REDUCTION OF KYMOGnAPHIC TRACINGS. 287 



cylinder be GO millimetres, and it revolve once in a minute, 

 each 5 millimHres of paper will correspond to five seconds' 

 revolution. 



Secondly, we must ascertain the blood-pressure at different 

 times. At the point where the injection took place, we draw 

 from the tracing a perpendicular to the abscissa, and another, 

 5, 10, or 15 seconds further back. The mean pressure is most 

 readily and exactly got by means of a planimeter ; but, as this 

 is an expensive instrument and possessed by few, we usually 

 employ ruder methods. The first is to determine the square 

 superficies of the irregular figure contained by the abscissa, the 

 two perpendiculars and the curve, and then divide it by the 

 length of {he abscissa ; this gives the mean height of the 

 pressure-curve. The size of the figure is ascertained l)y placing 

 over it a piece of tracing-paper or glass ruled in square milli- 

 mHres, and counting the number of squares contained in it. 

 Volkmann cuts the figure exactly out in paper of uniform 

 texture and weighs it. By then comparing its weight with that 

 of a square of given size, the superficies of the figure is easily 

 ascertained. The second method is still simpler, and^ though 

 not so exact, takes much less time, and is, therefore, frequently 

 employed. It consists in drawing a straight line from one per- 

 pendicular to the other along the curve, so as to cut the pulse- 

 and respiration-waves as nearly as possible in their middle, and 

 leave as much of their surface aboVe as below it. We then 

 measure the height of this line above the abscissa, double it, 

 and subtract from it the fraction of the whole, which represents 

 the column of carbonate or bicarbonate of soda solution which 

 entered one limb of the manometer and pressed on the mercury 

 in it as the mercury rose in the other limb. For a solution of 

 carbonate of specific gravity 1018, this fraction will be about ^V 

 of the whole. 



Passing along the curve taken after the drug has been 

 injected, we note the place where any change in pressure has 

 occurred, and here we draw another perpendicular and proceed 

 as before. 



Thirdly, we obtain the number of pulsations and respirations 

 in a minute by counting the pulse- and respiration- waves. 



