THE CIRCULATION OF THE BLOOD AND LYMPH 101 



mean blood-pressure, the dicrotic wave is always marked on 

 it, preceded by one or more oscillations falling within the 

 period of the systole, and followed by one or more within the 

 period of the diastole. When the blood-pressure is low, the 

 first or primary elevation is the highest of the whole curve 

 (Fig. 29). When the blood-pressure is high, the maximum 

 falls later, coinciding with one of the secondary systolic 

 waves, but always preceding the dicrotic wave ; and the 

 curve assumes an anacrotic character. 



That all the secondary oscillations, including the dicrotic 

 wavelet, are of central, and not of peripheral origin, may 

 be shown, just as in the sphygmographic method, by re- 

 cording the blood-pressure simultaneously at two points of 

 the arterial system at different distances from the heart 

 e.g., in the crural and carotid arteries. The secondary 

 waves are found, by measuring the tracings, to reach the 

 more distal point later than the more central. 



The increase of pressure during the systole, as indicated by the 

 height of the primary elevation, is always very large, much larger 

 than it appears in a tracing taken with a mercury manometer. In 

 the rabbit this pulsatory variation is one-third to one-fourth of the 

 minimum pressure. In the dog it is still greater, owing to the slower 

 rate of the heart, and oftens amounts to 50 mm. of mercury, while 

 under favourable conditions (low minimum pressure and slowly beat- 

 ing heart) the systolic increase of pressure may be actually more than 

 double the minimum (Hiirthle). Fick found also, by means of his 

 spring manometer, that the pulsatory variations of blood-pressure 

 were greater than the respiratory variations (p. 249), although in the 

 records of the mercury manometer the reverse appears often to be 

 the case. Landois, too, in the course of experiments in which a 

 divided artery was allowed to spout against a moving surface, and 

 to trace on it a sort of pulse-curve painted in blood (a haemautogram 

 as it is called), observed that the rate of escape of the blood was 

 nearly 50 per cent, greater during the systole, than during the diastole, 

 of the heart. The existence of the dicrotic wave on this tracing 

 was long looked on as the best proof that it was not an artificial 

 phenomenon. 



The wave of increased pressure, as it runs along the 

 arterial system, carries with it wherever it arrives an 

 increase of potential energy. But this excess of potential 

 energy is continually being worn down, owing to the friction 

 of the vascular bed ; and although in the comparatively 



