heat, the very motion of the lung, airlike and void, suffices by
itself to cool them for a considerable period.
These animals, speaking generally, are also distinguished from
others by their smaller bulk. For heat promotes growth, and
abundance of blood is a sure indication of heat. Heat, again, tends to
make the body erect; and thus it is that man is the most erect of
animals, and the vivipara more erect than other quadrupeds. For no
viviparous animal, be it apodous or be it possessed of feet, is so
given to creep into holes as are the ovipara.
The lung, then, exists for respiration; and this is its universal
office; but in one order of animals it is bloodless and has the
structure described above, to suit the special requirements There
is, however, no one term to denote all animals that have a lung; no
designation, that is, like the term Bird, applicable to the whole of a
certain class. Yet the possession of a lung is a part of their
essence, just as much as the presence of certain characters
constitutes the essence of a bird.
7
Of the viscera some appear to be single, as the heart and lung;
others to be double, as the kidneys; while of a third kind it is
doubtful in which class they should be reckoned. For the liver and the
spleen would seem to lie half-way between the single and the double
organs. For they may be regarded either as constituting each a
single organ, or as a pair of organs resembling each other in
character.
In reality, however, all the organs are double. The reason for
this is that the body itself is double, consisting of two halves,
which are however combined together under one supreme centre. For
there is an upper and a lower half, a front and a rear, a right side
and a left.
This explains why it is that even the brain and the several organs
of sense tend in all animals to consist of two parts; and the same
explanation applies to the heart with its cavities. The lung again
in Ovipara is divided to such an extent that these animals look as
though they had actually two lungs. As to the kidneys, no one can
overlook their double character. But when we come to the liver and the
spleen, any one might fairly be in doubt. The reason of this is, that,
in animals that necessarily have a spleen, this organ is such that
it might be taken for a kind of bastard liver; while in those in which
a spleen is not an actual necessity but is merely present, as it were,
by way of token, in an extremely minute form, the liver plainly
consists of two parts; of which the larger tends to lie on the right
side and the smaller on the left. Not but what there are some even
of the Ovipara in which this condition is comparatively indistinctly
marked; while, on the other hand, there are some Vivipara in which the
liver is manifestly divided into two parts. Examples of such
division are furnished by the hares of certain regions, which have the
appearance of having two livers, and by the cartilaginous and some
other fishes.
It is the position of the liver on the right side of the body that
is the main cause for the formation of the spleen; the existence of
which thus becomes to a certain extent a matter of necessity in all
animals, though not of very stringent necessity.
The reason, then, why the viscera are bilateral is, as we have said,
that there are two sides to the body, a right and a left. For each
of these sides aims at similarity with the other, and so likewise do
their several viscera; and as the sides, though dual, are knit
together into unity, so also do the viscera tend to be bilateral and
yet one by unity of constitution.
Those viscera which lie below the diaphragm exist one and all on
account of the blood-vessels; serving as a bond, by which these
vessels, while floating freely, are yet held in connexion with the
body. For the vessels give off branches which run to the body
through the outstretched structures, like so many anchorlines thrown
out from a ship. The great vessel sends such branches to the liver and
the spleen; and these viscera-the liver and spleen on either side with
the kidneys behind-attach the great vessel to the body with the
firmness of nails. The aorta sends similar branches to each kidney,
but none to the liver or spleen.
These viscera, then, contribute in this manner to the compactness of
the animal body. The liver and spleen assist, moreover, in the
concoction of the food; for both are of a hot character, owing to
the blood which they contain. The kidneys, on the other hand, take
part in the separation of the excretion which flows into the bladder.
The heart then and the liver are essential constituents of every
animal; the liver that it may effect concoction, the heart that it may
lodge the central source of heat. For some part or other there must be
which, like a hearth, shall hold the kindling fire; and this part must
be well protected, seeing that it is, as it were, the citadel of the
body.
