THE CIRCULATION OF THE BLOOD. 349 



1911). It consists in the interpretation of certain sounds heard with the 

 stethoscope, in the artery under observation when it is gradually released from a 

 pressure that has obliterated its lumen in a given region. 



In the employment of this method the brachial artery is selected and 

 compressed in the usual manner with a wide cuff in connection with a 

 graduated mercurial manometer. 



After the pulse has been obliterated the stethoscope is placed over the 

 artery below the cuff, care being taken to prevent undue pressure. On 

 releasing the pressure in the cuff very gradually as in the employment of 

 other methods a series of sounds corresponding with each arterial pulsation 

 is heard as the pressure falls from the systolic to the diastolic level. The series 

 of events are spoken of as phases of which five are recognized. 



The first phase is characterized by a loud clear-cut snapping sound : 

 the second phase is characterized by a series of murmurs; the third phase 

 by a succession of loud clear snapping sounds which resemble very closely 

 those of the first phase but are less loud; the fourth phase is inaugurated by 

 a sudden decrease in the intensity of the murmurs of the third phase giving 

 rise to what is described as a dull tone that rapidly becomes weaker and soon 

 fades away; the fifth phase is one of silence. 



These phases which are sharply defined and easily distinguishable are 

 believed to be associated with vibrations of the arterial walls. The first 

 sound is generally believed to be due to the sudden distention of the artery, 

 by the inrush of blood beneath the cuff, and indicates the systolic pressure 

 which can be at once observed by the height of the mercury in the manom- 

 eter. This sound lasts until the pressure falls about 14 millimeters. The 

 second sound, a succession of murmurs, is believed to be caused by whirl- 

 pool eddies in the blood stream as it is propelled from the ' partially con- 

 stricted artery into the non- constricted region below the cuff. These murmurs 

 last until the pressure falls about 20 millimeters. The third sound is at- 

 tributed to the vibration of the arterial wall but as the lumen of the artery 

 is so much greater than that of the compressed portion the rapidity of the 

 current is less and hence the sound is neither so sharp nor pronounced. 

 It lasts until the pressure falls about 6 millimeters. The transition from the 

 second to the third sound involves a fall of about 5 millimeters. The disap- 

 pearance of the sounds is coincident with the return of the artery to its nor- 

 mal size and hence a cessation of the vibration. It therefore indicates the 

 diastolic pressure, which can at once be observed by the height of the mer- 

 cury in the manometer. 



The systolic pressure obtained by this method corresponds to the first 

 sound that is heard over the brachial artery and is about 130 millimeters; 

 the diastolic pressure, corresponds with the cessation of all sounds and is 

 about 85 millimeters. The pulse pressure is therefore 45 millimeters. 



In pathologic states of the vascular apparatus the duration and intensity 

 of the sounds undergo considerable modification. In some diseases they are 

 quite characteristic and hence have both a diagnostic and therapeutic value. 



THE VELOCITY OF THE BLOOD. 



From the number of heart-beats per minute, 72, and the amount of blood 

 discharged from the left ventricle at each beat, 80 c.c., it is evident that the 



