Circulation of the Blood. 181 



Let a, b, c be a capillary tube filled with a certain 

 liquid, between which and the tube there are at dif- 

 ferent points affinities differing in intensity. Suppose 

 at a the affinity between the liquid and the tube is 

 intense, that it becomes feebler and feebler towards 

 b, and at c has ceased altogether. Under these cir- 

 cumstances there will be a continuous flow through 

 the tube from a to c. 



To make this quite plain, let us imagine the tube a c to be 

 formed of combustible matter of any kind, and at the point a 

 an oxidizing liquid enters it. The liquid, as it passes along 

 the tube, exerts its oxidizing agency, which at the expense of 

 the tube is gradually satisfied. In successive portions of such 

 a tube the affinity is constantly declining. It is greatest at a, 

 diminishes as it passes along, and ceases altogether at c. 

 Under these circumstances there will be a constant flow along 

 the tube. 



A tube with an included liquid which is thus incessantly 

 varying in its relations will give rise to a continuous move- 

 ment. At the point of entrance, the liquid, powerfully at- 

 tracted by the tube, rises with energy; but the chemical 

 changes that set in, satisfying and neutralizing that attraction, 

 to use a common expression, it loses its hold on the tube as it 

 goes, and new quantities, arriving behind, continuously press 

 out those which are before them. 



These various results may be expressed in the following 

 general terms. 



If a given liquid occupies a capillary tube, or a porous or 

 parenchymatous structure, and has for that tube or structure 

 at different points affinities which are constantly diminishing, 

 movement will ensue in a direction from the point of greater 

 to the point of less affinity. 



Or thus : 



If a given liquid occupies a capillary tube, or a porous or 

 parenchymatous structure, and whilst in that tube or structure 

 changes happen to it, which tend continually to diminish its 

 attraction for the surface with which it is in contact, move- 

 ment will ensue in a direction from the changing to the 

 changed fluid. 



Application of this 'principle to the Circulation of the Blood. 

 — Let us now apply these principles to some of the circula- 

 tions which take place in the human system, and select for 

 that purpose the four leading forms, the systemic, the pul- 

 monary, the portal and the placental circulation. 



The Systemic Circulation. — The arterial blood, which 

 moves along the various aortic branches, contains oxygen 



