The poem begins with wonder. The speaker marvels at how vast the body is, not as a machine or a container, but as a field where visions appear. Those visions come and go quickly. They can't be held onto or ordered around, and the short, plain lines mimic that slipperiness: each one arrives and is gone.
Then the tone shifts. The last two lines are a strange, almost casual request: if I go mad at night, "call me up a bear." The speaker seems to accept that these uncontrollable visions might tip into madness, and rather than ask for calm or a doctor, they ask for something wild and physical. A bear is heavy, warm, and animal, so it could be a comfort or a companion for the dark. The poem suggests the speaker would rather meet the unruly imagination with another vision than try to shut it down.
The mood is quiet, awed, a little uneasy, and oddly playful.