74 CIRCULATION OF THE BLOOD 



In addition to the double, or more rarely triple valves just 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. The 

 veins are adapted to the return of blood to the heart in a comparatively 

 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. 



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 on 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 

 ventricle is sufficient to account for the venous circulation. The force 

 of the heart, therefore, must be regarded as the prime cause of the 

 movement of blood in the veins. 



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

 is continuous and uniform. The intermittent impulse of the heart, 

 which progressively diminishes toward the periphery, but is still felt 

 even in the smallest 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 continually 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 sub- 

 ject to great variations, there must necessarily be corresponding varia- 

 tions in the current in the veins and in the quantity of blood which 

 these vessels receive. 



It often occurs that a vein becomes obstructed from some cause that 

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

 number of veins, as compared with the arteries, and their free com- 

 munications with each other, provide that the current, under these 

 conditions, simply is diverted, passing to the heart by another channel. 

 When any part of the venous system is distended, the vessels react 

 on the blood and exert a certain influence on the current, always press- 

 ing it toward the heart, for the valves oppose a flow in the opposite 

 direction. 



PRESSURE OF BLOOD IN THE VEINS 



The pressure in the veins is always much less than in the arteries, and 

 is variable in different parts of the venous system and in the same part 



