CAUSES OF THE VENOUS CIRCULATION. 97 



ration. The veins at the lower part of the neck are then seen to empty 

 themselves of blood during inspiration, and they become distended during 

 expiration, producing a sort of pulsation which is synchronous with respira- 

 tion. This can always be observed after exposure of the jugular in the lower 

 part of the neck in an inferior animal. Direct observations on the jugulars 

 show conclusively that the influence of inspiration can not be felt much 

 beyond these vessels. They are seen to collapse with each inspiratory act, a 

 condition which limits this influence to the veins near the heart. The flac- 

 cidity of the walls 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 con- 

 nections witb surrounding tissues, so that the force of aspiration is felt far- 

 ther than in experiments on vessels removed from the body. The blood, 

 as it approaches the thorax, impelled by other forces, is considerably accel- 

 erated in its flow ; but it is evident that beyond a certain point, and that 

 point 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 a very important ele- 

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

 arrangement which 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 point where it passes 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 tissue of the organ, 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 influ- 

 ence 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. Although this must be a very important element in 

 the production of 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. 



A farthei proof, if any were needed, of the suction force of inspiration is 

 found in an accident which is not infrequent in surgical operations on the 

 lower part of the neck. When the veins in this situation are kept open by a 

 tumor or by induration of the surrounding tissues, an inspiratory effort has 

 occasionally been followed by the entrance of air into the vessels, an acci- 

 dent which is likely to lead to the gravest results. This occurs only when a 

 divided vein is kept patulous ; and the accident proves both the influence of 

 inspiration on liquids in the veins near the chest and its restriction to the 

 vessels in this particular situation by the flaccidity of their walls. 



