52 SCIENCE PRIMERS. [§ V. 



rather firm stout walls; and the veins, which are 

 generally full of blood, and have thinner and fiabby 

 walls. The arteries when you cut them generally 

 gape and remain open ; the veins fall together and 

 collapse. The larger the arteries, the stouter and 

 firmer they are, and the greater the difference between 

 them and the veins. 



You have also studied the capillaries in the frog's 

 foot ; you have seen that they are minute channels, 

 with the thinnest and tenderest walls, forming a close 

 network in which the smallest arteries end, and from 

 which the smallest veins begin. 



You have moreover been told that all over your 

 own body, in every part, there are, though you cannot 

 see them, networks of capillaries like those in the 

 frog's foot which you can see ; that all the arteries of 

 your body end in capillaries, and all the veins begin 

 in capillaries. Let me repeat that, one or two 

 structures excepted, fehere is no pare of your "•body 

 in which, could you put it under a microscope, 

 you would not see a small artery branching out and 

 losing itself in a network of capillaries, out of which, 

 as out of so many roots, a small vein gathers itself 

 together again. 



In some places the network is very close, the capil- 

 laries lying closer together than even in the frog's foot; 

 in others the network is more open, and the capillaries 

 wider apart; but everywhere, with a few exceptions 

 which you will learn by and' by, there are capillaries, 

 arteries, and veins. ^; / :t^ 



Suppose you were a little lone red corpuscle, all by 

 yourself in the quite empty blood-vessels of a dead 

 body, squeezed in the narrow pathway of a capillary, say 



