It has three
cavities, as has been said: the right-hand one the largest of the
three, the left-hand one the least, and the middle one intermediate in
size. 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;
and there is no common passage, but the passages through their
having a common wall receive the breath and pass it on to the heart;
and one of the passages conveys it to the right cavity, and the
other to the left.
With regard to the great vein and the aorta we shall, by and
by, treat of them together in a discussion devoted to them and to them
alone. In all animals that are furnished with a lung, and that are
both internally and externally viviparous, the lung is of all organs
the most richly supplied with blood; for the lung is throughout spongy
in texture, and along by every single pore in it go branches from
the great vein.
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