GENERAL FEATURES OF THE CIRCULATION 



917 



tion of such long tubes is awkward, and the blood which escapes into the 

 tubes very soon clots and renders further observation impossible. It is 

 therefore customary when we desire to gain an idea of the average pressure 

 in any blood vessel, especially in an artery, to use a mercurial manometer 

 for this purpose. 



This instrument, which was first applied to physiological purposes by Ludwig, con- 

 sists of a U-tube with two vertical limbs about eighteen inches in height, which is half- 

 filled with clean mercury. On the surface of the mercury of one limb is a float of 

 vulcanite from which a stiff fine rod of straw, glass, or steel rises, bearing on its upper 

 end the writing-point. This point may be adjusted so as to write on the blackened 

 glazed surface of a moving sheet of paper (Fig. 385). (The arrangement for imparting 

 a continuous movement to a sheet of 

 glazed paper is known as a kymograph. ) 

 Instead of smoking the paper, a pen may 

 be fitted to the end of a rod and its 

 excursions recorded in ink on a moving 

 band of white paper. The other limb 

 of the manometer is connected by a 

 flexible inextensible tube with a small 

 tube or cannula which is tied into the 

 central end of an artery, a clip being 

 previously placed on the artery so as 

 to prevent the escape of blood during 

 the insertion of the cannula. To the 

 manometer is connected a three-way 

 tap by means of which the manometer 

 can be placed in communication with 



the artery alone, or with the artery and 



a pressure bottle. By means of the 



latter the whole system is filled with 



magnesium sulphate solution (25 per 



cent.) or a half -saturated solution of 



sodium sulphate, at a pressure of 150 



mm. Hg. The pressure bottle is then FIG. 385. Arrangement of an apparatus for. 



cut off so that the manometer remains takin g blood-pressure tracing. 



in connection only with the cannula, the a > art , er y (carotid); c, cannula; d, three 



i r-^ MI- wa Y cock; m, mercurial manometer; &, drum 



mercury m one limb being 150 milh- covered with smoked paper; x, tube to press^e" 



metres above that in the other. The bottle. 



clip is then taken off the artery. The 



pressure in the cannula being greater than that in the artery, a small amount of the 



fluid used to fill the tubes runs into the circulation. The mercury in the manometer 



drops to a height of 100 to 120 mm. Hg. and stays about that level, rising and falling 



slightly with each heart beat (Fig. 387). The blood which enters the cannula at each 



heart beat does not clot for a considerable time owing to its admixture with the saline 



fluid used for filling the cannula and connecting tubes. 







If a vein be ligatured, it swells up on the distal side of the ligature. 



the vein be cut across, blood escapes chiefly from the peripheral end, 

 and instead of spurting out to a considerable distance with each heart 

 beat it flows steadily, but with very little force, so that light pressure by 

 a bandage is sufficient to restrain the haemorrhage. If a mercurial mano- 

 meter be connected with the vein, the pressure in its interior is found to 



ount to only a few mm. Hg. 



am ' 



