16 from the cannula 9 to assess the backflow of cerebral spinal fluid csf presence of csf that the cannula 39 s lateral opening 6 39 is in the subarachnoid space 25 the obturator 16 may be reinserted into the cannula 9 so that in the event there is no csf backflow the cannula obturator device 17 may be advanced or repositioned until backflow is obtained the lineally placed markings on the shaft of the cannula 5 39 are utilized to assess the depth of tip 6 insertion typically the depth of the dural 5 membrane from the surface of the skin is approximately 4 centimeters the operator then identifies the orientation of the lateral opening 6 39 by visually identifying the locking tab 12 locking grove 4 assembly on the cannula obturator hub the operator then affirms that the lateral opening 6 39 is oriented either cephalad or caudad as desired in preparation for introduction of the catheter 19 through the hollow channel 8 of the cannula and its eventual advancement out the lateral opening 6 39 of the cannula tip 6 the operator may inject suitable medications into the subarachnoid space 25 or thread a suitably pliable and appropriately sized catheter 19 into the cannula hub 1 advancing it through.