102 CIKCULATION OF THE BLOOD. 



the venous circulation by contractions of muscles on their nutrition. It is apparently 

 necessary that the supply of blood should be increased in a muscle, in proportion to and 

 during its activity ; for at that time its disassimilation is undoubtedly augmented, and 

 there is an increased demand on the blood to supply the waste. It is apparently a pro- 

 vision of Nature that the activity of a muscle, facilitating the passage of blood in its 

 veins and consequently its flow from the capillaries, induces an increased supply of the 

 nutrient fluid. As the development of tissues is generally in proportion to their vascu- 

 larity, this may account for the increase in the development of muscles which is the al- 

 most invariable result of exercise. 



Force of Aspiration from the Thorax. During the act of inspiration, the enlarge- 

 ment of the thorax, by depression of the diaphragm and elevation of the ribs, affects 

 the movements of fluids in all the tubes in its vicinity. The air rushes in by the trachea 

 and expands the lungs, so that they follow the movements of the thoracic walls. The 

 flow of blood into the great arteries is somewhat retarded, as is indicated by a dimi- 

 nution in the arterial pressure ; and, finally, the blood in the great veins passes to the 

 heart with greater facility and in increased quantity. This last-mentioned phenomenon 

 can be readily observed, when the veins are prominent, in profound or violent inspira- 

 tion. 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 respiration. This can always be observed after ex- 

 posure of the jugular in the lower part of the neck in an inferior animal. After this 

 operation, if we cause the animal to make violent respiratory efforts, the vein will be 

 almost emptied and collapsed with inspiration and turgid with expiration. The move- 

 ments of the veins near the thorax have long been observed and have been described 

 with tolerable accuracy. Direct observations on the jugulars show conclusively that the 

 influence of inspiration cannot 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 flaccidity of the walls of the veins will not permit the extended 

 action of any suction force. If a portion of a vein removed from the body be attached 

 to the nozzle of a syringe and we attempt to draw a liquid through it, although the suc- 

 tion force be applied very gently, when the vessel has any considerable length its walls 

 will -be drawn together. 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 any experiment 

 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 seen by direct observation, 

 that beyond a certain point, and that very near the chest, ordinary aspiration has no in- 

 fluence, and violent efforts rather retard than favor the venous current. 



In the liver, the influence of inspiration becomes a very important element 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. This double capillary plexus between the 

 left and the right side 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 vascular 

 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 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 remain open under all con- 



