THE CIRCULATION. 



283 



such that all backward flow is 

 prevented. On filling the appara- 

 tus with water by means of a 

 funnel, it is ready for operation. 

 When any portion of this elastic 

 conduit is squeezed the liquid 

 immediately beneath the point 

 of pressure attempts to escape. 

 This it can do only in one direc- 

 tion (because of the valves), 

 thereby producing a forward 

 motion of the liquid. With each 

 compression there follows a cor- 

 responding wave, so that if 

 the compressions be numerous 

 enough the liquid will move 

 round and round within the con- 

 duit. This represents only very 

 imperfectly the circulation of 

 the blood ; in the living appara- 

 tus the impulse of the heart is 

 not at the end of the venous 

 system. 



From the operation of even 

 so simple a piece of apparatus, it 

 cannot but be noticed that the 

 circulation depends upon a dif- 

 ference of tension. Liquids al- 

 ways take the direction of the 

 pressure. The obstruction offered 

 to the blood in the presence of 

 the capillaries has a tendency to 

 increase arterial tension at the 

 expense of venous pressure. The 

 narrower and more difficult the 

 capillaries to be traversed are, 

 the greater is arterial pressure, 

 or vice versa.. The prime cause 

 of difference of pressure is ven- 

 tricular contraction, aided, how- 

 ever, by elasticity of vessels. 



Fig. 90. Weber's Schema. 



4-5 and 8-9 are two pieces of intestine of 

 the same size. 6, A piece of glass tubing. 11 

 and 2, Two wooden tubes. 1, A short piece of 

 intestine. 3, 12, Valves which open only in 

 one direction. 1 represents the ventricle. 10, 

 A funnel to let water enter the schema. 4-5, 

 The arterial system. 8-9, The venous system. 

 7, A sponge representing the capillaries. 3, 

 The semilunar valve. 12, The auriculo-ven- 

 tricular valve. 



