ARTERIAL PRESSURE. THE PULSE. 241 



since liquid in a set of horizontal tubes communicating freely, 

 as these do at D, always distributes itself so that the pressure 

 upon it is everywhere the same. Let the pump c now con- 

 tract once, and then dilate : during the contraction it will 

 empty itself into B and during the dilatation fill itself from 

 A. Consequently the pressure in B, indicated by the gauge 

 x, will rise and that in A will fall. But very rapidly the 

 liquid will redistribute itself from B to A through D, until 

 it again exists everywhere under the same pressure. Every 



FIG. 98. Diagram of Weber's Schema. 



time the pump works there will occur a similar series of 

 phenomena, and there will be a disturbance of equilibrium 

 causing a wave to flow round the tubing; but there will be 

 no steady maintenance of a pressure on the side B greater 

 than that in A. Now let the upper tube D be closed so that 

 the liquid to get from B to A must flow through the narrow 

 lower tubes D', which oppose considerable resistance to its 

 passage on account of their frequent branchings and the 

 great internal friction in them ; then if the pump works fre- 

 quently enough there will be produced and maintained in B 

 a pressure considerably higher than that in A, which may 

 even become negative. If, for example, the pump works 60 

 times a minute and at .each stroke takes 180 cubic centi- 

 meters of liquid (6 ounces) from A and drives it into B, the 

 quantity sent in at the first stroke will not (on account of the 

 resistance to its flow offered by the small branched tubes), 

 have all got back into A before the next stroke takes place, 

 sending 180 more cubic centimeters (6 oz.) into B. Conse- 

 quently at each stroke B will become more and more dis- 

 tended and A more and more emptied, and the gauge x will 



