e, while separate arterial trunks for the carotid and
subclavian arteries should spring from the aorta on the left of this
mid-line, thus making a remarkable exception to the rule of symmetry
which characterizes all the arterial vessels elsewhere, still I cannot
but regard this exceptional fact of asymmetry as in itself expressing a
question by no means foreign to the interests of the practical.

In the abstract or general survey of all those peculiarities of length
to which the innominate artery, A, Plate 9, is subject, I here lay it
down as a proposition, that they occur as graduated phases of the
bicleavage of this innominate trunk from the level of A, to the aortic
arch, in which latter phasis the aorta gives a separate origin to the
carotid and subclavian vessels of the right side as well as the left. On
the other hand, I observe that the peculiarities to the normal separate
condition of A and B, the carotid and subclavian arteries of Plate 10,
display, in the relationary aggregate, a phasial gradation of A and B
joining into a common trunk union, in which state we then find the aorta
giving origin to a right and left innominate artery. Between these two
forms of development--viz., that where the four vessels spring
separately from the aortic arch, and that where two innominate or
brachio-cephalic arteries arise from the same--may be read all the sum
of variation to which these vessels are liable. It is true that there
are some states of these vessels which cannot be said to be naturally
embraced in the above generalization; but though I doubt not that these
might be encompassed in a higher generalization; still, for all
practical ends, the lesser general rule is all-sufficient.

In many instances, the innominate artery, A, Plate 9, is of such
extraordinary length, that it rises considerably (for an inch, or even
more) above the level of C, the sternal end of the clavicle. In other
cases, the innominate artery bifurcates soon after it leaves the first
part of the aortic arch; and between these extremes as to length, the
vessel varies infinitesimally.

The innominate artery lies closer behind the right sterno-clavicular
junction than the left carotid or subclavian arteries lie in relation to
the left sterno-clavicular articulation; and this difference of depth
between the vessel of the right side and those of the left is mainly
owing to the form and direction of the aortic arch from which they take
origin. The aor