omes 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. The interauricular and interventricular septa, by
gradual development from without inwards, at length meet and coalesce,
thereby dividing the two cavities into four--two auricles and two
ventricles--a condition proper to the Avian and Mammalian classes
generally. In the centre of the interauricular septum of the human
heart, an aperture (foramen ovale) is left as being necessary to the
foetal circulation. While the septa are being completed, the arterial
bulb also becomes divided by a partition formed in its interior in such
a manner as to adjust the two resulting arteries, the one in connexion
with the right, the other with the left ventricle. The right ventricular
artery (pulmonary aorta) so formed, has assigned to it the fifth
(posterior) opposite pair of arches, and of these the right one
remaining pervious to the point where it gives off the right pulmonary
branch, becomes obliterated beyond this point to that where it joins the
descending aorta, while the left arch remains pervious during foetal
life, as the ductus arteriosus still communicating with the descending
aorta, and giving off at its middle the left pulmonary branch. The left
ventricular artery (systemic aorta) is formed of the fourth arch of the
left side, while the opposite arch (fourth right) is altogether
obliterated. The third and second arches remain pervious on both sides,
afterwards to become the right and left brachio-cephalic arteries. The
first pair of arches, if not converted into the vertebral arteries, or
the thyroid axes, are altogether metamorphosed. By these changes the
heart and primary arteries assume the character in which they usually
present themselves at birth, and in all probability the primary