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 becomes constricted in situations, marking out the future auricle and ventricle and arterial bulb, which still communicate with each other. From the artery are given off on either side symmetrically five branches (branchial arches), which arch laterally from before, outwards and backwards, and unite in front of the vertebrae, forming the future descending aorta. In this condition, the human heart and vessels resemble the Piscean pipe. The next changes which take place consist in the gradual subdivision, by means of septa, of the auricle and ventricle respectively into two cavities. On the separation of the single auricle into two, while the ventricle as yet remains single, the heart presents that condition which is proper to the Reptilian class.