CIRCULATION.] PHYSIOLOGY. 65 



and go on their way as if nothing had happened. 

 But suppose they were journeying the other way, 

 from the heart to the capii^tries. When they came 

 to the open mouth of a watch-pocket valve, some 

 of them would be sure to run into the pocket, and 

 then the pocket would bulge out, and the more it 

 bulged out the more blood would run into it, until 

 at last it would be so firil of blood that it would 

 press close against the top of the vein, as is shown 

 in Fig. 7 (or, if there were two or three, they 



1 ■ r • I 



I : C 



H " 



♦ " 



Fig. t .~-Diagrainntatic Sections of Vehis with Valves, 



In the upper, the blood is supposed to be flowing in the direction of the 

 arrow, towards the heart ; in the lower, the reverse w.iy. C, capillary side ; 

 H, heart side. 



would all meet together) and so quite block the vein 

 up. If you doubt this, make a watch-pocket out of a 

 piece of silk or cotton, fasten it on to a piece of 

 brown paper, and roll the paper up into a tube, so 

 that the valve is nicely inside the tube. If you 

 pour some peas down the tube with the mouth of the 

 valve looking away from you, they will run through 

 at once ; but if you try to pour them the other way, 

 your tube will soon be choked, and if you carefully 

 unroll the tube you will find the watch-pocket 

 crammed full of peas, - 



F 



