650 CEREBRAL COMMISSURES OF THE MAMMALIA, 



traced in the Mammalian series as the gyrus subcallosus of 

 Zuckerkandl* or the " peduncle of the corpus callosum." Neither 

 of these terms is applicable to the nonplacental mammal since 

 there is no corpus callosum. The area in question may be 

 distinguished as '' the precommissural area" a purely descriptive 

 term equally applicable to all vertebrates possessing a cortex. The 

 " precommissural ai^ea " from its connections and ontogeny is part 

 of the rhinencephalon. Now the only two commissures which are 

 known to have any connection with the rhinencephalon are the 

 anterior and hippocampal commissures — the corpus callosum 

 never. To distinguish some fibres of the hippocampal commissure 

 by the name of " corpus callosum " — as Herrick does — simply 

 because they go to the precommissural area is therefoi'e entirely 

 unwarranted, and opposed to all the accepted criteria of a corpus 

 callosum. Since the corpus callosum of certain reptiles is confined 

 to the homologous area, it is only natural to conclude that a 

 corpus callosum is absent in them as it is in Didelphys. 



Moreover, Osbornf in Amphibia speaks of a bundle of fibres 

 as *' a commissure of the dorsal portion of the mantle and homo- 

 logous, on this ground, with the corpus callosum," quite oblivious 

 of the fact that, by his own showing, the dorsal commissure in 

 Marsupials is anterior commissure. Besides, Edinger;}: shows 

 that the mesial and dorsal portions of the cortex of reptiles is 

 homologous with the Amnion's horn, and therefore its connecting 

 fibres must be hippocampal commissure. When it is upon such 

 flimsy and unscientific evidence that many writers base their belief 

 in the presence of a corpus callosum in Submammalia, one is fully 

 justified in doubting their conclusions, especially when there is so 

 much discordance as to what actually is the corpus callosum. Thus 

 many writers {e.g., Osborn, Gage, &c.) describe as corpus callosum 

 a structure which is undoubtedly hippocampal commissure. Evi- 

 ' dence is accumulating, which will go far to prove that the corpus 



* Lor. cit. 

 t Loc. cit. 

 t Loc. cit. 



