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OF THE DIVISIONS OF THE HUMAN BODY. 31
each having two cavities, an auricle and ventricle. The
heart is simply two force-pumps, joined together for
convenience. In some of the lower animals, as fishes,
there is but a —in single pump reptiles, there are
three chambers. Each pump is furnished with beau
tiful valves, which allow the current of blood to go
on, but prevent its re
turn. These valves
work constantly for
more than a hundred
in some cases,
years,
without getting out of
order.
Ideal section of mammalian
heart, a, arch of aorta ; 0, b',
pulmonary arteries; c, supe
rior vena cava ; d, df, pulmo
nary veins; e, right auricle;
/, tricuspid valves; g, inferior
vena cava ; h, right ventricle ;
ventriculorum ; k,
i, septum
descending aorta ; I, left ven
tricle ; m, mitral valve ; n, left
auricle.
Fig. 9.
Now, this is the course ofthe circulation. The blood,
as it comes from all parts of the system, by the veins,
is received into the right auricle, or receptacle, of the
heart, from which, by a muscular contraction, it is sent
into the right ventricle. The right ventricle contracts,
and throws the blood it contains, through the pulmonary
artery, into the lungs, where it is purified and changed
in its color and qualities. This is the actioh of the right
pump. The blood now goes back by the pulmonary
veins to the left auricle, thence into the left ventricle ;
which, contracting, foi ?es it into the great aorta, and so
b
t
ah'
k
THE HEART.