360 TEXT-BOOK OF PHYSIOLOGY 



an elastic pressure applied to the outside of an artery is just sufficient to 

 equalize the diastolic pressure within. Inasmuch as these pulsations can 

 be transmitted to, taken up and reproduced by a mercurial column in connec- 

 tion with the pressure appliances, it becomes possible, when the maximum 

 oscillation of the mercurial column is attained, to read off the diastolic 

 pressure. 



The truth of this suggestion was subsequently demonstrated by Howell 

 and Brush. These experimenters connected the right carotid artery of a 

 dog with a mercurial manometer, interposing along the course of the con- 

 necting tube a maximum and a minimum valve. The left carotid artery 

 was surrounded by a plethysmograph which was connected, with both a 

 mercurial and a spring manometer, the former for the purpose of indicating 

 the pressure necessary to obtain the greatest oscillation, the latter for the pur- 

 pose of magnifying and recording the pulsation. When the observations were 

 simultaneously made it was found that the diastolic pressure in the right 

 carotid measured by the minimum manometer was almost exactly equal 

 to the pressure measured by the manometer in connection with the sphyg- 

 momanometer surrounding the left carotid artery, when it was exhibiting 

 its maximum excursions. The difference in the results of the two sides 

 scarcely exceeded more than one or two millimerers of mercury. It was, 

 therefore, established that the greatest oscillations record diastolic pressure. 



Many forms of sphygmomanometers adapted for clinic purposes, with 

 which both the systolic and the diastolic pressures can be obtained, have been 

 devised. With all forms however, the pressure is applied to the arm by a rubber 

 armlet which should be at least 8 centimeters wide. This is the widest 

 armlet that can be adjusted to the average-sized arm and presents distinct 

 advantages over narrower armlets. This armlet is prevented from expand- 

 ing outward by a cuff composed of some unyielding material held in position 

 by straps which completely encircle the cuff. The rubber armlet is con- 

 nected by stiff-walled rubber tubes with a mercurial manometer on the one 

 hand and with a rubber bulb or air piston on the other hand. In using the 

 apparatus the pressure is raised by forcing air into the closed system dis- 

 tending the rubber armlet and with the same degree of force changing the 

 levels of the mercury in the manometer, until the pulse is no longer felt at 

 the wrist. By special devices the pressure is then carefully lowered until 

 the mercurial column begins to fall. At a given level it exhibits a consider- 

 able oscillation which may be mistaken for the actual systolic pressure but 

 which is probably due to the impact of the blood against the upper edge of 

 the rubber portion of the cuff. If the column of mercury be still further 

 lowered so that the pressure indicated is a trifle lower than the systolic pres- 

 sure the blood will be forced through the compressed artery and give rise 

 to a pulse wave, which may be felt at the wrist. The highest excursion of 

 the mercurial column noted by the eye at the moment the pulse reappears 

 is regarded as the systolic pressure. 



The pressure is then lowered 5 millimeters at a time and the oscillations of 

 the mercurial column noted. As the pressure is thus slowly lowered there 

 will come a moment when the oscillations will attain a maximum value and 

 beyond which the oscillations again diminish. The lowest level of the 

 mercury column at the time of the sudden termination of the greatest 

 oscillation is taken as the diastolic pressure. 



