How does the final scene of this novel, which is both poignant and controversial, illustrate Casy’s notion of the human soul?

Casey felt there is still the divine in people no matter how bad things get. The old man breast feeding was certainly controversial. I recall it was cut from the original 1940 John Ford film. Still, it appeals to humanity on the deepest of levels. When there is nothing to give, people have the capacity to sell give. There is nothing "gross" about giving life.