THE BRAIN 



345 



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

 commissure. Other fibers, as the 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 callosum (Fig. 333 B). 



Corpus callosum 



Body of fornix 



Hippocampal commissure 

 Anterior commissure 



Arit.piillan of fo 



Chorioid fissure 

 Tha.la.mus 



Body of fornix 

 jepfuni pellucidum 



Hippocampal commissure 



Corpus callosum 



Ant. commissure' 



Jhalamus 



V?n/. pillar of form* 



FIG. 333. Two stages in the development of the cerebral commissure. A, Median view of the 

 right hemisphere of an 83 mm. embryo; B, the same of a 120 mm. embryo. (Based on reconstructions 

 by His and Streeter). 



The fibers of the anterior commissure cross in the lamina terminalis ventral 

 to the hippocampal commissure. They arise as a cranial and a caudal division. 

 The fibers of the former take their origin from the olfactory 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. 



The corpus callosum appears cranial and dorsal to the hippocampal com- 



