CHAP, iv.] 



THE VASCULAR MECHANISM. 



169 



Various causes have been suggested as bringing about the 

 secondary waves, and much discussion has arisen especially con- 

 cerning the dicrotic wave. When the tube of the artificial scheme 

 bearing two levers is blocked just beyond the far lever, the primary 

 wave is seen to be accompanied by a second wave, which at the far 

 lever is seen close to, and often fused into, the primary wave 



Fra. 35. INFLUENCE OF CHANGES IN THE PRESSURE APPLIED TO THE EXTERIOR OF THE 



VESSEL ON THE FORM OF THE CURVE. 



a, From the Art. radialis of healthy man of 27 years of age with an extra arterial 

 pressure equal in a to 70 mm., in a' to 50mm., in a" to 30 mm. mercury. 



(Fig. 27, VI. a'), but at the near lever is at some distance from it 

 (Fig. 27, I. a), being the farther from it, the longer the interval 

 between the lever and the block in the tube. The second wave is 

 evidently the primary wave reflected at the block and travelling 

 backwards towards the pump. It thus of course passes the far 

 lever before the near one. And it has been argued that the 

 dicrotic wave of the pulse is really such a reflected wave, started 

 either at the minute arteries and capillaries, or at the points 

 of bifurcation of the larger arteries, and travelling backwards to 

 the aorta. But if this were the case the distance between the 

 primary crest and the dicrotic crest ought to be less in arteries 

 more distant from the heart than in those nearer, just as in 

 the artificial scheme the reflected wave is fused with a primary 



