The septum pellucidum consists of two layers or laminae of both white and gray matter.[1] During fetal development there is a space between the two laminae called the cave of septum pellucidum which, in ninety per cent of cases, disappears during infancy.[2][3] The cavum was occasionally referred to as the fifth ventricle but this is no longer used as the space is usually not continuous with the ventricular system.[4] The fifth ventricle is recognised as the terminal enlargement of the spinal cord.[5]