140 



THE VASCULAE MECHANISM. 



beats. Having reached a certain level, the mercury ceases to rise any more. 

 It does not, however, remain absolutely at rest, but undergoes oscillations ; 

 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. 38 will be 



FIG. 38. 



A/A 



FIG. 39. 



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. 



described. Each of the smaller curves {p p} corresponds to a heart-beat, 

 the rise corresponding to the systole and the fall to the diastole of the ven- 

 tricle. The larger undulations (r r) in the tracing, which are respiratory 



in origin, will be discussed hereafter. 

 In Fig. 39 are given two tracings 

 taken from the carotid of a rabbit ; in 

 the lower curve the recording surface 

 is travelling more rapidly than in the 

 upper curve ; otherwise the curves are 

 alike and repeat the general features 

 of the curve from the dog. 



Description of experiment. In a carotid, 

 or other bloodvessel, prepared as explained, 

 a small glass tube, of suitable bore, called 

 a cannla is introduced by the method 

 described above, and is subsequently con- 

 nected, by means of a short piece of India- 

 rubber tubing (Fig. 37 /,) and a leaden or 

 other tube t which is at once flexible and 

 yet not extensible, with the descending 

 limb, w, of the manometer or mercury 

 gauge. The canula, tube, and descending 

 limb of the manometer are all filled with some fluid, which tends to prevent clot- 

 ting 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 



more convenient plan. The tube t is connected with the leaden tube t, and the stopcock c with 

 the manometer, of which m is the descending and m' the ascending limb, and s the support. The 

 mercury in the ascending limb bears on its surface the float fl., a long rod attached to which is 

 fitted with the pen p, writing on the recording surface r. The clamp d. at the end of the tube t 

 has an arrangement shown on a larger scale at the right hand upper corner. 



The descending tube m of the manometer and the tube t being completely filled along its whole 

 length with fluid to the exclusion of all air, the canula c is filled with fluid, slipped into the 

 open end of the thick-walled India-rubber tube i, until it meets the tube t (whose position within 

 the India-rubber tube is shown by the dotted lines), and is then securely fixed in this position by 

 the clamp cl. 



The stopcocks cand c" are now opened, and the pressure bottle raised or fluid driven in by the 

 syringe until the mercury in the manometer is raised to the required height. The clamp c" is 

 then closed and the forceps bd removed from the artery. The pressure of the blood in the carotid 

 ca is in consequence brought to bear through t upon the mercury in the manometer. 



Blood-pressure Curves from the Carotid 

 of Rabbit, the time marker in each case 

 marking seconds. 



