The first line of this poem endows "Prose" with figurative possibilities; it could function as a metaphor for some dreary domestic or familial situation, a "Captivity" imposed on the speaker. But the comparison with a bird, one of Dickinson's familiar images for the poet, suggests that the speaker is also using "Prose" to mean a mode of writing. "They" have attempted, unsuccessfully, to confine her attention to prose. The final stanza could mean that the speaker can escape from prose into verse as easily as a bird can fly from the pound. As soon as her captors turn their backs, she can read or write poetry. But the lines "Still! Could themself have peeped-- / And seen my Brain--go round—" show that the speaker's liberation is an interior one, a liberation managed within the limits of her captivity. In other words, as shutting a child in a closet will not necessarily "still" that child, so confinement to prose will not necessarily shut out the structures of verse.
Pointing to Dickinson's mannerism of turning her prose "abruptly into metered expression" in her early letters to Higginson, Porter comments that she seems to be trying to demonstrate to him that the rhythms of verse "so pervade her consciousness that she cannot make the distinction between them and unmetered prose." Discussing the three "Master" letters, Gelpi remarks that their "diction and imagery are so much an extension of the poetry that these letters are best read (as are many of Dickinson's letters) as prose poems or free verse." Often, however, not only the diction and imagery of her letters but also their formal structures overlap with those of her verse. Some letters are far too metrical to be considered prose poems or free verse. For Dickinson, writing cannot be broken down into two separate modes, the unmetered language of prose and the metered language of verse. Instead, the metricality of her prose insists on the continuity and likeness of the two modes.
From Fictions of Form in American Poetry. Princeton: Princeton UP, 1993. Copyright © 1993 by Princeton UP.