As far as the kidneys, each of the two remaining undivided, the
aorta and the big vein extend; and here they get more closely attached
to the backbone, and branch off, each of the two, into a A shape,
and the big vein gets to the rear of the aorta. But the chief
attachment of the aorta to the backbone takes place in the region of
the heart; and the attachment is effected by means of minute and
sinewy vessels. The aorta, just as it draws off from the heart, is a
tube of considerable volume, but, as it advances in its course, it
gets narrower and more sinewy. And from the aorta there extend veins
to the mesentery just like the veins that extend thither from the
big vein, only that the branches in the case of the aorta are
considerably less in magnitude; they are, indeed, narrow and
fibrillar, and they end in delicate hollow fibre-like veinlets.