CIRCULATION OF THE BLOOD. 145 



The explanation of these tracings presents some difficulties, not, how- 

 ever, as regards the two primary factors, viz., the upstroke and down- 

 stroke, because they are universally taken to mean the sudden injection 

 of blood into the already full arteries, and that this passes through the 

 artery as a wave and expands them, the gradual fall of the lever signify- 

 ing the recovery of the arteries by their recoil. It may be demonstrated 

 on a system of elastic tubes, such as was described above, where a syringe 

 pumps in water at regular intervals, just as well as on the radial artery, 

 or on a more complicated system of tubes in which the heart, the arteries, 

 the capillaries and veins are represented, which is known as an arterial 

 schema. If we place two or more sphygmographs upon such a system 

 of tubes at increasing distances from the pump, we may demonstrate 



FIG. 127. Diagram of the formation of the pulse-tracing. A, percussion wave; B, tidal wave; 

 C, dicrotic wave. (Mahomed.) 



that the rise of the lever commences first in that nearest the pump, 

 and is higher and more sudden, while at a longer distance from the pump 

 the wave is less marked, and a little later. So in the arteries of the body 

 the wave of blood gradually gets less and less as we approach the periphery 

 of the arterial system, and is lost in the capillaries. By the sudden in- 

 jection of blood two distinct waves are produced, which are called the 

 tidal and percussion waves. The tidal wave occurs whenever fluid is 

 injected into an elastic tube (Fig. 127, B), and is due to the expansion of 

 the tube and its more gradual collapse. The percussion wave occurs 

 (Fig. 127, A) when the impulse imparted to the fluid is more sudden; 

 this causes an abrupt upstroke of the lever, which then falls until it is. 

 again caught up perhaps by the tidal wave which begins at the same time 

 but is not so quick. 

 VOL. I. 10. 



