The heart consists of at least three chambers, a sinus venosus which receives the venous blood from the body, an auricle and a ventricle, to which is added a conus arteriosus in the Elasmobranchs, certain Teleostomi (Crossopterygii, Chondrostei, and Holostei), and in the Dipnoi. Through these cardiac chambers the blood is forced in the order mentioned. In the Dipnoi the auricle is subdivided by a more or less complete interauricular septum into a right and left auricle,[[373]] the former receiving the venous blood from the sinus venosus, and the latter the aerated blood from the lung-like air-bladder.
The sinus venosus and the auricle have very thin walls; the ventricular walls, on the contrary, are very thick and in great measure are composed of a sponge-like network of muscular bundles which generally encroaches considerably on the ventricular cavity. Membranous valves, the sinu-auricular, and the auriculo-ventricular valves, are developed at the junctions of the sinus venosus with the auricle, and the auricle with the ventricle respectively. The conus arteriosus is muscular and contractile, and is interposed between the ventricle and the root of the ventral aorta. Internally, the conus is provided with several transverse rows of pocket-shaped or semilunar valves. In Teleosts the conus is non-muscular and vestigial, and has but a single row of valves, corresponding to the most anterior of the multiple rows of valves in the Elasmobranchs. In these Fishes the vestigial conus is succeeded by a non-contractile, bulb-like dilatation, or bulbus aortae, of the root of the ventral aorta. In only a single Teleost, viz. Albula, one of the Albulidae, is the vestigial conus muscular, and at the same time provided with two rows of valves.[[374]] In the Cyclostomata there is a bulbus with a single row of two valves, but no true conus.
In the Dipnoi (e.g. Protopterus) the heart, like the rest of the vascular system, exhibits certain interesting resemblances to the Amphibian heart. In addition to a more or less complete interauricular septum separating right and left auricles, there is a median longitudinal ridge, partly muscular and partly fibrous, which incompletely subdivides the cavity of the ventricle. The spirally-twisted conus arteriosus is furnished with several transverse rows of valves, certain of which coalesce longitudinally to form a complete septum dividing the cavity of the conus into two distinct lateral channels: with this septum there coalesces another septum, which cuts off the origins of the anterior two pairs from the remaining afferent branchial arteries. The formation of these septa has the physiological effect of subdividing the series of cardiac cavities into two parallel channels, of which one has its origin behind in the sinus venosus and transmits venous blood to the posterior afferent branchial vessels; while the other, commencing with the left auricle, conveys arterial blood to the first two pairs of afferent branchial arteries.[[375]] In Neoceratodus, however, the longitudinal septum in the conus is incomplete, and hence the blood which is sent to the anterior afferent vessels is mixed.[[376]]
The Arterial System.—The ventral aorta is a median artery situated beneath the floor of the pharynx, and having its origin, behind, either directly from the ventricle or from the conus arteriosus.
In the Cyclostomata[[377]] (e.g. Petromyzon) the ventral aorta (Fig. 194) is continued forwards from the heart as a single vessel to the fourth pair of gill-sacs, where it divides into right and left branches which extend as far as the anterior walls of the first pair of gill-sacs. Eight pairs of afferent branchial arteries arise from the ventral aorta and its two branches, of which the first and last supply the anterior walls of the first pair of sacs and the posterior walls of the last pair respectively. Each of the remaining afferent vessels extends into an interbranchial septum, and supplies the gill-lamellae of the posterior wall of one sac and those of the anterior wall of the next sac behind. The corresponding efferent branchial vessels have a similar distribution, and unite dorsally to form a median dorsal aorta. Beneath the base of the skull the latter vessel divides into two branches which, after receiving the first pair of efferent branchial vessels, pursue a divergent course forwards, but subsequently converge and unite to form a "circulus cephalicus," as in Teleosts. From the cephalic circle are given off on each side (1) an "internal carotid" artery for the brain and eye; (2) an "external carotid" for the lateral and ventral walls of the head; and (3) a large ventral branch which supplies the lingual apparatus; while from the abdominal portion of the dorsal aorta are derived, first, a coeliaco-mesenteric artery for the liver and alimentary canal, and subsequently branches for the myotomes, kidneys, and the gonad. The terminal portion of the aorta then enters the tail and forms the caudal artery.
Fig. 194.—Branchial arterial system of the Lamprey (Petromyzon fluviatilis). The ventral aorta and the afferent branchial arteries are seen on the left side, and the efferent branchial and dorsal aorta on the right (diagrammatic). af.b.a, ef.b.a, Afferent and efferent branchial arteries; a.o, auditory organ; b.c, branchial canal; c.c, cephalic circle; cm.a, coeliaco-mesenteric artery; d.a, dorsal aorta; e, eye; ex.c, "external carotid"; h, heart; i.b.s, interbranchial septum; in.c, internal carotid; oph.a, ophthalmic artery; r.v.a, l.v.a, right and left ventral aorta; v.a, median ventral aorta; v.c, "ventral carotid"; 1-7, gill-sacs. (Modified from Vogt and Yung.)
In Elasmobranchs[[378]] (e.g. Mustelus antarcticus) the undivided ventral aorta gives off five pairs of afferent branchial arteries which, on each side, ascend in succession the outer convex sides of the hyoid and first four branchial arches (Fig. 195).
Fig. 195.—The branchial arterial system of Mustelus antarcticus. Left lateral view. The ventral aorta and afferent branchial vessels are in solid black, the efferent arteries and their branches have double contours. The branchial clefts have fringed borders to indicate their hemibranchs, and the arches are in simple outline. a.c.a, Anterior carotid; a.d.a, anterior dorsal aorta; af.b.a, afferent branchial artery; br.a, brachial artery; c.m.a, coeliaco-mesenteric; d.a, dorsal aorta; E, eye; ep.a, epibranchial artery; H, heart; h.b.a, hypobranchial artery; hy.a, afferent pseudobranchial or hyoidean artery; md.a, mandibular artery; op.a, ophthalmic artery; p.c.a, posterior carotid; sb.a, subclavian; sp, spiracle; v.a, ventral aorta; 1-5, the hyobranchial and four succeeding branchial clefts. The hypobranchial artery is seen immediately beneath the ventral aorta. (After T. Jeffery Parker, diagrammatic.)