216 CIRCULATION OF THE BLOOD. 
of the tide brings back a large proportion of what is carried 
down at ebb, the waters speedily become so contaminated with 
hurtful and offensive matters as to be unfit for use, unless 
means be provided for getting rid of these as fast as they are 
poured in. The perfection with which this requirement is 
fulfilled in the Animal body, while it excites our admiration, 
should also incite us to imitation, so far as the art of Man 
can hope to imitate the works of the Divine Artificer. 
Circulation of the Blood. 
244. In some of the lower tribes of Animals, the blood 
appears to circulate in channels which are merely excavated 
in the substance of their tissues and organs. But among all 
the Yertebrata, and even in most of the Invertebrated classes, 
the movement of the blood takes place in a very complicated 
apparatus, which is composed, 1st, of a system of tubes or 
canals which serve to convey it through every part of the 
structure, and 2d, of a special organ for the purpose of 
giving motion to that liquid. These canals are known as the 
blood-vessels ; and this special organ is the heart. 
245. The Heart is the centre of the circulating apparatus. 
It is a kind of fleshy bag, communicating with the blood¬ 
vessels : and it alternately dilates to receive the blood, which 
is conveyed to it by one set of these ; and then contracts so as 
to force it out into another set of tubes. In this manner a 
continual current is kept up. All but the lowest animals 
have a heart, or something which represents it. Such an 
organ exists, not merely among all the Vertebrated classes, 
but in all the Mollusca, and in the higher Articulata. But, 
as will presently appear, there is a great diversity in its form, 
and in the complexity of its construction; for whilst, in its 
simplest condition, it possesses but one cavity, communicating 
with both sets of vessels,—it contains, in its highest forms, four 
different chambers, each of which has its own peculiar function. 
246. The two sets of blood-vessels just adverted-to are, 1st, 
the Arteries, which convey the blood from the heart into the 
several parts and organs of the body; and 2d, the Veins , 
which collect the blood that has been distributed through 
these, and return it to the heart. The Arterial system, as it 
issues from the heart, consists of one or more large trunks, 
which divide into branches, very much in the manner of the, 
