212 THE CIRCULATION OF THE BLOOD 



figure 185. Of the two free ends of the T-cannula one is connected with 

 the manometer, the other with the pressure bottle. The peripheral end of the 

 cut artery is tied to obviate the escape of blood. By this means, the pressure 

 communicated to the column of mercury is the forward, and not the lateral, 

 pressure of blood, but there is very little difference. 



As soon as the experiment is begun, the writing float is seen to oscillate 

 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 ilie 

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

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



FIG. 187. Tracing of Normal Arterial Pressure Taken from the Rabbit with a Hurthle 

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



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 purpose to use the Hurthle membrane manometer than the 

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

 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 appara- 

 tus 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 appa- 

 ratus 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 



