The answer is "He's sane."
"True - nervous - very, very dreadfully nervous had I been and am; but why will you say that I am mad? The disease had sharpened my senses not destroyed not build them. Above all was the sense of hearing acute. I heard all things in the heaven and the earth I heard many things in hell. How, then, am I mad? Harken! and observe how healthily - how calmly I can tell you the whole story."
Although he is mentally ill, he claims to be sane.
Change of tone. You can change the tone of your voice to make the story more compelling
An allusion is an expression designed to call something to mind without mentioning it explicitly; an indirect or passing reference.
