240 THE CIRCULATION OF THE BLOOD 



The explanation of these tracings present some difficulties, not, how- 

 ever, as regards the two primary factors, viz., the upstroke and downstroke, 



FIG. 198. Diagram of Pulse Tracing. A, upstroke or anacrotic limb; P, downstroke or 

 katacrotic limb; C, pre-dicrotic wave; D, dicrotic; E, post-dicrotic wave. 



because they are universally taken to mean the sudden injection of blood 

 into the already distended arteries, and the gradual recovery of the arteries 

 by their recoil. These points may be demonstrated on a system of elastic 

 tubes, with a pump to inject water at regular intervals, just as well as on the 

 radial artery, or on the arterial schema, a more complicated system of tubes 

 in which the heart, the arteries, the capillaries, and veins are represented. 

 If we place two or more sphygmographs upon such a system of tubes at in- 

 creasing distances from the pump, we may demonstrate, first, that the rise 



FIG. 199. Sphygmogram from the Radial Artery Taken with Marey's Sphygmograph. 



(Langendorff.) 



of the lever commences earliest in that nearest the pump, and, second, that 

 it is higher and more sudden. So in the arteries of the body the wave gradu- 

 ally gets less and less as we approach the periphery of the arterial system, 

 and it is lost in the capillaries. 



The origin of the secondary waves is to some extent a matter of uncer- 

 tainty. The anacrotic wave occurs when the peripheral resistance is high; 

 that is, when, for some time during the systole, the flow from the aorta toward 

 the periphery is slower than the flow from the ventricle into the aorta. Thus 

 it is seen in some cases of nephritis where the arteries are rigid and the periph- 

 eral resistance is high. 



The dicrotic wave is the most important of the secondary waves, and 

 has been the subject of much discussion. It is constantly present in pulse 

 tracings, but varies in height. In point of time the dicrotic wave occurs 

 immediately after the closure of the aortic semilunar valves. In certain 



