yet strangely enough the curious little African 



