THE PORTAL SYSTEM. 919 



vertical position of these veins ; (b) the length of the column of blood they carry, 

 extending, in the case of the long saphenous vein, from its beginning at the ankle to 

 the upper orifice of the inferior vena cava ; (r) in many cases to compression above, 

 as from abdominal or pelvic growths, or the gravid uterus, or from garters. (2) In 

 the superficial veins the frequency of varicosity is also due to the lack of adequate 

 external support to their thin and distensible walls, the saphenous veins, for example, 

 lying outside of the deep fascia in loose connective tissue. (3) To the increased 

 resistance that must be overcome at the points where the deep and superficial vessels 

 communicate, and where in many cases the varicosity seems to begin. At such 

 points the upward current of blood has to overcome — and the walls of the veins to 

 support — not only the downward pressure of the vertical column of blood in the 

 vessels above it, but also the resistance of the blood-stream driven out of the deep 

 vein by the contracting muscles between which it lies, and entering the superficial 

 vein at a right angle. The valve next below this point of entrance prevents the relief 

 that might be obtained from temporary distention of a long lower section of the vein 

 and limits these forces to a circumscribed area, which yields and becomes varicose. 



The venous plexus between the two layers of the muscles of the calf is often the 

 seat of varices of great size. The six chief veins which pass from the soleus muscle 

 alone to enter into the posterior tibial and peroneal trunks have a united diameter 

 of not less than one inch (Treves). 



The fact that each of the saphenous veins is accompanied by a sensory nerve 

 accounts for the aches and pains associated with varicosity. 



THE PORTAL SYSTEM. 



The portal system is composed of all the veins which have their origin in the 

 walls of the digestive tract below the diaphragm (with the exception of those of the 

 lower part of the rectum) and includes also the veins which return the blood from 

 the pancreas, spleen, and gall-bladder. It presents a marked peculiarity in that the 

 system begins and ends in capillaries, the blood which it contains having entered its 

 constituent veins from the capillaries of the intestine, stomach, and the other organs 

 mentioned above, and passing thence to the liver, where it traverses another set 

 of capillaries, by which it reaches the hepatic veins and so the heart. Coming as 

 it does principally from the intestine, the portal blood is more or less laden with 

 nutritive material, which has been digested and absorbed through the intestinal 

 walls, but is not yet in a condition, so far as some of its constituents are concerned, 

 suitable for assimilation by the tissues. To undergo the changes necessary for its 

 conversion into assimilable material it is carried by the portal vein to the liver, and as it 

 passes through the capillaries of that organ it undergoes the necessary modifications. 



In other words, the portal vein stands in a somewhat similar relation to the liver 

 that the pulmonary vein does to the lungs. Its purpose is not to convey material to 

 the organ for its nutrition, that being accomplished by the hepatic arteries for the 

 liver just as it is accomplished by the bronchial arteries for the lungs, but to carry to 

 the liver crude material upon which the organ may act, elaborating it and returning 

 it, as required, to the circulation in a purified and assimilable condition. 



The inclusion of the veins of the spleen, gall-bladder, and pancreas, or even of 

 those of the rectum, in the portal system is to be explained on the ground of topo- 

 graphic relationship rather than on the basis indicated above. 



The main stem of the portal system will first be described and then its tributaries 

 in succession. 



The Portal Vein. 



The portal vein (v. portae) (Figs. 772, 774) is formed behind the head of the 

 pancreas by the union of the superior mesenteric and splenic veins, the latter receiving 

 the inferior mesenteric vein shortly before its union with the superior mesenteric. 

 The two veins unite almost at a right angle, and from their point of union the portal 

 vein passes obliquely upward and to the right, along the free edge of the lesser 

 omentum, towards the transverse fissure of the liver. There it divides into two 

 trunks, of which the right is the larger and shorter and quickly bifurcates into 

 an anterior and a posterior branch. It is distributed to the whole of the right 



