THE BUMS IN THE EDENTATA. 325- 



the gyrus fornicatus in the rhinencephalon, in conformity with Broca's idea of a limbic 

 lobe, would, if adoj^ted, destroy the whole value of the term " rhinencephalon " as a 

 contradistinction to the term " pallium." 



In employing the term rhiuencejihalon we must always lieav in mind that this 

 heterogeneous collection of siu'face-areas is not exclusively a " smell-brain," but that 

 the association of all these parts with the olfactory ai)paratus is much more intimate 

 than it is with any other sensory organs. This will explain the apparently paradoxical 

 statement that smell-less animals, like certain Cetacea, possess a " smell-brain " or 

 rhinencejDhalon, since the pyriform lobe and hippocampus do not entirely vanish in such 

 animals. 



The pallium is the j)rogressive part of the cerebral hemisphere, and therefore the 

 region to Avhich we must attach most importance in the study of the mammalian brain. 

 Por in the lowest mammal the rhinencephalon has already reached as high a stage in 

 the evolutionary process as it ever will reach, and its essential features change but little 

 in mammals. The slight change that does occiu' is princij)ally a process of retrogression 

 in the more highly-organized members. 



In the following description I have departed from the usual custom by employing the 

 term " sulcus " lor all the pallial fui-rows instead of the more usual term " fissure," 

 not only because the former is a much more acciu'ate name, but also in order to introduce 

 a distinction between the strictly intrapallial furrows and the pallial boundaries, which 

 we still call " rhinal Jissu/-e," " hqjpocampal fissure," and " callosal fissure." To this 

 usual distinction the only exception will be made in the case of the Sylvian fissure, 

 which in its fully-developed form deserves the name " fissure." The mesial sm-face of 

 the pallium presents much greater ujiiformity in the arrangement of its sulci than 

 the cranial surfaces, and hence w e may preferably begin an examination of the pallium 

 in this reoion. 



Upon the inner aspect of the hemisphere of Orycteropus the pallium forms a broad 

 strip of cortex which extends from the corpus callosum to the dorsal edge of the 

 hemisphere. This strip of pallium bends downward in front of the corpus callosum, and 

 becomes continuous along an ill-defined line with the precommissural area. Behind the 

 corpus callosum it bends downward and becomes continuous with the pyriform lobe. 

 The whole extent of this strip of pallium is divided into two ajjproximately equal areas, 

 a central or circumcallosal and a peripheral or marginal respectively, by a single deep 

 and well-defined sulcus (fig. 4). This sulcus, which we may at present distinguish as a, 

 begins posteriorly in close proximity to the caudal extremity of the rhiual fissure : that 

 is, at the extreme ventral Hmit of the mesial pallium. As it ascends it follows a coiu'se 

 parallel to the peripheral margin of the hemisphere, and bends forward midway between 

 the corpus callosum and the upper edge of the hemisphere vintil it I'eaches the anterior j)ole ; 

 still following its course parallel to the peripheral margin, it bends downward in front, 

 then after a short course suddenly bends again upward and backward, I. e. jjarallel to 

 the palho-precommissural junction, and ultimately terminates in a triangular depression 

 formed by the opening up of the anterior extremity of the callosal fissure. From the 

 antero- superior bending of this sulcus one or two small but deep branches are usually 



SECOND SERIES. — ZOOLOGY, VOL. VII. 45 



