78 CIRCULATION OF THE BLOOD 



to the veins near the heart. The flaccidity of the wall of the veins will 

 not permit the extended action of any suction force. In the circulation, 

 the veins are moderately distended with blood by the vis a tergo, and, 

 to a certain extent, they are supported by connections with surrounding 

 tissues, so that the force of aspiration is felt farther than in experiments 

 on vessels removed from the body. The blood, as it approaches the 

 thorax, impelled by other forces, is considerably accelerated in its flow ; 

 but it is evident that beyond a certain distance, and that very near the 

 chest, ordinary aspiration has no influence, and violent efforts rather retard 

 than favor the venous current. 



In the liver the influence of inspiration becomes an important element 

 in the mechanism of the circulation. This organ presents a vascular 

 arrangement that is exceptional. 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 is 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. The three 

 hepatic veins open into the inferior vena cava near the diaphragm, where 

 the force of aspiration from the thorax would materially 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 hepatic tissue, that 

 when cut across, they remain patulous ; and it is evident that they must 

 remain open under all conditions. The thorax can therefore exert a 

 powerful influence on the hepatic circulation ; for it is only the flaccidity 

 of the walls of the vessels which prevents this influence from operating 

 throughout the venous system. Although this must be an important 

 element in the circulation in the liver, the fact that the blood circu- 

 lates in this organ in the foetus before any movements of the thorax 

 take place shows that it is not essential. 



Aside from the pressure exerted by the contraction of muscles and 

 the force of aspiration from the thorax, the influences that assist the 

 venous circulation are insignificant. There is a slight contraction in 

 the venae cavas in the immediate proximity of the heart, which is more 

 extended in many of the lower vetebrate animals and may be mentioned 

 as having an influence very small it is true on the flow of blood 

 from the great veins. 



In the veins that pass from above downward, the force of gravity 

 favors the flow of blood. This is shown by the turgescence of the veins 

 of the neck and face when the head is kept for a short time below the 

 level of the heart. If the arm is elevated above the head, the veins of 

 the back of the hand are reduced in size, from the greater facility with 



