BLOOD-PRESSURE AND BLOOD-VELOCITY. 513 



ing the diastole. The elasticity of the arteries, in connection with 

 the peripheral resistance, makes an important difference. As the 

 heart discharges into the aorta the pressure rises, but the walls of 

 the arterial system are distended by the increased pressure, and 

 during the following diastole the recoil of these distended walls 

 maintains a flow of blood through the capillaries into the veins. 

 With a certain rapidity of heart beat the distention of the arterial 

 walls is increased to such a point that the outflow through the capil- 

 laries into the veins is as great during diastole as during systole ; the 

 rhythmical flow in the arteries becomes converted by the elastic 

 tension of the overfilled arterial system into a continuous flow in 

 the capillaries and veins. This effect may be illustrated by a simple 

 schema such as is represented in Fig. 210. A syringe bulb (a), rep- 

 resenting the heart, is connected by a short piece of rubber tubing to a 

 glass tube (6), and also by a piece of distensible band tubing (e) with 



Fig. 210. Simple schema to illustrate the factors producing a constant head of pres- 

 sure in the arterial system: a, A syringe bulb with valves, representing the heart; 6, glass 

 tube with fine point representing a path with resistance alone, but no extensibility (the out- 

 flow is in spurts synchronous with the strokes of the pump) ; c, outflow with resistance and 

 also extensible and elastic walls represented by the large rubber bag, e ; the outflow is a 

 steady stream due to the elastic recoil of the distended bag, e. 



a similar glass tube drawn to a fine point (c). In the latter case the 

 distensible, elastic tubing represents the arterial system, and the 

 fine pointed glass tube the peripheral resistance of the capillary area. 

 If the syringe bulb is put into rhythmical play and the flow is directed 

 through tube b the discharges are in rhythmical spurts, but if 

 directed through tube c the discharge is a continuous stream, 

 since the force of the separate beats becomes stored as elastic tension 

 in the walls of the band tubing, and it is this constant force which 

 drives a steady stream through the capillary point. In a general 

 way, this schema gives us a true picture of the conditions in the cir- 

 culation. The rhythmical force of the heart beat is stored as elastic 

 tension in tjie walls of the arteries, and it is the squeeze of these 

 distended walls which gives the continuous driving force that is 

 responsible for the constant flow in the capillaries and veins- 

 Enumeration of the Factors Concerned in Producing Nor- 

 mal Pressure and Velocity. In the normal circulation we may 

 33 



