THE FLOW OF BLOOD IN THE VEINS 977 



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 

 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- 

 petence of the auriculo- ventricular valves. 



Besides the favourable influences exercised on the circulation through 

 the veins by the aspiration of the thorax, a considerable part is played in 

 the venous circulation by the contraction of the muscles of the body as 

 well as by the passive movements of different parts. The adjuvant effect 



RaoL. art. 



JLJUUUUUUJUU 



FIG. 429. Venous pulse tracing from jugular vein compared \vith the 

 arterial pulse tracing from the radial artery. 



of passive or active movement on the circulation through the veins is ren- 

 dered 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 backwards 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 v the heart plus 

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

 the heart, *. e. about a metre of blood. On measuring the pressure by 

 von Kecklinghausen's or by Hill's method in these veins, this is not found 



