Poe writes that Usher "entered, at some length, into what he conceived to be the nature of his malady." What exactly is his "malady" we never learn. Even Usher seems uncertain, contradictory in his description: "It was, he said, a constitutional and a family evil, and one for which he despaired to find a remedy--a mere nervous affection, he immediately added, which would undoubtedly soon pass off." The Narrator notes an "incoherence" and "inconsistency" in his old friend, but he offers little by way of scientific explanation of the condition. As a result, the line between sanity and insanity becomes blurred, which paves the way for the Narrator's own decent into madness. This madness is manifested not only in the breakdown of Usher's mind but in his decrepit body. The diseased rotting corps of his sister also illustrates this motif.
The author wants to show that even though the Monkey King thinks he is an important ruler and is equal to the others, the other deities just consider him a monkey (and he might not be as important as he thinks he is).
