546 



THE CEREBRUM. 



Immediately behind the anterior pillars, where they descend, the fornix, 

 which further back rests upon the optic thalami, the velum interpositum 

 alone intervening, has an interval on each side left between it and the 

 groove where the optic thalamus and corpus striatum meet. This interval 

 leads from the lateral ventricle to the third ventricle the space between the 

 thalami and beneath the velum interpositum. The openings of opposite 

 sides, passing downwards and backwards, meet in the middle line below, 

 and thus is produced a passage, single below, but dividing into two 

 branches above somewhat like the letter Y, and forming a communication 

 between the third ventricle and both lateral ventricles. This passage is 

 named the foramen of Monro, or foramen commune anterius. 



Fig. 373. 



Fig. 373, A. LOWER AND BACK PART OF THE 

 CEREBRUM OP THE LEFT SIDE, SHOWING THE 

 POSTERIOR AND MIDDLE CORNUA OP THE 

 LATERAL VENTRICLE OPENED (altered from 

 Hirschfeld and Leveille). | 



1, 1', inner convolution of the temporal lobe 

 turning round into the convolution of the gyrus 

 fornicatus, and showing on its surface the 

 reticulated structure; 2, cut surface of the 

 cerebral hemisphere ; 3, point of the posterior 

 cornu of the lateral ventricle ; 3', eminentia 

 collaterals ; 4, cut surface of the lower and 

 back part of the corpus callosum divided near 

 the middle ; 4', placed on the extension of the 

 corpus callosum into the cerebral hemisphere, 

 points by a line to the hippocampus minor in 

 the posterior cornu ; 5, cut edge of the posterior 

 pillar of the fornix passing down at 5', into 

 the hippocampus major and corpus fimbriatum ; 

 6, continuation of the corpus fimbriatum or 

 taenia hippocampi ; 6', pes hippocampi ; 7, fascia 

 dentata on the inside of the white substance of 

 the tfenia. 



Fig. 373, B. SECTION OP THE HIPPOCAMPUS 

 MAJOR TO snow THE ARRANGEMENT OP THE 

 GREY AND WHITE SUBSTANCE (from Mayo). 



a, white layer on the surface of the hippo- 

 campus ; b, grey substance which is involuted 

 from the surface of the neighbouring convolution ; 

 c, fascia dentata ; d, white reticulated substance 

 of the lower part of the gyrus fornicatus; 

 e, cavity of the lateral ventricle. 



The posterior crura or pillars of the 

 fornix are the diverging continuations 

 backwards of the two flat lateral bands 

 of which the body is composed. At 



first they adhere to the under surface of the corpus callosum, then curving 

 outwards, each crus enters the descending cornu of the corresponding 

 lateral ventricle, and is prolonged as a narrow band of white matter, named 

 tcenia hippocampi or corpus fimbriatum, which is situated on the inner 

 margin of the hippocampus major, and extends to the extremity of that 

 structure. 



On examining the under surface of the fornix and corpus callosum, there are 

 seen posteriorly the thickened border or pad, and in front of it the diverging 



