THE CAT 255 



the roof of which is formed by the corpus callosum. Now cut 

 through the corpus callosum into the ventricle and carefully chip 

 away the entire roof of the latter, but do not remove the fornix. 

 The cavity is thus fully exposed ; its anterior end is called the 

 anterior horn, and the posterior portion the inferior or posterior 

 horn. The former passes ventrally and ends near the medial plane, 

 in the anterior portion of the hemisphere; the latter is a long, 

 narrow passage, semicircular in form, which lies in the posterior 

 lobe of the hemisphere and parallel to its outer surface. 



The floor of the anterior horn projects into the cavity and is 

 called the corpus striatum. The floor of the posterior horn like- 

 wise projects prominently into the cavity and is called the hippo- 

 campus. Along the entire anterior border of the hippocampus is a 

 band called the fimbria, which is covered by a vascular membrane. 



The fornix, as we have already seen, is a narrow, median band 

 of longitudinal fibers, which lies between the hemispheres just 

 below the corpus callosum. It consists of several parts, which 

 cannot be observed if the section is not exactly median. Its for- 

 ward end is split, forming the anterior pillars of the fornix, which 

 are medial to the corpus striatum. Just behind the pillars, and 

 between them and the middle commissure, is the foramen of 

 Monro, a small opening which joins the two ventricles of the 

 hemispheres with the third ventricle. The posterior portion of 

 the fornix is also split, forming the posterior pillars, which lie 

 medial to the hippocampi of the hemispheres. The fornix, the 

 hippocampus, and the fimbria, it will be seen, lie in the ventral 

 wall of the hemisphere and immediately in front of and dorsal to 

 the third ventricle. 



Exercise 42. Draw a sketch showing these structures. 



Carefully remove the hippocampus and the fimbria. Beneath 

 them are two oblique ridges, the paired optic thalami, which form 

 the sides of the diencephalon and between which is the third ven- 

 tricle. Each thalamus extends laterally and ventrally to the ven- 

 tral side of the brain, where it becomes the optic tract. The two 

 optic tracts form the optic chiasma; from this the optic nerves 

 proceed. 



