by Aristotle
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.
10
Let us now make, as it were, a fresh beginning, and consider the
heterogeneous parts, taking those first which are the first in
importance. For in all animals, at least in all the perfect kinds,
there are two parts more essential than the rest, namely the part
which serves for the ingestion of food, and the part which serves
for the discharge of its residue. For without food growth and even
existence is impossible. Intervening again between these two parts
there is invariably a third, in which is lodged the vital principle.
As for plants, though they also are included by us among things that
have life, yet are they without any part for the discharge of waste
residue. For the food which they absorb from the ground is already
concocted, and they give off as its equivalent their seeds and fruits.
Plants, again, inasmuch as they are without locomotion, present no
great variety in their heterogeneous parts. For, where the functions
are but few, few also are the organs required to effect them. The
configuration of plants is a matter then for separate consideration.
Animals, however, that not only live but feel, present a greater
multiformity of parts, and this diversity is greater in some animals
than in others, being most varied in those to whose share has fallen
not mere life but life of high degree. Now such an animal is man.
For of all living beings with which we are acquainted man alone
partakes of the divine, or at any rate partakes of it in a fuller
measure than the rest. For this reason, then, and also because his
external parts and their forms are more familiar to us than those of
other animals, we must speak of man first; and this the more fitly,
because in him alone do the natural parts hold the natural position;
his upper part being turned towards that which is upper in the
universe. For, of all animals, man alone stands erect.
In man, then, the head is destitute of flesh; this being the
necessary consequence of what has already been stated concerning the
brain. There are, indeed, some who hold that the life of man-would
be longer than it is, were his head more abundantly furnished with
flesh; and they account for the absence of this substance by saying
that it is intended to add to the perfection of sensation. For the
brain they assert to be the organ of sensation; and sensation, they
say, cannot penetrate to parts that are too thickly covered with
flesh. But neither part of this statement is true. On the contrary,
were the region of the brain thickly covered with flesh, the very
purpose for which animals are provided with a brain would be
directly contravened. For the brain would itself be heated to excess
and so unable to cool any other part; and, as to the other half of
their statement, the brain cannot be the cause of any of the
sensations, seeing that it is itself as utterly without feeling as any
one of the excretions. These writers see that certain of the senses
are located in the head, and are unable to discern the reason for
this; they see also that the brain is the most peculiar of all the
animal organs; and out of these facts they form an argument, by
which they link sensation and brain together. It has, however, already
been clearly set forth in the treatise on Sensation, that it is the
region of the heart that constitutes the sensory centre. There also it
was stated that two of the senses, namely touch and taste, are
manifestly in immediate connexion with the heart; and that as
regards the other three, namely hearing, sight, and the centrally
placed sense of smell, it is the character of their sense-organs which
causes them to be lodged as a rule in the head. Vision is so placed in
all animals. But such is not invariably the case with hearing or
with smell. For fishes and the like hear and smell, and yet have no
visible organs for these senses in the head; a fact which demonstrates
the accuracy of the opinion here maintained. Now that vision, whenever
it exists, should be in the neighbourhood of the brain is but what one
would rationally expect. For the brain is fluid and cold, and vision
is of the nature of water, water being of all transparent substances
the one most easily confined. Moreover it cannot but necessarily be
that the more precise senses will have their precision rendered
still greater if ministered to by parts that have the purest blood.
For the motion of the heat of blood destroys sensory activity. For
these reasons the organs of the precise senses are lodged in the head.
It is not only the fore part of the head that is destitute of flesh,
but the hind part also. For, in all animals that have a head, it is
this head which more than any other part requires to be held up.
But, were the head heavily laden with flesh, this would be impossible;
for nothing so burdened can be held upright. This is an additional
proof that the absence of flesh from the head has no reference to
brain sensation. For there is no brain in the hinder part of the head,
and yet this is as much without flesh as is the front.
In some animals hearing as well as vision is lodged in the region of
the head. Nor is this without a rational explanation. For what is
called the empty space is full of air, and the organ of hearing is, as
we say, of the nature of air. Now there are channels which lead from
the eyes to the blood-vessels that surround the brain; and similarly
there is a channel which leads back again from each ear and connects
it with the hinder part of the head. But no part that is without blood
is endowed with sensation, as neither is the blood itself,
but only
some one of the parts that are formed of blood.
The brain in all animals that have one is placed in the front part
of the head; because the direction in which sensation acts is in
front; and because the heart, from which sensation proceeds, is in the
front part of the body; and lastly because the instruments of
sensation are the blood-containing parts, and the cavity in the
posterior part of the skull is destitute of blood-vessels.
As to the position of the sense-organs, they have been arranged by
nature in the following well-ordered manner. The organs of hearing are
so placed as to divide the circumference of the head into two equal
halves; for they have to hear not only sounds which are directly in
line with themselves, but sounds from all quarters. The organs of
vision are placed in front, because sight is exercised only in a
straight line, and moving as we do in a forward direction it is
necessary that we should see before us, in the direction of our
motion. Lastly, the organs of smell are placed with good reason
between the eyes. For as the body consists of two parts, a right
half and a left, so also each organ of sense is double. In the case of
touch this is not apparent, the reason being that the primary organ of
this sense is not the flesh or analogous part, but lies internally. In
the case of taste, which is merely a modification of touch and which
is placed in the tongue, the fact is more apparent than in the case of
touch, but still not so manifest as in the case of the other senses.
