CT scan emptying

Operating system: MAC OS MOJAVE
Slicer version: 4.11.2
Expected behavior:
Actual behavior:I uploaded a CT scan of a brown bear skull. the CT scan contains only the bone tissues and I need to keep only the external surface of the skull and eliminate all internal contents. how can I do? after loading the dicom I created a segment in the segment editor and used the threshold to give it an appropriate representation but in doing so the internal contents were also reproduced.

Please install the WrapSolidify effect for the segment editor from the extension manager and try to use it.