CH. XXII.] ELASTICITY OF THE BLOOD-VESSELS 265 



Use of the Elasticity of the Vessels. 



If a puinp 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 continued 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. If now the orifice be diminished, the dura- 

 tion 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 outflow is of necessity the same as that 

 of the inflow, whereas in the second case this time is prolonged. If 

 the constriction of the orifice of the elastic tube is sufficiently 

 increased, a point is at last reached at which the outflow lasts 

 throughout the whole cycle of the pump, and here therefore some of 

 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, and a point will ultimately be reached 

 at which the outflow will become not only continuous but also 

 constant. The degree of constriction necessary to produce this effect 

 will depend upon the distensibility of the elastic tube. The more 

 distensible this is, the earlier will this stage be reached, and the lower 

 will be the mean pressure. This is the condition we find in the 

 circulatory system. 



Let us now apply this to the body. 



