The
blood is contained in a vast system of pipes, spreading through the
whole body, connected with a force pump,--the heart,--which, by its
position and by the contractions of its valves, keeps the blood
constantly circulating in one direction, never allowing it to rest; and
then, by means of this circulation of the blood, laden as it is with
the products of digestion, the skin, the flesh, the hair, and every
other part of the body, draws from it that which it wants, and every
one of these organs derives those materials which are necessary to
enable it to do its work.
The action of each of these organs, the performance of each of these
various duties, involve in their operation a continual absorption of
the matters necessary for their support, from the blood, and a constant
formation of waste products, which are returned to the blood, and
conveyed by it to the lungs and the kidneys, which are organs that have
allotted to them the office of extracting, separating, and getting rid
of these waste products; and thus the general nourishment, labour, and
repair of the whole machine is kept up with order and regularity.
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