THE CIRCULATION 121 



order is now to be indicated. The blood which has trav- 

 ersed the small vessels of the digestive tract (including the 

 stomach, both intestines, the pancreas, and the spleen) 

 would be expected to make its way back to the heart 

 through branches of the systemic veins. The actual ar- 

 rangement is as follows: A large vessel receives all the 

 blood from this region and carries it into the liver, where 

 a second set of capillaries is entered. When these unite 

 it is to form short veins discharging into the chief vein of the 

 body, just below the diaphragm and practically within the 

 boundaries of the liver. The channel leading from the 

 organs of digestion to the liver occupies a unique position. 

 Inasmuch as it is formed from a capillary system it seems 

 to be a vein, but since it also supplies a capillary system 

 it may be contended that it is an artery. Its structure 

 favors the view that it is a vein and it is so called. The 

 vessels which gather the blood from the digestive tract 

 and distribute it inside the liver are said to form the portal 

 system; the chief conductor described above is called the 

 portal vein. An important consequence of this arrange- 

 ment is that the absorbed products of digestion, so far as 

 they are in the blood rather than the lymph, are brought 

 under the influence of the liver before they go elsewhere. 



The question will naturally be raised whether the liver 

 is supplied exclusively with venous blood. Provision is 

 made for a supplementary supply through what is known 

 as the hepatic artery, a vessel which is an offshoot of the 

 general arterial system, and which, of course, introduces 

 into the liver blood rich in oxygen. An analogous condi- 

 tion may be noted in the lungs, where the main currents 

 are the venous streams coming from the right side of the 

 heart, but where there are also interwoven in the tissue 

 much smaller vessels which take their rise in the arterial 

 tree springing from the left ventricle. 



The problems of the circulation fall into two classes: 

 There are those which are purely physical and which can 

 be approximately reproduced and more or less successfully 

 studied in lifeless models. There are also those which have 



