THE CIRCULATION OF THE BLOOD. 103 



CHAPTEK XIII.* 



THE CIRCULATION OF THE BLOOD. 



The Blood is kept in continual motion in order to 



nourish and purify the body arid itself. For as life means 

 work, and work brings waste, there is constant need of 

 fresh material to make good the loss in every part of the 

 system, and of the removal of matter which is no longer 

 fit for use. 



In the very lowest animals, where every part of the 

 structure is equally capable 

 of absorbing the digested 

 food and is in contact with 

 it, there is no occasion for 

 any circulation, although 

 even in them the digested 

 food is not allowed to stag- 

 nate. But in proportion as 

 the power of absorption is 

 confined to certain parts, 

 the more is the need and 

 the greater the complexity 

 oi an apparatus for convey- 

 ing the nutritive fluid to 

 the various tissues. 



In nearly all animals, 

 the nutritive fluid is con- 

 veyed to the various parts 

 of the body b}' a system 

 of tubes, called blood-ves- 



,_ . , FIG. 67. Venous Valves. They usually oc- 



Ine higher lOrmS cur in pairs, as represented. 



* See Appendix. 



