STRUCTURE OF THE BRAIN 265 



There remains, then, no dispute as to the resemblance, 

 in fundamental characters, between the ape's brain ana 

 man's: nor any as to the wonderfully close similarity be- 

 tween 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 seri- 

 ous question as to the nature and extent of these differences. 

 It is admitted that the man's cerebral hemispheres are abso- 

 lutely and relatively larger than those of the orang and chim- 

 panzee; that his frontal lobes are less excavated by the up- 

 ward protrusion of the roof of the orbits; that his gyri and 

 sulci are, as a rule, less symmetrically disposed, and present a 

 greater number of secondary plications. And it is admitted 

 that, as a rule, in man the temporo-occipital or "external per- 



J)endicular" fissure, which is usually so strongly marked a 

 eature of the ape's brain, is but faintly markra. But it is 

 also clear that none of these differences constitute a sharp 

 demarcation between the man's and the ape's brain. In 

 respect to the external perpendicular 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 in 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 definition 

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

 with its remarkable distinctness in the brain of most Qaadru- 

 mana, is owing to the presence, in the former, of certain su- 

 perficial, well-marked, secondary convolutions which bridge 

 it over and connect the parietal with the occipital lobe. The 

 closer the first of these bridging gyri lies to the longitudinal 

 fissure, the shorter is the externST 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 con- 

 stant character of the higher ape's brain. For, in the chim- 

 panzee, the more or less extensive obliteration of the exter- 



n "ConTolutioDS of the Human Cerebrum Topographically Considerad," 

 1866, p. 12. 

 Descent— ToL. 1—12 



