What is the simile in Let us go then you and I when evening spread out against sky like a patient etherized upon table?

The simile in the line "Let us go then, you and I, When the evening is spread out against the sky Like a patient etherized upon a table" is:

"Like a patient etherized upon a table"

This compares the evening sky to a patient lying on a table, anesthetized and therefore still and passive.