476 PHYSIOLOGICAL SBK1K>. 



Anthropoidea the central sulcus sharply marks the exact caudal limit 

 of the area of excitable cortex, whereas in the Garni vora (so the 

 physiologists tell us) the crucial sulcus lies in the midst of the excitable 



If we admit the homology of the central and crucial sulci we shall 

 (by comparison with the behaviour of the latter) find an explanation 

 of many features of the former. According to such an hypothesis a 

 glance at a Bear's brain will at once make intelligible the meaning of 

 the superior genu, the caudal bend in the mesial extremity, and the 

 tendency of the central sulcus in the Anthropoid Apes and Man to 

 extend on to the mesial surface in front of the upturned end of the 

 calloso-marginal sulcus. 



In the features of its central sulcus (the relative positions of the 

 genua aud the behaviour of the mesial extremity of the sulcus) the 

 AnthropopitJitci approach much nearer to Man than does the Orang or 

 any other Ape *. 



The human brain is distinguished from those of the Apes by the 

 abundance of sulci between these stable and constant elements. 



The superior frontal and especially the middle frontal sulci are much 

 better developed than in the Apes, and innumerable sulci develop in 

 connection with these. The inferior transverse sulcus (so constant 

 in the Simiidae and larger Cereopithccidae) is longer and deeper ; ami 

 the diagonal sulcus, rarely or never seen in a well-developed form 

 in the Apes, is now almost constantly present as a deep, extensive 

 sulcus, lying between the anterior ascending limb of the Sylvian and 

 the inferior precentral sulcus. 



The parietal area is notably much more variable and much richer 

 in secondary sulci than it is in the Apes. 



In the temporo-occipital region the " Affenspalte " of the Apes has 

 disappeared, and the depth and extent of the dorsal end of the parallel, 

 the transverse occipital and lateral occipital sulci are correspondingly 

 increased. The inferior occipital, inferior temporal, occipi to-temporal, 

 and collateral sulci are usually all present in a well-developed form. 

 In the Apes the deepening and lengthening of any one of these snk-i 

 involved a dwindling of its neighbour a highly developed occipitu- 

 t< inporal sulcus often led to the abortion of the inferior temporal, the 

 disappearance of the anterior end of the collateral, or the curtailment 



* As the result of further investigations since, the above was written, I 

 have come to the conclusion that the crucial sulms rr]>rr>.-ni.s the dorsal part 

 of the central sulcus and thnt tin- ventral part of the latter is forim-d Hth.-r 

 from or lit tin- r.\p !!- "I (iii.-chiinicjilly) the candid r.xtivinity of tin- minimi 



8U1CU8. 



