Chapter CXXI: Section 5: The circulation presents, in many respects, an

approximation to the state of affairs in the developing embryos of the higher types. The heart (Figure 3, Sheet -14- {Error in First Edition} [16]) is roughly, Z shape, and transmits only venous blood. It lies in a cavity, the pericardial cavity (P.c.c.), cut off by a partition from the general coelome. At one point this partition is imperfect, and the two spaces communicate through a pericardio-peritoneal canal (p.p.c.), which is also indicated by an arrow (p.p.) in the position and direction in which the student, when dissecting, should thrust his "seeker," in Figure 1 Sheet 15. A sinus venosus (s.v. in Figure 3, Sheet 16) receives the venous trunks, and carries the blood through a valve into the baggy and transversely extended -auricle- [atrium] (au.), whence it passes into the muscular ventricle (Vn.), and thence into the truncus arteriosus. This truncus consists of two parts: the first, the conus or pylangium (c.a.), muscular, contractile, and containing a series of valves; the second, the bulbus or synangium (b.a.), without valves and pulsatile. In the rabbit both sinus and truncus are absent, or merged in the adjacent parts of the heart.

Section 6. From the bulbus there branch, on either side, four arterial trunks, the first of which forks, so that altogether there are five afferent branchials (a.br.) taking blood to be aerated in the gills, here highly vascular filamentary outgrowths of the internal walls of the gill slits.

{Lines from Second Edition only.}
[There are altogether nine vascular outgrowths (demi-branchs), one
on each wall of each gill slit except the last, on the hind wall of
which there is none. (In the spiracle is a miniature demibranch, the
pseudo-branch. This suggests that the spiracle is really a somewhat
modified gill slit.)]

Four efferent branchials (e.br.) carry the aerated blood on to the dorsal aorta (d.ao.). A carotid artery runs forward to the head, and a hypo-branchial artery supplies the ventral side of the pharyngeal region. There are sub-clavian, coeliac, mesenteric, and pelvic arteries, and the dorsal aorta is continued through the length of the tail as the caudal artery (Cd.A.).

Section 7. A caudal vein (Cd.V.), bringing blood back from the tail, splits behind the kidneys (K.), and forms the paired renal portal veins (r.p.v.), breaking up into a capillary system in the renal organ. A portal vein brings blood from the intestines to the liver.