THE BLOODFLOW IN THE ARTERIES 203 



precede this notch can not be related to definite changes occurring in 

 the heart. Evidently, then, the secondary pulse waves are not all of 

 equal significance, by far the most important being that which occurs 

 immediately after the second sound, called the dicrotic wave (c), the 

 notch in front of it being called the dicrotic notch. Any secondary 

 waves occurring before the dicrotic are called predicrotic, or if they 

 occur on the ascending limb of the main pulse wave, as they sometimes 

 do, they are called anacrotic. Waves occurring after the dicrotic are 

 called postdicrotic. 



The relative importance of the dicrotic, in comparison with the pre- 

 dicrotic and postdicrotic waves, is further evidenced by the fact that 

 it alone is seen on a so-called hemataugram, which is the tracing ob- 

 tained by allowing a fine stream of blood, escaping from a pinhole made 

 in the wall of an artery, to impinge upon a moving sheet of white blot- 

 ting paper. That such a tracing shows a dicrotic but no secondary wave, 

 indicates that only the former is present in the blood stream itself, and 

 that the other secondary waves must be produced by some condition 

 arising either in the elastic tissue of the walls of the blood vessels, or 

 in the elastic properties of the instruments used for taking the pulse 

 tracing. 



The Dicrotic Wave. Because of its obviously greater significance, we 

 shall first of all consider the exact cause of the dicrotic wave and of the 

 notch preceding it. Theoretically, two possible causes might explain 

 the wave: either it is due to some secondary wave set up at the heart, 

 or it i$ dependent upon waves reflected from the periphery of the cir- 

 culation back along the blood stream, just as secondary waves are re- 

 flected from the walls of a tub of water when a stone is thrown in the 

 center. In considering this second possibility, we are of course making 

 the assumption that at the ends of the arterial system there is a sudden 

 resistance to the onward movement of blood. The frequent branching 

 which occurs when the arterioles open into the capillaries no doubt of- 

 fers many opportunities for the reflection of pulse waves back to the 

 heart, but these waves must be reflected at such varying distances along 

 the arterial system that there can be little opportunity for them to be- 

 come added together so as to form a wave of sufficient magnitude to 

 make itself perceptible in the blood flowing in the larger arteries. These 

 waves are relatively so small and they occur at such different times that 

 the net result of their addition, so far as the production of a larger 

 wave is concerned, must be practically nil. Notwithstanding these con- 

 siderations, it is possible that under some conditions, such as in cases 

 of high arterial tension, certain of the predicrotic or postdicrotic waves 

 may be due to the above causes. 



