On The Parts Of Animals   

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