PRINCIPAL VEINS. IOQ 



greatest resistance. The third and fourth coats form folds, 

 which when extended partially close the vessel. These valves 

 are disposed in such a manner as that when the blood moves 

 toward the heart it presses them close to the walls of the veins, 

 and they are no obstacle to its course; while, if it moves in the 

 contrary direction, they close and prevent its return toward 

 the extremities. The veins are disposed in two planes. 

 Some are deeply placed, and accompany the arteries, of which 

 they are called the companions (vena comites arteriaruni)\ 

 others are superficial, and creep along under the skin without 

 any coincidence with the direction of the arterial vessels. 

 Springing from the capillaries, by means of which they com- 

 municate with the arteries, the venous radicles unite more 

 rapidly than those of the arteries into considerable branches, 

 superior in numbers and total capacity to the arterial trunks. 

 Many of the arteries, in fact, are accompanied by two veins 

 as satellites, of at least equal calibre; and the superficial veins 

 show still greater disproportion. In the interior of the 

 cranium the veins are transformed into sinuses or canals 

 formed by the dura mater, which receive the venous branches 

 of the brain. The veins are enveloped in their course by 

 numerous lymphatic vessels. 



Principal veins. The superior and inferior vena cava are 

 to the venous system what the aorta is to the arterial system. 



The superior vena cava receives the blood from the head, 

 neck, the upper extremities, and the walls of the chest; it 

 opens into the right auricle of the heart. It is formed by the 

 two brachio-cephalic trunks and the azygos vein. Each of 

 these brachio-cephalic venous trunks unites, like the arterial 

 trunks of the same name, all the principal veins of the head 

 and the arms, which are the two jugulars, internal and ex- 

 ternal, and the subclavian. 



The internal jugular corresponds to the carotid artery; it 

 receives the blood from the sinuses of the dura mater, from 

 the veins of the head, of the neck, and part of the shoulder. 

 The external jugular carries the blood from a part of the 

 superficial veins of the head to the subclavian vein. 



The subclavian vein corresponds to the artery of the same 

 name, and receives the companion veins, and veins corre- 



