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Medical Definition of Sixth ventricle
1. An inconstant, horizontal, slitlike space between the posterior one-third of the corpus callosum and the underlying commissura fornicis (commissura hippocampi; psalterium) resulting from failure of these two commissural plates to fuse completely during foetal development; like the cavity of the septum pellucidum, the space is not a true ventricle in the sense that it did not develop from the central canal of the neural tube. Synonym: cavum psalterii, cavum vergae, sixth ventricle. (05 Mar 2000)