[Origin of the common pulmonary vein, septation of the primary sinus venosus atrial situs and theory of the "sinus man"].

Situated at the entry to the heart, the sinus venosus regulates at an early stage the distribution of the veins. Originally symmetrical, it receives on either side an omphalomesenteric vein, a common cardinal vein (duct of Cuvieri, ductus cuvieri) and a common pulmonary vein. This symmetrical pattern disappears with the obliteration of the rough right pulmonary vein and the invagination of the left ductus cuvieri into the sinusal cavity. Thus, the pulmonary venous blood is kept on the left side and the systemic venous blood is transferred to the right side. This is the usual situs solitus arrangement. Situs inversus is the opposite arrangement. In situs ambiguus the original symmetry is preserved. A sufficiently early cauterization of the left wall of the sinus venosus prevents the left ductus cuvieri from invaginating and results in "absence of coronary sinus"; this arrangement, where part of the original symmetry is preserved, is in fact similar to situs ambiguus. The situs of the liver and stomach is thought not to be determined by these organs but imposed to them by the sinus venosus, more precisely by the invagination--or lack of invagination--of a ductus cuvieri. This would explain the concordance between their situs and that of the sinus venosus and atria. It would appear that two errors are frequently made: the common pulmonary vein is said to originate from the left atrium, whereas it originates from the sinus venosus and only belongs to the left atrium when the sinus is incorporated in the atrium; the transverse septation of the sinus is incorporated to a shift to the right of the left sinoatrial fold which separates the sinus from the primitive atrium. This fold is indeed displaced to the right, but it is more distal and corresponds, in fact, to the cephalic border of the left ductus cuvieri, and its shift is produced by the invagination of that duct.