472 THE SYSTEMIC VEINS. 



THE SYSTEMIC VEINS. 



The systemic veins commence by small branches wliich receive the 

 blood from the capillaries throughout the body, and unite to form 

 larger vessels, Avhich end at last by pouring their contents into the 

 right auricle of the heart through two large venous trunks, the supe- 

 rior and inferior venae cav^. The blood which nourishes the sub- 

 stance of the heart itself, is also returned by the coronary veins to the 

 right auricle. 



The veins, however, which bring back the blood from the stomach, 

 intestines, spleen, and pancreas, have an exceptional destination, not 

 conveying the blood directly to the heart, but joining to form a single 

 trunk — the poi'tal vein, which ramifies after the manner of an artery 

 in the substance of the liver, and carries the blood within it to the 

 capillaries of that organ. From these the blood passes into the ulti- 

 mate twigs of the hepatic veins, and is conveyed by these veins into the 

 inferior vena cava. The veins thus passing to the liver constitute the 

 'ported sijsfem. 



The anastomoses of veins are much larger and more numerous than 

 those of arteries. The veins of many parts of the body consist of a 

 subcutaneous and a deep set, which have very frequent communications 

 with each other. In some parts of the body, chiefly in the limbs and 

 at the surface, the veins are provided with valves, whilst in others no 

 valves exist. 



The systemic veins are naturally divisible into two groups: firstly, those 

 from which the l^lood is carried to the heart by the superior vena cava, 

 viz., the veins of the head and neck and upj^er limbs, together with 

 those of the spine and a part of the walls of the thorax and abdomen, 

 with which may be associated also the veins of the heart ; and secondly, 

 those from which the blood is carried to the heart by the inferior vena 

 cava, viz., the veins of the lower limbs, the lower part of the trunk, and 

 the abdominal viscera. (For a general representation of the venous 

 system, see fig. 240, at p. 344.) 



StrPERIOR VEWA CAVA. 



The superior vena cava conveys to the heart the blood which is 

 returned i'rom the head, the neck, the upper limbs, and the thorax. It 

 is formed by the union of the right and left brachio-cephalic veins. It 

 extends from a little below the cartilage of the first rib on the right 

 side of the internum to the base of the heart, where it opens into the 

 right auricle. Its course is slightly curved, the convexity of the curve 

 being turned to the right side. It has no valves. At about an inch 

 and a half above its termination, it is invested by the fibrous layer of 

 the pericardium, the serous membrane being reflected over it. The 

 npper vena cava lies immediately in front of the right pulmonary 

 vessels, and l)clwccn the right lung and the aorta, which partly overlap 

 it. It receives several small veins from the pericardium and the medias- 

 tinum ; and lastly, it is joined by the right azygos vein, immediately 

 above the place where it becomes invested by the pericardium. 



Varieties.— In several instances, the two innominate veins, which usually 

 join to form the vena cava superior, have been seen to open separately into the 

 right auiicle. This peculiarity is explained by reference to the development of 



