The only accurate method of determining both the systolic and diastolic pressure, but especially the diastolic, is by the so-called auscultatory method. (See Fig. 25.) The cuff is adjusted in the usual way and one places the bell of a binaural stethoscope over the brachial artery from one to two centimeters below the lower edge of the cuff.[3] Care must be taken that the bell is not pressed too firmly against the arm and that the edge of the bell nearest the cuff is not pressed more firmly than the opposite end. For this purpose, one can not use the ordinary Bowles stethoscope or any of the other much lauded stethoscopes, because the surface of the bell is too large. The diameter of the bell must not be more than twenty-five millimeters, twenty is still better. It is advisable before beginning the observation to locate with the finger the pulse in the brachial artery just above the elbow, so that the stethoscope may be placed over the course of the artery. (Fig. 26.) The first wave which comes through is heard as a click, and occurs at a point on the manometer or dial scale from 5-10 mm. higher than can usually be palpated at the radial artery. This is the true systolic pressure. By keeping the bell of the stethoscope over the brachial artery while the pressure is falling, one comes to a point when all sound suddenly ceases. This is said to be the diastolic pressure. This is incorrect as will be shown later. [3] A firm makes a stethoscope so that the bell is clamped on the arm leaving both the operator's hands free. [Illustration: Fig. 25.--Observation by the auscultatory method and a mercury instrument. One hand regulates the stop cock which releases air gradually.] [Illustration: Fig. 26.--Observation by the auscultatory method and a dial instrument. The right hand holds the bulb and regulates the air valve.] =Arterial Pressure= The arterial pressure in the large arteries undergoes extensive fluctuations with every heart beat. The maximum pressure produced by the systole of the left ventricle of the heart is known as the =maximum= or =systolic pressure=. It practically equals the intraventricular pressure. The minimum pressure in the artery, the pressure at the end of diastole, is called the =diastolic pressure=. The difference between the systolic and diastolic pressures is known as the =pulse pressure=. There is yet another term known as the =mean pressure=. For convenience, this may be said to be the arithmetical mean of the systolic and diastolic pressures. Actually, however, this can not be the case, owing to the form of the pulse wave, which is not a uniform rise and fall--the upstroke being a straight line, but the downstroke being broken usually by two notches. We do not make use of the mean pressure in recording results. It is of experimental interest and needs only to be mentioned here.