h it, or as low down as the
fifth lumbar vertebra. The occasional existence of a sixth lumbar
vertebra also causes a variety in the length, not only of the aorta, but
of the two common iliac vessels and their branches.[Footnote]

[Footnote: Whatever may be the number of variations to which the
branches arising from both extremes of the aorta are liable, all
anatomists admit that the arrangement of these vessels, as exhibited in
Plate 25, is by far the most frequent. The surgical anatomist,
therefore, when planning his operation, takes this arrangement as the
standard type. Haller asserts this order of the vessels to be so
constant, that in four hundred bodies which he examined, he found only
one variety--namely, that in which the left vertebral artery arose from
the aorta. Of other varieties described by authors, he observes--"Rara
vero haec omnia esse si dixero cum quadringenta nunc cadavera humana
dissecuerim, fidem forte inveniam." (Iconum Anatom.) This variety is
also stated by J. F. Meckel (Handbuch der Mensch Anat.), Soemmerring (De
Corp. Hum Fabrica), Boyer (Tr. d'Anat.), and Mr. Harrison (Surg. Anal.
of Art.), to be the most frequent. Tiedemann figures this variety
amongst others (Tabulae Arteriarum). Mr. Quain regards as the most
frequent change which occurs in the number of the branches of the aortic
arch, "that in which the left carotid is derived from the innominate."
(Anatomy of the Arteries, &c.) A case is recorded by Petsche (quoted in
Haller), in which he states the bifurcation of the aorta to have taken
place at the origin of the renal arteries: (query) are we to suppose
that the renal arteries occupied their usual position? Cruveilhier
records a case (Anal. Descript.) in which the right common iliac was
wanting, in consequence of having divided at the aorta into the internal
and external iliac branches. Whether the knowledge of these and numerous
other varieties of the arterial system be of much practical import to
the surgeon, he will determine for himself. To the scientific anatomist,
it must appear that the main object in regard to them is to submit them
to a strict analogical reasoning, so as to demonstrate the operation of
that law which has produced them. To this end I have pointed to that
analogy which exists between the vessels arising from both extremities
of the aorta. "Itaque convertenda plane est opera ad inquirendas et
notandas rerum similitudines et analoga tam integralibus quam partibus;