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.
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