en. XXL] 



TIME OF A COMPLETE CIRCULATION 



285 



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 maxi- 

 mum before the pressure curve ; this 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 

 sinks below the base line oo. This negative effect is naturally much 



FIG. 285. 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.) 



more marked in the aorta and its first large branches than in situa- 

 tions further 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 

 changes has also been observed. Thus Lortet found that the carotid 

 flow is five or six times greater when the horse is actively masticating 

 than when it is at rest. After section of the cervical sympathetic, 

 the lessening of the peripheral resistance raised the velocity from 

 540 to 750 mm. per second. 



The Time of a Complete Circulation. 



Among the earliest investigators of the question how long an 

 entire circulation takes, was Hering. He injected a solution of 

 potassium ferrocyanide into the central end of a divided jugular 

 vein, and collected the blood either from the other end of the same 



