WEBERS SCHEMA. 235 



example, the pump works 60 times a minute and at each 

 stroke takes 180 cubic centimeters 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. Consequently at each 

 stroke B will become more and more distended and A more 

 -and more emptied, and the gauge x will indicate a much 

 higher pressure than that on A. As B is more stretched, 

 however, it squeezes harder upon its contents, until at last 

 -a time comes when this squeeze is powerful enough to force 

 through the small tubes just 180 cubic centimeters (6oz.) in 

 a second. Then further accumulation in B ceases. The 

 pump sends into it 10,800 cubic centimeters (360 ounces) 

 in a minute at one end and it squeezes oat exactly that 

 -amount in the same time from its other end; and so long 

 .as the pump works steadily the pressure in B will not rise, 

 nor that in A fall, any more. But under such circumstances 

 "the flow through the small tubes will be nearly constant 

 since it depends upon the difference in pressure prevailing 

 between B and A, and only indirectly upon the pump 

 which serves simply to keep the pressure high in B and 

 low in A. At each stroke of the pump it is true there will 

 be a slight increase of pressure in B due to the fresh 180 

 cub. cent. (6 oz.) forced into it, but this increase will be 

 but a small fraction of the total pressure and so have but an 

 insignificant influence upon the rate of flow through the 

 small connecting tubes. 



Arterial Pressure. The condition of things just de- 

 scribed represents very closely the phenomena presented in 

 the blood-vascular system, in which the ventricles of the 

 lieart, with their auriculo-ventricular and semilunar valves, 

 represent the pump, the smallest arteries and the capil- 

 laries the resistance at D', the large arteries the elastic 

 tube B, and the veins the tube A. The ventricles con- 

 stantly receiving blood through the auricles from the veins, 

 send it into the arteries, which find a difficulty in emptying 

 themselves through the capillaries, and so blood accumu- 



