214 ORIGINAL ARTICLES. 



the reader in the shape of a short account of the dissection which dis- 

 closed them. 



The right cerebral hemisphere was removed down to the level of the 

 corpus callosuni, as seen in Pig. 4. At a point relatively much further 

 distant from its posterior edge, 14, than is the case in man, we see the 

 internal perpendicular fissure, 16. Posteriorly again to this fissure, 

 and running nearly parallel with it, we see a second, 17, the " scissure 

 des hippocampes" ol'M. Gratiolet. Corresponding with this indenta- 

 tion, we have within the cavity of the ventricle an eminence, 19, the 

 lesser hippocampus, bounded by an arm or creek running up, 18, along 

 its outer surface from the central ventricular expanse. This arm or 

 creek was called, by another metaphor than those we have used, the 

 third cornu of the lateral ventricle, in the phraseology of the old anato- 

 mists. The large smooth headland into which the hippocampus swells 

 at 19, justifies the expression we find at page 19 of M. Gratiolet' s 

 work — "L' anfractuosite d' ergot . . . qui est plus evidente encore 

 dans les Singes que dans V Homme" In the brain of a cercopithecus 

 now before us, its proportions are very much larger. The width of 

 this third cornu was at its commencement three-eighths of an inch ; 

 and the similar cavity in a human brain examined at the same time 

 was of the same width. But the cavity narrows much more rapidly in 

 the orang than in man; and before reaching its termination, at a distance 

 of one inch from its commencement, it becomes almost a linear cavity ; 

 but, as our figure shows, the distinctness of its limiting walls and the 

 continuity of its lining membrane were unambiguously visible up to its 

 very extremity. The length of this third cornu is as great absolutely, 

 and relatively, therefore much greater in the cercopithecus, than in the 

 orang. In the human brain it was but half an inch longer than in the 

 orang, scooped out though it was in a posterior lobe relatively very 

 much longer. Neither in the cercopithecus, nor in the orang, does the 

 bourrelet or posterior rounded edge of the corpus callosum extend 

 nearly so far back as to allow us to take it as " the best measure of the 

 position" of the third cornu;* indeed, when we find Tiedemann speak- 

 ing of the pedes hippocampi minores as "Processusf duo medullares 

 qui a posteriore corporis callosi margine proficiscuntur," it is easy to 

 understand how he came to overlook their existence altogether, " in 

 cerebro Simiarum desunt," being so far in error as to their relations to 

 neighbouring parts. 



This relation of the posterior edge of the corpus callosum to the 

 commencement of the third cornu is of importance, not merely as a 

 guide to the discovery of that fissure, but also as, when coupled with 

 the relations which the corpus callosum holds to the internal occipital 

 figure 16, laterally, and to the corpora quadrigcmina posteriorly, speak- 

 ing unambiguously of great diminution of the antero-posterior diameter 

 of the simious corpus callosum. 



Nat. Hist. Rev., 1. c, p. 79. f Icones, p. 51. 



