ANATOMY AND PHYSIOLOGY OF THE CIRCULATORY APPARATUS A. The Heart.—The primary cardiac tube, Will Cil lies alone in the cervical region, behind the last visceral arch, possesses two ends, a pos terior for the reception of venous blood, and an anterior end which serves to supply the blood to the body. In amniotic vertebrates, the heart, which consists of two ventricles and two auricles, arises gradu ally by constrictions and twists of this tube in many places, as well as by the formation of internal septa. The cardiac tube, originally straight, becomes an S-shaped loop, differentiated into three parts: the upper or aortic portion with the bulbus, the lower venous portion, and the middle or ventricular portion. This central part soon bends upon itself, into the shape of the stomach, so that what was originally the posterior, venous portion of the heart is directed more dorsally and the arterial portion more ventrally, By- the lateral outgrowth of bEnd sacs from the venous portion, to both right and left, the beginnings of the auricles develop, while that part of the central portion corresponding to the greater curvature is divided in its centre by a shallow groove, the sulcus interventricularis.
The earliest processes of ventricular division appear between the fourth and fifth foffal weeks, when a perpendicular septum grows up inside, from the groove just mentioned, while transverse constriction, canalis auricularis or auricular canal, is formed between the atrium and the ventricular portion. The atrioventricular valves develop within the auricular canal. By the eighth week this upward growing inter ventricular septum has reached the auricular canal, so that one common atrimn and two ventricles are formed at this time. The single atrium freely communicates with each ventricle through the right and left atrioventricular ostia. Only then does the truncus arteriosus communis divide into aorta and pulmonary artery, by the development of two winglike septa which gradually grow together to form the septum trunci, dividing the originally single tube into two tubes, lying parallel to one another like the barrels of a double-barreled gun. Now, as the septum trunci is growing downward in the same direction in which the interventricular septum grows upward, it penetrates the ventricular cavity until both septa meet, with the final result that the aorta com municates directly with the left ventricle and the pulmonary artery with the right ventricle. The separation of the single truncus arteriosus, however, first occurs in its lowest part, while further upward pulmonary artery and aorta still remain united, forming later the ductus Botalli.
The development of the auricular septum forms the last stage in the partition of the cardiac cavities. A septum grows from above and behind, ending within the cavity in a concave border. To the right of this fold opens the superior vena cava, while the inferior vena cava enters the auricle just beyond the edge of this fold. In its centre re mains an opening, the foramen ovate. Inside arise the valve of the foramen ovate and the Eustachian valve which direct the fietal blood stream into the auricles (see Fmtal Circulation, page 417).
The valves on the venous ostia (atrioventricular valves) grow from tumor-like thickenings of the ostial edge and connect the ventricles by bundles of muscle which are grouped with the subsequent papillary muscles (Bernays). According to Rose, this is true only of the median valves, while the lateral valves arise directly from the ventricle wall by the separation of true muscle which is transformed into connective tissue later.
The valves upon the arterial ostia develop from peculiar cushions of endothelium, are at first coarse and thick and later become delicate, acquiring tliin walls.
B. The Arteric,.—The aortic arches, originally but. a single one. become multiplied into five for each side with the development of the five visceral arches, and join to form a common trunk directed down ward (Ratlike). In human beings the two upper visceral arches on each side atrophy first. With the division of the truncus arteriosus communis into aorta and pulmonary artery the lowest arch on the right side becomes the pulmonary artery, and therefore arises from the right side of the heart, while the arch on the left becomes transformed partly into the ductus Botalli and partly into the main branches of the pul monary artery. The middle arch on the left side becomes the perma nent aortic arch, into which the ductus Botalli empties; the middle arch on the right becomes the right subclavian artery. The upper most arch on each side becomes the origin of the carotid artery.
C. The Veikie.—It should be noted, in regard to the venous sys tem, that four cardinal veins are formed, an anterior and a posterior for each side, and that, before emptying into the heart, the two on each side unite to form a common trunk, the duct of Cuvier. The large veins of the body, the superior and inferior venat", arise by the formation of numerous anastomoses and divisions of these two sets of cardinal veins and empty into the right ventricle, as will be shown later.