288 



THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXII. 



recording tambour C. If the mass of the pendulum is small, the 

 accuracy of the instrument is considerable. Fig. 254 shows the 

 tracing obtained from the carotid artery of the horse. The pressure 

 curve is placed below it for purpose of comparison. The tracing 

 shows the effects during the time corresponding to one cardiac cycle. 

 On both curves the upstroke is the effect of the ventricular systole ; 

 this terminates at the apex of the first small curve (between the 

 vertical lines 3 and 4) on the down stroke of the pressure curve, the 

 rest of the downstroke until the commencement of the next systole 

 (line 5) corresponds with the ventricular diastole. Beyond the line 4 

 is a larger secondary wave, which is known as the dicrotic wave ; the 

 smaller post-dicrotic waves are due to elastic vibrations. We shall 

 be studying all these points more in detail when we come to the 

 pulse. When we compare the two curves together we note that the 

 velocity curve reaches its maximum before the pressure curve ; this 



FIG. 254. Velocity curve (V), and pressure curve (P) from the carotid artery of the horse ; oo, abscissa 

 of velocity curve; 1, 2, 3, 4 show simultaneous points on both curves. (Chauveau and Marey.) 



is because, as the arteries become overfilled, the heart cannot maintain 

 the initial velocity of output. The blood is thus forced along the 

 arteries ; then comes the diastole, and the recoil of the elastic arteries 

 not only forces the blood onwards, but also produces a back-swing 

 against the closed aortic valves ; this produces the notch before the 

 dicrotic wave; the blood is reflected from the aortic valves, once 

 more producing a positive wave (the dicrotic wave). This affects 

 both speed and pressure. It will be noticed that during the dicrotic 

 notch the pressure falls comparatively little, but in the velocity curve 

 the fall is considerable, and the curve may sink below the base line oo. 

 This negative effect is naturally much more marked in the aorta and 

 its first large branches than in situations farther from the heart. 



In actual values Chauveau found that the velocity in the horse's 

 carotid reached 520 mm. per second during systole ; it sank to 220 

 at the time of the dicrotic wave, and to 150 during diastole. 



The effect on the blood-flow of functional activity or vaso-motor 



