THE PULSE. 



181 



Since the pulse is not due to an actual movement of blood 

 along the arteries, but rather to changes in tension producing 

 an expansion of the vessel wall, it follows that the transmission 

 of the wave may be much more rapid than the movement of 

 blood. This may be explained by reference to the motion im- 

 parted to a row of billiard balls when the one on the end is hit 

 with the cue. The one hit actually moves very little, but imparts 

 its energy of movement to the others, so that the ball at the end 

 of the row moves away with some velocity, while the others move 

 slowly. The wave of energy spreads in a fraction of a second 



Fig. 23. Jaquet Sphygmocardiograph. 



from ball to ball. By simultaneously taking tracings of the caro- 

 tid and the radial pulses, for example, it has been computed that 

 the pulse wave is transmitted at the rate of ten metres a second, 

 and that it may be six metres long. This means that the pulse 

 wave reaches the peripheral vessels before the systole of the 

 heart is completed. Any local change in the vessel may slow 

 down the rate of transmission, and if there is a difference in the 

 appearance of the pulse in the two arms or legs, it is indicative 

 of some obstruction or change in one of the vessels. 



When we analyze the pulse wave obtained by a sphymograph 

 taken, for example, from the radial artery, it is seen that the 



