334 TEXT-BOOK OF PHYSIOLOGY. 



sp. gr. 1060, and under a pressure approximately equal to that in the vessel 

 of the animal as determined in previous experiments. When communication 

 is established between the vessel and the cannula, the mercurial column 

 adjusts itself to the pressure in the artery and at once exhibits the same 

 cardiac oscillations and respiratory undulations as did the column of blood 

 in the previous experiment. 



The height of the mercurial column kept in equilibrium by the pressure 

 of the blood within, and the pressure of air without the vessel is that 

 between the lower level of the mercury in the proximal, and the higher level 

 in the distal limb of the manometer, both of which can be read off on a scale 

 placed between the two limbs. 



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 



FIG. 155. 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. 



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. 154 and 155, 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 

 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 



