THE CIRCULATION OF THE BLOOD. 



103 



CHAPTER XIII. 



THE CIRCULATION OF THE BLOOD. 



The Blood is kept in continual motion in order to 

 nourish and purify the body and 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 

 in them even the blood is 

 not allowed to stagnate. 

 But in proportion as the 

 power of absorption is con- 

 fined to certain parts, the 

 more is the need and the 

 greater the complexity of 

 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 by a sj^stem 

 of tubes, called blood-ves- 



, mi , . , „ Fig. 67. —Venous Valves. They usually oc- 



SelS. 1 he higher IOrmS cur in pairs, as represented. 



