186 STUDIES IN ADVANCED PHYSIOLOGY. 



may be compressed by forcing it into a hose of very elastic 

 rubber and then closing both ends. If slight openings into 

 such a distended rubber hose would be made, the water 

 would at once be forced out by the elasticity of the walls. 

 The pressure of the blood, which, of course, is but the 

 amount of compression due to the elastic walls, is, as has 

 been stated, 200 millimetres of mercury, or four pounds to 

 the square inch (in the aorta) . It is this pressure which 

 forces the blood onward. Even when the heart is not 

 beating at all, the pressure still continues in the vessels 

 keeping the stream in motion. When the heart throws in 

 its amount of blood at each beat this arterial pressure is 

 raised but very little, only three to four millimetres. Such 

 a slight change in the pressure would not affect the flow 

 through the capillaries at all, any more than the fluctuation 

 of a few pounds of pressure in the central pump of a water- 

 works station could be detected by the individual who is 

 watching the stream issue from the nozzle of his hose in a 

 distant section of the city. 



THE RATE AND TIME OF THE BLOOD FLOW. 



1. Rate. The rate at which the blood moves forward in 

 these vessels varies at different points along its course. It 

 is fastest near the heart and becomes gradually slower towards 

 the peripheral ramifications. This different rate of flow, is, 

 of course, compensated by the fact that the combined lumen 

 of the arteries and veins becomes gradually larger as we ap- 

 proach the periphery. This is easily understood by watch- 

 ing the flow of water in a river. Where the river is wide the 

 flow is sluggish, but where the banks of the river approxi- 

 mate each other the river may have quite a swift current. 

 Thus, the forward motion of the water on Lake Erie is prob- 

 ably not perceptible, but when the forward flow of the lake 

 becomes narrowed between the banks of the Niagara River it 

 may be changed into a torrent. The fact that the blood does 

 not flow as rapidly in the veins as it does in the arteries is due 

 to the same thing, the veins being correspondingly larger. 



