THE GREAT VESSELS 315 



the clavicle by the union of the internal jugular and subclavian 

 veins. On the right side it descends vertically on the medial 

 aspect of the apex of the right lung. It is only i inch in 

 length and, therefore, it is rarely involved alone, as tumours 

 which are of sufficient size to compress it are also large 

 enough to affect the left innominate vein, either directly or 

 indirectly through the superior vena cava. 



The left innominate vein passes to the right and downwards 

 behind the upper half of the manubrium. It is about 3 inches 

 long, and is consequently more exposed to pressure than the 

 corresponding vein of the right side. In its course, it crosses 

 the left subclavian, the left carotid and the innominate 

 arteries close to their origin from the aortic arch, and it may 

 therefore be compressed by aneurisms affecting this part of 

 the aorta or the branches mentioned. It is placed behind 

 the remains of the thymus, and it is characteristic of 

 mediastinal tumours which originate from this developmental 

 remnant that their pressure symptoms are first discovered in 

 the engorgement of the veins of the left upper limb and the 

 left side of the head and neck. 



The absence of valves from the superior vena cava and the 

 innominate veins, and the venous jugular pulse, which is thus 

 rendered possible, are referred to on page 301. 



The Inferior Vena Cava is formed at the right side of the 

 fifth lumbar vertebra by the union of the two common iliac 

 veins, which return the blood from the lower limbs and the 

 pelvis. It ascends through the abdomen behind the peritoneum 

 on the posterior abdominal wall, and, in its lower part, it is 

 related anteriorly to the root of the mesentery (p. 240) and 

 the coils of the small intestine. Opposite the third lumbar 

 vertebra, the inferior vena cava is crossed by the third portion 

 of the duodenum and, immediately above this level, it lies 

 behind the head of the pancreas. In the latter situation, the 

 vena cava may be compressed against the vertebral column by 

 malignant tumours of the gland. Above the pancreas, the 

 vena cava passes successively behind the first portion of the 



