perfect creatures the cause is its failure in the organ containing the

source of the creature's essential nature. This member is situate,

as has been said, at the junction of the upper and lower parts; in

plants it is intermediate between the root and the stem, in

sanguineous animals it is the heart, and in those that are bloodless

the corresponding part of their body. But some of these animals have

potentially many sources of life, though in actuality they possess

only one. This is why some insects live when divided, and why, even

among sanguineous animals, all whose vitality is not intense live

for a long time after the heart has been removed. Tortoises, for

example, do so and make movements with their feet, so long as the

shell is left, a fact to be explained by the natural inferiority of

their constitution, as it is in insects also.

The source of life is lost to its possessors when the heat with

which it is bound up is no longer tempered by cooling, for, as I

have often remarked, it is consumed by itself. Hence when, owing to

lapse of time, the lung in the one class and the gills in the other

get dried up, these organs become hard and earthy and incapable of

movement, and cannot be expanded or contracted. Finally things come to

a climax, and the fire goes out from exhaustion.

Hence a small disturbance will speedily cause death in old age.

Little heat remains, for the most of it has been breathed away in

the long period of life preceding, and hence any increase of strain on

the organ quickly causes extinction. It is just as though the heart

contained a tiny feeble flame which the slightest movement puts out.

Hence in old age death is painless, for no violent disturbance is

required to cause death, and there is an entire absence of feeling

when the soul's connexion is severed. All diseases which harden the

lung by forming tumours or waste residues, or by excess of morbid

heat, as happens in fevers, accelerate the breathing owing to the

inability of the lung to move far either upwards or downwards.

Finally, when motion is no longer possible, the breath is given out