pellucidum

Pellucidum is the neuter form of the Latin adjective pellucidus, meaning clear or transparent. In anatomical usage, it appears most often in the name septum pellucidum, a thin, translucent membrane within the brain.

The septum pellucidum is a midline structure that contributes to the walls of the lateral ventricles. It consists of two delicate membranes that extend from the corpus callosum anteriorly toward the fornix posteriorly and typically fuse along the midline. The space between the two leaflets is called the cavum septi pellucidi (CSP). In fetal and neonatal life the CSP is common and usually closes during infancy; in some individuals it may persist into adulthood as a normal variant.

Developmentally, the septum pellucidum forms during early brain development as the leaflets separate and then, in most people, fuse to form a single midline partition. The CSP represents a potential space that can be visible on imaging when the leaflets remain unfused or incompletely fused. A persistent CSP can be detected by ultrasound or MRI and is generally considered a benign variant, though its presence can be noted in the context of other neurodevelopmental findings.

Clinical significance is primarily imaging-related. A persistent or abnormally large CSP may draw attention during prenatal or neurologic imaging, but it is not itself a disease. Abnormal absence or thinning of the septum pellucidum can occur with certain brain malformations and requires clinical correlation. Related anatomical structures include the corpus callosum and the fornix, and the CSP is often discussed alongside these in neuroanatomy and radiology contexts.