CIECULATION IN THE VEINS. 93 



In addition to the double, or more rarely triple valves which have just 

 been described, there is another variety, found in certain parts, at the point 

 where a tributary vein opens into a main trunk. This consists of a single 

 fold, which is attached to the smaller vessel but projects into the larger. Its 

 action is to prevent regurgitation, by the same mechanism as that by which 

 the ileo-caecal valve prevents the passage of matters from the large into the 

 small intestine. 



The veins are adapted to the return of blood to the heart in a compara- 

 tively slow and unequal current. Distention of certain portions is provided 

 for ; and the vessels are so protected with valves, that whatever influences the 

 current must favor its flow in the direction of the heart. 



Course of the Blood in the Veins. The experiments of Hales and Sharpey, 

 showing that defibrinated blood can be made to pass from the arteries into 

 the capillaries and out at the veins by a pressure less than that which exists 

 in the arterial system, and the observations of Magendie upon the circulation 

 in the leg of a living dog, showing that ligation of the artery arrests the flow 

 in the vein, have established the fact that the force exerted by the left ven- 

 tricle is sufficient to account for the venous circulation. The heart must be 

 regarded as the prime cause of the movement of blood in the veins. Ee- 

 garding this as definitely ascertained, there remain to consider, in the study 

 of the course of the blood in the veins, the character of the current, the influ- 

 ence of the vessels themselves, the question of the existence of forces which 

 may assist the vis a tergo from the heart, and conditions which may interfere 

 with the flow of blood. 



As a rule, in the normal circulation, the flow of blood in the veins is con- 

 tinuous and uniform. The intermittent impulse of the heart, which pro- 

 gressively diminishes toward the periphery but is still felt even in the small- 

 est arteries, is lost in the capillaries. Here, for the first time, the blood 

 moves in a constant current ; and as the pressure in the arteries is continu- 

 ally supplying fresh blood, that which has circulated' in the capillaries is 

 forced into the venous radicles in a steady stream. As the supply to the 

 capillaries of different parts is regulated by the action of the small arteries, 

 and as this supply is subject to great variations, there must necessarily be 

 corresponding variations in the current in the veins and in the quantity of 

 blood which these vessels receive. Consequently, the venous circulation is 

 subject to very great variations due to irregularity in the supply of blood, 

 aside from any action of the vessels themselves or any external disturbing 

 influences. 



It often happens that a vein becomes obstructed from some cause which is 

 entirely physiological, such as the action of muscles. The great number of 

 veins, as compared with the arteries, and their free communications with each 

 other, provide that the current, under these conditions, is simply diverted, 

 passing to the heart by another channel. When any part of the venous sys- 

 tem is distended, the vessels react on the blood and exert a certain influence 

 on the current, always pressing it toward the heart, for the valves oppose a 

 flow in the opposite direction. 

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