In Alex Ayers book THE WIT AND WISDOM OF MARK TWAIN he quotes from Freud's
work
entitled JOKES AND THEIR RELATION TO THE UNCONSCIOUS (translated by J.
Strachey.  W.W. Norton, 1963.)

Evidently Freud felt that anger was a frequent theme in Twain's humor and he
illustrated his point with the story Twain told of the cow falling through
the subterranean dwelling.

Barb