All sanguineous animals, then, need these two parts; and this
explains why these two viscera, and these two alone, are invariably
found in them all. In such of them, however, as breathe, there is also
as invariably a third, namely the lung. The spleen, on the other hand,
is not invariably present; and, in those animals that have it, is only
present of necessity in the same sense as the excretions of the
belly and of the bladder are necessary, in the sense, that is, of
being an inevitable concomitant. Therefore it is that in some
animals the spleen is but scantily developed as regards size. This,
for instance, is the case in such feathered animals as have a hot
stomach. Such are the pigeon, the hawk, and the kite. It is the case
also in oviparous quadrupeds, where the spleen is excessively
minute, and in many of the scaly fishes. These same animals are also
without a bladder, because the loose texture of their flesh allows the
residual fluid to pass through and to be applied to the formation of
feathers and scales. For the spleen attracts the residual humours from
the stomach, and owing to its bloodlike character is enabled to assist
in their concoction. Should, however, this residual fluid be too
abundant, or the heat of the spleen be too scanty, the body becomes
sickly from over-repletion with nutriment. Often, too, when the spleen
is affected by disease, the belly becomes hard owing to the reflux
into it of the fluid; just as happens to those who form too much
urine, for they also are liable to a similar diversion of the fluids
into the belly. But in those animals that have but little
superfluous fluid to excrete, such as birds and fishes, the spleen
is never large, and in some exists no more than by way of token. So
also in the oviparous quadrupeds it is small, compact, and like a
kidney. For their lung is spongy, and they drink but little, and
such superfluous fluid as they have is applied to the growth of the
body and the formation of scaly plates, just as in
birds it is applied
to the formation of feathers.
On the other hand, in such animals as have a bladder, and whose lung
contains blood, the spleen is watery, both for the reason already
mentioned, and also because the left side of the body is more watery
and colder than the right. For each of two contraries has been so
placed as to go together with that which is akin to it in another pair
of contraries. Thus right and left, hot and cold, are pairs of
contraries; and right is conjoined with hot, after the manner
described, and left with cold.
The kidneys when they are present exist not of actual necessity, but
as matters of greater finish and perfection. For by their special
character they are suited to serve in the excretion of the fluid which
collects in the bladder. In animals therefore where this fluid is very
abundantly formed, their presence enables the bladder to perform its
proper office with greater perfection.
Since then both kidneys and bladder exist in animals for one and the
same function, we must next treat of the bladder, though in so doing
we disregard the due order of succession in which the parts should
be enumerated. For not a word has yet been said of the midriff,
which is one of the parts that environ the viscera and therefore has
to be considered with them.
8
It is not every animal that has a bladder; those only being
apparently intended by nature to have one, whose lung contains
blood. To such it was but reasonable that she should give this part.
For the superabundance in their lung of its natural constituents
causes them to be the thirstiest of animals, and makes them require
a more than ordinary quantity not merely of solid but also of liquid
nutriment. This increased consumption necessarily entails the
production of an increased amount of residue; which thus becomes too
abundant to be concocted by the stomach and excreted with its own
residual matter. The residual fluid must therefore of necessity have a
receptacle of its own; and thus it comes to pass that all animals
whose lung contains blood are provided with a bladder. Those
animals, on the other hand, that are without a lung of this character,
and that either drink but sparingly owing to their lung being of a
spongy texture, or never imbibe fluid at all for drinking's sake but
only as nutriment, insects for instance and fishes, and that are
moreover clad with feathers or scales or scaly plates-all these
animals, owing to the small amount of fluid which they imbibe, and
owing also to such residue as there may be being converted into
feathers and the like, are invariably without a bladder. The
Tortoises, which are comprised among animals with scaly plates, form
the only exception; and this is merely due to the imperfect
development of their natural conformation; the explanation of the
matter being that in the sea-tortoises the lung is flesh-like and
contains blood, resembling the lung of the ox, and that in the
land-tortoises it is of disproportionately large size. Moreover,
inasmuch as the covering which invests them is dense and shell-like,
so that the moisture cannot exhale through the porous flesh, as it
does in birds and in snakes and other animals with scaly plates,
such an amount of secretion is formed that some special part is
required to receive and hold it. This then is the reason why these
animals, alone of their kind, have a bladder, the sea-tortoise a large
one, the land-tortoises an extremely small one.
9
What has been said of the bladder is equally true of the kidneys.
