206 BLOOD PRESSURE. [BOOK i. 



might directly measure the lateral pressure in the carotid by some- 

 what modifying the procedure described above. We might connect 

 the carotid with a tube the end of which was not straight but 

 made in the form of a | piece, and might introduce the | piece 

 in such a way that the blood should flow along one limb (the 

 vertical limb) of the | piece from the proximal to the distal part 

 of the carotid, and at the same time by the other (horizontal) limb 

 of the I piece into the main upright part of the glass tube. The 

 column of blood in the tube would then be a measure of the 

 pressure which the blood as it is flowing along the carotid is 

 exerting on a portion of its walls corresponding to the mouth of 

 the horizontal limb of the H piece. If we were to introduce 

 into the aorta, at the place of origin of the carotid, a similar 

 (larger) | piece and to connect the glass tube with the horizontal 

 limb of the h- piece by a piece of elastic tubing of the same length 

 and bore as the carotid, the column of blood rising up in the tube 

 would be the measure of the lateral pressure exerted by the blood 

 on the walls of the aorta at the origin of the carotid artery and 

 transmitted to the rigid glass tube through a certain length of 

 elastic tubing. And indeed what is measured in the experiment 

 previously described is not the lateral pressiire in the carotid itself 

 at the spot where the glass tube is introduced, but the lateral 

 pressure of the aorta at the origin of the carotid modified by the 

 influences exerted by the length of the carotid between its origin 

 and the spot where the tube is introduced. 



115. Such an experiment as the one described has the dis- 

 advantages that the animal is weakened by the loss of the blood 

 which goes to form the column in the tube, and that the blood in 

 the tube soon clots and so brings the experiment to an end. 

 Blood pressure may be more conveniently studied by connecting 

 the interior of the artery (or vein) with a mercury gauge or 

 manometer, Fig. 26, the proximal, descending limb of which, m, 

 is filled above the mercury with some innocuous fluid, as is also 

 the tube connecting the manometer with -the artery. Using such 

 an instrument we should observe very much the same facts as in 

 the more simple experiment. 



Immediately that communication is established between the 

 interior of the artery and the manometer, blood rushes from the 

 former into the latter, driving some of the mercury from the de- 

 scending limb, m, into the ascending limb, m', and thus causing 

 the level of the mercury in the ascending limb to rise rapidly. 

 This rise is marked by jerks corresponding with the heart beats. 

 Having reached a certain level, the mercury ceases to rise any 

 more. It does not, however, remain absolutely at rest, but under- 

 goes oscillations ; it keeps rising and falling. Each rise, which is 

 very slight compared with the total height to which the mercury 

 has risen, has the same rhythm as the systole of the ventricle. 

 Similarly, each fall corresponds with the diastole. 



