:!XXX.] THE BEAIN. 317 



/. Carefully pass a seeker underneath the edge of 

 the fornix and of the fimbria ; note that they 

 can be raised from the subjacent parts and 

 that the choroid plexus dips underneath them. 



1 3. Gently raise the corpus callosum, a thin membrane 

 the septum lucidum will be seen stretching 

 from its under surface to the fornix and sepa- 

 rating the lateral ventricles of the two sides. 



14. Cut through the corpus callosum near its poste- 

 rior end and turn it forwards, cutting through 

 the septum lucidum ; indications of the forma- 

 tion of the septum from two membranes will be 

 seen, in its anterior portion a narrow space the 

 fifth ventricle may be seen. Cut through the 

 corpus callosum anteriorly and remove it. 



The parts of the fornix previously noticed in 

 each lateral ventricle are now seen to be joined 

 in the median line forming a triangular lamina. 

 Anteriorly this dips down some little distance 

 behind the genu of the corpus callosum ; as it 

 dips down it divides into two round cords, the 

 anterior pillars of the fornix (these will be 

 better seen when the fornix is cut through 

 (§ 15)) ; the part of the fornix where these 

 pillars are joined is called the body, this is 

 small in lower mammals since the pillars soon 

 diverge posteriorly ; where they diverge they are 

 called the posterior pillars of the fornix, these 

 run on to form the fimbriae of the hippocampus 

 (cp. § 12, e) and also send fibres which run over 

 the inner surface of the hippocampus, 



