Modifications among the vertebrate classes. Mammals
Mammals also evolved from reptiles, but not from the
same group as did birds, and must have developed their
double circulation independently from early reptiles.
Nevertheless, several parallel changes occurred, such
as the common incorporation of the sinus venosus into
the right auricle. The most striking manifestation of
different origins is seen in the mammalian aorta, which
leaves the left ventricle and curves to the left. The
aorta corresponds to the left half of the systemic arch,
while the right is missing. The carotid arteries arise
from the left systemic arch (aorta), though their precise
position varies among mammals. The arterial system is
asymmetric, as in birds, but in the opposite way.
The heart of both mammals and birds is a double pump,
powering two systems of vessels with different characteristics.
The left ventricle has a thicker layer of muscle around
it, a necessary adaptation for powering its beat against
the high resistance of the extensive systemic circulation
throughout the body. The right ventricle has a thinner
wall, consistent with its role in pumping blood to the
lungs against a much lower resistance.
