74 



EXPERIMENTAL PHYSIOLOGY. 



heart and fall to zero during the relaxation. The flow in such a 

 case would be spasmodic. On the other hand if the walls of the 

 vessels are elastic, they are stretched by each spurt of blood from 

 the heart, and then as the heart begins to relax the distended vessel 

 wall begins to contract, thus forcing the blood onward in spite of 

 the inactivity of the heart. The elastic wall therefore keeps the 

 blood pressure from falling to zero during diastole. 



If the blood meets with resistance somewhere in the path the 

 vessels on the near side are distended more than they would be if 



FIG. 22. Circulation scheme. The bulb (a) which represents the heart can 

 have its output varied by either a change in rate of compresssion or by sliding the 

 block (&) and thus changing the amplitude. The openings to the bulb are provided 

 with valves so that flow can take place only in one direction. The resistance which 

 corresponds to the capillaries is the part (c), (d). This maybe varied by means of 

 the clamp (c). The elasticity of the rubber tubing corresponds to the elasticity 

 of the arterial wall. The "venous" and "arterial" pressures are measured by mano- 

 meters (e) and (/) respectively. 



the resistance were decreased or absent. The greater the distension 

 of the tubes the longer they require to contract so that a steady flow 

 would be kept up for a longer period. When the peripheral resist- 

 ance is constricted venous flow is regular ; great dilatation produces 

 an intermittent venous flow. 



Experiment 33. Circulation Scheme. Many of the foregoing 

 principles can be demonstrated by a simple system of rubber 

 tubes (Fig. 22). A rubber bulb represents the heart. The 



