According to the more detailed views of Galen and his school the blood was perfected and had its central source not in the heart, but in the liver, to which the portal vein brought a cruder liquid derived from the products of digestion. In the liver the veins also originated, while the arteries originated at the heart. The blood left its source in the liver, by way of the roots of the venous system, that is, by the hepatic veins of modern anatomy. From these it entered the great venous trunk, the vena cava, a vessel which comprised the inferior cava, the right auricle, and the superior cava of our present nomenclature. Upon leaving the liver the blood at once divided into two sharply diverging streams, one flowing directly downward through the vena cava, the belly, and the lower extremities; the other stream flowing directly upward through the vena cava to the chest, the upper extremities, and the head. Therefore, that part of the vena cava which we call the right auricle simply formed a part of the upward pathway of the blood, at a place where some of the blood left this upward pathway and flowed through a side opening into the right ventricle. This ventricle, therefore, received only a fraction of that portion of the blood which ascended from the liver. The rest of the ascending blood mounted in the vena cava past the right opening which led into the ventricle and, having traversed thus what we call the right auricle, entered and traversed what we call the superior vena cava, to be distributed to the veins and tissues of the arms and head. Of the fraction of the blood that entered the right ventricle a part went to the lungs simply for their nutrition, by the "arterial vein"--the pulmonary artery of modern parlance--and a part percolated in a refined condition through pores of the septum from the right ventricle to the left, to be worked up there with the vital spirits and thus become the basis of the spirituous blood of the arteries. From the left ventricle this spirituous blood went to the body at large by way of the arteries. There is no evidence that Galen believed any blood to pass from the right to the left ventricle otherwise than through the pores of the septum. As he says, however, that the branches of the "venous artery" (our pulmonary vein) "transmit thin and pure and vaporous blood in abundance" to the lungs for their nutrition,[145] we may infer that he held this supply to be derived from the left ventricle like that of the rest of the body. This was possible, according to Galen's system, because he held to the irrational opinion that what is now called the mitral valve closed less perfectly than the other valves, inasmuch as it possessed only two segments instead of three.