In the Selachii an arterial cone is developed from the foremost end of
the ventricle, as a special division, cut off by valves. It passes
into the enlarged base of the trunk of the branchial artery (Figure
2.363 abr). On each side 5 to 7 arteries proceed from it. These rise
between the gill-clefts (s) on the gill-arches, surround the gullet,
and unite above into a common trunk-aorta, the continuation of which
over the gut corresponds to the dorsal vessel of the worms. As the
curved arteries on the gill-arches spread into a network of
respiratory capillaries, they contain venous blood in their lower part
(as arches of the branchial artery) and arterial blood in the upper
part (as arches of the aorta). The junctures of the various aortic
arches on the right and left are called the roots of the aorta. Of an
originally large number of aortic arches there remain at first six,
then (owing to degeneration of the fifth arch) only five, pairs; and
from these five pairs (Figure 2.364) the chief parts of the arterial
system develop in all the higher Vertebrates.

(FIGURE 2.371. Heart of a rabbit-embryo, from behind, a vitelline
veins, b auricles of the heart, c atrium, d ventricle, e arterial
bulb, f base of the three pairs of arterial arches. (From Bischoff.)

FIGURE 2.372. Heart of the same embryo (Figure 2.371), from the front.
v vitelline veins, a auricle, ca auricular canal, l left ventricle, r
right ventricle, ta arterial bulb. (From Bischoff.))

The appearance of the lungs and the atmospheric respiration connected
therewith, which we first meet in the Dipneusts, is the next important
step in vascular evolution. In the Dipneusts the auricle of the heart
is divided by an incomplete partition into two halves. Only the right
auricle now receives the venous blood from the veins of the body. The