Herrick, Cerebniin of the Opossum. 9 



is not direct along the very mesal line, but here the tela is distended 

 to form a homologue of the dorsal sac of fishes which extend caudad 

 to the epiphysis and is distinct from the aula except along the median 

 line. Its walls give rise to abundant plexus. (2) The hippocampal 

 commissure and fornix body form an oval mass dorsad of the anterior 

 commissure and are attached to it. From the lateral aspects of the for- 

 nix body cephalo-ventrad the descending fornix tracts appear. In exact- 

 ly median sections the fornix body is circumscribed on all sides except 

 ventrad by the ventricle, while the tela springs from a special promi- 

 nence, but laterad the tela adheres to the fornix body and can be 

 traced to the free margin of the fascia dentata (gyrus uncinatus). (3) 

 The anterior commissure itself is obscurely composed of three portions 

 which are medianly rolled into a compact cylinder. Each has a sheath 

 which can be seen under favorable circumstances. The dorsal and 

 ventral parts are crescentic in section, while the median portion is 

 oval. The dorsal crescent overlaps the ventral cephalad. (4) The 

 lamina terminalis is medianly very thin but contains some gray mat- 

 ter. (5) The callosal fibres are. too few to be very obvious in this 

 view. 



Dorsal and Ventral regions of the Cerebrum. We think there are 

 good morphological and practical reasons for distinguishing the dorsal 

 and ventral portions of the cerebrum as structures essentially distinct. 

 The limits of the two regions are easily drawn in the opossum. Ce- 

 phalad the olfactory crus with its cortex is very sharply distinguished 

 from the pre-crucial portion dorsad of it. (Plate A, Fig. 2.) Here 

 the cortex of the ventral portion is very largely covered by the olfac- 

 tory fibres. Laterad, the two regions are limited, as we proceed caud- 

 ad, by the rhinalis fissure, mesad the splenial fissure is an equally dis- 

 tinct boundary. Cephalad, these two fissures occupy nearly the same 

 horizontal plane but caudad the former passes ventrad and the latter 

 dorsad, a change which may be ascribed to the interposition of the 

 thalamus. The ventral portion is distinct in cellular structure and 

 presumably in function from the dorsal region. In the former two 

 prominences have their origin, the pyriform lobe of either side caudad. 

 and the post-rhinal lobe cephalad. At the mesal union of the ventral and 

 dorsal regions the hippoca'upus has its origin as a curious convolution 

 at the splenial fissure. The hippocampus has been carefully described 

 in rodents by C. Judson Herrick in Bulletin Denison Univ. Vol. 

 VI. The relations are still^more simple in marsupials and, because 



