The heart is formed by the blending in the median fine of two longitudinal endothelial tubes lying ventral to the fore-gut of the early embryo. Each tube is partially surrounded laterally by the splanchnic mesoderm which forms a septum between the right and left sides of the coelomic cavity. The blended endothelial tubes form the endocardium. The splanchnic mesoderm in relation to the endocardium becomes the myoepicardium, and the double layer connecting the heart dorsally and ventrally with the somatic mesoderm becomes the (temporary) dorsal and ventral mesocardia. The somatic mesoderm of the heart region becomes the pericardium.
The originally straight heart-tube grows rapidly and becomes tortuous on account of its increasing length between the limits assigned by its fixed arterial and venous ends. Its arterial end is continued into the truncus arteriosus, which is later divided into the pulmonary artery and the ascending aorta. Its venous end receives the viteline and umbilical veins, and, later on, the common cardinals also. By the formation of a series of alternate bulgings and constrictions the heart becomes differentiated into the sinus venosus, atrium, ventricle and conus arteriosus, counting from the venous to the arterial end. These parts, after going through a process of progressive differentiation and shifting take up relative positions somewhat approaching those of the adult.
The sinus venosus lies on the dorsal wall of the atrium, and is composed of right and left horns united by a transverse portion. The sinus is separated from the atrium by a sagitally directed slit-like opening, guarded by right and left lateral valves which project into the atrium. The atrium is wide, being prolonged into a ventrally projecting pouch on either side, the future right and left auricles. The ventricle is situated caudal and somewhat ventral to the atrium. The right limb of the common ventricle, which leads into the conus arteriosus, is the future right ventricle; the left limb, connected with the atrium, is the future left ventricle. The communication between the atrium and the ventricle, known as the atrial canal, is indicated on the exterior by a constriction; its interior consists of a transversely placed slit. The conus arteriosus is continued from the ventricle without obvious constriction and passes over into the truncus arteriosus.
The sinus venosus early loses its bilateral symmetry owing to the rapid enlargement of the right horn. This horn soon receives, through the proximal portion of the right vitelline vein {inferior vena cava), all the blood coming from the left vitelline and both umbilical veins. The right common cardinal also gains ascendency over the left and becomes the superior vena cava.
The left horn and transverse part, now only draining the dwindling left common cardinal, (left superior cava) and the coronary veins, become the coronary sinus. The right horn gradually becomes absorbed into the right end of the atrial cavity until the superior and inferior cava; and the coronary sinus acquire separate openings into that chamber. Between the opening of the coronary sinus and that of the inferior cava there is a ridge, the sinus-septum (between the right horn and transverse parts of the sinus), which becomes attached to the lower part of the right sinus valve.
In the atrium a septum begins early to grow from the ventro-cephalic wall of the atrium, toward the atrial canal. As the interatrial communication around the edge of the septum (ostium primum) is becoming narrow, a perforation occurs near the attached margin of the septum (ostium secundum). This first septum (septum primum) is incomplete because when its edge reaches the atrial canal the atria still communicate through ostium secundum. To the right of the septum primum another septum (s. secundum) is formed later; this never stretches completely across the atrium and is rather a crescentic ridge than a true septum. Until the free edges of the two septa overlap one another there is a direct passage leading from one side of the atrium to the other; eventually they do overlap and the communication becomes oblique but persists until birth. The cavities resulting from the division of the common atrium are the right and left atria of the adult. The oblique channel connecting the atria (foramen ovale) is bounded on the right side by the s. secundum the free edge of which forms the limbus fossae ovalis. The channel is bounded on the left by the s. primum which slants into the left atrium. The free edge of the s. primum becomes the valvula foraminis ovalis; the remainder, the membranous atrial septum of the adult.'
The portion of the dorsal wall of the right atrium immediately adjoining the septa is derived from the sinus venosus. This part of the atrium (the sinus venarum) receives the great venous openings. The left side of the left sinus-valve is attached to both septa and assists the septum secundum in the formation of the limbus foraminis ovalis. The cephalic part of the right sinus- valve disappears along the line of the (adult) crista terminalis, which therefore limits the right portion of the right atrium derived from the sinus venosus. The caudal portion of this valve persists as the inferior caval and coronary valves. These are drawn out of their original alignment by the adhesion between the caudal part of the right sinus-valve and the sinus-septum.
The left atrium receives, through the dorsal mesocardium, the originally single pulmonary vein. This common stem is absorbed into the atrial wall; later, the primitive right and left tributaries are absorbed in a similar way, leaving the four pulmonary veins of the adult opening separately into the left atrium. The area of the left atrium adjacent to the pulmonary veins, therefore, is not part of the original atrial wall.
