Animals, then, are composed of homogeneous parts, and are also
composed of heterogeneous parts. The former, however, exist for the sake
of the latter. For the active functions and operations of the body are
carried on by these; that is, by the heterogeneous parts, such as the eye,
the nostril, the whole face, the fingers, the hand, and the whole arm.
But inasmuch as there is a great variety in the functions and motions not
only of aggregate animals but also of the individual organs, it is necessary
that the substances out of which these are composed shall present a diversity
of properties. For some purposes softness is advantageous, for others hardness;
some parts must be capable of extension, others of flexion. Such properties,
then, are distributed separately to the different homogeneous parts, one
being soft another hard, one fluid another solid, one viscous another brittle;
whereas each of the heterogeneous parts presents a combination of multifarious
properties. For the hand, to take an example, requires one property to
enable it to effect pressure, and another and different property for simple
prehension. For this reason the active or executive parts of the body are
compounded out of bones, sinews, flesh, and the like, but not these latter
out of the former.
...
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.
...
Very nearly resembling the bones to the touch are such parts as
nails, hoofs, whether solid or cloven, horns, and the beaks of birds, all
of which are intended to serve as means of defence. For the organs which
are made out of these substances, and which are called by the same names
as the substances themselves, the organ hoof, for instance, and the organ
horn, are contrivances to ensure the preservation of the animals to which
they severally belong. In this class too must be reckoned the teeth, which
in some animals have but a single function, namely the mastication of the
food, while in others they have an additional office, namely to serve as
weapons; as is the case with all animals that have sharp interfitting teeth
or that have tusks. All these parts are necessarily of solid and earthy
character; for the value of a weapon depends on such properties. Their
earthy character explains how it is that all such parts are more developed
in four-footed vivipara than in man. For there is always more earth in
the composition of these animals than in that of the human body. However,
not only all these parts but such others as are nearly connected with them,
skin for instance, bladder, membrane, hairs, feathers, and their analogues,
and any other similar parts that there may be, will be considered farther
on with the heterogeneous parts. There we shall inquire into the causes
which produce them, and into the objects of their presence severally in
the bodies of animals. For, as with the heterogeneous parts, so with these,
it is from a consideration of their functions that alone we can derive
any knowledge of them. The reason for dealing with them at all in this
part of the treatise, and classifying them with the homogeneous parts,
is that under one and the same name are confounded the entire organs and
the substances of which they are composed. But of all these substances
flesh and bone form the basis. Semen and milk were also passed over when
we were considering the homogeneous fluids. For the treatise on Generation
will afford a more suitable place for their examination, seeing that the
former of the two is the very foundation of the thing generated, while
the latter is its nourishment.
...
All animals that have hairs on the body have lashes on the eyelids;
but birds and animals with scale-like plates, being hairless, have none.
The Libyan ostrich, indeed, forms an exception; for, though a bird, it
is furnished with eyelashes. This exception, however, will be explained
hereafter. Of hairy animals, man alone has lashes on both lids. For in
quadrupeds there is a greater abundance of hair on the back than on the
under side of the body; whereas in man the contrary is the case, and the
hair is more abundant on the front surface than on the back. The reason
for this is that hair is intended to serve as a protection to its possessor.
Now, in quadrupeds, owing to their inclined attitude, the under or anterior
surface does not require so much protection as the back, and is therefore
left comparatively bald, in spite of its being the nobler of the two sides.
But in man, owing to his upright attitude, the anterior and posterior surfaces
of the body are on an equality as regards need of protection. Nature therefore
has assigned the protective covering to the nobler of the two surfaces;
for invariably she brings about the best arrangement of such as are possible.
This then is the reason that there is no lower eyelash in any quadruped;
though in some a few scattered hairs sprout out under the lower lid. This
also is the reason that they never have hair in the axillae, nor on the
pubes, as man has. Their hair, then, instead of being collected in these
parts, is either thickly set over the whole dorsal surface, as is the case
for instance in dogs, or, sometimes, forms a mane, as in horses and the
like, or as in the male lion where the mane is still more flowing and ample.
aristotle /// on the parts of animals
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