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. The mouth then, its duty done, passes over the food to the
stomach, and there must necessarily be something to receive it in turn
from this. This something is furnished by the bloodvessels, which
run throughout the whole extent of the mesentery from its lowest
part right up to the stomach. A description of these will be found
in the treatises on Anatomy and Natural History. Now as there is a
receptacle for the entire matter taken as food, and also a
receptacle for its excremental residue, and again a third
receptacle, namely the vessels, which serve as such for the blood,
it is plain that this blood must be the final nutritive material in
such animals as have it; while in bloodless animals the same is the
case with the fluid which represents the blood. This explains why
the blood diminishes in quantity when no food is taken, and
increases when much is consumed, and also why it becomes healthy and
unhealthy according as the food is of the one or the other
character. These facts, then, and others of a like kind, make it plain
that the purpose of the blood in sanguineous animals is to subserve
the nutrition of the body. They also explain why no more sensation
is produced by touching the blood than by touching one of the
excretions or the food, whereas when the flesh is touched sensation is
produced. For the blood is not continuous nor united by growth with
the flesh, but simply lies loose in its receptacle, that is in the
heart and vessels
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