THE VASCULAR MECHANISM. 



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balance the pressure of the blood in the artery, and to prevent 

 the blood escaping into the cannula. A writing point .attached to 

 a float resting on the' mercury in the other limb of the manometer 

 can be used to record the pressure on a moving blackened surface 

 (kymograph). 



The following method is used (fig. 55). An artery (such as the 

 carotid or femoral) is exposed in an anaesthetised animal, and the flow 

 of blood is shut off by a clip. The artery is ligatured about 2-3 cm. 

 beyond the clip, opened between the clip and the ligature, and a 

 cannula containing a half-saturated solution of sodium sulphate is tied 

 into it. This cannula is connected by thick rubber tubing with a 



FIG. 55. Apparatus for taking a blood-pressure tracing. 

 A, cannula inserted in an artery ; B, pressure bottle ; C, mercurial manometer. 



bottle containing a half-saturated solution of sodium sulphate, and with 

 one limb of the manometer. By raising the bottle the cannula and 

 connecting tubing can be filled with sodium sulphate solution under 

 such pressure that the column of mercury in the limb to which the 

 float is attached rises from 100 to 150 mm. higher than the column in 

 the limb connected with the artery. The connection between the pres- 

 sure bottle and the manometer is then shut off by a screw-clamp, and 

 the clip is removed from the artery. The column of mercury rises or 

 falls slightly until the pressure counter-balances that of the blood, and 

 the writing lever remains at a constant level, except for slight oscilla- 

 tions with each heart beat and with the respiratory movements (fig. 56). 

 The difference in height of the columns of mercury in the two limbs 

 represents the mean arterial blood pressure. 



