METHODS OF MEASURING ARTERIAL BLOOD PRESSURE 191 



in a regular manner, and a curve of blood pressure is traced upon the smoked 

 paper by the style (or, if a continuous roll of unsmoked paper be used, the 

 trace is made by an inked pen) when a figure similar to figure 186 will be 

 obtained. This indicates two main variations of the blood pressure. The 

 smaller excursions of the lever correspond with the systole and diastole of the 

 heart, and the larger curves correspond with the respirations, being called 

 the respiratory undulations of blood pressure, to which attention will be directed 

 in the next chapter. Of course, the undulations spoken of are seen only in 

 records of arterial blood pressure. They are more clearly marked in the ar- 

 teries nearer the heart than in those more remote. The amount of the 

 pressure in the smaller arteries as well as the indication of the systolic rise 

 of pressure is, comparatively speaking, small. 



In order to record the details of the undulations of arterial pressure, it is 

 better for some purposes to use the Hiirthle membrane manometer than the 

 mercurial manometer. Two views of this instrument are shown in figure 166. 



FIG. 187. Tracing of Normal Arterial Pressure Taken from the Rabbit with a Hiirthle Manom- 

 eter. The horizontal lines show zero pressure. Time in seconds. (Dreyer.) 



The instrument consists of a hollow tube and cup covered with rubber sheet 

 against which a disc supported by a metal spring is adjusted. The apparatus 

 is filled with fluid, the interior of which is connected with the artery by means 

 of a metal tube and cannula. The pressure transmitted to the apparatus 

 tends to stretch the rubber and bend the spring, and the movement thus 

 produced is communicated by means of a lever to a writing style and so to 

 a recording apparatus. This instrument obviates the errors which might 

 be caused by the inertia of the mercury in the mercurial manometer; it also 

 shows in more detail the variations of the blood pressure in the vessel during 

 and after each individual beat of the heart. 



As regards the actual amount of blood pressure, from observations which 

 have been made by means of the mercurial manometer, it has been found 



