LATERAL VEl^miCLES. 



481 



surface rests on the velum interpositum. The anterior pillars 

 descend in front of the anterior commissure of the brain, and, 

 becoming inflected downwards and backwards by the sides of the 

 third ventricle, pass to the corpus albicans, of which they form 

 the external fibres. 



Behind the anterior pillars of the fornix on each side, small 

 openings pass downwards and backwards, and, imiting in the 

 middle line, lead into the third ventricle. This is the anterior 

 common opening or foramen of Monro, which forms a communi- 

 cation between the two lateral and the third ventricles. 



The posterior pillars of the 

 fornix pass downwards and out- 

 wards to the descending comua 

 of the lateral ventricles, and 

 then become continuous wi4:h the 

 hippQcampi. The thin pro- 

 longation behind the choroid 

 plexu3 is the corpus Jlmh^- 

 tum (tcsnioy hippocampi), and 

 the transverse white fibres which 

 pass, like the strings of a harp, 

 between the- pUlars, form the 

 Lyra. 



The hippocampus (comu 

 Ammonis) is the large eminence, 

 constructed of white matter exter- 

 nally and grey internally, which 

 occupies the posterior part of 

 the floor of the lateral ventricle. 

 The hippocampi are continuous 

 with the posterior pillars of the 

 fornix, and are separated from the optic thalami by the choroid 

 plexxises. Posteriorly they diverge and enter the descending 

 comua of the ventricles, occupying the cid-de-sacs. Towards 

 their terminations they become enlarged, and each presents a 

 number of rounded elevations, with indented edges somewhat 

 resembling the foot of an animal, and called the pes hippocampi. 

 The external white matter of the hippocampi is continuous, 

 through the corpora fimbricta, ivith the fornix and corpus 

 callosum. 



The ccn^pnz striatum, is the pear-shaped prominence which 



2i 



Fio. 179. 

 Lateral ventricles opened from above. I, 

 Grey matter; 2, White matter; ot. Septum 

 lucidum; b. Part of corpus callosum; cc. 

 Hippocampus ; d, Taenia semicircularis ; e. 

 Choroid plexus ; //, Corpus striatum ; g. An- 

 terior coma of the ventricle ; h, FxirroTV lead- 

 ing backwards to descending comu. 



