VELOCITY AND PRESSURE OF BLOOD-FLOW. 505 



chamber communicates above with a sensitive tambour, h, and by means of the 

 stopcock g can be placed in communication with the outside air. The systolic 

 pressure may be determined in two ways: By one method only the mercury 

 manometer is necessary, the instrument corresponding with the Riva-Rocci 

 apparatus described above. By means of the pressure bag, c, the bag, a, up 

 the arm is blown up until the pressure is above the systolic pressure and the 

 radial pulse below disappears. By turning stopcock i properly the system is 

 allowed to communicate with the air through a capillary opening, k. Conse- 

 quently the pressure upon the artery in the arm falls slowly, and by palpating 

 the radial artery one can determine the pressure, as measured by the mercury 

 manometer, at which the pulse just gets through. This pressure will measure 

 approximately the systolic pressure. The second method (method of v. 

 Recklinghausen) gives higher and doubtless more accurate results. In this 

 method the pressure is at first raised above systolic pressure with stopcocks 

 d and g open, a, e, and b are under the same pressure. If stopcock g is now 

 turned off, the pulsations in a are transmitted to e and through it to the 

 tambour, h, and the lever of the tambour writes these pulsations on a kymo- 

 graphion. It should be explained that pulsations are obtained even when 

 the pressure on the arm is much more than sufficient to completely obliterate 

 the brachial artery. The reason for this is that the pulsations of the central 

 stump of the closed artery will be communicated to bag a. When the pressure 

 is suprasystolic these pulsations are small. If now the pressure in the system 

 is diminished slowly by turning stopcock i so as to communicate with the 

 capillary opening, fc, it will be found that at a certain point the pulsations 

 suddenly increase in height (Fig. 205). This point marks the moment when 

 the pulse wave is first able to break through the brachial artery, and it gives, 

 therefore, the systolic pressure. In many cases this method of determining 

 the point of systolic pressure is not satisfactory, since the pulse waves increase 

 gradually in amplitude without a sudden break, or perhaps there is more 

 than one place at which a sudden increase occurs. A more reliable method 

 according to Erlanger is to note the point at which the ascending and descend- 

 ing limbs of the pulse wave show a noticeable separation (Fig. 205). "At 

 the moment the pressure on the artery falls below systolic, blood succeeds in 

 making its way beneath the cuff. This must be squeezed out before the lever 

 can return to the base line, whereas at higher pressures the lever is raised 

 only through the hydraulic-ram action of the pulse wave upon the upper 

 edge of the cuff." After finding the systolic pressure the diastolic pressure 

 is obtained by allowing the pressure to drop still further. The pulsations 

 increase in height to a maximum size and then decrease. The pressure at 

 which the maximum pulse wave is obtained marks the diastolic pressure. 

 It is better perhaps in dropping the pressure for this last purpose to manipu- 

 late stopcock i so as to drop the pressure 5 mms. at a time, recording the pulse 

 wave at each pressure. In this way a record is obtained such as is given in 

 Fig. 202. It should be added, also, that in order to keep the lever of the 

 tambour horizontal while the pressure in the system is being lowered there 

 is a minute pinhole in the metal bottom of the tambour. Through this 

 pinhole the pressure in the tambour and chamber, /, is kept atmospheric 

 throughout, except during the quick changes caused by the pulse waves. 

 By means of this instrument one can determine within a minute or so the 

 amount of the systolic and diastolic pressure in the brachial artery, and also, 

 of course, the difference between the two, the pulse pressure, which may be 

 taken as an indication of the force of the heart-beat. 



The Normal Arterial Pressure in Man and Its Variations. 



By means of one or other of the instruments devised for the 

 purpose, numerous results have been obtained regarding the 

 blood-pressure in man at different ages and under varying normal 

 and abnormal conditions. Unfortunately, the methods used have 

 not always been complete. Some authors give only systolic 

 pressures, for example. In such experiments also a troublesome 



