276 PHYSIOLOGY CHAP. 



at the commencement of systole, on the other hand, it increases 

 more in the central than in the peripheral arteries. 



It is no less interesting to examine the features presented by 

 tachygrains as compared with sphygmograms simultaneously 

 recorded. Fig. 1 14 gives an instructive example of these tracings 

 as obtained by Lortet, another of Chauveau's pupils. Point 1 

 corresponds to the moment in which the primary systolic wave, on 

 reaching the carotid, produces a simultaneous rise in pressure 

 and in velocity. At point 2 the pressure reaches its maximum 

 when the velocity has already begun to fall, in accordance with the 

 increase of elastic tension in the distended artery, by which the 

 velocity of the movement of the blood is proportionately moderated 



FIG. 114. Tachygram (V) and sphygmogram (P) registered simultaneously on carotid artery of 

 horse, with Chauveau's haemodroinogiaph. (Lortet.) 



and depressed. Point 3 probably corresponds to the moment at 

 which the systolic efflux ceases and the closure of the semilunar 

 valves ensues, and point 4 to the moment at which the latter are 

 thrown into tension owing to the beginning of diastole, which 

 makes the blood column gravitate and recede against the already 

 closed semilunar valves, and in the sphygmogram determines 

 the negative wave that precedes the positive dicrotic wave. This 

 interpretation is in fact confirmed by the course of the tachy- 

 graphic curve, which at points 3 and 4 drops below the zero line 

 indicating a backward movement of the current, followed rapidly 

 by the dicrotic rise. In the entire interval from 4 to 1 (which 

 corresponds approximately to the periods of peri- and pre-systole), 

 the pressure curve shows a slowly falling line, which expresses the 

 decrease of arterial pressure in proportion as the elastic reaction 

 of the artery drives the blood into the capillaries and veins. 

 Velocity also decreases at the same time, but more slowly. 



XL During the passage of the pulse wave through the arteries, 



