220 A MANUAL OF ANATOMY. 



posteriorly with the pia, from which it originally springs. 

 The velum interposition forms the covering or roof for the 

 third ventricle. Its margins project into the cavity of the 

 lateral ventricles and are composed of masses of capillaries 

 called the choroid plexuses of these ventricles. 



In a similar manner from the under surface of the velum 

 festoons of capillaries hang downward into the cavity of 

 the third ventricle and furnish the choroid plexuses for that 

 ventricle. 



As stated above, the velum is a portion of the pia which 

 seems to be contained within the ventricular cavity of the 

 brain. But such is not really the case, for though it 

 projects into the ventricles of the brain it carries in front 

 of it a covering of epithelium which is the attenuated 

 remains of the roof of the primitive vesicles of the brain. 

 This layer covers the velum and the choroid plexuses and 

 becomes continuous with the walls forming the boundaries 

 of the ventricles and so shuts the velum and choroid 

 plexuses out of the cerebral ventricles. Diag. I 5. 



The velum interpositum with its lining layer of epithe- 

 lial cells (which represents the primitive roof of the brain) 

 is called the tela choroidea superior, and the portion of 

 the pia which reaches from the under surface of the cere- 

 bellum to the upper surface of the medulla, and forms the 

 covering for the posterior portion of the fourth ventricle, 

 is called the tela choroidea inferior. 



The Deep Cerebral Veins. 



(a) The vein of the corpus striatum collects the blood 

 from the corpus striatum, optic thalamus, fornix, septum 

 lucidum, courses forward and inward in the groove between 

 the corpus striatum and the optic thalamus, and terminates 

 at the entrance of the foramen of Monro. 



