we must be
prepared to meet the "varieties."

The veins assume an arrangement similar to that of the arteries, and the
above remarks will therefore equally apply to the veins. In the same way
as the arteries, H I K, may present in the condition of two common or
brachio-cephalic trunks, and thereby simulate the condition of the
common iliac arteries, so we find that the normal forms of the veins
above and below actually and permanently exhibit this very type. The
brachio-cephalic veins, D B, Plate 26, exactly correspond to each
other, and to the common iliac veins, S T; and as these latter
correspond precisely with the common iliac arteries, so may we infer
that the original or typical condition of the vessels I K, Plate 25, is
a brachia-cephalic or common-trunk union corresponding with its
brachio-cephalic vein. When the vessels, I K, therefore present of the
brachio-cephalic form as the vessel H, we have a perfect correspondence
between the two extremes of the aorta, both as regards the arteries
arising from it, and the veins which accompany these arteries; and this
condition of the vascular skeleton I regard as the typical uniformity.
The separate condition of the vessels I K, notwithstanding the frequency
of the occurrence of such, may be considered as a special variation from
the original type.

The length of the aorta is variable in two or more bodies; and so,
likewise, is the length of the trunk of each of those great branches
which springs from its arch above, and of those into which it divides
below, The modes in which these variations as to length occur, are
numerous. The top of the arch of the aorta is described as being in
general on a level with the cartilages of the second ribs, from which
point it descends on the left side of the spinal column; and after
having wound gradually forwards to the forepart of the lumbar spine at
C, divides opposite to the fourth lumbar vertebra into the right and
left common iliac arteries. The length of that portion of the aorta
which is called thoracic, is determined by the position of the pillars
of the diaphragm F, which span the vessel; and from this point to where
the aorta divides into the two common iliac arteries, the main vessel is
named abdominal. The aorta, from its arch to its point of division on
the lumbar vertebrae, gradually diminishes in caliber, according to the
number and succession of the branches derived from it.

The varieties as to length exhibited by