On the Burning of the "Twigs"

 The different regions of the bronchial tree are compared to the different parts of a tree. The branch region probably ends with the "twigs," the terminal bronchioles. it's probably the p. arteriolar conduits at the level of the acinus that is referred to as the "fire" as there occurs a functional change in the conduit at this level; earlier it was "wind," now it's "fire."