. Comparative anatomy. Anatomy, Comparative. THE VASCULAR SYSTEM 371 Soon after the two halves of the heart are united in the mid-ventral line, the heart itself becomes S-shaped as a consequence of its elongation in a confined space. The dorsal curve is posterior and connects directly with the paired vitelline (omphalomesenteric) and umbilical veins. The ventral curve is anterior and extends forward beneath the pharynx as the truncus arteriosus. Circulation has already begun when the heart is in this tubular condition. By the time the embryo is two months old, the heart, although its size is m
. Comparative anatomy. Anatomy, Comparative. THE VASCULAR SYSTEM 371 Soon after the two halves of the heart are united in the mid-ventral line, the heart itself becomes S-shaped as a consequence of its elongation in a confined space. The dorsal curve is posterior and connects directly with the paired vitelline (omphalomesenteric) and umbilical veins. The ventral curve is anterior and extends forward beneath the pharynx as the truncus arteriosus. Circulation has already begun when the heart is in this tubular condition. By the time the embryo is two months old, the heart, although its size is minute, has reached its adult form and structure. (Fig. 308) The processes involved in converting a tubular heart into a four- chambered one include: i. The increased flexion of the heart so that the PRECARDINAL VEIN AORTIC ARCHII AORTIC ARCHlv,^^^ \,,ii-'rT7T7-*r-^ ,SINO-ATRIAL REGION OF HEART ^POSTCARDINAL VEIN 'INTERSEGMENTAL ART. â MESONEPHRIC VESSELS. ALLANTOIC VASCULAR PLEXUS YOLK SAC'^---.:r:rr^::iâ'' ^allantois Fig. 309.âThe circulatory system of a young swine embryo. The arteries are shown in black, veins are stippled. All blood vessels at this stage are paired, but those of the left side only are shown in the figure. (Redrawn after Patten.) posterior atrial portion becomes anterior, while the morphologically anterior ventricular portion lies posteriorly. 2. The formation of a longitudinal septum which divides the heart into right and left chambers. 3. The relative hypertrophy of the two atria, that of the right side enlarg- ing the more rapidly. 4. The separation of atria and ventricles by the growth of the atrio-ventricular valves. 5. The inclusion of the posterior division of the heart, the sinus venosus, within the right atrium. 6. The division of the anterior portion of the heart, the conus, into aorta and pulmonary artery. The changes thus briefly summarized are best understood from diagrams of the successive stages, (Figures 298, 308.) During intra-uterine lif
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