266 THE CiKCULATION IN THE BLOOD-VESSELS [CH. XXI. 



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 

 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 

 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 

 the force and frequency of the heart's beat causes a rise of arterial 

 pressure. 



Again, if more fluid is poured into R, there is a correspond- 

 ing rise in fluid in the manometers. This illustrates the rise of 

 pressure produced by an increase in the contents of the vascular 

 system. 



And this schema, rough though it is, also serves to illustrate the 

 third important factor in the maintenance of the blood-pressure, 

 namely, the peripheral resistance. This is done by means of the clip 

 E ; if the clip is tightened, one imitates increased constriction of the 

 arterioles ; if it is loosened, one imitates dilatation of the arterioles. 

 If it is closed entirely, the fluid in A and B rises to the same level as 

 that in R ; the pressure of R is not felt at all by C and D, which 

 empty themselves, and the flow ceases. If the clip E is only tightened 

 so as not to be quite closed, the arterial pressure (in A and B) rises, 



