PHYSICAL FACTORS OF THE CIRCULATION 



183 



On rhythmically shutting and opening the compressor, it will be 

 found that the outflow from the elastic tube is continuous, from the 

 rigid it is intermittent. The increased and continuous flow is due 

 to the potential energy stored up in the stretched wall of the elastic tube. 

 'The elastij tube also delivers more fluid per minute than the former. 



In the circulatory system we are concerned with a system of elastic 

 tubes. The conditions appertaining in the circulation can best be repre- 

 sented, therefore, by a schema having elastic tubes, such as Fig. 88. 



Here H, the bulb of a Higginson syringe, represents the head of 

 pressure the heart. -Two valves, representing the mitral and aortic 

 valves, regulate the flow in one direction only. Coming away from 

 the syringe is a rubber tube (about J inch diameter), which divides 

 into two channels the arteries, leading (1) to two lamp glasses filled 



To 



Manometer 



To 

 Manameter 



To 



Mznometer 



To 



Manometer 



FIG. 88. ARTIFICIAL SCHEMA. 



with chopped rubber sponge (the capillaries), and (2) to a rubber tube 

 shut with a clamp (the short circuit). This clamp represents the 

 muscle wall of the arterioles. These are connected with the inner 

 tube of a bicycle tyre, representing the capacious venous system, and 

 this in turn to the Higginson syringe again. 



Inserted in the circuit are manometers connected by T-pieces to 

 the artery and to the vein. The whole system is filled with water, 

 air being removed by tilting the board to which the schema is fixed 

 and working the pump, but only so far that the vein is not distended, 

 and there is no positive pressure in the system when at rest. When 

 the screw clip (the arterioles) is widely open, there is little resistance 

 to flow. The outflow from the artery into the vein then ceases during 

 diastole, the conditions being the same as if the artery were a rigid tube. 

 'The variations of pressure are great both in artery and veins, and both 



