ELEMENTARY EXPERIMENTAL PHYSIOLOGY 133 



inserted in the course of a glass tube will act better than a rubber tube. 

 The middle of a piece of glass tube is drawn out into capillary size 

 and divided, so that outflow orifices are obtained of the same size. 

 Rhythmically open and shut the compressor. The flow from the rigid 

 tube is intermittent, while from the elastic tube it is continuous. The 

 latter delivers more fluid in one minute than the former. Observe that 

 the outflow from the elastic tube becomes intermittent when the outflow 

 orifice is enlarged. The increased and continuous flow from the elastic 

 tube is due to the potential energy stored up in the stretched wall of 

 the tube, which maintains the flow during diastole. 



The Artificial Schema. The two ends of a Higginson syringe B are 

 connected with a soft rubber tube about \ inch in diameter and a yard 

 long. The tube divides into two channels ; ( 1 ) a short length of wide 

 glass tube (a lamp chimney with a cork will do) filled with shot repre- 

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

 screw-clip represents the muscular wall of the arterioles. These are 

 connected with the inner tube of a bicycle tyre, which represents the 

 capacious venous system. A mercury manometer 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 system is filled with 

 water, and air removed by tilting the board to which the schema is 

 fixed and working the pump; the air is allowed to escape through a 

 side tube at the top of the schema. The schema is filled only so far 

 that the vein is not distended and there is no positive pressure when 

 the pump is at rest, so as to represent the conditions in the dead body. 

 The bulb of the syringe may be worked by hand. 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 manometer. The systolic and diastolic 

 variations of pressure become greatly reduced. The systolic variation 

 disappears in the venous manometer. 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 



