TRANSVERSE FISSURE OF THE CEREBRUM. 



547 



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. 



The transverse fissure of the cerebrum is the passage by which the pia 

 mater passes from the surface 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 undis- 

 turbed velum interpositum below. It will then be found that, in like manner, 

 the posterior and middle lobes 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 



Fig. 374. 



Fig. 374. VIEW OP THE UPPER SURFACE OP THE VELUM INTERPOSITTJM, CHOROID PLEXTJS, 

 AND CORPORA STRIATA (from Sappey after Vicq-d'Azyr). f 



1, fore part of the tela choroidea or velum interpositum ; 2, choroid plexus; 3, left 

 vein of Galen partly covered by the right ; 4, small veins from the front of the corpus 

 callosum and the septum lucidum ; 5, veins from the corpus striatum ; 6, convoluted 

 marginal vein of the choroid plexus ; 7, vein rising from the thalamus opticus and corpus 

 striatum ; 8, vein proceeding from the inferior cornu and hippocampus major ; 9, one 

 from the posterior cornu ; 10, anterior pillars of the fornix divided in front of the 

 foramen of Monro ; 11, fornix divided near its forepart and turned backwards ; 12, lyra; 

 13, posterior pillar united with, 14, the corpus callosum behind, and covered by the 

 choroid plexus as it descends into the inferior cornu. 



