216 CIRCULATION OF THE BLOOD 



farthest arteries is just about equal to this wave length. Only the very longest 

 arterial paths in the body, therefore, are long enough to include the entire 

 length of a pulse wave; for the end of a wave enters the aorta only after the 

 beginning of it has already reached the periphery. 



Since the movements of the contact surface of the instrument are caused 

 by fluctuations of pressure inside the artery, the pulse curve gives expression 

 to the rise and fall of this pressure. But it does not represent the variations 

 of arterial pressure exactly, for the arterial pressure is exerted not only 

 against the contact surface but also against the arterial wall and the neigh- 

 boring soft parts. 



The sphygmograph is affected also by other movements than those of the 

 blood in the arteries e. g., by changes of turgor of that part of the body where 

 it is applied. If the return flow of the blood from the veins is hindered, the 



FIG. 88. Radial pulse curve recorded with Marey's sphygmograph, after Langendorff. To 



be read from left to right. 



entire series of curves in the sphygmogram rises because the swelling skin 

 increases the tension of the spring. One dare not infer a rise of blood pressure, 

 therefore, from such a rise in the series of curves. 



When suitable apparatus is employed the pulse curve presents a number of 

 peculiarities, some of which constantly recur more or less well marked, what- 

 ever the artery from which the curve is taken. 



The pulse curve (Fig. 88, cf. also Fig. 11) begins with a rather steep 

 ascent which corresponds to the positive wave caused by the inflow of blood 

 into the aorta. This line usually reaches its highest point without interrup- 

 tion, whence begins immediately the descending limb of the curve. The latter 

 shows several irregularities, one of which at least, the second mound, occurs 

 in all pulse curves (Fig. 88). This mound is designated as the dicrotic eleva- 

 tion. That it is not an artifact has been shown by the above-mentioned 

 tests to which the sphygmograph has been subjected. 



The dicrotic elevation is without doubt a positive wave running in the 

 centrifugal direction; but opinions differ as to the way in which it arises. 

 Of the two hypotheses which at present are worthy of discussion, one accounts 

 for the elevation by supposing that the primary pulse wave is reflected as a 

 positive wave from the periphery of the arterial system. This reflected wave 

 comes into the aorta, strikes against the closed semilunar valves, and is once 

 more reflected without change of sign. The second reflection (that from the 

 semilunar valves) is the cause of the dicrotic elevation (v. Kries, v. Frey). 



According to the other hypothesis the dicrotic elevation arises in the 

 following manner. When the cardiac contraction ceases, and the semilunar 

 valves are no longer supported by the blood in the heart, or by their own 



