x] Frontal and Prefrontal Ar<* 247 



very diminutive the frontal lobe is in these animals. And while the ape shows a marked 

 advance cm the lower vertebrate, its frontal development, even in the anthropoid, is still far 

 behind that of man. 



But, as was pointed out in describing the histology of the frontal lobe, it is possible in 

 the case of the anthropoid ape to effect a subdivision of the frontal cortex on lines similar 

 to those in the human being, and on further analysing the various areas in the two brains 

 we notice that the chief difference in regard to partition is that in the anthropoid far more 

 of the frontal lobe in the common sense of the term is covered by " precentral " and 

 " intermediate precentral " cortex than in man, and vice versa, far less of the frontal lobe is 

 left over. And of the remainder it seems to me that the ape possesses much less " frontal " 

 cortex than man, while with respect to " prefrontal " cortex although the inferiority is not so 

 great it is still apparent. Hence if Hitzig's hypothesis be correct the deduction to be drawn 

 is that the cortex of the "frontal" area constitutes the focus of the intellectual sphere. 



Looking from the platform of phylogenesis there is another point which has long had 

 a special fascination for me as a histologist, because I have thought that it might help to 

 explain the structural peculiarities of the frontal lobe. It has reference to the question 

 whether the feeble structural representation and the relative poverty of signs of specialisation 

 in this " prefrontal " region are attributable to the truth that it is the very last to make its 

 appearance in the course of phylogenetic development. The idea seems plausible. It further 

 occurs to one that as the " prefrontal " stretch of cortex is represented in the ape as well 

 as in man, as it is so inferior to other regions in its nerve cell and fibre supply, as it is 

 quite inexcitable by electricity, and as it is doubtful whether lesions confined to it give rise 

 to any symptoms, therefore it cannot share the functional importance of the better developed 

 " frontal " cortex placed further back : in short although it may have a future in front of it, 

 at present its evolution both structural and functional is incomplete. 



That the last-named area is of importance there seems little doubt, its major development 

 is the feature which specially distinguishes the human from the ape's brain, and experimental 

 interference as well as natural lesions are followed by positive results. Finally, I anticipate 

 that the facts of histology which I have narrated will prove acceptable evidence to those 

 who maintain that the direction followed by the growth producing the massive frontal lobe 

 of man has been downwards and outwards. 



SUMMARY. 



1. That part of the frontal lobe uncovered by "intermediate precentral" and "limbic" 

 cortex can be split up into two fields ; the hinder of these, forming a skirt to the " inter- 

 mediate precentral" area, I have called "frontal"; the anterior centred on the frontal pole, 

 I have called " prefrontal." 



2. On the mesial surface of the hemisphere the " frontal " area occupies a small portion 

 of the marginal gyrus lying above the level of the callosal genu ; on the lateral surface 

 it occupies the anterior half of the superior frontal gyrus, a large slice of the middle frontal 

 and the anterior extremity of the inferior frontal gyrus ; on the orbital surface it occupies 

 a small area lateral to the external sagittal sulcus. 



3. The general fibre wealth of the " frontal " area is less than that of the " intermediate 

 precentral," but greater than that of the "prefrontal." Constitutionally it differs from the 



