left auricle receives the arterial blood from the pulmonary veins. The
two auricles have a common opening into the simple ventricle, where
the two kinds of blood mix, and are driven through the arterial cone
or bulb into the arterial arches. From the last arterial arches the
pulmonary arteries arise (Figure 2.365 p). These force a part of the
mixed blood into the lungs, the other part of it going through the
aorta into the body.

From the Dipneusts upwards we now trace a progressive development of
the vascular system, which ends finally with the loss of branchial
respiration and a complete separation of the two halves of the
circulation. In the Amphibia the partition between the two auricles is
complete. In their earlier stages, as tadpoles (Figure 2.262), they
have still the branchial respiration and the circulation of the
fishes, and their heart contains venous blood alone. Afterwards the
lungs and pulmonary vessels are developed, and henceforth the
ventricle of the heart contains mixed blood. In the reptiles the
ventricle and its arterial cone begin to divide into two halves by a
longitudinal partition, and this partition becomes complete in the
higher reptiles and birds on the one hand, and the stem-forms of the
mammals on the other. Henceforth, the right half of the heart contains
only venous, and the left half only arterial, blood, as we find in all
birds and mammals. The right auricle receives its carbonised or venous
blood from the veins of the body, and the right ventricle drives it
through the pulmonary arteries into the lungs. From here the blood
returns, as oxydised or arterial blood, through the pulmonary veins to
the left auricle, and is forced by the left ventricle into the
arteries of the body. Between the pulmonary arteries and veins is the
capillary system of the small or pulmonary circulation. Between the
body-arteries and veins is the capillary system of the large or