veins
corresponded in form, number, and distribution with the arterial
vessels, and underwent, at the same time, a similar mode of
metamorphosis. One point in respect to the original symmetrical
character of the primary veins is demonstrable--namely, that in front of
the aortic branches the right and left brachio-cephalic veins, after
joining by a cross branch, descend separately on either side of the
heart, and enter (as two superior venae cavae) the right auricle by
distinct orifices. In some of the lower animals, this double condition
of the superior veins is constant, but in the human species the left
vein below the cross branch (left brachio-cephalic) becomes obliterated,
whilst the right vein (vena cava superior) receives the two
brachio-cephalic veins, and in this condition remains throughout life.
After birth, on the commencement of respiration, the foramen ovale of
the interauricular septum closes, and the ductus arteriosus becomes
impervious. This completes the stages of metamorphosis, and changes the
course of the simple foetal circulation to one of a more complex
order--viz., the systemic-pulmonary characteristic of the normal state
in the adult body.

VIII.--Such being the phases of metamorphosis of the primary (branchial)
arches which yield the vessels in their normal adult condition, we
obtain in this history an explanation of the signification not only of
such of their anomalies as are on record, but of such also as are
potential in the law of development; a few of them will suffice to
illustrate the meaning of the whole number:--lst, The interventricular
as well as the interauricular septum may be arrested in growth, leaving
an aperture in the centre of each; the former condition is natural to
the human foetus, the latter to the reptilian class, while both would be
abnormal in the human adult. 2nd. The heart may be cleft at its apex in
the situation of the interventricular septum--a condition natural to the
Dugong, A similar cleavage may divide the base of the heart in the
situation of the interauricular septum. 3rd. The partitioning of the
bulbus arteriosus may occur in such a manner as to assign to the two
aortae a relative position, the reverse of that which they normally
occupy--the pulmonary aorta springing from the left ventricle and the
systemic aorta arising from the right, and giving off from its arch the
primary branches in the usual order. [Footnote 1] 4th. As the two aortae
result from a d