40 



SCIENCr PRIMERS, 



div. 



all over the body, round and round in the torrent of the 

 circulation ; as it sweeps past them, or rather through 

 them, the muscle, the brain, the nerve, the skin pick out 

 new food for their work and give back the things they 

 have used or no longer want. As they all have dif- 

 ferent works, some use up what others have thrown 

 away. There are, besides, scavengers and cleaners 

 to pick up things no longer wanted anywhere and to 

 throw them out of the body. Thus the blood is kept 

 pure as well as fresh. Through the blood thus ever 

 brought to them, each part does its work : the muscle 

 contracts, the brain feels and wills, the nerves carry 

 the feeling and the willing, and the other organs of 

 the body do their work too, and thus the whole body 

 is kept alive and well. r • 



■■,■_■■'■' , ■ . \ ' 



THE NATURE OF BLOOD. § IV. ' - ^ 



21. What, then, is this blood which does 



* 



so much? , 



Did you ever look through a good microscope at 

 the thin transparent web of a frog's foot, and watch 

 the red blood coursing along its narrow channels? 

 If not, go and look at it at once; you will never 

 understand any physiology till you have done so. 

 There you will see a network of delicate passages far 

 finer than any of your own hairs, and through those 

 passages a tumbling crowd of tiny oval yellow 

 globules hurrying and jostling along. Some of the 

 passages are wider than others, and through some of 

 the wider ones you will see a thick stream of globules 

 rushing onwards towards the smaller channels, and 

 spreading out among them. The globules which 



