All these cavities,

even the two small ones, are connected by
passages with the lung, and this fact is rendered quite plain in one
of the cavities. And below, at the point of attachment, in the largest
cavity there is a connexion with the great vein (near which the
mesentery lies); and in the middle one there is a connexion with the
aorta.

Canals lead from the heart into the lung, and branch off just
as the windpipe does, running all over the lung parallel with the
passages from the windpipe. The canals from the heart are uppermost;