CH. XXL] 



SCHEMA OF THE CIRCULATION 



267 



and the venous pressure (in D) falls ; if the clip is freely opened, the 

 arterial pressure falls, and the venous pressure rises. 



These same facts can be demonstrated by a more perfect circula- 

 tion schema, such as is represented in fig. 268. 



The heart (H) is represented by a Higginson's syringe, which is 

 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 (J-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 (J is at the same level, 



FIG. 268. Schema of the circulation. 



the pressure of the fluid in connection with one limb is exactly equal to 

 that exerted by 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 atmos- 

 phere ; this is therefore called positive pressure, and the total amount 

 of pressure is measured by the difference between the levels a and a. 

 The manometer B attached to the vein, however, indicates a negative 

 pressure (b &'), 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 kymoscope (fig. 269) is a more complete schema. 

 It consists of a long leaden tube filled with fluid, the two ends of 

 which are connected by an india-rubber tube on which is a valved 

 syringe to represent the heart. On the course of the tube are a large 

 number of open-mouthed upright manometers which indicate the pres- 

 sure when the syringe is worked, and confer on the tube the elasticity 



