CIRCULATION OF THE BLOOD. 



57 



" stretching back." That is, by the elastic reaction of the rubber tube 

 the jerky action of the bulb is converted into the steady flow that we 

 see. In the first experiment we had a rigid tube and practically no 

 resistance. In the second, although the tube was elastic, there was no 

 resistance, so the elasticity was not brought into play. In the third, 

 there was resistance, but the tube was inelastic. In the fourth, the 

 resistance brought into play the elasticity of the rubber tube, and the 

 elastic reaction of the tube continues (so to speak) the action of the bulb 

 between two successive strokes. In this experiment the pulse can be 

 felt in the tube. 



The Veins. The capillaries, after penetrating the tis- 

 sues, reunite to form small veins, which in turn reunite 

 to form larger ones, till finally two great veins, the caval 

 veins, precaval and postcaval, return the blood to the 

 heart. The veins, like the arteries, are smooth inside and 

 elastic (though less elastic than the arteries). They are 

 thinner than the arteries, and, in consequence, collapse 

 when the blood flows out of them, whereas the larger 

 arteries stand open, after they are emptied of blood. 



The Valves in the Veins. The only valves in the arte- 

 ries are those which we have seen at the beginning of the 

 aorta and pulmonary artery. 

 Many of the veins have 

 similar pocket-like valves, 

 though less strong than 

 those of the arteries. They 

 are usually in pairs, but some- 

 times single or in threes. It 

 is important to note that they 

 all have the mouths of the 

 pockets toward the heart, so 

 that the blood flows freely 

 toward the heart, but is prevented from flowing the other 

 way on account of the filling of the valves by the reflow 



Open Shut 



Fig. 27. Venous Valves. 



