Segment Skeletal thorax (Ribs, Thoracic spine and sternum)

You will find it after installing the “Segment Editor Extra Effects” extension from the extensions manager.

More information on the effect see here: New Segment Editor effect: Local threshold - #7 by sfglio

It is important to remember that - after selection of a “bone” threshold - you need to left click into one of the slices (onto the bone) to add to the new segmentation.