<unk>  seen on CT The<unk>  (CSP), cavum septi pellucidi, or cavity of<unk><unk>  is a slit-like space in the<unk><unk>  that is present in fetuses but usually fuses during infancy. The<unk><unk>  is a thin, laminated translucent vertical membrane in the midline of the brain separating the anterior horns of the right and left ventricles. It lies posterior to the corpus callosum. Persistence of the<unk>  after infancy has been loosely associated with neural maldevelopment and several mental disorders that correlate with decreased brain tissue. The<unk>  is bounded anteriorly by the genu of the corpus callosum, superiorly by the body of the corpus callosum, posteriorly by the anterior limb and pillars of the fornix, inferiorly by the anterior commissure and the rostrum of the corpus callosum, and laterally by the leaflets of the<unk><unk> . In prenatal development of the fetus, the laminae of the<unk><unk>  separate to form a small cavity – the<unk> . This is an important normal structure to identify in the sonographic assessment of the fetal brain. In post-natal life, the laminae of the<unk><unk>  usually fuse, which obliterates the cavum. The cavity contains cerebrospinal fluid (CSF) that filters from the ventricles through the septal laminae. There are individual differences in the degree of CSP; whereas some have complete closure of the cavum, others present with a small degree (4–6 mm wide, in the coronal plane) of incomplete closure. The most common type of CSP is noncommunicating; that is, it does not connect to the brain's ventricular system. Because of this lack of communication, the previous use of the term "fifth ventricle" is no longer used, and the fifth ventricle is the name often used for the terminal ventricle. CSP is present in 100% of fetuses, but over 85% of them fuse by 3–6 months after birth.
Cave of septum pellucidum
Zona pellucida
Sinus pericranii
Interventricular foramina 
Medial eminence of floor of fourth ventricle
Pocock boundary
Cortical homunculus
Kernohan's notch