The embryo's vein system develops out of a very irregular network of capillaries, from which finally individual ones transform themselves definitively into veins while others disappear again. The result of this is that the venous system is not very uniform, and in the adult far more variants of venous outflows than on the arterial side exist. One can nevertheless distinguish among three basic systems: cardinal, umbilical and omphalomesenteric. The pulmonary veins cannot be counted as belonging to these three systems and are treated separately.
|
|
|