On The Parts Of Animals   

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.
Part 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.
Part 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