For these also are wanting in all animals that are clad with
feathers or with scales or with scale-like plates; the sea and land
tortoises forming the only exception. In some of the birds, however,
there are flattened kidney like bodies, as though the flesh allotted
to the formation of the kidneys, unable to find one single place of
sufficient size, had been scattered over several.
The Emys has neither bladder nor kidneys. For the softness of its
shell allows of the ready transpiration of fluid; and for this
reason neither of the organs mentioned exists in this animal. All
other animals, however, whose lung contains blood are, as before said,
provided with kidneys. For nature uses these organs for two separate
purposes, namely for the excretion of the residual fluid, and to
subserve the blood-vessels, a channel leading to them from the great
vessel.
In the centre of the kidney is a cavity of variable size. This is
the case in all animals, excepting the seal. The kidneys of this
animal are more solid than those of any other, and in form resemble
the kidneys of the ox. The human kidneys are of similar shape; being
as it were made up of numerous small kidneys, and not presenting one
unbroken surface like the kidneys of sheep and other quadrupeds. For
this reason, should the kidneys of a man be once attacked by
disease, the malady is not easily expelled. For it is as though many
kidneys were diseased and not merely one; which naturally enhances the
difficulties of a cure.
The duct which runs to the kidney from the great vessel does not
terminate in the central cavity, but is expended on the substance of
the organ, so that there is no blood in the cavity, nor is any
coagulum found there after death. A pair of stout ducts, void of
blood, run, one from the cavity of each kidney, to the bladder; and
other ducts, strong and continuous, lead into the kidneys from the
aorta. The purpose of this arrangement is to allow the superfluous
fluid to pass from the blood-vessel into the kidney, and the resulting
renal excretion to collect by the percolation of the fluid through the
solid substance of the organ, in its centre, where as a general rule
there is a cavity. (This by the way explains why the kidney is the
most ill-savoured of all the viscera.) From the central cavity the
fluid is discharged into the bladder by the ducts that have been
mentioned, having already assumed in great degree the character of
excremental residue. The bladder is as it were moored to the
kidneys; for, as already has been stated, it is attached to them by
strong ducts. These then are the purposes for which the kidneys exist,
and such the functions of these organs.
In all animals that have kidneys, that on the right is placed higher
than that on the left. For inasmuch as motion commences from the
right, and the organs on this side are in consequence stronger than
those on the left, they must all push upwards in advance of their
opposite fellows; as may be seen in the fact that men even raise the
right eyebrow more than the left, and that the former is more arched
than the latter. The right kidney being thus drawn upwards is in all
animals brought into contact with the liver; for the liver lies on the
right side.
Of all the viscera the kidneys are those that have the most fat.
This is in the first place the result of necessity, because the
kidne
ys are the parts through which the residual matters percolate.
For the blood which is left behind after this excretion, being of pure
quality, is of easy concoction, and the final result of thorough
blood-concoction is lard and suet. For just as a certain amount of
fire is left in the ashes of solid substances after combustion, so
also does a remnant of the heat that has been developed remain in
fluids after concoction; and this is the reason why oily matter is
light, and floats on the surface of other fluids. The fat is not
formed in the kidneys themselves, the density of their substance
forbidding this, but is deposited about their external surface. It
consists of lard or of suet, according as the animal's fat is of the
former or latter character. The difference between these two kinds
of fat has already been set forth in other passages. The formation,
then, of fat in the kidneys is the result of necessity; being, as
explained, a consequence of the necessary conditions which accompany
the possession of such organs. But at the same time the fat has a
final cause, namely to ensure the safety of the kidneys, and to
maintain their natural heat. For placed, as these organs are, close to
the surface, they require a greater supply of heat than other parts.
For while the back is thickly covered with flesh, so as to form a
shield for the heart and neighbouring viscera, the loins, in
accordance with a rule that applies to all bendings, are destitute
of flesh; and fat is therefore formed as a substitute for it, so
that the kidneys may not be without protection. The kidneys, moreover,
by being fat are the better enabled to secrete and concoct their
fluid; for fat is hot, and it is heat that effects concoction.
Such, then, are the reasons why the kidneys are fat. But in all
animals the right kidney is less fat than its fellow. The reason for
this is, that the parts on the right side are naturally more solid and
more suited for motion than those on the left. But motion is