214 A MANUAL OF ANA TOMY. 



hippocampus major the posterior pillar of the fornix, 

 the fimbria (the free edge of this portion of the fornix), and 

 the choroid plexus. 



The floor is taken up by the eminentia collateralis. 



The hippocampus major is an elongated roll of brain 

 matter along the inner wall of the descending horn of the 

 lateral ventricle, which is produced by the outward involu- 

 tion of the cerebral cortex of the temporal lobe to accom- 

 modate the hippocampal or dentate fissure. Figs. 35, 45, 

 46, 47. 



The hippocampus major terminates at the anterior 

 extremity of the descending horn in a broadened indented 

 end which resembles (slightly) the paw of an animal 

 hence the name of pes hippocampi. 



DISSECTION. 



Remove the corpus callosum, exposing both lateral ventricles. Trace the 

 descending horn on one side by cutting the brain away from the outside. 

 Save the septum lucidum (between the halves of which find the fifth ventricle) 

 and the fornix. 



The Septum Lucidum. Figs. 31, 32, 33, 38, 39, 41 to 

 44 inclusive. 



This will be seen on transverse sections to be a thin, 

 double layer, and on anteroposterior sections to have a 

 triangular shape and to be attached in front and above to 

 the inner surface of the corpus callosum, and below and 

 behind to the fornix. 



It is described as the attenuated wall of the hemispheres, 

 which was imprisoned within the brain by the development 

 of the fibres of the corpus callosum above it. But such is 

 not its derivation, for it is a thinned and broadened lamina- 

 tion of the central portion of the lamina terminalis, from 

 which it comes, the outer portion of the lamina terminalis 

 being developed into the corpus callosum and the inner 



