NERVOUS SYSTEM. VERTEBRATA. 159 



A well-defined rhinal fissure begins anteriorly in the 

 cleft between the olfactory bulb and the apex of the 

 hemisphere, and separates the pyriform lobe from the neo- 

 pallium. 



Upon the mesial surface (fig. 46) the olfactory bulb 

 may be distinctly seen to be connected by a short cylindrical 

 peduncle to the precommissural area of the mesial wall. 

 The tuberculum olfactorium is seen immediately below the 

 latter. 



In the caudal region of the mesial wall the fascia dentata 

 and fimbria are seen presenting features such as we find in 

 most mammals. The prominent, broad fimbria at once 

 distinguishes this hippocampal formation from that of the 

 Monotremes. 



Fig. 46. (Nat. size.) 



HIP. F. SULC.CALC. 



FASC. DENT. 



COMML D 

 OLF.BULB 



OLF. TUBER: 'COMM.V. 



This fimbria is seen to pursue a regular arcuate course 

 upward and forward to reach a point immediately above 

 the large ventral (modified anterior) commissure ; and 

 many of its fibres then cross the middle line in close 

 relation to the upper extremity of the lamina terrninalis, so 

 as to form a smaller dorsal commissure, which is thus 

 derived by means of the fornix (fimbria) from the hippo- 

 campus. Histological examination shows that the fibres 

 of this dorsal commissure are not grouped irregularly to 

 form a round bundle, as in the Monotremes, but are 

 compactly arranged in a crescentic form. In some 

 Marsupials this dorsal commissure becomes more acutely 

 bent so as to be distinctly bilaminar. Some of the fibres 

 of the fimbria do not enter the dorsal commissure and are 

 divided into two groups, a scattered anterior group of pre- 

 commissural fibres and a posterior group the so-called 

 anterior pillar of the fornix -which proceeds behind the 

 ventral commissure and passes through the optic thalamus 

 toward the corpora mammillaria. 



