222 THE CIRCULATION OF THE BLOOD 



cent, magnesium sulphate, to fill the apparatus from a bottle suspended at 

 a height about that of the pressure to be measured, and capable of being 

 raised or lowered as required for the purpose. 



The cannula inserted and tied into the artery may be of several different 

 kinds. A glass T-tube with the end drawn out and cut so that it is oblique, 

 and provided with a slightly constricted neck to prevent its coming out of 

 the artery easily, is a very convenient form, figure 184. 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 presssure communicated to the 

 column of mercury is the forward, and not the lateral, pressure of blood, 

 but there is very little if any 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 the 

 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 purposes to use the Hurthle membrane manometer than the 

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



