38 CAUSE OF THE PORTAL CIRCULATION. 



From this circumstance, they require some arrangement by which they can be filled or 

 emptied. The auricles of the heart relieve all their corresponding veins, to a very 

 great extent, from pressure ; the ventricles serve, in their turn, to fill thoroughly the 

 aorta and the pulmonary artery. 



124. In the last place, let us turn to the portal circulation, and show how, in it, the 

 same physical principles apply. The blood which flows towards the liver, along the 

 portal vein, has been obtained by that vein from the chylopoietic viscera ; it has, there- 

 fore, the same relation to the blood furnished from the different and corresponding aor- 

 tic branches as has the general systemic venous blood. The arterial blood, therefore, 

 drives it before it, in the same way that the general systemic circulation takes place, 

 and, passing along the portal vein, it is now distributed to the liver. In this organ it 

 also receives the blood which has been brought by the hepatic artery, and which has 

 served for the purposes of the liver. t 



125. The process of biliary secretion now takes place, and compounds of carbon 

 and hydrogen along with soda are separated as bile, and pass along the biliary tubes. 

 In its final effect, therefore, the chemical action of the liver closely resembles the chem- 

 ical action of the lungs. Compared with the resulting blood which passes along the 

 branches of the hepatic veins, and finds its way into the ascending vena cava, the por- 

 tal blood differs by containing the elements of bile. 



126. Two systems of forces now conspire to drive the portal blood out of the liver 

 into the ascending cava. Let us consider them in succession. 



1st. The blood which is coming along the capillary portal veins, and that which is 

 receding by the hepatic veins, compared together, as to their affinities for the structure 

 of the liver, obviously have this relation ; the portal blood is acted upon bv the liver, 

 and there is separated from it the constituents of the bile ; the affinities which have 

 been at work in producing this result have all been satisfied, and the residual blood, 

 over which the liver can exert no action, constitutes that which passes into the hepatic 

 veins. Between the portal blood and the structure of the liver there is an energetic 

 affinity, betrayed by the circumstance that a chemical decomposition takes place, and 

 bile is separated ; and that change completed, the residue, which is no longer acted 

 on, forms the venous blood of the hepatic veins. In the same manner, therefore, that 

 in the systemic circulation arterial blood, in its passage along the capillaries, becomes 

 deoxydized in consequence of an affinity between its elements and those of the struc- 

 tures with which it is brought in contact, and drives the inert venous blood before it, 

 so, too, in the portal circulation, in consequence of the chemical affinities and reactions 

 which obtain between the portal blood and the substance of the liver affinities and 

 reactions which are expressed by the separation of the bile that blood drives before it 

 the inert blood which is found in the hepatic veins. 



2d. But in the liver there is a second agency at work, which, conspiring in its re- 

 sultant with the former, produces motion in the same direction. As we have said, the 

 blood of the hepatic artery, after serving for the economic purposes of the liver, is 

 thrown into the portal plexus. Hence arises a second force. The pressure of the ar- 

 terial blood in the hepatic capillaries upon this, is sufficient not only to impel it into 



