Objective To identify the configuration of the fascial sheath of the sciatic nerve in the adult pelvis . Methods A total of 14 cadaveric pelvic specimens and 20 living subjects were used for cadaveric examination and magnetic resonance imaging ( MRI) observation , respectively .The cadaveric specimens were investigated by the local latex injection method ( n =10 ) sectional anatomy ( n=2 ) or histological staining methods ( n =2 ) .The MR images were collected and analyzed from twenty patients without pelvic disorders .Results The injected latex did not diffuse along the pelvic segment of the sciatic nerve .Sectional anatomical and histological data showed that the architecture of the fascial sheath of the sciatic nerve varied at different levels .An intact circular sheath around the nerve was rarely observed .There was no fascia barrier between the obturator and sciatic nerves .Appearance on MRI was consistent with the cadaveric observation.Conclusion No intact fascial sheath exists along the pelvic segment of the sciatic nerve , thus sciatic anesthesia may block the obturator nerve .