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. 



■MESONEPHRIC VESSELS 



ALLANTOIC 



VASCULAR 



PLEXUS 



YOLK SAC '^---.:r:rr^::i—'' ^allantois 



Fig. 309. — The 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 life, the septa which divide the heart into right 

 and left halves are perforated to permit blood from the right side to pass 

 through foramina into the left side, and thus to make possible a systemic 

 circulation even when the pulmonary circulation is small. In the atrium, 

 two longitudinal septa arise, the one first formed (Septum I) being on 

 the left. Since relatively little blood comes from the lungs, the blood 



