504 CIRCULATION OF BLOOD AND LYMPH. 



size of the pulse or by a sudden spreading of the limbs of the 

 pulse wave. As the pressure is dropped in steps below this 

 point the size of the pulse wave increases to a maximum, and then 

 again declines (Fig. 202). The pressure at which the maximum 

 pulse waves are obtained indicates diastolic pressure, according 

 to the principle first suggested by Marey. This principle assumes 

 that when the pressure on the outside of an artery is just equal to 

 the diastolic pressure on the inside, that is to say, to the pressure 

 in between the pulse beats, the arterial wall is in a position in 

 which it will give the widest excursion for any given force of heart 

 beat. The correctness of this principle has been demonstrated 

 upon the exposed artery. In the living person a certain amount 

 of soft compressible tissue intervenes between the cuff and the 



Fig. 205. To show the method of detecting the systolic pressure upon the tracing given 

 by the Erlanger sphygmomanometer. The pressure upon the arm is raised above systolic 

 pressure and is then dropped 5 mm. at a time, a short record being taken after each drop. 

 Records are shown for 130, 125, 120, 115, and 110 mm. At 115 mm. it will be seen that 

 the limbs of the pulse-wave show the separation or spreading which indicates the first pulse- 

 wave to get through the occluded artery, and therefore the systolic pressure. 



brachial artery, and a portion of the external pressure is used in 

 overcoming the resistance of this tissue. The readings as made 

 are, therefore, probably somewhat above the actual diastolic pres- 

 sure by an amount that has been estimated at from 6 to 10 mms. 

 of mercury. As these instruments are usually employed, measure- 

 ments are made upon the brachial artery at the level of the heart, 

 and consequently no correction need be made for a hydrostatic 

 factor, such as would be necessary, for example, if the deter- 

 minations were made on an artery in the leg in a person standing 

 erect. 



The way in which the Erlanger apparatus is used may be understood from 

 the schematic Fig. 203. a is the rubber bag, which is buckled upon the arm 

 by a leather strap. This bag communicates with the mercury manometer, b, 

 with a pressure bag, c, through the two-way stopcock, i, and through the 

 stopcock d with a rubber bag, e, contained in a glass chamber, /. This glass 



