In Vertebrates the organs of circulation are the heart, arteries, veins. and lymphatics. The heart is the ventral organ of the circulatory system. To it the blood is drawn by suction and i, again forced out from it. The heart, as well as the rest of the vascular system. is of mesoblastie origin and appears dose behind the gill-clefts. Like the other large blood-vessels, it is eomposed of layer-, the outer (serous), the middle tmuscularl, and the inner (epithelium). Hence in origin the heart is a strongly developed blood-vessel. Its eomplicated structure, such as occurs in the higher vertebrates, is the result of folding and swelling,. The heart in the lowest of the vertebrates and in embryonic life is at first divided into two chambers with valves be tween them to prevent the blood from flowing backward again. One chamber, the auricle, re ceives the blood. The other, the ventricle, expels it to the body again. By further moditieation the sinus venosus develops at the venous end and the Iodine, arteriosus at the arterial end. Such a condition of heart occurs at some time in the embryonic development of all vertebrates, and it is the permanent condition in many fishes. With the development of lungs, and consequently of the pulmonary artery and veins, the primitive heart is much iltanged. This change from gilled to lung-bearing condition may be studied in the development of all lung-bearing verte brates. The heart of fishes is simple. and, in structure. like the primitive heart described. It receives venous blood only, which it forces to the In Dipnoi, where both gills and lungs exist side by side, the heart is half way between that of fishes and amphibia. In amphibia the ventricle is single, and hence the blood in it is mixed. In many of the amphibia the ventricle is eontinued into a emus arteriosns, which is spirally twisted and contains a transverse row of valves. Among reptiles an incomplete yen trieular septum exists in lizards, snakes, and turtles. but there is a complete one in croco diles. The blood from the right ventricle passes into the pulmonary artery and into the left aortic arch. There are fewer valves in the heart and only one row at the beginning of the aorta and pulmonary artery. In birds and mammals there are t auricles and ventricles—that is to say, auricular and ventricular septa are com plete. The ventricles are the larger, and have more strongly developed walls. The blood from the head. heart, and body passes into the right
auricle. In birds and mammals there is only one aortic arch. In birds it is the fourth right arch which persists, and in mammal, the fourth left. III the embryological development of birds and mammals the auricles are, for a titne, in eommunicat ion through the foramen ovale. 3Ineh variation exists among mammals in the mode of origin of the carotids and solid:id:1ns from the arch of the aorta. In hranchial verte brates the dorsal aorta is formed by the union above the gills of the brambial vessels. The allantois vein, which plays so important a part in reptiles and amphibia, is functional in birds and mammals only for a time in embryonic life.
and from birds onward the hepatic-portal system supplants the renal-portal.
The veins of vertebrates are provided with valves to prevent a back flowing of the blood.
The lymphatic system of elasmobranchs, am phibia, and, to some extent, of birds and rep tiles, is provided with lymph-hearts. In fishes and amphibia there are large lymph-spaves. but from birds onward lymph-vessels with well-de fined trunks are present. The main lymph vessel is the thoracic duct which empties in mammals into the left and in the Sauropsida into both the right and left btanehio-cephalie vein, The lymph-vessels like the veins. are provided with valves which prevent a reflux of the lymph fluid. The lymph, like the blood, is composed of fluid and corpuscles. Lymphatic tissue oc curs in fishes and amphibia, but lymph-glanils proper appear along the course of the lymph vessels, probably first in birds. in lymphatic tissue or glands the leueocytes, or white cor puscles of the blood, and lymph develop.
Bietioenoeire. Sabatier, L'ttnks tar le recur dans la sc'rie des rerfrbr('s (Paris, 1873) ; Mena gaux. Recherches tar la circulation dcs bra aches marines (Besancon, 1;00 Bojanus, "Leber die -them- und Kreislaufwerkzeuge der zweischaligen Muscheln," in Isis (1s17, Isse2.0 1S27) ; Deluge. "Contribution ViAnde de Pap pareil eireulatoire des Mri.ophthalmes marines" in Archire de zoologic expf"rimentalc, Vol. IX. (Paris. 1S.Si1); Cmmot, "Etude sur le sang it les glandes lymphatiques clans la st;rie ani mal e." Archire de zoologic exprimentale (Paris, ISSS-911 ; "Ceber das Celiisssys tem der Fische," in Abhundlangcn der Berliner Al ademie (Berlin, 1839). ;See ANATOMY, COM PARATIVE; and consult the authorities referred to there.