nomena resulting in
both reciprocate and balance each other. In the pulmonary circulation,
the blood is aerated, decarbonized, and otherwise depurated; whilst in
the systemic circulation, it is carbonized and otherwise deteriorated.

VI.--The circulation through the lungs and the system is carried on
through vessels having the following form and relative position, which,
as being most usual, is accounted normal. The two brachio-cephalic veins
joining at the root of the neck, and the two common iliac veins joining
in front of the lumbar vertebrae, form the superior and inferior venae
cavae, by which the blood is returned from the upper and lower parts of
the body to the right auricle, and thence it enters the right ventricle,
by which it is impelled through the pulmonary artery into the two lungs;
and from these it is returned (aerated) by the pulmonary veins to the
left auricle, which passes it into the left ventricle, and by this it is
impelled through the systemic aorta, which branches throughout the body
in a similar way to the systemic veins, with which the aortic branches
anastomose generally. On viewing together the system of vessels proper
to each heart, they will be seen to exhibit in respect to the body a
figure in doubly symmetrical arrangement, of which the united hearts
form a duplex centre. At this centre, which is the theatre of
metamorphosis, the principal abnormal conditions of the bloodvessels
appear; and in order to find the signification of these, we must retrace
the stages of development.

VII.--From the first appearance of an individualized centre in the
vascular area of the human embryo, that centre (punctum saliens) and the
vessels immediately connected with it, undergo a phaseal metamorphosis,
till such time after birth as they assume their permanent character. In
each stage of metamorphosis, the embryo heart and vessels typify the
normal condition of the organ in one of the lower classes of animals.
The several species of the organ in these classes are parallel to the
various stages of change in the human organ. In its earliest condition,
the human heart presents the form of a simple canal, similar to that of
the lower Invertebrata, the veins being connected with its posterior
end, while from its anterior end a single artery emanates. The canal
next assumes a bent shape, and the vessels of both its ends become
thereby approximated. The canal now being folded upon itself in
heart-shape, next bec