Septum pellucidum

The septum pellucidum (also called the septum lucidum) is a thin, triangular, vertical membrane that separates the lateral ventricles of the brain. It separates the anterior horn of the left and right lateral ventricles. Runs as a sheet from the corpus collosum down to the fornix. And when the hemispheres are cut apart, the septum remain on one hemisphere (usually the left one)

Layers
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, referred to as cavum septum pellucidum, cavum septi pellucidi, or "fifth ventricle". The last term has lost favor in recent years, as the space is usually not continuous with the ventricular system and does not contain cerebrospinal fluid. Indeed fifth ventricle has been used for other purposes in recent years. .

Location
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.

Pathology
Absence of the septum pellucidum or corpus callosum, caused by mutations in the gene, is associated with septo-optic dysplasia. This may result in hypothalamic dysfunction and hypopituitarism, as well as problems of vision, coordination, and intelligence, among other abnormalities.

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.