the aorta itself, and by the
principal branches which spring from it, occur under the following
mentioned conditions:--When the arch of the aorta rises above or sinks
below its ordinary position or level,--namely, the cartilages of the
second ribs, as seen in Plate 25,--it varies not only its own length,
but also that of the vessels H I K; for if the arch of the aorta rises
above this level, the vessels H I K become shortened; and as the arch
sinks below this level, these vessels become lengthened. Even when the
aortic arch holds its proper level in the thorax, still the vessels H I
K may vary as to length, according to the height to which they rise in
the neck previously to their division. When the aorta sinks below its
proper level at the same time that the vessels H I K rise considerably
above that point at which they usually arch or divide in the neck, then
of course their length becomes greatly increased. When, on the other
hand, the aortic arch rises above its usual level, whilst the vessels H
I K arch and divide at a low position in the neck, then their length
becomes very much diminished. The length of the artery H may be
increased even though the arch of the aorta holds its proper level, and
though the vessels H I K occupy their usual position in the neck; for it
is true that the vessel H may spring from a point of the aortic arch A
nearer to the origin of this from the ventricle of the heart, whilst the
vessel I may be shortened, owing to the fact of its arising from some
part of H, the innominate vessel. All these circumstances are so
obvious, that they need no comment, were it not for the necessity of
impressing the surgeon with the fact that uncertainty as to a successful
result must always attach to his operation of including in a ligature
either of the vessels H I K, so as to affect an aneurismal tumour.

Now whilst the length of the aorta and that of the principal branches
springing from its arch may be varied according to the above-mentioned
conditions, so may the length of the aorta itself, and of the two common
iliac vessels, vary according to the place whereat the aorta, C,
bifurcates. Or, even when this point of division is opposite the usual
vertebra,--viz., the fourth lumbar,--still the common iliac vessels may
be short or long, according to the place where they divide into external
and internal iliac branches. The aorta may bifurcate almost as high up
as where the pillars of the diaphragm overarc