ELEMENTS EY DEMONSTRATIONS 



121 



capillaries, (2) a rubber tube closed by a screw-clip. The screw-clip 

 represents the muscular wall of the arterioles. A mercury mano- 

 meter is connected by a J_ tube with the artery and another with the 

 vein. A loose cotton wad plug is placed in the open end of each 

 manometer to prevent the mercury being accidentally forced out. 

 The whole system is filled with water. The bulb of the syringe is 

 placed under a hinged board which lies just beneath the shaft of the 

 motor. The shaft is provided with a projecting screw, which 

 depresses the board on each revolution, and compresses the syringe. 

 When the motor is set going the water is pumped by the syringe round 



JAIRD 3 TA7LOCK. LONDON. 



FIG. 100. Artificial schema of the circulation. 



the schema. The valves act as the mitral and aortic valves. When the 

 screw-clip is widely open, there is little resistance to flow. The outflow 

 from the artery into the vein ceases during the diastole of the syringe. 

 The conditions are the same as if the artery were a rigid tube. The 

 diastolic and systolic variations of pressure are very great, and affect 

 both manometers to a like extent. Screw up the clip. The flow, as 

 the resistance increases, becomes less and less intermittent and finally 

 continuous. The mean pressure rises in the arterial and falls in the 

 venous manometer. The systolic and diastolic variations of pressure 

 become greatly reduced in the former. The systolic variation disappears 

 in the latter. When the vascular system is formed of a wide tube 

 free from constrictions, each systolic pulse-wave travels with so great a 

 velocity that the whole system reaches the same pressure before the 

 next systole of the heart occurs. The conditions are otherwise when 

 the clip is screwed up, for the friction of the blood flowing through the 

 narrow channels prevents the blood from passing with anything like 



