=====Velum interpositum===== {{ http://www.bartleby.com/107/Images/large/image565.gif}} Small membrane containing a potential space just above and anterior to the [[pineal gland]] which can become enlarged to form a [[cavum velum interpositum]]. It contains the posterior medial choroidal arteries and internal cerebral veins The velum interpositum is formed by an invagination of [[pia mater]] forming a triangular membrane the apex of which points anteriorly. The membranous roof of the third ventricle comprises two layers of the tela choroidea and two arachnoid layers. These two arachnoid layers are derived from the arachnoid envelope over the pineal region ((Zhang XA, Qi S, Fan J, Huang G, Peng J, Xu J. The distribution of arachnoid membrane within the velum interpositum. Acta Neurochir (Wien). 2012 Sep;154(9):1711-5. doi: 10.1007/s00701-012-1436-8. Epub 2012 Jul 11. PubMed PMID: 22782652. )). ====Boundaries==== superiorly - the columns of the fornices and hippocampal commisure (psalterium) reaching as far forward as the foramen of Monro. Inferiorly - the [[internal cerebral vein]]s and tela choroidea of the third ventricle. [[Posteromedial choroidal artery]]. inferolaterally - the thalamus. posteriorly- the narrow base of the triangle abuts the splenium of the corpus callosum It varies in shape from persion to person, sometimes interposed between the internal cerebral veins and splenium, and depending on whether or not there is a cavum vergae (in which case the columns of the fornices are displaced inferiorly, narrowing the velum interpositum). When somewhat distended by fluid it forms a small triangular (in axial section) space and is referred to as a cavum velum interpositum. If larger and exterting mass effect it is known as a cavum velum interpositum cyst. The space between the two leaves of pia contains the posterior medial choroidal arteries. ====Pathology==== [[Velum interpositum meningioma]]