1404 



A MANUAL OF ANATOMY 



matter, which is covered by grey matter on the surface looking 

 towards the fifth ventricle. 



The two laminse are formed from portions of the mesial walls of the two 

 cerebral hemispheres which have become detached in the course of the 

 development of the corpus callosum and fornix. 



Fifth Ventricle. — ^The fifth ventricle, or ventricle of the septum, 



is the narrow cleft-like interval between the two laminse of the 

 septum lucidum. It is a closed space, and has therefore no com- 

 munication with the other ventricles. It is destitute of any 

 ependymal lining, and contains a very little fluid. 



Caudate Nucleus •*■-—.. 



Left Vein of Galen 



Vein of Corpus Striatum i 



Choroid Vein " 

 Veins of Galen 



Choroid Plexus 



entering De- 

 scending Cornu 

 of Lateral 

 Ventricle 



Septum Lucidum 



Fifth Ventricle 

 Fornix 



..Velum Inter - 

 positum 



Vena ^fagna Galeni 



— Lyra 



Posterior Pillar of Fornix 



Fig. 590. — The Velum Interpositum and Veins of Galen. 



As regards development, it differs from the other ventricles, 

 having been originally a part of the great longitudinal fissure. 



Velum Interpositum. — ^This is also known as the tela choroidea 

 superior, in contradistinction to the tela choroidea inferior, which 

 is the pia mater forming the roof of the lower part of the fourth 

 ventricle. It lies immediately beneath the fornix, and rests upon 

 the ependymal roof of the third ventricle, and also upon the ad- 

 jacent portions of the optic thalami. It consists of two layers 

 of pia mater, and is triangular, the apex being situated behind the 

 anterior pillars of the fornix at the foramina of Monro and the 

 base lying beneath the splenium of the corpus callosum. In the 

 latter situation the two layers of the velum interpositum become 



