THE BRAIN 



347 



portion of the rhinencephalon and received from it. In this region, 

 fibers crossing the midline form the hippocampal commissure. Other 

 fibers, as the diverging anterior pillars x>f 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 callosum (Fig. 351 B}. 



Corpus callosum 



Body of fornix 



Hippocampal commissure 



Anterior commissure 



Chorioid fissure 

 Ant. pillar of fornix Thalamus 



B 

 Body of fornix Hippocampal commissure 



Septum pellucidu 



Corpus callosum 



Ant. commissure __ _ . . _ _. ... 



' Thalamus 

 Ant. pillar of fornix 



FIG. 351. Two stages in the development of the cerebral commissure. (Based on recon- 

 structions by His and Streeter). A, Median view of the right hemisphere of an 83 mm.Jetus; 

 B, of a 120 mm. fetus. 



The fibers of the anterior commissure cross in the lamina terminalis 

 ventral to the hippocampal commissure. They arise in a cranial and a 

 caudal division. The fibers of the former take their origin from the olfac- 

 tory stalk and the adjacent cortex. The fibers of the caudal division pass 

 ventrally about the corpus striatum, between it and the cortex, and may 

 be derived from one or both of these regions. 



