The formation of this
membrane we shall find to be the result of necessity, as is that of
the other [similar] parts. What, however, is the final cause of its
existence in sanguineous animals is manifest on reflection. For it
is necessary that animals shall get nutriment from without; and,
again, that this shall be converted into the ultimate nutriment, which
is then distributed as sustenance to the various parts; this
ultimate nutriment being, in sanguineous animals, what we call
blood, and having, in bloodless animals, no definite name. This
being so, there must be channels through which the nutriment shall
pass, as it were through roots, from the stomach into the
blood-vessels. Now the roots of plants are in the ground; for thence
their nutriment is derived. But in animals the stomach and
intestines represent the ground from which the nutriment is to be
taken. The mesentery, then, is an organ to contain the roots; and
these roots are the vessels that traverse it. This then is the final
cause of its existence. But how it absorbs nutriment, and how that
portion of the food which enters into the vessels is distributed by
them to the various parts of the body, are questions which will be
considered when we come to deal with the generation and nutrition of
animals.
The constitution of sanguineous animals, so far as the parts as
yet mentioned are concerned, and the reasons for such constitution,
have now been set forth.
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