The blood vessels studied were the vena cava inferior, the portal vein, and several segments of their tributaries. Different structural configurations were observed from blood vessel to blood vessel, and from individual to individual. The finer structural configuration of a certain vein also often shows greater or lesser variation from point to point on its circumference. Generally speaking, a slight decrease in circular and a greater increase in longitudinal musculature is observable from the spleen or intestine toward the liver. The longitudinal musculature of the inferior vena cava in the hepatic and subhepatic sections exceeds that of the portal vein by far, whereas the circular muscle layer of the inferior vena cava is strongly reduced. In all the veins mentioned the elastic fibers course habitually in a longitudinal direction. Exceptions are the media and often also the inner adventitia of the portal vein, in which the mostly circularly arranged elements are also well developed. The collagenous fibers run in a nearly circular direction in the media, which is modified in the adventitia from inside to outside from a flat spiral to a steep spiral course.