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 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 
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 
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 system 
of tubes, called Uood-ves- 
’ t Fig. 67. —Venous Valves. They usually oc- 
sels. The higher forms cur in pairs, as represented. 
* See Appendix. 
