What is the symbolism?

A black sack is used as a symbol of death and rebirth.

Till about three in the morning he was in a state of stupefied misery. It seemed to him that he and his pain were being thrust into a narrow, deep black sack, but though they were pushed further and further in they could not be pushed to the bottom. And this, terrible enough in itself, was accompanied by suffering. He was frightened yet wanted to fall through the sack, he struggled but yet cooperated. And suddenly he broke through, fell, and regained consciousness.