CHAP, iv.] THE VASCULAR MECHANISM. 207 





FIG. 27. BLOOD-PKESSURE 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 tilled with some fluid, which tends to prevent clotting of the 

 blood, the one chosen being generally a strong solution (sp. gr. 1083) 

 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 sufficient 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 are 

 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 

 ascending limb of which falls a little, or sinks a little at first, or 

 may do neither, according to the success with which the probable 

 mean pressure has been guessed, and continues to exhibit the cha- 

 racteristic 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 a brush and ink on a continuous roll of paper, 

 as in the more complex kymograph (Fig. 28). 



116. 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, 



