The development of veins.
At the beginning of the placental circulation, when the heart is in the cervical region and not yet divided by partitions into the venous and arterial halves, the venous system has a relatively simple device. Large veins pass along the body of the embryo: in the region of the head and neck, the anterior cardinal veins (right and left) and in the rest of the body, the right and left posterior cardinal veins. Approaching the venous sinus of the heart, the anterior and posterior cardinal veins on each side merge to form common cardinal veins (right and left), which, having at first strictly transverse course, flow into the venous sinus of the heart. Along with the paired cardinal veins, there is another unpaired venous trunk – the primary vena cava inferior, which also flows into the venous sinus in the form of a small vessel. Thus, at this stage of development, three venous trunks flow into the heart: paired common cardinal veins and unpaired primary inferior vena cava.
Further changes in the location of the venous trunks are associated with the displacement of the heart from the cervical region downwards and the division of its venous part into the right and left atria. Due to the fact that after the separation of the heart, both common cardinal veins flow into the right atrium, the blood flow in the right common cardinal vein is in more favorable conditions. In this regard, an anastomosis appears between the right and left anterior cardinal veins, through which blood flows from the head into the right common cardinal vein. As a result, the left common cardinal vein ceases to function, its walls collapse and it is obliterated, with the exception of a small part, which becomes the coronary sinus of the heart, sinus coronarius cordis. The anastomosis between the anterior cardinal veins gradually increases, turning into a vena brachiocephalica sinistra, and the left anterior cardinal vein below the anastomosis is obliterated. Two vessels form from the right anterior cardinal vein: a part of the vein above the confluence of the anastomosis turns into a vena brachiocephalica dextra, and a part below it together with the right common cardinal vein is converted into the superior vena cava, collecting blood from the entire cranial half of the body. If an anastomosis is described as underdeveloped, an abnormal development is possible in the form of two superior vena cava.