CIRCULATION. 



369 



It must be noted here that a particle of blood which traverses a capillary 

 of the spleen, of the pancreas, of the stomach, or of the intestines, and enters 

 the portal vein, must next traverse a series of venous branches of diminishing 

 size, and a capillary of the liver, before entering the succession of veins which 

 will conduct the particle to the ascending vena cava (compare Figs. 93 and 94). 



Most of the blood, therefore, which 

 leaves the liver has traversed two sets 

 of capillaries, connected with one 

 another by the portal vein, since quit- 

 ting the arterial system. This ar- 





FIG. 93. General diagram of the circulation : 

 the arrows indicate the course of the blood: PA, 

 pulmonary artery; PC, pulmonary capillaries: 

 P V, pulmonary veins ; L A, left auricle ; L V, left 

 ventricle ; A, systemic arteries ; C, systemic capil- 

 laries ; V, systemic veins ; R A, right auricle ; R V, 

 right ventricle. 



FIG. 94. Diagram of the portal system : the ar- 

 rows indicate the course of the blood: A, arterial 

 system ; V, venous system : C", capillaries of the 

 spleen, pancreas, and alimentary canal ; P V, portal 

 vein ; C", capillaries of the liver ; C, the rest of the 

 systemic capillaries. The hepatic artery is not 

 represented. 



rangement is of extreme importance for the physiology of nutrition. An 

 arrangement of the same order, though less conspicuous, exists in the 

 kidney. 



Causes of the Blood-flow. The force by which the blood is driven from 

 the right to the left side of the heart through the capillaries which are related 

 to the respiratory surface of the lungs, is nearly all derived from the contrac- 

 tion of the muscular wall of the right ventricle, which narrows the cavity 

 thereof and ejects the blood contained in it ; the force by which the blood is 

 driven from the left to the right side of the heart through all the other capil- 

 laries of the body, often called the " systemic " capillaries, is derived nearly 

 all from the contraction of the muscular wall of the left ventricle, which nar- 

 rows its cavity and ejects its contents. The contractions of the two ventricles 

 are simultaneous. The force derived from each contraction is generated by 

 the conversion of potential energy, present in the chemical constituents of the 

 muscular tissue, into energy of visible motion ; a part also of the potential 

 energy at the same time becoming manifest as heat. In the maintenance of 

 the circulation the force generated by the heart is to a very subordinate degree 

 supplemented by the forces which produce the aspiration of the chest and by 



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