How does Jack Henry Abbott use imagery in In the Belly of the Beast: Letters from Prison?

Imagery:

"I learned a little secret in this period. A convict over sixty years old passed it on to me: cockroaches are a source of protein. Mash the day's catch all together in a piece of bread and swallow it like a big pill."

"Time descends in your cell like the lid of a coffin in which you lie and watch it as it slowly closes over you. When you neither move nor think in your cell, you are awash in pure nothingness."