196 THE DESCENT OF MAN. 



"for four-tenths of an Incti outwards, and then extended down- 

 "wards, as far as the lower margin of the outer surface of the 

 "hemisphere. The imperfect definition of this fissure in the ma- 

 "jorlty of human brains, as compared with its remarkable distinct- 

 "ness in the brain of most Quadrumana, is owing to the presenoe 

 "in the former, cf certain superficial, well marked, secondary con- 

 "volutlons which bridge it over and connect the parietal with the 

 "occipital lobe. The closer the first of these bridcing gyri lies to 

 "the longitudinal fissure, the shorter is the external parieto-occip- 

 "ital fissure" (1. c. p. 12). 



The obliteration of the external perpendicular fissure of Grat- 

 iolet, therefore, is not a constant character of the human brain. 

 On the other hand, its full development is not a constant character 

 of the higher ape's brain. For, in the chimpanzee, the more or 

 less extensive obliteration of the external perpendicular sulcus by 

 "bridging convolutions," on one side or the other, has been noted 

 over and over again by Prof. Rolleston, Mr. Marshall, M. Broca 

 and Professor Turner. At the conclusion of a special paper on 

 this subject the latter writes :'= 



"The three specimens of the brain of a chimpanzee just de- 

 "scribed, prove, that the generalization which Gratiolet has at- 

 "tempted to draw cf the complete absence of the first connecting 

 "convolution and the concealment of the second, as essentially 

 "characteristic features in the brain of this animal, is by no means 

 "universally applicable. In only one specimen did the brain, in 

 "these particulars, follow the law which Gratiolet has expressed. 

 "As regards the presence of the superior bridging-convolution, I 

 "am inclined to think that it has existed In one hemisphere, at 

 "least, in a majority df the brains of this animal which have, up 

 "to this time, been figured or described. The superficial position 

 "of the second bridging convolution is evidently less frequent, and 

 "has as yet, I believe, only been seen in the brain (A) recorded in 

 "this communication. The asymmetrical arrangement in the con- 

 "volutions of the two hemispheres, which previous observers have 

 "referred to In their descriptions is also well illustrated in these 

 "cjoecimens" (pp. 8, 9). 



Even were the presence cf the temporo-ocoipital, or external per- 

 pendicular, sulcus a mark of distinction between the higher apes 

 and man, the value of such a distinctive character would be ren- 

 dered very doubtful by the structure of the brain in the Platy- 

 rhine apes. In fact while the temporo-occipital is one of the most 

 constant of sulci in the Catarhine, or Old World, apes, it is never 

 very strongly developed in the New "World apes; it is absent in 



'- Notes more especially on the bridging: convolutions in the Brain 

 of the Chimpanzee, 'Proceedings o£ the Royal Society of Edinburgh,' 

 186B-6. 



