322 CIRCULATION. 



a certain point, and tliat very near the chest, ordinary aspi- 

 ration has no influence, and violent efforts rather retard than 

 favor the current. 



In the liver, the influence of inspiration becomes a very 

 important element in the production of the circulation. 

 This .organ presents a vascular arrangement which is excep- 

 tional. The blood, distributed by the arteries in a capillary 

 plexus in the mucous membrane of the alimentary canal and 

 in the spleen, instead of being returned directly to the heart 

 by the veins, is collected into the portal vein, carried to the 

 liver, and there distributed in a second set of capillary vessels. 

 It is then collected in the hepatic veins, and carried by the 

 vena cava to the heart. This double capillary plexus be- 

 tween the left and right sides of the heart has been cited as 

 an argument against the fact that the left ventricle is capable 

 of sending the blood through the entire circuit of the vascu- 

 lar system. The three hepatic veins open into the inferior 

 vena cava near the point where it passes the diaphragm, 

 where the force of aspiration from the thorax would mate- 

 rially assist the current of blood. On following these vessels 

 into the substance of the liver, it is found that their walls are 

 so firmly adherent to the tissue of the organ, that, when cut 

 across, they remain patulous ; and it is evident that they re- 

 main open under all conditions. The thorax can therefore 

 exert a powerful influence upon the hepatic circulation; 

 for it is only the flaccidity of the walls of the vessels which 

 prevents this influence from operating throughout the entire 

 venous system. 



Though this must be a very important element in the 

 production of the circulation in the liver, the fact that the 

 blood circulates in this organ in the foetus before any move- 

 ments of the thorax take place, shows that it is not absolute- 

 ly essential. All of the influences which we have thus far 

 considered are merely supplementary to the action of the 

 great central organ of the circulation. 



A further proof, if any were needed, of the suction force 



