sulcus spiralis externus sulcus spiralis externus the basilar crest gives attachment to the outer edge of the basilar membrane; immediately above the crest is a concavity, the sulcus spiralis externus. sulcus spiralis externusfloor of ductus cochlearis.detailsidentifierslatinsulcus spiralis externusta98a15.3.03.124fma77851anatomical terminology