480 PEOP. G. ELLIOT SMITH ON THE MOEPHOLOGT OF THE 



vol. xxxii.), and form the fasciculus marginalis. Olfactory impulses coming from the olfactory bulb are 

 thus poured into the ventral fringe of the hippocampus, and this fringe becomes specially modified to 

 form a large mass of " receptive cells " — the fascia dentata. In other regions of the brain modifications 

 of superficial cells are found in regions in which sensory paths impinge upon the cerebral cortex. This 

 is very slightly shown in the general cortex of higher Mammals because the sensory fibres are widely 

 scattered; but it is most pronounced upon the surface of the lobxis jjyriforniis {vide supra) : here a very 

 rich supply of incoming olfactory fibres covers the surface of the lobe, and the superficial cells become 

 specially modified to form the layer of " double pyramids," which is such a characteristic feature of this 

 region. In a similar manner a peculiar cortex is developed in the tuberculum olfactorium {vide supra). 



It would seem, therefore, not unreasonable to suppose that as the fibres coming from the olfactory 

 bulb began to collect at the ventral margin of the hippocampus, the latter region became specially 

 modified to form as the fascia deutata a receptive organ for olfactory impulses. 



In the Saurian brain a similar tendency is shown, and hence it follows that the small-celled ventral 

 portion of the Saurian hippocampus should be regarded at least as the physiological, if not the strict, 

 homologue of the Mammalian fascia dentata. In all the Saurians studied, the hippocampal formation 

 comes to an abrupt termination upon the dorsal surface of the hemisphere ; and the characteristic 

 column of cells ceases, and its lateral extremity is separated from the rest of the cortical region of the 

 brain by a pronounced hiatus. There is no reason for regarding the regions which are placed beyond 

 this hiatus as constituting part of the hippocampus. The mesial half of the roof of the hemisphere is 

 considered to be hippocampus because it is part of the one circumscribed histological formation, 

 concerning the homology of the mesial part of which there can be no question ; and there is no valid 

 reason for not extending this interpretation to the whole formation. But when this typical structure 

 comes to an end, it is only logical to infer that a region beyond the hippocampus has been attained. 

 Now, by comparison with the I\Iammalian condition, it is clear that this cortical area beyond the hiatus 

 must represent the neopallium, to which I have already referred in the Mammalian brain. 



In the Mammal, however, the transition from hippocampus to neopallium is usually a more gradual 

 process than this abrupt method which characterizes the Saurian ; and if an analogy for this be desired, 

 it will be found in the Chelonia and Rhynchocephalia, where the regular hippocampal column (jrudualhj 

 gives place in the roof of the hemisphere to the scattered arrangement which is characteristic of the 

 neopallium. In the earlier figures illustrating coronal sections through the hemispheres of Sphenodon, 

 this gradual transition from hippocampus to neopallium has been shown. 



The fibres which arise from or terminate in this hippocampus must be regarded as the fornix (in the 

 sense in which I defined this term : " The Relation of the Fornix to the Margin of the Cerebral 

 Cortex," Journal of Anatomy and Physiology, vol. xxxii. 1897) : the Reptilian fornix, like its 

 ^Mammalian representative, consists of sl fornix transversus (Forel) or hippocampal commissure [psalterium, 

 lijra,fornia.'-commissurc), and a form u- longus (Forel) composed of descending post-commissural fibres 

 [columna fornicis) and " pre-commissural fibres" of Huxley. 



The uncrossed fibres (or fornix longus) need not be further considered at present, but the crossing 

 fibres {fornix transversus) are of such great morphological importance in the discussion of the arrange- 

 ment of the cortex, that they cannot be dismissed in this summary fashion. 



The fibres which spring from the hippocampus and proceed across the mesial plane to enter the 

 hippocampus of the other hemisphere cannot be regarded otherwise than as the homologues of the 

 commissure variously known in mammals as psalteriwn, lyra, cmmnissura fornicis, commissura hippo- 

 campi, commissura cornu Amnionis, fornix transversus, as well as by other names. This commissure is 

 regarded as " hippocampal," not by reason of any particular relation it may present to the lamina 

 terminalis [compare Meyer's reasoning], to the recessus superior [comijare the writings of Osborn, 

 Herrick, and many American and German writers], to the roof of the forebrain or to the foramen of Monro, 

 but because its jib res are derived from the hippocampus. Stated thus directly, this truism may seem too 



