CH. XXII.] 



SCHEMA OF THE CIRCULATION 



269 



C, a clip E, which can be tightened or loosened at will, and which 

 will roughly represent the peripheral resistance produced by the 

 arterioles. The far end of the tube is provided with a stop-cock. If 

 this stop-cock is closed there will naturally be no flow of fluid, and 

 the fluid will rise to equal heights indicated by the dotted line in all 

 the upright tubes. This shows that the pressure in all parts of the 

 tube is the same. The upright tubes which measure the lateral 

 pressure exerted by the fluid on the wall of the main tube, are called 

 pizometers, manometers, or pressure measurers. 



If now the stop-cock is opened, the fluid flows on account of the 

 difference of pressure brought about by gravitation ; the height of the 



FIG. 236. Schema to illustrate blood-pressure. 



fluid in the manometers indicates that the pressure is greatest in R, 

 less in A, less still in B, and least of all in D. 



On account of the peripheral resistance of the arterioles and 

 capillaries, the pressure is very small in the veins, as indicated by the 

 height of the fluid in the manometer D. The difference between D 

 and B is much more marked than the difference between B and A. 

 If the fluid which flows out of the end of the tube is collected in a 

 jug and poured back into R, we complete the circulation. But the 

 schema is an extremely rough one, and is especially faulty in that the 

 pressure which starts at R is nearly constant and not intermittent. 

 This may be remedied by taking R in the hand, and raising and lower- 

 ing it alternately. The fluid in the manometers bobs up and down 

 with every rise and fall of R : this is least marked in D. The greater 

 and the faster the movement of R, the greater is the rise of arterial 

 pressure. This is a rough illustration of the fact that increase in 



