54 



SCIENCE PRIMERS. 



[§v. 



you, poor little corpuscle, would seem quite lost in 

 it. Had you anyone to ask, they would tell you it was 

 the main artery of the arm. Toiling onwards through 

 this, and passing a few but for the most part large 

 openings, you would suddenly tumble into a space 

 so vast that at first you would hardly be able to 

 realize that it was the tunnel of an artery like those 

 in which you had been joume)dng. This you would 

 learn to be the aorta, the great artery of all ; and 

 a little further on you would be in the heart. 



Suppose now you retraced your steps, suppose you 

 returned from the aorta to the main artery of the 

 arm, and thus back through narrower and narrower 

 tunnels till you came again to the spot from which 

 you started, and then tried the other end of the 

 capillary. You would find that that led you also, 

 in a very similar way, inio wider and wider passages. 

 Only you could not help noticing that though the 

 inside of all the passages was as smooth as before, 

 the walls were not nearly so thick and stout. You 

 would learn from this that you were in the veins, and 

 not in the arteries. You would meet too with some- 

 thing, the like of which you did not see in the arteries 

 (except perhaps just close to the heart). Every now 

 and then you would come upon what for all the world 

 looked like one of those watch-pockets that some- 

 times are hung at the head of a bedstead, a watch- 

 pocket with its opening turned the way you were 

 going. This you would find was called a valve, and 

 was made of thin but strong membrane or skin. 

 Sometimes in the smaller veins you would meet with 

 one watch-pocket by itself, sometimes with two or 

 even three abreast, and I dare say you would notice 



