is this a descent into madness?

I felt a funeral in my brain,
        And mourners, to and fro,
Kept treading, treading, till it seemed
        That sense was breaking through.

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,
        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--