BLOOD PRESSURE AT PARTS OF VASCULAR CIRCUIT 921 



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 pressure). As the pressure 

 is still further raised, the excursions of the manometer tend to diminish in size, first 



FIG. 390. Erlanger's apparatus for recording systolic and diastolic 

 blood pressures. 



slowly and then rapidly, and the point of rapid diminution corresponds to the systolic 

 pressure. Above this point the manometer still shows small oscillations, due to the 

 impact of the unoccluded stump of the artery on the upper border of the india-rubber 

 bag. 



Many different methods have been introduced for the purpose of recording the 

 pressure oscillations in the bag. In Erlanger's apparatus the rubber bag is put into 

 connection with a thick-walled rubber ball PS contained in a glass chamber. The 

 chamber (Fig. 390) communicates with a sensitive tambour and also, by means of a 

 capillary opening provided with a stop-cock, with the external air. By this means the 

 slow expansion of the ball PS is not recorded by the tambour, which moves only with 

 the sudden oscillations of pressure due to each heart beat. With this instrument it 

 is easy to read on the accompanying mercurial manometer the point at which the 