The ventricles are divided by a septum (s. musculare ventriculorum) growing from the caudal wall of the common ventricular cavity toward the atrial canal. The canal moves to the right, and the dorsal part of the septum blends with the dorsal lip of the canal. The free ventral edge of the interventricular septum helps to bound the foramen through which blood from the left ventricle must enter the right on its way to the conus arteriosus. The foramen persists until (the free margin of the interventricular septum having been joined by the aortic septum) it becomes the circumference of the aortic ostium.
The aortic septum is a composite structure formed partly by a septum growing between tlie fourth and sixth pairs of aortic arches, and partly by swellings growing in the interior of the conus and truncus arteriosus. When fully formed it extends spirally along the truncus and conus, and enters the right half of the common ventricular cavity, where it joins the right side of the free edge of the interventricular septum. The septum is arranged in such a way that the blood from the left ventricle passes no longer through the right ventricle but along its own channel {the aorta) through the conus and truncus to the first four pairs of aortic arches. The blood from the right ventricle passes through the pulmonary division of the conus and truncus arterio- sus, anterior and to the left of the aorta, into the sixth arches. Further differentiation brings about the external separation of the aorta from the pulmonary artery, but their common covering of epicardium persists as such in the adult. The lower end of the aortic septum persists in the adult as the septum membranaceum ventriculorum and the crista supraventricularis. During the formation of the aortic septum four endocardial swellings appear at the distal part of the interior of the conus. These are arranged as smaller and larger opposite pairs; the smaller and larger swellings, therefore, alternating around the lumen. The larger pair of swellings assists (by partial blending) in the formation of the aortic septum. When the septum is complete, half of each of the larger swellings is contained in the aorta and half of each in the pulmonary artery. One of the smaller swellings remains in the aorta and one in the pulmonary artery, so that there are now three swellings in each vessel. Each of the six swellings becomes undermined to form a semilunar valve of the adult.
The atrioventricular valves. The interior of the ventricular cavity, which is at first smooth, becomes undermined in an irregular way, to form a system of myocardial trabeculae. The Ups of the transversely directed atrial canal become thickened into prominent anterior and posterior endocardial cushions; these project into the ventricular cavity and become involved in its myocardial trabecular system. The atrial canal, which has now moved to the right, be- comes divided sagittally, into right and left venous osiia, by the septum primum. The inter- ventricular septum joins the ventricular side of the posterior endocardial cushion. The anterior and posterior endocardial cushions, where they blend with one another and with the septum primum on the medial side of each venous ostium, form an atrioventricular valve-cusp on either side, viz., the anterior cusp of the mitral in the left ostium, and the medial cusp of the tricuspid in the right. The posterior cusp of the mitral and the anterior and posterior of the tricuspid are formed later, partly, by lateral tubercles developing in either ostium, and partly by undermining of the ostia from the ventricular side. The atrial musculature extends into the atrioventricular valves and, until a late stage, is continuous with the trabecular system of the ventricles. Gradually, however, this connection between atrial and ventricular musculature is lost, leaving only the chordae tendinese connecting the papillary muscles with the valves Muscle is found at the basal region of the valve-cusps in the adult, and occasionally persists in the chordae tendinese.
The connection between the atrial and ventricular musculature is not confined to that occurring by means of the valves and trabecular system. The original myocardial connection between the atrial and ventricular portions of the heart remains complete until the embryo has reached the length of about 11 mm. From that time on the epicardium begins to blend with the fibrous annuli of the venous ostia. Meanwhile the atrial musculature rapidly loses its connection with that of the ventricles until they are connected in one place only, i. e., the site of the atrioventricular bundle.
The pericardial cavity is the original cephalic end of the intraembryonic coelom. The somatic mesoderm of the pericardial region forms the adult pericardium. The splanchnic mesoderm persists only in the part which furnishes the myo-epicardium. The ventral and dorsal mesocardia, both of which are formed by the splanchnic mesoderm, are, in the main, transitory. The early disappearance of the ventral mesocardium unites the right and left sides of the pericardial coelom ventral to the heart. The dorsal mesocardium persists at the arterial and venous ends of the heart only. The loss of the dorsal mesocardium between the latter points gives rise to the sinus transversus pericardii of the adult.
During development, the heart and pericardium migrate from a point opposite the cephalic end of the pharynx to one opposite the caudal end of the esophagus; in fact, from the neck well into the thorax. In the adult, instead of being at the cephalic end of the coelom, the heart and pericardium are contained between the right and left layers of the ventral mesentery of the esophagus; the pericardial pleura of the adult.
The cranio-caudal migration is evidenced in the adult by the course of the recurrent and of the cardiac nerves, and also by the apparent migration of the vessels derived from some of the dorsal segmental arteries.
