228 THE DESCENT OF MAN. 



the wonderfully close similarity between the chimpanzee, orang and 

 man, in even the details of the arrangement of the gyri and sulci of 

 the cerebral hemispheres. Nor, turning to the differences between 

 the brains of the highest apes and that of man, is there any serious 

 question as to the nature and extent of these differences. It is ad- 

 mitted that the man's cerebral hemispheres are absohitely and rela- 

 tively larger tban those of the orang and chimpanzee; that his frontal 

 lobes are less excavated by the upward protrusion of the roof of the 

 orbits; that his gyri and sulci are, as a rule, less symmetrically dis- 

 posed, and present a greater number of secondary plications. And it 

 is admitted that, as a rule, in man, the tempore occipital or "exter- 

 nal perpendicular " fissure, which is usually so strongly marked a 

 feature of the ape's brain is but faintly marked. But it is also clear, 

 that none of these differences constitutes a sharp demarcation between 

 the man's and the ape's brain. In respect to the external perpendic- 

 ular fissure of Gratiolet, in the human brain for instance, Prof. Turner 

 remarks : * 



" In some brains it appears simply as an indentation of the margin 

 of the hemisphere, but in others it extends for some distance more or 

 less transversely outward. I saw it in the right hemisphere of a 

 female brain pass more than two inches outward; and on another 

 specimen, also the right hemisphere, it proceeded for four-tenths of 

 an inch outward, and then extended downward as far as the lower 

 margin of the outer surface of the hemisphere. The imperfect defi- 

 nition of this fissure in the majority of human brains, as compared 

 with its remarkable distinctness in the brain of most Quadrumana, is 

 owing to the presence in the former of certain superficial, well- 

 marked, secondary convolutions which bridge it over and connect the 

 parietal with the occipital lobe. The closer the first of these bridg- 

 ing gyri lies to the longitudinal fissure the shorter is the external 

 parieto-occipital fissure " (1. c, p. 12). 



The obliteration of the external perpendicular fissure of Gratiolet, 

 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 exten- 

 sive 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 Prof. Turner. 

 At the conclusion of a special paper on this subject the latter writes :f 



*' The three specimens of the brain of a chimpanzee just described, 

 prove that the generalization which Gratiolet has attempted to draw 

 of 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 of the brains of this 



* " Convolutions of the Human Cerebrum Topogi-aphically Considered," 

 18(56, p. 12. 



t Notes more especially on the bridffinf? convolutions in the brain of the 

 chimpanzee, " Proceedinffs of the Royal Society of Edinburgh," 1865-66. 



