THE MAIN FACTS OF THE CIRCULATION. 



141 



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 transmit- 

 ted 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 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 manometer may 

 be taken either on a smoked surface of a revolving cylinder ( Fig. 11 ), or by means 

 of a brush and ink on a continuous roll of paper, as in the more complex kymo- 

 graph (Fig. 40). 



105. 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 further from 



FIG. 40. 



Ludwig's Kymograph for Recording on a Continuous Roll of Paper. 



the heart ; it diminishes, in fact, along the arterial tract from the heart 

 toward 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 toward the heart. In the large 

 veins near the heart it may be negative, that is to say, the pressure of blood 

 in the vein bearing on the proximal descending limb of the manometer may 

 be less than the pressure of the atmosphere on the ascending distal limb, so 



