THE VELUM INTERPOS1TUM. 



185 



and the double fold only distinct in the larger sulci. The pia mater also at the 

 transverse fissure is invaginated into the lateral ventricles and over the third 

 ventricle (covered however by the epithelium of those cavities), and there forms the 

 velum interpositum or tela choroidea superior and choroid plexuses. It is also 

 prolonged over the posterior wall or roof of the fourth ventricle, where it forms the 

 so-called tela choroidea inferior and choroid plexuses of that ventricle. 



The velum interpositum. or tela choroidea superior, is a triangular fold 

 of pia mater, between the two layers of which arachnoidal tissue and blood-vessels 

 are contained, which lies immediately underneath the fornix, and can therefore only 

 be properly seen when this structure is cut through and raised (as in fig. 130). The 

 velum interpositum is then seen to cover in the third ventricle and to extend over 



Fig. 130. VIEW OF THE UPPER SUR- 

 FACE OF THE VELUM INTERPOSITUM, 

 CHOROID PLEXUSES, AND CORPORA 



STRIATA (from Sappey after Vicq- 

 d'Azyr). f 



1, fore-part of the tela choroidea 

 or velum iuterpositum ; 2, 2, choroid 

 plexus ; 3, 3, left vein of Galen partly 

 covered by the right ; 4, anterior 

 pillars of the fornix divided in front 

 of the foramen of Monro ; on either 

 side are seen small veins from the 

 front of the corpus caliosum and the 

 septum lucidum ; 5, vein of the 

 corpus striatum ; 6, convoluted 

 marginal vein of the choroid plexus ; 

 7, vein rising from the thalamus opti- 

 cus and corpus striatum ; 8, vein 

 proceeding from the inferior cornu 

 and hippocampus major ; 9, one from 

 the posterior cornu ; 11, fornix 

 divided near its middle and turned 

 backwards ; 12, lyra ; 13, posterior 

 pillar of the fornix ; 14, the splenium 

 of the corpus calJosum. 



the adjacent upper surface of 

 the optic thalamus on each 

 side as far as the oblique 

 groove which marks that sur- 

 face (p. 110, and fig. 82). 

 The base of the triangle is 

 continuous with the general pia mater at the back and sides of the brain, the apex 

 ends just behind the anterior pillars of the fornix (at the foramen of Monro). Each 

 side of the triangle is bordered by a choroidal plexus which projects from under the 

 edge of the fornix into the respective lateral ventricle ; behind, these plexuses are 

 continued along the mesial border of the descending cornu of the ventricle, where 

 they are invaginated into the choroidal fissure, projecting over the fimbria ; in front 

 they converge, becoming gradually smaller, to the foramina of Monro, between 

 which they become united. From this united part two other smaller plexuses pass 

 backwards along the under surface of the velum interpositum, close to the middle 

 line in front, but diverging behind (choroid plexuses of the third ventricle). 



The choroid plexuses are covered where they project into the cavities by the 

 epithelium of the ventricles, as is also that part of the velum interpositum which 

 roofs in the third ventricle. Along the choroid plexuses a prominent vein, the 

 choroid vein (fig. 89, p. 125), courses from behind forwards to join the vein of the 

 corpus striatum and form the corresponding vein of Galen at the foramen of Monro : 



