THE BEAIN IN THE EDENTATA. 315 



thau a promiuoiit boss of inverted hippocampus covered with, an exceedingly attenuated 

 coat of extraventricukr alveus, and therefore serially homologous with the flattened 



.-v..^°-"'' 



KO"^''' 



4*8 



fimbria f ; ,• " ^.f^^^^j^ Jentata 



alveus 



W^ '■ fiss. hippocampi 



tubercul. ,.■%. J/r^^^ 

 hippocampi xii^:^^^ 'ob. pyriform. 



"x 



Dissection to expose ventral extremity of the hippocampal formation in Ori/cterojnts. Nat. size. 

 .!-. Attenuatcfl lower extremit_v of fascia dentata. 



band of inverted hippocampus which separates the fascia dentata from the fimbria. But 

 the lower extremity of the fascia dentata has not entirely disappeared, for it is repre- 

 sented by an extremely attenuated band which separates the hipjiocampal tubercle from 

 the pyriform lobe just as the "band of Giacomiui " does in the human brain. The 

 tuhercidum hippocampi is exactly analogous to the tip of the uncus in human anatomy, 

 which E,etzius has recently called the gyrus iiitralimhicus* . 



Such an arrangement, so far as I am aware, exists nowhere else outside the Primates. 



As the hippocampal formation proceeds upward toward the psalteriiim and splenium 

 of the corpus callosum it at the same time rapidly approaches the mesial plane, so that 

 a perspective view, such as a representation of the mesial surface of the hemisphere 

 affords, gives a very distorted picture of the hippocampal region. To gain an accurate 

 idea of this region we must examine from below those large opercula-like cortical folds 

 which form a dome-like roof al)ove the optic thalamus and corj)ora quadrigemina. Such 

 a view may be obtained by making a horizontal section immediately above tlie anterior 

 commissure and inverting the upper part of the cerebral hemispheres (fig. 21). In this 

 view we may start from the typical section of the hippocampus which has been described 

 above (fig. 20), and Avhich is now exposed once again. We see tlie fimbria extending 

 obliquely forward and inward toward the columna fornicis, the elliptical section of which 

 is seen on each side of the mesial plane. Many fibres of the fimbria enter the columna 

 fornicis of tlte corresponding side ; many other fibres of the fimbria cross the mesial 

 plane just behind the coliimncc fornicis and thus form the psalterium veiitrale. 



The area of inverted hippocampus which in the cut surface is placed immediately 

 behind the fimbria may be observed to rapidly taper and soon disappear as the mesial 

 plane is approached. 



But as the fascia dentata approaches the mesial plane it increases considerably in 



* Gustav lietzius, op. cU., ' Das Mensclieuhiru.' 



