THE OmCULATIOK OF THE BLOOD. 209 



of which the mercury may be made to rise in the tube of the manometer 

 to the level corresponding to the mean pressure of the artery experi- 

 mented with, so that the writing style simply records the variations of 

 the blood pressure above and below the mean pressure. This is done by 

 causing the saline solution, generally a saturated solution of sodium 

 carbonate or sulphate, to fill the apparatus from a bottle suspended at a 

 height, and capable of being raised or lowered as required for the pur- 

 pose, or by injecting the saline solution into the tube by means of a 

 syringe. The canula inserted and tied into the artery may be of two 

 kinds. In one case a fine glass tube is used with the end drawn out and 

 cut so that its end is oblique, and provided with a shoulder to prevent 

 its coming out easily, the peripheral end of the cut artery being 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, or a T-canula may be employed and may be tied into the two 

 ends of a divided artery, and the free arm of the T piece being made 



Fig. 175. Normal tracing of arterial pressure in the rabbit obtained with the mercurial kymo- 

 graph. The smaller undulations correspond with the heart beats; the larger curves with the respi- 

 ratory movements. (Burdon-Sanderson.) 



to communicate with the manometer. This communicates the lateral 

 blood pressure. 



As soon as the experiment is completed, 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 instead, by an inked pen), when a figure similar to fig. 

 175 will be obtained. 



This indicates two main variations of the blood pressure; the smaller 

 excursions of the lever corresponds with the systole and diastole of the 

 hearty and the large 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 only seen in records of arterial blood pressure; they are more clearly 

 marked in the arteries nearer the heart than in those more remote, in 

 the smaller arteries the amount of the pressure as well as the indication 

 of the systolic rise of pressure, being, comparatively speaking, small. 



In order to record the undulations of arterial pressure, for some pur- 

 poses it is better to use Fick's Spring Kymograph than the mercurial 

 manometer. Two forms of this instrument are shown in figs. 176 and 

 14 



