Chap, iv.] THE VASCULAR MECHANISM. 159 



w^ 



Tig. 27. Blood Pressure Curves from the Carotid of Rabbit, the Time 

 Marker in each case marking Seconds. 



Description of Experiment. Into a carotid, or other blood vessel, 

 prepared as explained, a small glass tube, of suitable bore, called a 

 cannula, is introduced by the method described above, and is subse- 

 quently connected by means of a short piece of india rubber tubing (Fig. 

 25 i), and a leaden or other tube t, which is at once flexible and yet not 

 extensible, with the descending limb, m, of the manometer or mercury 

 gauge. The cannula, tube, and descending limb of the manometer are 

 all filled with some fluid which tends to prevent clotting of the 

 blood, the one chosen being generally a strong solution of sodium 

 bicarbonate, but other fluids may be chosen. In order to avoid loss 

 of blood, a quantity of fluid is injected into the flexible tube suf- 

 ficient to raise the mercury in the ascending limb of the manometer 

 to a level a very little below what may be beforehand guessed at 

 as the probable mean pressure. When the forceps bd is removed, 

 the pressure of the blood in the carotid is transmitted through the 

 flexible tube to the manometer, the level of the mercury in the ascend- 

 ing limb of which rises a little, or sinks a little at first, or may do 

 neither, according to the success with which the probable mean pres- 

 sure has been guessed, and continues to exhibit the characteristic 

 oscillations until the experiment is brought to an end by the blood 

 clotting or otherwise. 



Tracings of the movements of the column of mercury in the mano- 

 meter may be taken either on a smoked surface of a revolving cylinder 

 (Fig. 1), or by means of ink on a continuous roll of paper, as in the 

 more complex kymograph (Fig. 28). 



§ 98. By the help of the manometer applied to various 

 arteries and veins we learn the following facts : 



(1) The mean blood pressure is high in all the arteries, but 

 is greater in the larger arteries nearer the heart than in the 

 smaller arteries farther from the heart ; it diminishes, in fact, 

 along the arterial tract from the heart towards the capillaries. 



(2) The mean blood pressure is low in the veins, but is greater 

 in the smaller veins nearer the capillaries than in the larger veins 

 nearer the heart, diminishing, in fact, from the capillaries towards 

 the heart. In the large veins near the heart it may be negative, 



