COURSE OF THE BLOOD IN THE VEINS. 311 



The veins form a system which is adapted to the re- 

 turn of blood to the heart in a comparatively slow and 

 unequal current. Distention of certain portions is pro- 

 vided for ; and the vessels are so protected with valves, 

 that whatever influences the current must favor its flow in 

 the direction of the heart. It is a system which is cal- 

 culated to receive the blood from the parts after it has 

 become unfit for nutrition, and pass it in the requisite 

 quantity to the lungs, through the right side of the heart, 

 for regeneration. 



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, points which have 

 already been fully discussed in treating of the causes of the 

 capillary circulation, have established, beyond question, the 

 fact that the force exerted by the left ventricle is sufficient to 

 account for the venous circulation. The heart must be con- 

 sidered the prime cause of all movement in these vessels. 

 Regarding this as definitely ascertained, there remain to con- 

 sider, in the study of the course of the blood in the veins, the 

 character of the current, the influence of the vessels them- 

 selves, and the question of the existence of forces which may 

 assist the vis a tergo from the heart, and circumstances which 

 may interfere with the flow of blood. 



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

 the veins is continuous. The intermittent impulse of the 

 heart, which progressively diminishes as we recede from this 

 organ, but is still felt even in the smallest arteries, is lost, as 

 we have seen, 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 



