NATURAL SUBDIVISION OF THE CEREBRAL HEMISPHERE. 447 



Ij'^Upon histological grounds the hippocampus and the neo- 

 pallium ought not to be regarded as one formation. But even 

 if their structure were similar, the behaviour of the two regions, 

 as their evolution is traced through the Vertebrate series, 

 stands in such marked contrast that their inclusion under the 

 one term "pallium" (Turner, His, and others) in the higher 

 Mammalia can hardly be regarded as a happy grouping, or one 

 conducive to accurate comparison.^ 



The pyriform lobe shares far more in common with the neo- 

 pallium in structure, in its phylogenetic history, in its behaviour 

 in relation to the ganglionic masses and the fibre systems of the 

 hemisphere, than the hippocampus does; and yet one cannot 

 hesitate to acknowledge the wisdom of separating the pyriform 

 lobe from the neopallium in a primary subdivision of the hemi- 

 sphere. But if the pyriform lobe is thus separated from the 

 " pallium " of Turner and His (as Turner has separated it and as 

 His believed he had done), the hippocampal formation should, 

 a fortiori, be similarly e .eluded ; for is it not the earliest cortical 

 formation to be elaborated ; a formation which throughout the 

 whole Vertebrate series retains its own peculiar fibre-system— 

 the fornix — separate from the other fibre-systems of the hemi 

 sphere; a cortex, moreover, which attains the height of its mor 

 phological importance in the Vertebrata when the neopallium 

 is yet an altogether insignificant area ? Furthermore, in the 

 Mammalian series, in which for the first time the neopallium 

 attains to any noteworthy functional significance or anatomical 

 importance, the hippocampus steadily declines in relative pro- 

 portions as this non-hippocampul pallium increases. For the 

 hippocampus does not share in the progressive greatness of the 

 mammalian brain. What legitimate reason, then, can there be 

 for harnessing together as " pallium " these two regions, the 

 behaviour of which contrasts so markedly that it might almost 

 with truth be said that they vary inversely in size and import- 

 ance ? If it be justifiable to separate the pyriform lobe from the 

 " pallium," how much greater reason is there for also excluding 

 the hippocampus ! If any critic objects (as he has good reason 



^ I have elsewhere pointed out the inevitable confusion which results 

 from this usage in Comparative Anatomy {this Journal, vol. xxxii. pp. 

 243-246). 



VOL. XXXV. (n.S. vol. XV.) — JULY 1901. 2 H 



