CHAPTER XVI* 



THE CIRCULATION OF THE BLOOD 



The Blood is kept in continual motion 

 nourish and purify the body and itself. 



means 

 , . brings 



in order to 

 For as life 

 work, and work 

 waste, there is 

 constant need of fresh 

 material to make good 

 the loss throughout the 

 system, and of the re- 

 moval of matter which 

 is no longer fit for use. 



In the very lowest ani- 

 mals, where all parts of 

 the structure are equally 

 capable of absorbing the 

 digested food and are in 

 contact with it, there is 

 no occasion for any cir- 

 culation, although even 

 in them the digested 

 food is not allowed to 

 stagnate. But in propor- 

 tion as the power of ab- 

 sorption is confined to 

 certain parts, the more is 

 the need and the greater the complexity of an apparatus 

 for conveying the nutritive fluid to the various tissues. 



* See Appendix. 



FIG. 264. Venous Valves. They usually oc- 

 cur in pairs, as represented. 



