204 Conditions in the Anthropoid Ape [CHAP. 



the sulcus post-limbicus (sulcus subparietalis) may be taken as the division between the two, 

 although in different cases either type may overstep this sulcus for a short distance. 



On the convexity and lateral surface of the hemisphere the boundaries are less de- 

 terminate. In the anterior direction it comes in contact with the " intermediate postcentral " 

 area, and the superior division of the postcentral sulcus may be called a limit. Posteriorly 

 it joins the " visuo-psychic " area, and the variations in this position have already been 

 mentioned. 



The inferior boundary on this surface is the hardest of all to settle ; approximately the 

 ramus horizon talis (interparietal fissure) along with the ramus occipitalis of the intraparietal fissure 

 form a dividing line, but these sulci certainly do not constitute a precise limit, for although 

 it is exceedingly difficult, almost impossible, to determine the exact point where " parietal " 

 cortex ends and " temporal " begins, on account of confusion of type, yet I think it correct to 

 say that the " parietal " type tends to cross the horizontal sulcal line and to trespass on 

 the upper part of the angular gyrus, as well as on the upper and anterior part of the 

 supramarginal convolution. 



PARIETAL AREA IN THE ANTHROPOID APE. (Plate II.) 



It is interesting to find that an area bearing topographical and structural resemblances to 

 that in man can be made out in the parietal lobe of the anthropoid ape. 



In reference to boundaries several points of considerable importance have to be mentioned. 

 Thus, on the outer surface of the hemisphere, the great " Affenspalte " and the deep and 

 apparently constant ramus horizontalis of the intraparietal sulcus, coupling the " Affenspalte " 

 with the sulcus postcentralis, form occipital and temporal limits much more definite than the 

 corresponding limits in the human brain. Then, on the mesial surface, the posterior boundary 

 gives no trouble, as it is formed by the upper end of the parieto-occipital fissure ; but the lower 

 or limbic boundary is of a less determined character, for the sulci in the anthropoid quadrate 

 lobe differ from corresponding sulci in the human cerebrum in being irregularly placed, and 

 especially in inclining to assume a radiate instead of an arcuate position in relation to 

 the corpus callosum ; therefore they cannot be used as limits, and in consequence an imaginary 

 arcuate line placed between the corpus callosum and the surface of the hemisphere, some- 

 what nearer the latter than the former, has to be thought of as a division. 



There is a further variation which concerns the extent of the area on the mesial surface. 

 In the three anthropoid brains which I have examined, a relative shortening of the interval 

 between the hinder upturned part of the calloso-marginal fissure and the parieto-occipital 

 fissure has produced a corresponding narrowing of the quadrate lobule, and in association with 

 this the extent of the parietal area on the mesial surface has presented a pronounced diminution 

 in comparison with the human arrangement. 



As structural guides to differentiation the features which serve in the human brain can 

 again be used. Passing backwards from the intermediate postcentral area there is a marked 

 drop in the number of large medullated fibres, and as soon as the "Affenspalte" is reached 

 unmistakeable visual characters appear. The absence of large fibres in the " common temporal " 

 and in the " limbic " cortex proclaim these types. 



