BRACniO-CEPHALIC VEINS. 453 



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. 224 at 

 p. 298.) 



UPPER VENA CAVA. 



The upper vena cava conveys to the heart the blood which is returned 

 from 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 sternum 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 upper cava lies immediately in front of the 

 right pulmonary vessels, and between the right lung and the aorta, which 

 partly overlap it. It receives several small veins from the pericardium and 

 the mediastinum ; and lastly, it is joined by the right azygos vein, imme- 

 diately above the place where it becomes invested by the pericardium. 



PECULIARITY. 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 

 auricle. This peculiarity is explained by reference to the development of the parts, 

 and will be more fully referred to at p. 485, in connection with the description of the 

 great cardiac vein. 



INNOMINATE OR BRACHTO-CEPHALIC VEINS. 



The blood returned from the upper limbs through the subclavian veins, 

 and from the head and neck by the jugular veins, is poured into two trunks, 

 named the brachio-cephalic or innominate veins. These vessels, resulting 

 from the union of the subclavian with the internal jugular vein at each side, 

 commence opposite the inner ends of the clavicles, and terminate a little 

 below the cartilage of the first rib on the right side, where, by uniting, they 

 form the upper vena cava. The right vein is very short, and nearly vertical 

 in its direction ; it is in apposition, on the right side, with the pleura and 

 the upper part of the right lung. The vein of the left side, about three 

 times longer than the right vein, pursues a course from left to right, at 

 the same time inclining somewhat downwards : it crosses behind the upper 

 part of the first bone of the sternum, separated from it by the sterno- 

 hyoid and sterno-thyroid muscles, and by the thymus gland or its remains ; 

 it lies in front of the three primary branches given off from the arch of the 

 aorta, and rests upon the highest part of the arch. The innominate veins 

 have no valves. 



LATERAL TRIBUTARIES. (a) The inferior thyroid veins emerge from a 

 venous plexus situated on the thyroid body those of opposite sides commu- 

 nicating by small branches across the trachea. The vein of the left side 

 descends in front of the trachea, behind the sterno-thyroid muscles, and 

 ends in the left brachio-cephalic or innominate vein : that of the right side 

 inclines outwards in some degree, and opens into the corresponding brachio- 

 cephalic vein, or into the angle of union between it and the vessel of the 

 opposite side. 



(6) The internal mammary veins follow exactly the course of the arteries 

 of the same name two veins accompanying each of the arteries. The 

 two companion veins of the artery arise by small branches, derived from 



