104 Anatomy of the Nervous System 



fornix. This bed of the commissure belongs morphologically to the 

 dorsal part of the septum. Continuous with it, a little mass of cells 

 just where the anterior end of the membranous roof of the third ventricle 

 meets the posterior surface of the commissure has been described in the 

 rat and other forms by Johnston as the noduliis marginalis. 



Another longitudinal band of hippocampal projection fibres 

 may be seen tp separate from the alveus in its postero-dorsal 

 part and to take up a position near the middle line between 

 the corpus callosum and the dorsal hippocampal commissure. 

 There it is joined by small bundles which cross through 

 the corpus callosum from behind and above. This is thefornix 

 superior, a structure which is inconspicuous in man, but is 

 very prominent in the rat and in many other mammals 

 (Pis. XIX. -XXL). This band increases in size as one passes 

 forward to the region of the lamina terminalis, where many of 

 its fibres join the descending columns of the fornix, while 

 others mingle with the praecommissural system in the septum. 

 A much reduced remnant of the fornix superior may be 

 traced some distance farther forward, when it will be found 

 to come from the olfacto-cortical tract (Pis. XXII., XXIII.). 

 The perforating fibres which run through the corpus callosum 

 to the fornix superior are derived partly from the indusium, 

 but many come from the region of the cingulum (p. 109), 

 after originating in the interhemispheric cortex (chiefly from 

 its posterior and middle regions in the rat). These, with the 

 fornix superior itself, are frequently called fornix longus, a 

 term which seems to have been used in somewhat different 

 ways by different writers. A few fibres of the fornix system, 

 after traversing the septum, join the medial forebrain bundle 

 for the hypothalamus. 



A diffuse tract running backwards in the molecular layer 

 of the cortex and mingling with the external hippocampal 

 fibres connects the deeper part of the corresponding layer of 

 the hippocampus with the subsplenial gyrus and the neo- 

 pallium caudal to it (Pis. XV.-XIX., XXVL). 