However, even in taste it is evident enough; for in some animals the
tongue is plainly forked. The double character of the sensations is,
however, more conspicuous in the other organs of sense. For there
are two ears and two eyes, and the nostrils, though joined together,
are also two. Were these latter otherwise disposed, and separated from
each other as are the ears, neither they nor the nose in which they
are placed would be able to perform their office. For in such
animals as have nostrils olfaction is effected by means of
inspiration, and the organ of inspiration is placed in front and in
the middle line. This is the reason why nature has brought the two
nostrils together and placed them as the central of the three
sense-organs, setting them side by side on a level with each other, to
avail themselves of the inspiratory motion. In other animals than
man the arrangement of these sense-organs is also such as is adapted
in each case to the special requirements.
11
For instance, in quadrupeds the ears stand out freely from the
head and are set to all appearance above the eyes. Not that they are
in reality above the eyes; but they seem to be so, because the
animal does not stand erect, but has its head hung downwards. This
being the usual attitude of the animal when in motion, it is of
advantage that its ears shall be high up and movable; for by turning
themselves about they can the better take in sounds from every
quarter.
12
In birds, on the other hand, there are no ears, but only the
auditory passages. This is because their skin is hard and because they
have feathers instead of hairs, so that they have not got the proper
material for the formation of ears. Exactly the same is the case
with such oviparous quadrupeds as are clad with scaly plates, and
the same explanation applies to them. There is also one of the
viviparous quadrupeds, namely the seal, that has no ears but only
the auditory passages. The explanation of this is that the seal,
though a quadruped, is a quadruped of stunted formation.
13
Men, and Birds, and Quadrupeds, viviparous and oviparous alike, have
their eyes protected by lids. In the Vivipara there are two of
these; and both are used by these animals not only in closing the
eyes, but also in the act of blinking; whereas the oviparous
quadrupeds, and the heavy-bodied birds as well as some others, use
only the lower lid to close the eye; while birds blink by means of a
membrane that issues from the canthus. The reason for the eyes being
thus protected is that nature has made them of fluid consistency, in
order to ensure keenness of vision. For had they been covered with
hard skin, they would, it is true, have been less liable to get
injured by anything falling into them from without, but they would not
have been sharp-sighted. It is then to ensure keenness of vision
that the skin over the pupil is fine and delicate; while the lids
are superadded as a protection from injury. It is as a still further
safeguard that all these animals blink, and man most of all; this
action (which is not performed from deliberate intention but from a
natural instinct) serving to keep objects from falling into the
eyes; and being more frequent in man than in the rest of these
animals, because of the greater delicacy of his skin. These lids are
made of a roll of skin; and it is because they are made of skin and
contain no flesh that neither they, nor the similarly constructed
prepuce, unite again when once cut.
As to the oviparous quadrupeds, and such birds as resemble them in
closing the eye with the lower lid, it is the hardness of the skin
of their heads which makes them do so. For such birds as have heavy
bodies are not made for flight; and so the materials which would
otherwise have gone to increase the growth of the feathers are
diverted thence, and used to augment the thickness of the skin.
Birds therefore of this kind close the eye with the lower lid; whereas
pigeons and the like use both upper and lower lids for the purpose. As
birds are covered with feathers, so oviparous quadrupeds are covered
with scaly plates; and these in all their forms are harder than hairs,
so that the skin also to which they belong is harder than the skin
of hairy animals. In these animals, then, the skin on the head is
hard, and so does not allow of the formation of an upper eyelid,
whereas lower down the integument is of a flesh-like character, so
that the lower lid can be thin and extensible.
The act of blinking is performed by the heavy-bodied birds by
means of the membrane already mentioned, and not by this lower lid.
For in blinking rapid motion is required, and such is the motion of
this membrane, whereas that of the lower lid is slow. It is from the
canthus that is nearest to the nostrils that the membrane comes. For
it is better to have one starting-point for nictitation than two;
and in these birds this starting-point is the junction of eye and
nostrils, an anterior starting-point being preferable to a lateral
one. Oviparous quadrupeds do not blink in like manner as the birds;
for, living as they do on the ground, they are free from the necessity
of having eyes of fluid consistency and of keen sight, whereas these
are essential requisites for birds, inasmuch as they have to use their
eyes at long distances. This too explains why birds with talons,
that have to search for prey by eye from aloft, and therefor
e soar
to greater heights than other birds, are sharpsighted; while common
fowls and the like, that live on the ground and are not made for
flight, have no such keenness of vision. For there is nothing in their
mode of life which imperatively requires it.
Fishes and Insects and the hard-skinned Crustacea present certain
differences in their eyes, but so far resemble each other as that none
of them have eyelids. As for the hard-skinned Crustacea it is
utterly out of the question that they should have any; for an
eyelid, to be of use, requires the action of the skin to be rapid.
These animals then have no eyelids and, in default of this protection,
their eyes are hard, just as though the lid were attached to the
surface of the eye, and the animal saw through it. Inasmuch,
however, as such hardness must necessarily blunt the sharpness of
vision, nature has endowed the eyes of Insects, and still more those
of Crustacea, with mobility (just as she has given some quadrupeds
movable ears), in order that they may be able to turn to the light and
catch its rays, and so see more plainly. Fishes, however, have eyes of
a fluid consistency. For animals that move much about have to use
their vision at considerable distances. If now they live on land,
the air in which they move is transparent enough. But the water in
which fishes live is a hindrance to sharp sight, though it has this
advantage over the air, that it does not contain so many objects to
knock against the eyes. The risk of collision being thus small,
nature, who makes nothing in vain, has given no eyelids to fishes,
while to counterbalance the opacity of the water she has made their
eyes of fluid consistency.
14
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