Rhinexcephalox 103 



with a much more gradual curve In the medial part of the 

 thalamus. A large proportion of the fornix fibres {tractus 

 cortico-mamillaris) continue backwards in this way until they 

 reach the mamillary body, where, as already observed, many 

 end while a few decussate and pass farther down in the brain 

 stem. In the earlier part of their course, these fibres are 

 accompanied by hippocampal fibres for the habenula {tractus 

 cortico-hahenularis medialis), which separate off just after the 

 fornix turns backwards, postero-dorsal to the anterior com- 

 missure, and pass posteriorly and dorsally in the stria medul- 

 laris thalami. This bundle we have seen to accompany the 

 habenular nuclei and end in them on the same and the opposite 

 sides. 



The hippocampus also gives rise to a large number of 

 commissural fibres which run to the same region of the other 

 hemisphere, forming the hippocampal commissure or psalterium 

 icommissura hippocampi, lyra). In man, this is a thin plate 

 of transverse fibres crossing between the bodies of the fornix 

 of the two hemispheres and merging at either side into the 

 alveus. In rodents, there is a very large commissure {ventral 

 hippocampal commissure, commissure of the fimbria, psalterium 

 ventrale) just behind the ventrally directed columns of the 

 fornix in the lamina terminalis, and this has a thin dorsal 

 extension {dorsal hippocampal commissure, psalterium dorsale) 

 which stretches back ventral to the corpus callosum until it 

 reaches the splenium, with which it seems to merge. This 

 extension is thickest in its posterior part, which is really a 

 distinct structure, and which Cajal believes not really to be 

 commissural in character, terming it the crossed temporo- 

 hippocampal tract. Some other authors seem to consider the 

 latter portion as part of the splenium, though it is clearly 

 different from the rest of that structure, taking a deeper stain 

 in Weigert preparations (PI. XXVL). 



The fibres of the ventral hippocampal commissure partly run through 

 a small-celled mass of gray matter lying between it and the columns of the 



