The septum pellucidum, also called the septum lucidum, is a thin, triangular, vertical membrane that separates the lateral ventricles of the brain.
The septum pellucidum actually consists of two layers or laminae of both white and gray matter, called the laminae septi pellucidi. These layers are normally fused; however, in approximately one-tenths of humans, there is a slit-like cavity between them. This space is occasionally called the fifth ventricle, although is usually not continuous with the ventricular system and does not contain cerebrospinal fluid.
The septum pellucidum is located in the midline of the brain, between the two cerebral hemispheres. It is attached superiorly (above), anteriorly (in front), and inferiorly (below) to the corpus callosum, the large collection of nerve fibers that connect the two hemispheres. Inferiorly and posteriorly (in back), it is attached to the anterior part of the fornix. On either side are the two lateral ventricles, pockets of cerebrospinal fluid within the cerebral hemispheres.
One famous reference to an abnormality of the septum pellucidum would be the movie Rocky V. In the movie, the main character Rocky Balboa is forced to retire due to brain damage sustained throughout his career.
Gray, Henry & Clemente, Carmine D. (1984). Gray's Anatomy of the Human Body (30th ed.). New York: Lippincott Williams & Wilkins.
Kasper, Dennis L.; Braunwald, Eugene; Fauci, Anthony S.; Hauser, Stephen L.; Longo, Dan L.; Jameson, J. Larry; & Kurt J. Isselbacher, (Eds.) (2004). Harrison's Principles of Internal Medicine (16th ed.). New York: McGraw-Hill.