Chapter CVI: Section 8: It might be inferred from this that pure and impure blood

mix in the ventricle, and that a blood of uniform quality flows to lungs, head, and extremities; but this is not so. The spongy nature of the ventricle sufficiently retards this mixing. It will be noted that the opening of pulmonary arteries lies nearest to the heart, next come the aortic and carotid arches, which have a common opening at A. Furthermore, at c.g.l. [the carotid artery, repeatedly divides to form a close meshwork of arterioles, the carotid gland, forming a sponge-like plug in this vessel.] is a spongy mass of matter, the carotid gland inserted upon the carotid. Hence the pulmonary arteries yawn nearest for the blood, and, being short, wide vessels, present the least resistance to the first rush of blood-- mainly venous blood for the right auricle. As they fill up, the back resistance in them becomes equal, and then greater, than the resistance at A, and the rush of blood, now of a mixed quality passes through that aperture. It selects the dorsal aorta, because the carotid arch, plugged by the carotid gland, offers the greater resistance. Presently, however, the back resistance of the filled dorsal aorta rises above this, and the last flow of blood, from the ventricular systole-- almost purely oxygenated blood for the left auricle-- goes on towards the head.

Section 9. At the carotid gland the carotid artery splits into -an- [a] -external carotid- [lingual] (e.c.), and a deeper internal carotid. The dorsal aorta passes round on each side of the oesophagus, as indicated by the dotted lines in Figure 2, Sheet 11, and meets its fellow dorsal to the liver. Each arch gives off subclavian arteries to the limbs, and the left, immediately before meeting the right, gives off the coeliaco-mesenteric artery [to the alimentary canal]. This origin of the coeliaco-mesenteric artery a little to the left, is the only asymmetry (want of balance) in the arterial system of the frog, as contrasted with the very extensive asymmetry of the great vessels near the heart of the rabbit. [Posteriorly the dorsal aorta forks into two common iliac arteries (right and left) supplying the hind limbs.]