NATURAL HISTORY.

"Blest son of foresight, lord of fate!

That awful independent of to-morrow!

Whose work is done; who triumphs in the past;

Whose yesterdays look backward with a smile."-YouNG.

27

taining the denser fluids, and reflects the rays of light in a different manner.

71. How is the blood in the human body circulated and purified?

First, the heart sends the blood to all the capillaries through one set of blood-vessels called arteries. Then the blood is brought back Next by

to the heart by another set of blood-vessels called veins.
the blood which returns by the veins is impure, and is sent by
another set of arteries from the heart into the lungs, to be purified
by the air we breathe. Then it is brought back to the heart by
another set of veins. Thus there are two sets of arteries and veins
through which the blood is all made to pass; first, through the
body and back to the heart for the purpose of nourishment, and
then through the lungs and back for the purpose of purification.

72. The right side of the heart receives and sends out the impure blood. The upper

division receives it from the body
through two large veins. Then the
'blood passes to the lower division of
the heart, where it is sent to the
lungs through one large artery
with two branches, one for each
lung. This is the pulmonary artery.
The left side of the heart receives
and sends out the pure blood. The
upper portion receives it from the
lungs through three large veins.
Then the blood passes to the lower
division of the heart when it is sen.
to the capillariet all over the body
through one large artery called the
aorta.

73. Why is the blood regarded as so important an agent in animal economy?

Because the blood is the vehicle of life to every atom of our organization. By properties peculiar to itself, all the various fluids of our body are produced from it, and every particle of bone, muscle, membrane, nerve, and vessel must have existed as an ingredient of the blood, and have been conveyed to its appropriate