BY G. ELLIOT SMITH. 655 



In man and microsmatic mammals the temporal part of the 

 hippocampus retains its primitive contour, the fascia dentata 

 ending just behind the splenium of the corpus callosum. Hill,* 

 not recognising the association between the disappearance of part 

 of the hippocampus and the appearance of the corpus caUosum, 

 attributes this fact to " accident." But what happens in the 

 macrosmatic mammal 1 For, in order to prepare for the advent 

 of a corpus callosum, the whole of the upper and anterior part of 

 the hippocampus has disappeax'ed, and only the small temporal 

 segment is left to carry on the functions of the whole. To 

 compensate for this restriction, the hippocampus, as it grows, 

 becomes bent upon itself in an S-shaped manner (fig. 6) and 

 becomes accommodated tinder the corpus callosum, in which 

 situation it may extend forwards almost as far as the foramen of 

 Monro. This bending can be readily seen in the brain of the 

 rabbit or foetal pig. Hill, in discussing the question " whether 

 the fascia dentata is continuous with the nervus Lancisii?" comes 

 to the conclusion that there is no continuity between the two 

 structures, and that the fascia dentata is essentially a subcallosal 

 structure. His principal argument is that in the ox "he fails to 

 see any indication of the return of the fascia dentata to the under- 

 surface of the splenium in order that it may round the splenium 

 and sweep forward in the nervus Lancisii in the manner required 

 by the theory" [of Honegger and Zuckerkandlf]. This view, that 

 the fascia dentata is essentially subcallosal, is not only directly 

 opposed to the facts of its development as described and figured 

 by Mihalkovics, Blumenau and Marchand, but also to the facts 

 elicited by a comparison with the Marsupial. The corpus callosum 

 occupies the same relative position to the Randbogen that the 

 superior limb of the hippocampal commissure does in Metatheria, 

 and hence the fascia dentata is essentially supracallosal. 



If the suggestions concerning the striae Lancisii advanced in 

 this paper are correct— and the whole weight of comparative 



* " The Hippocampus," ?oc. ct<. 

 t Loc. cit. 



