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 system 
of tubes, called blood-ves - TT „ „ 
sels. The higher forms cur in pairs, as represented. 
