On The Parts Of Animals   

follows as a necessary consequence that every animal must have some
homogeneous parts; for these alone are capable of sensation, the
heterogeneous parts serving for the active functions. Again, as the
sensory faculty, the motor faculty, and the nutritive faculty are all
lodged in one and the same part of the body, as was stated in a former
treatise, it is necessary that the part which is the primary seat of
these principles shall on the one hand, in its character of general
sensory recipient, be one of the simple parts; and on the other hand
shall, in its motor and active character, be one of the heterogeneous
parts. For this reason it is the heart which in sanguineous animals
constitutes this central part, and in bloodless animals it is that
which takes the place of a heart. For the heart, like the other
viscera, is one of the homogeneous parts; for, if cut up, its pieces
are homogeneous in substance with each other. But it is at the same
time heterogeneous in virtue of its definite configuration. And the
same is true of the other so-called viscera, which are indeed formed
from the same material as the heart. For all these viscera have a
sanguineous character owing to their being situated upon vascular
ducts and branches. For just as a stream of water deposits mud, so the
various viscera, the heart excepted, are, as it were, deposits from
the stream of blood in the vessels. And as to the heart, the very
starting-point of the vessels, and the actual seat of the force by
which the blood is first fabricated, it is but what one would
naturally expect, that out of the selfsame nutriment of which it is
the recipient its own proper substance shall be formed. Such, then,
are the reasons why the viscera are of sanguineous aspect; and why in
one point of view they are homogeneous, in another heterogeneous.
Part 2
Of the homogeneous parts of animals, some are soft and fluid, others
hard and solid; and of the former some are fluid permanently, others
only so long as they are in the living body. Such are blood, serum,
lard, suet, marrow, semen, bile, milk when present, flesh, and their
various analogues. For the parts enumerated are not to be found in all
animals, some animals only having parts analogous to them. Of the hard
and solid homogeneous parts bone, fish-spine, sinew, blood-vessel, are
examples. The last of these points to a sub-division that may be made
in the class of homogeneous parts. For in some of them the whole and a
portion of the whole in one sense are designated by the same term-as,
for example, is the case with blood-vessel and bit of
blood-vessel-while in another sense they are not; but a portion of a
heterogeneous part, such as face, in no sense has the same designation
as the whole.
The first question to be asked is what are the causes to which these
homogeneous parts owe their existence? The causes are various; and
this whether the parts be solid or fluid. Thus one set of homogeneous
parts represent the material out of which the heterogeneous parts are
formed; for each separate organ is constructed of bones, sinews,
flesh, and the like; which are either essential elements in its
formation, or contribute to the proper discharge of its function. A
second set are the nutriment of the first, and are invariably fluid,
for all growth occurs at the expense of fluid matter; while a third
set are the residue of the second. Such, for instance, are the faeces
and, in animals that have a bladder, the urine; the former being the
dregs of the solid nutriment, the latter of the fluid.
Even the individual homogeneous parts present variations, which are
intended in each case to render them more serviceable for their
purpose. The variations of the blood may be selected to illustrate
this. For different bloods differ in their degrees of thinness or
thickness, of clearness or turbidity, of coldness or heat; and this
whether we compare the bloods from different parts of the same
individual or the bloods of different animals. For, in the individual,
all the differences just enumerated distinguish the blood of the upper
and of the lower halves of the body; and, dealing with classes, one
section of animals is sanguineous, while the other has no blood, but