138 



THE VASCULAR MECHANISM. 



in the tube, will be a very low one, not more than a very few centimetres 

 high, and that while the level of the column may vary a good deal, owing, 

 as we shall see later, to the influence of the respiratory movement, there will 

 not, as in the artery, be oscillations corresponding to the heart-beats. 



"We learn, then, from this simple experiment, that in the carotid of the 

 rabbit the blood while it flows through that vessel is exerting a considerable 

 mean pressure on the arterial walls, equivalent to that of a column of mer- 

 cury about 70 mm. high, but that in the jugular vein the blood exerts on 

 the venous walls a very slight mean pressure, equivalent to that of a column 

 of mercury 3 or 4 mm. high. We speak of this mean pressure exerted by 

 the blood on the walls of the bloodvessels as blood-pressure, and we say that 

 the blood- pressure in the carotid of the rabbit is very high (70 mm. Hg), 

 while that in the jugular vein is very low (only 3 or 4 mm. Hg.). 



In the normal state of things the blood flows through the carotid to the 

 arterial branches beyond, and through the jugular vein toward the heart; 

 the pressure exerted by the blood on the artery or on the vein is a lateral 

 pressure on the walls of the artery and vein, respectively. In the above ex- 

 periment the pressure measured is not exactly this, but the pressure exerted 

 at the end of the artery (or of the vein) where the tube is attached. We 

 might directly measure the lateral pressure in the carotid by somewhat 

 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 

 T-piece, and might introduce the T-piece in such a way that the blood 

 should flow along one limb (the vertical limb) of the T-piece from the 

 proximal to the distal part of the carotid, and at the same time by the other 

 (horizontal) limb of the T-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 

 T-piece. If we were to introduce into the aorta, at the place of origin of the 

 carotid, a similar (larger) T-piece, and to connect the glass tube with the 

 horizontal limb of the T-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 pressure 

 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. 



104. Such an experiment as the one described has the disadvantages 

 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. 37) 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 descending 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- 



