410 J. B. JOHNSTON 



fimbria system). This boundary line is marked by the fimbrio- 

 dentate sulcus. This adds needed emphasis to the fact that 

 the corpus callosum does not itself mark this boundary. 



GENERAL RELATIONS OF FIBER TRACTS AND COMMISSURES IN 

 THE SEPTUM OF VERTEBRATES 



These relations are readily understood in view of the consid- 

 erations in the last section and of the conditions in selachians. 

 In these fishes (fig. 91) the pallial commissures are imbedded in 

 the hippocampal primordium, the corpus callosum dorsal to the 

 hippocampal commissure. Interwoven with both commissures 

 are longitudinal fibers of the tractus ol/acto-corticalis, while the 

 columns of the fornix emerge from among the fibers of the hippo- 

 campal commissure to descend at either side of the neuroporic 

 recess as in reptiles and matnmals. When in these higher forms 

 a part of the hippocampal primordium becomes hippocampus 

 the commissures remain imbedded in the undifferentiated pri- 

 mordium and the longitudinal fibers reach the hippocampus 

 through the residual primordium, retaining essentially the same 

 relations among themselves. As the hemisphere elongates, the 

 olfacto-cortical system takes on the form of a more compact bun- 

 dle, the precommissural fimbria. This is joined by the fibers 

 from the nuclei of the lateral olfactory tract running by way of 

 the anterior perforated space and helping to form the fasciculus 

 precommissuralis. This bundle contains according to Elliot Smith 

 ('97) and Cajal ('04) many fibers which arise and others which 

 end in the septum. The efferent fibers go in part at least to 

 the hypothalamus. The fibers which end in the septum belong 

 chiefly to the olfacto-cortical system. A study of the precom- 

 missural system in the opossum and the rabbit shows that the 

 fibers come chiefly from the olfactory tubercle, anterior perfor- 

 ated space, nucleus of the lateral olfactory tract and perhaps 

 the whole pyriform lobe. It is the great system of fibers by 

 which impulses are carried from the area olfactoria to the hippo- 

 campal formation. Since many of these fibers end in the sep- 

 tum, the fiber relations of the septum are not unlike those of 

 the hippocampus itself. 



