988 



PHYSIOLOGY 



elasticity of the tissues themselves we may take the pressure in the bag as repre- 

 senting the pressure in the tissue fluids surrounding the artery, so that the pulse - 

 obliterating pressure in the bag will correspond to the maximum or systolic 

 pressure in the artery. By a slight modification of the apparatus it is possible 

 to determine also the diastolic pressure. For this purpose the rubber bag is 

 connected also with a manometer of small inertia, giving a true representation 



FIG. 375. Erlanger's apparatus for recording systolic and diastolic 

 blood pressures. 



of the actual changes of pressure. It is evident that when the pressure in the bag 

 and in the tissues surrounding the artery exactly corresponds to the diastolic 

 pressure, the artery will be completely collapsed when the pressure arrives at 

 its lowest point and will then dilate almost to the utmost with the systolic rise 

 of pressure. If we are taking a record of the pressure changes in the bag in this 

 way, the pulse -waves as recorded by the manometer will slowly increase in size 

 as the pressure in the bag is gradually raised. At one point the waves rapidly 

 increase and reach a maximum, marking the pressure at which the artery is just 

 completely collapsed at the lowest point of each pulse -wave (the diastolic pres- 

 sure). As the pressure is still further raised the excursions of the manometer 

 tend to diminish in size, first slowly and then rapidly, and the point of rapid 

 diminution corresponds to the systolic pressure. Above this point the manometer 



