Escape
Physiological response
Notice the physical reaction—not intellectual but bodily. The word itself triggers actual blood flow, like a fight-or-flight response.
Prisons broad
Dickinson inverts the expected—prisons should be narrow, confining. 'Broad' suggests she's thinking of something larger than a jail cell.
Childish at my bars
The adverb matters: not 'childishly' (foolishly) but 'childish' (like a child). She tugs with a child's hope despite adult knowledge it won't work.