NATUKAL SUBDIVISION OF THE CEREBItAL HEMISPHERE. 451 



adoption.^ It may be employed to designate the olfactory bulb 

 and peduncle as Owen used it, and as such is unnecessary, and 

 therefore superfluous; or it may be used to include all those 

 regions which are pre-eminently olfactory in function, and have 

 become deji7iitely specialised in structure in consequence. Such a 

 definition will include the olfactory bulb, its peduncle, the tuber- 

 culum olfactorium and locus perforatus, the pyriform lobe, the 

 paraterminal body, and the whole hippocampal formation. The 

 paradoxical statement that " if this definition be adopted, a 

 smell-less animal will possess a rhinencephalou," is no argument 

 and only a very " partial truth." For though the brain of an 

 anosmatic Cetacean apparently possesses a tuberculum olfacto- 

 rium, a pyriform lobe, and a small hippocampus, histological 

 examination will show that the two former present little else 

 than the areas corresponding to the tuberculum olfactorium and 

 pyriform lobe from which most of the typical elements have 

 vanished, and that the hippocampus is a mere wreck of "its 

 former self," as exhibited in osmatic mammals. Ziehen's objec- 

 tion is therefore little else than a verbal quibble. We might 

 evade the difficulty of using these terms "rhinencephalou" 

 and "neopallium" by coining such expressions as "^;a?'s 

 limhica [hemisjjJuei'ii] " and "^jrn'-s crescens [hemisjjhoirii] " 

 respectively. 



It is a curious and instructive fact that many writers who, in 

 name at least, adopt the teaching of Turner and His, are driven 

 by force of circumstances to include (apparently unconsciously) 

 the hippocampus in their " rhinenceplialon." Several of the 

 most prolific writers on Comparative Neurology of recent years 

 fail to draw a sharp line of distinction between the hippocampus 

 (which they sometimes erroneously call the " hippocampal con- 

 volution ") and the uncinate [or hippocampal] gyrus (which they 

 often call the "hippocampus" or "cornu ammonis"). Under 

 such circumstances it is not surprising that having recognised 

 the pyriform lobe (which forms the cephalic extremity of the 

 human uncinate gyrus) as part of the "rhinencephalou" of 

 Turner, they also include the hippocampal formation. In other 



^ There does not seem to me to be any evidence to svibstantiate the views of 

 several writers, more especially in America, that this term may be given a seg- 

 mental significance. 



