CH. XXI.] ELASTICITY OF THE BLOOD-VESSELS 261 



Use of the Elasticity of the Vessels. 



If a pump is connected to a rigid tube, such as a glass tube, 

 filled with water, and a certain amount of water is forced into the 

 tube, an exactly equal amount of water is driven out from the open 

 end. During the intervals of pumping the flow ceases, accurately at 

 the instant the inflow stops. If in the next place the open orifice is 

 constricted and the pumping >con tinned as before, the outflow is still 

 restricted to the time during which water is being driven into the 

 tube. The only difference is that a greater force of pumping will be 

 required if the pump is to empty itself in the same time as before, 

 and the force required will increase in proportion to the degree of 

 constriction of the orifice, until with a fairly considerable constriction 

 the force required will be enormous. 



If the rigid tube is replaced by an elastic one with a wide free 

 opening, the outflow will again be intermittent but not quite restricted 

 to the time of the pumping. This latter difference is because the 

 elastic wall of the tube will stretch a little at each output of the 

 pump, and this continues after the pump has ceased to discharge, and 

 will then recover, at the same time driving out the extra small amount 

 of fluid it contained, after the pump has ceased to act. The flow will 

 thus be intermittent, but the outflow will last for a short time 

 longer than the inflow. This persistence will increase with the 

 resistance of the tube, with the velocity of the inflow, and lastly with 

 the mass of the column of fluid lying in the tube. If now the 

 orifice be diminished, the duration of the outflow will begin to 

 increase still further, and, as the constriction is increased more and 

 more, will gradually extend over the diastolic period of the pumping. 

 The amount of work required to drive the fixed volume of fluid 

 through the constricted orifice is the same with a rigid and with an 

 elastic tube. In the former case, however, the duration of the out- 

 flow is of necessity the same as that of the inflow, whereas in the 

 second case this time is prolonged. Consequently the rate of working 

 in the first case must exceed that in the second in proportion to this 

 difference of time, or the maximum pressure set up in the former 

 case is far greater than in the latter. If the constriction of the 

 orifice of the elastic tube is still further increased, a point is at last 

 reached at which the outflow lasts throughout the whole cycle of the 

 pump, and here therefore the energy imparted to the fluid by the 

 pump is converted into a pressure energy represented by the tension 

 of the elastic walls of the tube, and this energy is given out again 

 after the fluid has ceased to enter the tube and is just sufficient to 

 exactly drive out the stored fluid during the resting period. If the 

 constriction be carried still further, the tube will not be able to empty 

 itself during the diastolic period, and when the second inflow begins, 



