284 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



worked with the hand ; the tube from it represents the arterial 

 system, the clip E the resistance of the arterioles ; C is the 

 capillary lake, from which the vein (larger than the artery) leads 

 back to the heart H. A and B are two manometers which 

 respectively indicate arterial and venous pressures. Only in place 

 of straight tubes, mercurial manometers are used. Each of these 

 is a U-tube about half filled with mercury, and united to the 

 artery or vein by a tube containing fluid. If the mercury in the 

 two limbs of the U is at the same level, the pressure of the fluid 

 in connection with one limb is exactly equal to that exerted by 



!Jnnnnnnnnnnnnnnnnnnnnnnnnnff" 



Fig. 273. Anderson Stuart's Sphygmoscope. 



the atmospheric pressure on the other. The mercury, however, 

 is pushed up in the far limb of the manometer connected to the 

 artery, the pressure there being greater than that of the atmo- 

 sphere ; this is therefore called positive pressure, and the total 

 amount of pressure, usually measured in millimetres, is the 

 difference between the levels a and a'. The manometer B attached 

 to the vein, however, indicates a negative pressure (h 6'), that is, 

 a pressure less than that of the atmosphere, so that the mercury 

 in the limb nearest the vein is sucked up. 



Anderson Stuart's sphyc/moscope (fig. 273) is a much more 

 complete schema. It consists of a long leaden tube filled with 



