Sentence:  Turning back, then, toward the basement staircase, she began to grope her way through blinding darkness, but had taken only a few uncertain steps when, of a sudden, she stopped short and for a little stood like a stricken thing, quite motionless save that she quaked to her very <WSD>marrow</WSD> in the grasp of a great and enervating fear. 
Meaning of the word marked with <WSD> tags:  The substance inside bones which produces blood cells.