[TA] the deep depression of the basal surface of the forebrain that corresponds in position to the anterior perforated substance. Bounded medially by the optic tract and rostrally by the orbital surface of the frontal lobe, it extends laterally around the overhanging pole of the temporal lobe into the sylvian fissure (sulcus lateralis). SYN: fossa lateralis cerebri [TA], fossa of Sylvius, lateral fossa of brain, vallecula sylvii.