THE FLOW OF BLOOD IN THE VEINS 933 



of as the carotid elevation, and has been ascribed by Mackenzie to direct 

 propagation to the jugular vein from the underlying carotid artery. He 

 has come to this conclusion because he has not found it in tracings of the 

 liver pulse in cases of incompetent tricuspid valves. There is no doubt, 

 however, that the elevation can be seen on tracings from the inferior vena 

 cava. The explanation of its absence from liver tracings is probably to 

 be ascribed to the fact that the great mass of the liver substance is unable 

 to transmit the very rapid oscillation of pressure due to the closure of 

 the auriculo-ventricular valves. These venous pulsations are much more 



FIG. 419. Venous pulse-tracing from jugular vein compared with the 

 arterial pulse -tracing from the radial artery. 



marked in cases of heart disease, where there is partial failure of the heart 

 pump and overfilling of the venous system, often combined with incom- 

 petency of the auriculo-ventricular valves. 



Besides the favourable influences exercised on the circulation through 

 the veins by the aspiration of the thorax and the momentary aspiration 

 of the heart-beat itself, a considerable part is played in the venous circula- 

 tion by the contraction of the muscles of the body as well as by the passive 

 movements of different parts. The adjuvant effect of passive or active 

 movement on the circulation through the veins is rendered possible by the 

 existence in these vessels of valves, which are semilunar folds of the intima 

 projecting into their lumen, and so arranged that they allow the passage 

 of blood only towards the heart. Two such valves are as a rule situated 

 opposite to each other. Every movement of a limb, active or passive, 

 causes an external pressure on the veins, and therefore empties them 

 towards the heart. Thus, in walking, each time the thigh is moved back- 

 wards the femoral vein becomes empty and collapses, and fills again as 

 soon as the leg is brought forward to its former position or is flexed in front 

 of the body. When muscular movements become general, as in walking 

 or running, the active compression of the veins thus brought about plays 

 an important part in hurrying the blood into the right heart, so that the 

 output of this organ is increased and the arterial blood-pressure is raised. 



Since the blood in the vessels is subject to the influence of gravity, we 

 should expect to find that the pressure in the veins of the foot was equal 

 to the pressure in the veins, say, of the hand at the level of the heart plus 

 the pressure equivalent to the column of blood between these veins and 



