Septum pellucidum

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Ventricular system of the brain with neighboring structures.

The septum pellucidum is a thin triangular membrane that is found in the midline septal area of the brain. It is positioned between the two cerebral hemispheres and separates the anterior horns of the lateral ventricles. This area is also where the septal nuclei are located. The septum pellucidum is also connected to the lower part of the corpus callosum. 

Comprising two layers of both white and gray matter, the septum pellucidum has a space between these layers known as the cave of septum pellucidum. This space typically disappears during infancy. Although the term "fifth ventricle" was sometimes used for this space, it's no longer accurate due to its lack of continuous connection with the ventricular system. 

The septum pellucidum is made up of bilateral fiber laminae, with sparse grey matter and neuroglia. By dividing these layers and moving downward between the fornices, access to the third ventricle can be achieved. Below this upper segment, the septal region contains four major nuclear groups:

• Dorsal group: primarily featuring the dorsal septal nucleus.
• Ventral group: encompassing the lateral septal nucleus.
• Medial group: comprising the medial septal nucleus and the nucleus of the diagonal band of Broca.
• Caudal group: involving the fimbrial and triangular septal nuclei.