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Medical Definition of Velum interpositum
1. A double fold of pia mater, enclosing subarachnoid trabeculae, between the fornix above and the epithelial roof of the third ventricle and the thalami below; at each lateral margin is a vascular fringe projecting into the choroidal fissure of the lateral ventricle; on its undersurface are several small vascular projections filling the folds of the ependymal roof of the third ventricle. Synonym: tela choroidea ventriculi tertii, tela choroidea superior, triangular lamella, velum interpositum, velum triangulare. (05 Mar 2000)