MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 371 



placed ? Must it not be that the arterial blood bearing 

 its oxygen has an intense affinity for those structures, 

 but, those affinities being satisfied, that which was arte- 

 rial passes into the condition of venous blood ? The af- 

 finities it had for the structures with which it was in 

 contact are satisfied and have come to an end. The 

 arterial blood presented a highly energetic force, which 

 in the venous has diminished to zero. Under these cir- 

 cumstances, in accordance with the general principle, the 

 arterial blood must press the venous blood before it, and 

 the flow must be from the artery to the vein. 



2. The Pulmonary. In this the venous blood pre- 

 sents itself in the air-cells to receive oxygen. The sys- 

 temic circulation deoxidized arterial blood, the pulmo- 

 nary oxidizes venous. The latter, therefore, is the con- 

 verse of the former. The venous blood has an affinity 

 for the oxygen dissolved in the tissues with which it is 

 in contact, and the arterial blood has none. Movement, 

 therefore, must ensue; but as the conditions of the affin- 

 ity are reversed, so also is the direction of the motion, 

 for now the venous blood drives the arterial before it, 

 and drives it to the heart. 



3. The Portal Circulation. In this 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, therefore, the 

 same relation to the blood furnished from the different 

 and corresponding aortic branches as has the general 

 systemic venous blood. The arterial blood, therefore, 

 drives it before it in the same way that the general sys- 

 temic circulation takes place ; and, passing along the por- 

 tal vein, it is now distributed to the liver. In this or- 

 gan it also receives the blood which has been brought 

 by the hepatic artery. 



The process of biliary secretion now takes place, and 



