Circulation of the Blood. 183 



which are taking effect in the system and in the lungs ; in the 

 former, as respects the blood, it is a de-oxidation, in the latter 

 an oxidation. 



The Portal Circulation. — Two systems of forces con- 

 spire to drive the portal blood out of the liver into the as- 

 cending cava. 



1st. The blood which is coming along the capillary portal 

 veins, and that which is receding by the hepatic veins, com- 

 pared together as to their affinities for the structure of the liver, 

 have obviously this relation — the portal blood is acted upon 

 by the liver, and there are separated from it the constituents 

 of the bile ; the affinities which have been at work in produ- 

 cing 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, be- 

 trayed by the circumstance that a chemical decomposition 

 takes place, and bile is separated ; and that change completed, 

 the residue, which is no longer acted upon, 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 deoxidized, in consequence of 

 an affinity between its elements and those of the structures 

 with which it is brought in contact, and drives the inert ve- 

 nous blood before it, so too, in the portal circulation, in con- 

 sequence of the chemical affinities and reactions which obtain 

 between the portal blood and the substance of the liver, affi- 

 nities and reactions which are expressed by the separation of 

 the bile, that blood drives before it the inert blood of the he- 

 patic veins. 



2nd. The blood of the hepatic artery, after serving for the 

 occonomic purposes of the liver, is thrown into the portal plexus. 

 Hence arises a second force. The pressure of the arterial 

 blood in the hepatic capillaries upon this is sufficient not only 

 to impel it into the capillaries of the portal veins, but also to 

 give it a pressure in a direction towards the hepatic veins ; 

 for any pressure which arises between the arterial blood 

 of the hepatic, and its corresponding venous blood, must give 

 rise to motion towards the hepatic veins, no regurgitation can 

 take place backward through the portal vein upon the blooa 

 arriving from the chylopoietic viscera, because along that 

 channel there is a pressure in the opposite direction, arising 

 from the arterial blood of the aortic branches. The pressure 

 therefore arising from the relations of the hepatic arterial blood 

 conspires with that arising from the portal blood, and both to- 

 gether join in giving rise to motion towards the ascending cava. 



