i I 



544 THE CEPvEBRUM. 



Inimediately behind the anterior pillars is an interval on each side 

 between the anterior part of the fornix and the groove where the optic 

 thalamus and corpus striatum meet. This interval leads from the 

 lateral ventricle to the third ventricle. The openings of opjiosite 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 forameji of llonro, ov foramen commune anterius. 



H\\Q posterior crura or pillars of the fornix (fig. 378, 6) 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 cornn 

 of the corresponding lateral ventricle, where part of its fibres are 

 bestowed on the surface of the great hippocampus and the remainder are 

 prolonged as a narrow band of white matter, named fcmia hippocamjn 

 or corpus fimhriatinn, which skirts the concave margin of the hippo- 

 campus major, and extends to its extremity. 



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 halves of the fornix, between which a triangular portion 

 of the corpus callosum appears, marked with transverse, longitudinal, 

 and oblique lines. To this part the term lyra has been applied 

 (fig. 382, 12). 



The transverse fissure of the cerebrum is the passage by which 

 the pia mater passes from the surfoce into the ventricles of the brain to 

 form the choroid plexus. It may be laid open in its whole extent, after 

 the lateral ventricles have been opened, by completely dividing the 

 fornix and corpus callosum in the middle line, and raising the divided 

 parts from the undisturbed velum interpositum below. It will then be 

 found that, in like manner, the posterior and middle portions of the brain, 

 including the hippocampus major and corpus fimbriatum, may be raised 

 from the subjacent parts as far as the extremity of the descending 

 cornu of the lateral ventricle. The transverse fissure is, therefore, a 

 fissure extending from the extremity of the descending cornu on one 

 side, over the constricted part of the cerebrum, to the extremity of the 

 descending cornu of the other side. It is bounded above by the 

 corpus callosum and fornix in the middle, and more externally on each 

 side by a free margin of the hemisphere : inferiorly it is bounded near 

 the middle line by the corpora quadrigemina, and on each side by the 

 posterior part of the optic thalamus. 



In the free margin of the hemisphere, brought into view by opening 

 out the part of the transverse fissure which leads into the descending 

 cornu of the lateral ventricle, there are seen (fig. 381) (1st) the ribbon- 

 like ledge formed by the corpus fimbriatum, along the hippocampus 

 major : (2nd) beneath this, a smaU. grey indented ridge, the fascia 

 clentata ; and (3rd) beneath the fascia dentata, the gyrus hippocampi. 

 On making a transverse section (fig. 381, B), it is seen that the corpus 

 fimbriatum forms the free margin of the white substance of the hemi- 

 sphere, and that the fascia dentata is the free margin of the cortical 

 substance, and is continuous with the grey matter of the hippocampus 

 major, and that thus the hippocampus major is the swelling in reverse 

 of the sulcus between the fascia dentata and gyrus hippocampi (uncinate 



