SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 409 



to the caudal part of the hippocampus that it is preserved at 

 all in higher mammals. Certainly it curves about the splenium 

 to enter the fully developed hippocampus in all the forms that 

 I have studied. It is undoubtedly made up chiefly of the very 

 primitive system of olfacto-cortical fibers which are already seen 

 in (cyclostomes and) selachians and which constitute most of 

 the fimbria bundle in the rostral part of the reptilian hemisphere. 

 The fibers of this system which enter the rostral part of the 

 hippocampus nearly all disappear in mammals possessed of a 

 large corpus callosum so that the stria lateralis Lancisii consists 

 chiefly of hippocampal association fibers and is sometimes diffi- 

 cult to recognize in these, higher forms. 



Elliot Smith ('97, p. 55) suggested that the fasciculus mar- 

 ginalis described by him in Ornithorhynchus is represented in 

 higher mammals by the stria medialis. The fasciculus margin- 

 alis in the opossum goes in part into the stria medialis and in 

 part into the '^ association bundle" in the stratum zonale of the 

 hippocampus. Since the latter is practically lost in higher mam- 

 mals it would be true to say that the fasciculus marginalis is 

 represented by the stria medialis. On the other hand, the stria 

 medialis is not wholly made up of the fasciculus marginalis, 

 since it receives fibers from the other parts of the precommissural 

 system. 



Two other points are worthy of notice in this connection. 

 First, the relations in the bear show clearly that the sulcus 

 corporis callosi in higher mammals is the fissura hippocampi, 

 strictly comparable to that of the marsupial or the bat. And 

 this suggests that when the perforating fibers of the fornix supe- 

 rior appear to go to the cingulum they are in reality only taking 

 a position along the lateral border of the reduced hippocampus. 

 The hippocampus is sometimes enlarged and the hippocampal 

 fissure better developed in front of the corpus callosum or beneath 

 the genu, as in the bear and the rat (figs. 58, 77). Second, the 

 recognition that the stria medialis is independent of the hippo- 

 campus proper affords us a definite boundary line in most mam- 

 mals between the hippocampal formation and the residue of the 

 hippocampal primordium (i.e., septum, pallial commissures and 



