3 o4 TEXT-BOOK OF PHYSIOLOGY. 



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. 140 

 and 141, in which the larger curves represent the respiratory, the 

 smaller curves the cardiac oscillations. 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 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 at it rises 

 in the distal limb of the manometer. 



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

 into two components : viz., (i) the pressure in the arteries during the 

 period of the cardiac diastole, which is termed the arterial pres- 

 sure; and (2) that additional pressure occurring at the time of the 

 cardiac systole, which is termed the cardiac pressure. The arte- 

 rial pressure is represented by the distance between the base-line 

 and the points of the curve corresponding to the diastolic rest; the 

 cardiac pressure, by the increase of distance between these points 

 and the apices of the smaller oscillations. The relation of these two 

 components varies in different animals and in the same animal at 

 different times. If the arterial pressure is low, the cardiac increase 

 may be considerable; if the former is high, the latter may be slight. 

 The relation, however, of these components is not so accurately shown 

 by the mercurial manometer, owing to the inertia of the mercury, as 

 by one of the various forms of quickly responsive spring manometers 

 used in determining the rapid variations of intra-cardiac pressure. 

 These instruments show a much larger rise of pressure during the 

 systole, often amounting to as much as one-third or one-fourth of the 

 arterial pressure. 



In a series of experiments it will be found that the arterial pres- 

 sure, though rising and falling a certain number of millimeters, yet 

 retains a fairly constant general average, the result of an adjustment 

 between the number of heart-beats per minute and the amount of the 

 resistance^offered to the escape of blood into the capillaries and veins. 

 In a. tracing in which the respiratory undulations are absent, the 

 arterial pressure, plus one-half of the cardiac increase, represents the 

 mean arterial pressure. If the respiratory undulations are present, 

 as is generally the case, the mean pressure may be represented by a 

 line drawn horizontally across the tracing midway between the apex 

 and trough of the undulation. 



Estimates of the Blood-pressure. By means of the kymo- 

 graphic methods previously mentioned the pressure in the larger 

 arteries has been determined for all classes of animals. In the carotid 



