Emily Dickinson's poem, "I felt a Funeral in my Brain" is an extended metaphor for the speaker's progressive descent into madness. The lines
And mourners to and fro
Kept treading, treading, till it seemed
That sense was breaking through
indicate that the speaker senses that she is beginning to lose her hold on sanity. With the images of the funeral ritual, Dickinson then marks the steady progression of the speaker's losing hold of reality and entering the "solitary" realm of her own unstable mind. When "a plank in reason broke," the speaker feels herself plunging into a separate realm, the one of her own unstable mind.
Emily Dickinson's poem exemplifies her unique ability to "replicate human consciousness in a controlled poetic form" as the rituals of a burial are used in her extended metaphor to symbolize the mental confusion of the speaker. And, because there is enough ambiguity in the imagery and symbolism for other interpretations, the poem's metaphors can also represent the speaker's being tortured simply by an idea that comes into conflict with her previously conceived ones, rather than total madness. So, in this case, "the mourners" are the challenging concepts and ideas. But, at any rate, the speaker's mind terminates its angst of madness or confusion in the last line as it "finished knowing then."