Answer:
In this passage, Whitman is celebrating how the death and life of his self and his body are interconnected with the natural world.
Step-by-step explanation:
When we die, the physical substance of the body—literally the molecules of the flesh—rot away to become once again a part of the natural world. But the same thing is true when we are living. We breathe in the molecules of the air, which become a part of us, even as they began as a part of other things. "Song of Myself" is all about these kinds of transcendent connections. Whitman is celebrating his "self" ("I celebrate myself, and sing myself"), but he's doing so by acknowledging the ways his self relies on the forces and energies and bodies of the natural and human worlds around him.