"It is a highly vascular layer containing blood vessels that supply the underlying surface of the brain particularly over the cerebrum 3. At the point where blood vessels enter the underlying neural tissue, extensions of the pia mater are carried with them creating a perivascular space 1. Because the brain does not have a true lymphatic system this perivascular space is important in allowing the movement of substances back into the CSF."
"At certain points in the third, fourth and lateral ventricles the pia mater forms a double layer and is covered by ependymal cells to create the tela choroidea. In the third ventricle a double-layered tela choroidea forms the velum interpositum 4."