282 THE CIKCTJLATION IN THE BLOOD-VESSELS. [CH. XXI. 



reservoir, R (fig. 271), along a tube, which we will imagine is 

 open at the other end. 



In the course of the tube we will suppose three upright glass 

 tubes (A, B, and D) are inserted at equal distances. Between 

 B and D there is a bladder, which may be divided into a number 

 of channels by packing it with tow to represent the capillaries, and 

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

 will, and which will roughly represent the peripheral resistance 

 produced by the constricted 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 



Fig. 271. Schema to illustrate blood-pressure. 



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 manometers or pressure measurers. 

 The lateral pressure of a fluid is equal to the forward pressure. 



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

 the difference of pressxtre brought about by gravitation ; the 

 height of the fluid in the manometers indicates that the pressiire 

 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 



