A flexible surgical sheath and multi-part insertion cannula for inserting the flexible sheath in a cranial surgical tunnel for gaining access to a ventricle or other surgical target. An obturator or other elongated insertion member drives a sheath assembly into a surgical recess, typically a hole drilled through the skull and brain of the surgical patient. The inserted sheath assembly includes two or more rigid sheath portions disposed around the obturator. The obturator has a tapered or angled tip for guiding the sheath assembly through the surgical tunnel to a target region such as a ventricle or other brain structure for which surgical intervention is sought. Following insertion, the obturator is withdrawn and a flexible sheath inserted in an insertion tunnel defined by the rigid sheath portions.
