THE CIRCULATION OF THE BLOOD 345 



The height of the mercury as well as its oscillations in the distal limb 

 may be recorded by placing on the top of the mercury a light float, the upper 

 end of which carries a writing point. When the latter is placed in contact 

 with the moving blackened surface of a recording cylinder or kymograph, 

 the height and the oscillations are recorded in the form of a tracing similar 

 to that shown in Figs. 152 and 153, in which the smaller oscillations represent 

 the changes in pressure due to the systole and diastole of the heart and the 

 larger oscillations to variations in the average pressure due to the respiratory 

 movements. The height of the mercurial column kept in equilibrium at any 

 particular moment is determined by measuring the distance between a base- 

 line or abscissa, which represents the position of the mercury at atmospheric 



FIG. 153. A PORTION or A BLOOD-PRESSURE TRACING OBTAINED FROM THE CAROTID ARTERY 

 OF THE RABBIT WITH A MERCURIAL MANOMETER. The small oscillations are due to the heart-beat; 

 the large oscillations are due to the respiratory movements. 



pressure, and any given point on the trace above, and multiplying it by 2, 

 for the reason that the mercury sinks in the proximal limb as high as it rises 

 in the distal limb of the manometer and hence the column of mercury sup- 

 ported is that observed between the upper and lower levels of the mercury 

 in the distal and proximal limbs of the manometer. 



The blood-pressure as revealed by the tracing may be resolved into two 

 components: viz., (i) a more or less constant element represented by the 

 pressure in the arteries during the period of the cardiac diastole, which is 

 termed the diastolic or minimum pressure; and (2) a variable element 

 represented with certain limitations by that additional pressure occurring 

 at the time of the cardiac systole, which is termed the systolic or maximum 

 pressure. The diastolic pressure is represented by the distance between the 

 base-line and the points of the curve corresponding to the diastolic pause; the 

 systolic pressure, by the distance between the base-line and the apices of 



