BY G. ELLIOT SMITH. 639 



by the fascica dentata. The hitter structure in Platypus throughout 

 forms the upper border of the choroid fissure, but in Perameles and 

 others, where the ventricle turns downwards, and the descending 

 cornu thus formed bends forwards, the fascia dentata and fimbria 

 come, of course, to occupy a position at the lower margin of the 

 choroid fissure in its descending limb, and the fimbria lies above 

 the fascia dentata in this region. The destination of the alveus 

 fibres, after they have arrived at the region of the foramen of 

 Monro, varies. 



(a) The great majority of the fibres cross the middle line to enter 



the hippocampal region of the opposite side, so that they 

 effect an exchange of fibres between the two hippocampi. 

 In the alveus of the opposite hippocampus they give ofi" 

 numerous collaterals which enter the white matter and 

 come into relationship with the pyramidal cells. These 

 fibres constitute therefore the hippocampal commissure 

 proper of Owen and Symington — the coi'pus calloaum of 

 Flower, Sander, Osborn and almost all recent writers — the 

 hippocampal decussation (in Platypus) of Hill. Its origin 

 from cells of the hippocampus (including the subiculum and 

 fascia dentata) indicates without the slightest possibility 

 of doubt its homology with the psalterium of Eutheria. 



(b) A large bundle of fibres dips downwards behind the anterior 



commissure to enter the optic thalamus. This is the 

 anterior pillar of Oie jornix. 



(c) A third smaller and more scattered bundle arches forwards 



in the mesial wall of the hemisphere in front of the 

 anterior commissure — the precomiinissural fibres of Huxley. 



(rt) A considerable number of alveus fibres pass into other 

 regions of the same hippocampus from which they arise. 

 These may be called hippocampal association fibres. 



The present paper will only deal with the first of these sets of 

 fibres — the hippocampal commissure— and will only refer to the 

 others in so far as they are related to the commissural fibres. It 



