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The velum interpositum is a small membrane containing a potential space just above and anterior to the pineal gland which can become enlarged to form a cavum velum interpositum.
The velum interpositum is formed by an invagination of pia mater forming a triangular membrane the apex of which points anteriorly.
- superiorly: the body of the fornix and hippocampal commissure (psalterium) reaching as far forward as the foramen of Monro
- inferiorly: the internal cerebral veins and tela choroidea of the third ventricle
- inferolaterally: the thalamus
- posteriorly: the narrow base of the triangle abuts the splenium of the corpus callosum 1
It varies in shape from person 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 2.
- 1. Chen CY, Chen FH, Lee CC et-al. Sonographic characteristics of the cavum velum interpositum. AJNR Am J Neuroradiol. 1998;19 (9): 1631-5. AJNR Am J Neuroradiol (abstract) - Pubmed citation
- 2. L. M. Ketonen. [et al.]. Pediatric Brain and Spine. Springer. (2005) ISBN:3540264361. Read it at Google Books - Find it at Amazon