BY DR. BURDON-S ANDERSON. 223 



soda solution enters the artery and produces a slight and transi- 

 tory disturbance of the circulation. If, indeed, the previously 

 existing pressure in the artery tube is somewhat less than that 

 of the artery, no such effect occurs ; but inasmuch as we have 

 no means of knowing the arterial pressure of any particular 

 animal beforehand, it is usually unavoidable. 



A kymographic observation may last a few minutes or several 

 hours, according to the question to be investigated. In the 

 latter case, tracings are taken at intervals. Two persons are 

 required, one of whom performs the experiment, while the other 

 undertakes the charge of the. writing apparatus, and notes on 

 the paper-roll, with a soft pencil, the events as they occur and 

 the times of beginning eacli tracing. In this way the roll 

 stands in the place of a protocol, and is less liable to errors of 

 time'and order than any other kind of record. 



35. Measurement of absolute Arterial Pressure at 

 any given moment during the period of observation. 

 For this purpose it is necessary to draw the abscissa of the 

 pressure curve, i. e., the horizontal line which the writer would 

 have drawn had the arterial pressure been equal to that of the 

 atmosphere. This is accomplished immediately after the ter- 

 mination of the experiment, by closing the stem of the canula 

 and then removing it from the artery, and immersing it in a 

 capsule containing soda solution, standing at a level equal to 

 that of the artery. The clip having been opened, the clock- 

 work is set in motion for a moment, and a horizontal line drawn 

 which coincides with the abscissa required. In this line the 

 paper is then pierced with a pointed instrument in such a way 

 us to perforate the several layers of paper at the same level. 

 By removing the roll from the cylinder and connecting the 

 holes, a horizontal straight line is obtained which runs from 

 end to end of the record. By drawing an ordinate from any 

 point in the tracing to this line, measuring its length in milli- 

 metres and doubling the result, the absolute arterial pressure 

 at the corresponding moment is obtained in millimetres of 

 mercury. 



The mean arterial pressure is obtained by drawing ordi- 

 nates at regular intervals and measuring the length of each. 

 The mean of the lengths corresponding to the period investi- 

 gated, multiplied by two, is the mean pressure required. [I 

 never use paper divided into squares in other words, with the 

 ordinates readj^ measured finding by experience that they do 

 not tend to accuracy. Moreover, such paper is expensive, and 

 thereby furnishes an inducement for an undesirable economy 

 in its use.] In all normal kymographic records it is seen that 

 the arterial expansions due to the contractions of the left ven- 

 tricle are indicated by oscillations which differ very materially 

 in form, and that these differences are dependent on their ire- 



