THE CIRCULATION OF TH3 BLOOD 549 



are recorded by a lever. In the record just taken 

 from the radial artery, the pressure was made by 

 the elastic tension of the rubber membrane clos- 

 ing the thistle-tube. In the case of the carotid 

 artery, this membrane is replaced by the skin of 

 the neck. 



In every instance, the sphygmograph records 

 the changes of blood-pressure in a section of the 

 artery so short in comparison with the length of 

 the whole arterial tree as to be practically a 

 cross-section. 



Low Tension Pressure-Pulse. 1. In the arti- 

 ficial scheme open slightly the side-branch that 

 permits the liquid in the arterial tubes to flow 

 out without passing through the resistance. The 

 arterial pressure will fall in consequence of the 

 diminished peripheral resistance. Normally this 

 effect is produced by a dilatation of the smaller 

 arteries. Let the arterial pressure fall to about 

 20 mm. Hg. Eecord a series of pulse curves. 



Note that the oscillations of the mercury 

 column with each ventricular beat are much 

 higher than with normal pressure (120-150 mm.). 

 Feel the pulse with the finger. With each beat 

 the artery quickly expands and as quickly re- 

 laxes. The artery is "softer" than usual. 



2. Feel the normal pulse in the radial artery. 

 Note the normal " hardness." Let the subject 



