THE VEINS 



nriHE veins, like the arteries, are divided into the pulmonary and the systemic. 

 I 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 sys- 

 temic 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 substance, 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. 



1. THE VEINS OF THE THORAX 



The veins of the thorax are: the pulmonary, which carry the blood from the 

 lung to the left side of the heart; and the superior vena cava and its tributaries, 

 which return the venous blood from the head and neck, the upper extremities, and 

 the walls of the thorax, to the right side of the heart. The inferior vena cava, 

 which brings back the blood from the abdomen and pelvis and lower extremities, 

 is described with the veins of the abdomen, in Avhich cavity it lies throughout by 

 far the greater part of its course, somewhat less than half an inch of its upper end 

 only being situated in the thorax. 



The pulmonary veins are contained in the middle mediastinum. The superior 

 vena cava and the right and k'ft innominate veins course through the superior 

 mediastinum. The azygos veins, the larger of which opens into the superior vena 

 cava, lie on either side of the thoracic vertebra in the posterior mediastinum. Tliey 

 receive the intercostal, the bronchial, and the oesophageal veins. 



THE PULMONARY VEINS 



The pulmonary veins (fig. 320) return the aerated blood from the kmgs to the 

 heart. They are usually four in number, two right and two left. Occasionally, 

 however, there are three pulmonary veins on the right side, the result of the vein 



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