548 THE OUTGO OF ENERGY 



however, is not a measure of the absolute blood- 

 pressure, because it is not possible ~o estimate 

 accurately how much of the blood-pressure is 

 still held in equilibrium by the elastic tension 

 of the arterial wall and the surrounding tissues. 

 The pulse curve does give with approximate 

 correctness the variations in the blood-pressure. 

 The correctness would be complete were it not 

 that the part of the blood-pressure held in 

 equilibrium by the elastic tension of the arterial 

 wall varies with the size of the vessel, and the 

 size of the vessel increases as the blood-pressure 

 increases. Thus the portion of the blood-pres- 

 sure which fails of record constantly varies. 

 The error thus introduced is not important. 

 The sphygmograph, therefore, gives a practically 

 true record of the form of the pulse, i. e. the 

 time-relations of the changes in blood-pressure. 

 This knowledge cannot possibly be secured by 

 the palpation of the pulse. The sphygmograph, 

 it may be repeated, does not give a true record 

 of the absolute blood -pressure (hardness) or of 

 the amplitude (size) of the pulse. Both hardness 

 and amplitude are better measured by the pal- 

 pating finger. 



In many sphygmographs, for example, Marey's 

 and Dudgeon's, the pressure on the artery is 

 made by a metal spring, the movements of which 



