304 



PHYSIOLOGY. 



artery. The pressure is increased until the radial pulse disappears 

 at the wrist. The moment the radial pulse disappears, the mercurial 

 column indicates the systolic pressure in the artery of the arm. 



The instrument of Eiva-Rocci gives the systolic pressure. Mosso, 

 however, invented an instrument which gives the diastolic pressure. 

 The principle involved in Mosso's instrument is a registration of the 

 pulsations of the artery under different pressures, and rinding out 

 under what pressure maximal pulsations are obtained. This pressure 

 should be equal to the diastolic pressure inside the artery. Howell 

 and Brush have proved this on the exposed artery of a dog. 



Fig. 102. Riva-Rocci Sphygmomanometer. 



THE SPHYGMOMANOMETER OF Mosso. In this instrument the 

 two middle fingers of each hand are placed in rubber capsules inside 

 metallic tubes, and the pressure regulated to obtain the greatest 

 excursion of the mercury manometer. In this instrument water- 

 pressure is used outside an artery, and increased so as to exactly bal- 

 ance the internal pressure; then the oscillation of the arterial walls 

 will be greatest when they are fre& to move. There are several 

 sphygmomanometers in this country, and the best is that of Dr. 

 Erlanger. 



The maximum systolic and minimum diastolic pressure can be 

 determined by the Erlanger apparatus. To measure the maximum 

 systolic pressure the arm is compressed by the air-cuff until no pulse 



