VELOCITY AND PRESSURE OF BLOOD-FLOW. 499 



needle valve the pressure on the artery is allowed to drop slowly. 

 The moment that the pressure falls to a point at which a pulse 

 wave can break through the constricted area a distinct sound 

 will be heard in the stethoscope. If the mercury manometer is 

 read at this point, it will give the value for the svstolic pressure. 

 As the outside pressure continues to fall the sound alters in char- 

 acter. According to the description of Ettinger* it passes through 

 five phases: 1, The initial sharp clear sound; 2, muffled with the 

 character of a murmur; 3, clear and loud; 4, dull; 5, cessation. 

 Some observers have thought that the fifth phase, the cessation of 

 the sound, marks the point of minimal or diastolic pressure, but 

 later workers seem to agree that this pressure is indicated more 



Fig. 202. — Record (Erlanger) to show the maximum size ol the recorded pulse 

 wave when the outside or extravascular pressure is equal to the internal diastolic pressure. 

 The artery is compressed first with a pressure above systolic, sufficient to obliterate the 

 lumen. As this pressure is lowered in steps of 5 mms. the recorded pulse wave increases in 

 size to a maximum and then again becomes smaller. The outside pressure with which the 

 maximum pulse is obtained measures the amount of the internal diastolic pressure (Marey s 

 principle). 



accurately by the transition from phase 3 to phase 4, at the begin- 

 ning of phase 4. The difference in pressure between this point 

 and that at which the sound disappears may be slight ordinarily, 

 but in some cases may be considerable.! If, however, for any 

 reason it is desirable to obtain a record of the systolic and dias- 

 tolic pressures, some form of sphygmomanometer must be employed 

 in which the pulse from the artery under the cuff is recorded upon 

 the smoked surface of a kymograph ion. Several instruments of 

 this kind have been described, but the one devised by Erlanger J 

 is probably the most complete. This instrument is illustrated in 

 Figs. 203 and 204, and is described in some detail below. When 

 the pressure in the cuff is raised above the systolic level the brachial 

 artery is completely closed, but the pulsations of the stump above 



* Ettinger, "Weiner Klin. Wochenschrift," 20, 992, 1907. 



t Weysse and Lutz, "American Journal of Physiology," 32, 427, 1913; and 

 Melvin and Murray, "Quarterly Journal of Experimental Physiology," 8, 125, 

 1914. 



t" American Journal of Physiology," "Proceedmgs of the American 

 Physiological Society," 6, xxii., 1902; and "Johns Hopkins Hospital Reports, 

 12, 53, 1904. 



