the Disease as Sergeant at Arms
the tall slumped extremely fast wraith might represent the Sergeant at Arms, the Disease, exploiting the loose security of Gately’s fever-addled mind, getting ready to fuck with his motives and persuade him to accept Demerol just once, just one last time, for the totally legitimate medical pain.
Context
The white-knuckle test continuing inside the fever, with the temptation reconceived as an enemy officer: Gately can't rule out that the wraith is the Disease itself, worming through his weakened defenses to make the case for Demerol "just once, just one last time" — and the trap's whole cunning is that the pain really is legitimate and medical. The Sergeant at Arms doesn't argue for relapse; it argues for a reasonable exception, which is the only form the argument ever takes.