THE VASCULAR MECHANISM. 



183 



of blood in the arteries into a continuous flow in the capillaries 

 and veins. 



They are (1) the beat of the heart,. (2) the elasticity of the arteries, 

 and (3) the peripheral resistance. 



The action of these factors is a purely mechanical one, and can be 

 reproduced in an artificial scheme such as is shown in fig. 58. A 

 reservoir R containing a coloured fluid is attached to a horizontal 

 rubber tube S T open at its other end, and a number of vertical glass 

 tubes open at the top are connected with this tube. As the fluid flows 

 from the reservoir, it rises in the vertical tubes to a height correspond- 

 ing with the pressure at that point, and the line A B joining the top of 



FIG. 58. 



the columns of fluid in the tubes shows that there is a uniform fall of 

 pressure along the rubber tube. If a screw clip is placed on the rubber 

 tube at C and gradually tightened, thereby introducing a resistance to 

 the flow of fluid along the tube, the pressure rises on the proximal side 

 and falls on the distal side of the clip, the pressure gradient along the 

 tube being indicated by the dotted line A, D, B. 



When the flow of fluid from the reservoir is made intermittent by 

 alternately compressing and releasing the connection between it and 

 the tube at short intervals, the fluid in the vertical tubes K, L, M, and 

 N, between the reservoir and the resistance at C, shows corresponding 

 oscillations in height, whereas in the tubes and P, beyond C, these 

 oscillations are absent, and fluid flows from the end of the rubber tube 

 in a steady stream. 



[n the body the reservoir is represented by the heart, which at each 

 beat sends into the aorta a certain quantity of blood (in man about 



