The veins, like the arteries, are divided into the pulmonary and the systemic.
The pulmonary return the aerated blood from the lungs to the left side of the heart, and are the only veins that contain arterial blood. The systemic veins bring back to the right side of the heart the impure venous blood from the rest of the body. All the systemic veins terminate ultimately either in the superior or inferior vena cava, except the cardiac veins, which return the blood from the heart's sub- stance, and open directly into the right auricle.
The veins from the stomach and intestines, the spleen, and the pancreas, before opening into the inferior vena cava, are collected into a large trunk vein called the portal vein, which breaks up, like an artery, into capillaries in the substance of the liver. From these capillaries the blood is again collected by the hepatic veins, which finally open, as two or more large-sized vessels, into the inferior vena cava.
The veins are described under the heads of: - 1. The veins of the thorax; 2. the veins of the head and neck; -3. the veins of the spine; 4. the veins of the abdomen; 5. the veins of the upper extremity; and -6. the veins of the lower extremity.
The superior or descending vena cava carries to the heart the blood returned from the head and neck and upper extremities through the right and left innominate veins, and from the walls of the thorax, either directly through the greater azygos vein, or indirectly through the innominate veins.
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