GENERAL FEATURES OF THE CIRCULATION 985 



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. 372). 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 peri- 

 pheral 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 hsemorrhage. 

 If a mercurial manometer be connected with the vein the pressure 

 in its interior is found to amount to only a few mm. Hg. 



FIG. 371. Scheme of blood pressure in A, the arteries ; c, capillaries ; 



and v, veins. 



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

 of blood pressure. 



By taking the pressure at different parts of the circulation we 

 obtain a distribution which is represented roughly in the accompanying 

 diagram (Fig. 371 ). 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. 



