CHAP, iv.] THE VASCULAR MECHANISM. 121 



beats, though the flow does not cease between the jets. The blood 

 is ejected with considerable force; thus, in Dr Stephen Hales' 

 experiments, when the crural artery of a mare was severed, the jet, 

 even after much loss of blood, rose to the height of two feet. The 

 larger the artery and the nearer to the heart, the greater the force 

 with which the blood issues, and the more marked the intermittence 

 of the flow. The flow from the distal section may be very slight, 

 or may take place with considerable force and marked intermittence, 

 according to the amount of collateral communication. 



Arterial pressure. If a mercury (or other) manometer, 

 Fig. 17m, m, be connected with a large artery, e.g. the carotid, in 

 such a way that while the blood is allowed to flow uninterruptedly 

 along the artery, there is free communication between the interior 

 of the artery and the proximal (descending) limb of the manometer, 

 the following facts are observed. 



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 into the ascending limb, 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 undergoes oscilla- 

 tions ; 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. 



If a float, swimming on the top of the mercury in the ascending 

 limb of the manometer, and bearing a brush or other marker, be 

 brought to bear on a travelling surface, some such tracing as that 

 represented in Fig. 18 will be described. Each of the smaller 



FIG. 18. TRACING OF ARTERIAL PRESSURE WITH A MERCURY MANOMETER. 



The smaller curves p p are the pulse-curves. The space from r to r embraces 

 a respiratory undulation. The tracing is taken from a dog, and the irregularities 

 visible in it are those frequently met with in this animaL 



curves (p, p) corresponds to a heart-beat, the rise corresponding to 

 the systole and the fall to the diastole of the ventricle. The larger 

 undulations (r, r) in the tracing, which are respiratory in origin, 



