GENERAL FEATURES OF THE CIRCULATION 873 



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 mano- 

 meter 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 

 rilled 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 cut off so that the manometer remains in connection only 

 with the cannula, the mercury in one limb being 150 millimetres above 

 that in the other. The 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 



FIG. 375. Scheme of blood-pressure in A, the arteries ; c, capillaries ; and v, veins. 



oo, line of no pressure; LV, left ventricle; RA, right auricle; BP, height of 



blood-pressure. 



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. 376). 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. 

 If 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 man- 

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

 amount to only a few mm. Hg. 



By taking the pressure at different parts of the circulation we obtain 

 a distribution which is represented roughly in the accompanying diagram 

 (Fig. 375). The blood pressure, which is about 100 to 120 mm. Hg in 

 the large arteries near the heart, falls only slowly in these arteries, so that 

 in the radial artery it is not very much below that in the aorta. Between 

 the medium- sized arteries and capillaries there is a very extensive fall of 

 pressure as the blood passes through the arterioles, so that in the capillaries 

 the pressure on an average may be taken as 20 to 40 mm. Hg ; from the 

 capillaries to veins the blood-pressure falls steadily until in the big veins 



near the heart it may be negative. 



28* 



