THE BRAIN 



347 



enclose a cavity, the so-called fifth ventricle, or space of the septum pellucidum 

 (Fig. 351). 



The fornix takes its origin early, chiefly from cells in the hippocampus. 

 The fibers course along the chorioidal side of the hippocampus cranially, pass- 

 ing dorsal to the foramen of Monro (Fig. 351 ^). In the cranial portion of the 



Corpus callosum 



Body of fornix 



Ant. piUai cffanix Thalamm 



B 



Body of fornix Hippocampal commissure 



'um pellucidum 



Corpus callosum 



Ant LOm>}!is\u}e 



' Thalamus 

 Ant. pillar of fornix 

 Fig. 351. — Two stages in the development of the cerebral commissure. (Based on reconstructions 

 by His and Streeter.) A, Median view of the right hemisphere of an 83 mm. fetus; B, the same of a 120 

 mm. fetus. 



lamina terminaHs fibers are given off to, and received from, the basal portion of the 

 rhinencephalon. In this region, fibers crossing the midline form the hippocampal 

 commissure. Other fibers, as the diverging anterior pillars of the fornix, curve 

 ventrally and end in the mammillary body of the hypothalamus. The commissure 

 of the hippocampus, originally cranial in position, is carried caudalward with the 

 caudal extension of the corpus caUosum (Fig. 351 B). 



