And when they all were seated
A Service, like a Drum
Kept beating—beating—till I thought
My Mind was going numb
And then I heard them lift a Box
And creak across my Soul
With those same Boots of Lead, again
Then Space—began to toll
As all the Heavens were a Bell
And Being, but an Ear
And I, and Silence, some strange Race
Wrecked, solitary, here
And then a Plank in Reason, broke
And I dropped down, and down
And hit a World, at every plunge
And Finished knowing—then
Karen Ford of Harvard College states that “The ‘Plank in Reason’ that breaks in the final stanza is anticipated in the shift from interior to exterior space, as though the walls, floor, and ceiling of the room (or the sides, lid, and bottom of the coffin), all made of planks, suddenly disappear, plunging the speaker into limitless and terrifying space.” 
 Dickinson here tries to express her feelings of complete loss, and the end of the road for her. All hope has been lost and she is near the end of her journey. 
 Ford, Karen. “On 280 ("I Felt a Funeral, in My Brain”).“ On 280 ("I Felt a Funeral, in My Brain”). Harvard College, 1997. Web. 03 Feb. 2014. 

I felt a Funeral in my Brain class page