Sentence:  ... he laments that he can find no physiological phenomenon answering to his subject's winning a race, or losing it. Between his terminal output of energy and his victory or defeat there is a mysterious <WSD>crevasse</WSD>. Physiology is baffled. 
Meaning of the word marked with <WSD> tags:  A discontinuity or “gap” between the accounted variables and an observed outcome.