MEDIASTINAL GKOWTHS. 337 



of one innominate vein, it is said that the external jugular 

 vein of the obstructed side remains distended during inspira- 

 tion, whilst the vein on the opposite side is emptied by the 

 thoracic suction. 



When the superior vena cava or its large tributaries are 

 obstructed, the feeding veins become distended, and relief can 

 only be obtained by diversion of blood into the inferior cava. 

 This necessitates the opening up of collateral channels. These 

 channels lie, some within the body cavity, some in the depth of 

 the body walls and some on the surface. In rare instances the 

 deeper collateral channels are sufficient to relieve the congestion, 

 and no enlargement of the surface veins can be detected ; in 

 others a congested network of superficial veins appears carrying 

 blood from above downwards. The following are the chief 

 possible channels of collateral circulation (Fig. 47, p. 338) : 



1. Within the body cavity the azygos veins, which receive the 

 blood from most of the intercostal spaces, are connected below 

 with the ascending lumbar veins and so communicate with the 

 lumbar, renal, or iliac tributaries of the inferior vena cava. 



2. Within the rectus sheath the deep superior epigastric 

 veins which accompany the epigastric branches of the internal 

 mammary artery, communicate with the deep inferior epigastric 

 veins which accompany the deep epigastric artery. Thus the 

 innominate vein is linked with the external iliac vein. The 

 lumbar tributaries of the inferior vena cava also communicate 

 with both the superior and inferior epigastric veins. 



3. On the surface of the chest and abdomen, mesial, lateral, 

 and posterior groups of anastomosing veins exist. These 

 when enlarged form a striking clinical picture. The mesial 

 superficial veins consist above that is, on the chest and 

 epigastrium of a plexus of vessels derived from the anterior 

 intercostal tributaries of the internal mammary veins, and near 

 the free margin of the thorax, from tributaries of the deep 

 superior epigastric veins. A large vein may often be seen 

 running vertically from the ensiform cartilage to the umbilicus. 

 This may be called the superficial superior epigastric vein. It 



C.A.A. 22 



