17
Every animal in order to exist requires nutriment, in order to
prevent itself from dying, refrigeration; and so Nature employs the
same organ for both purposes. For, as in some cases the tongue
serves both for discerning tastes and for speech, so in animals with
lungs the mouth is employed both in working up the food and in the
passage of the breath outwards and inwards. In lungless and
non-respiring animals it is employed in working up the food, while
in those of them that require refrigeration it is the gills that are
created for this purpose.
We shall state further on how it is that these organs have the
faculty of producing refrigeration. But to prevent their food from
impeding these operations there is a similar contrivance in the
respiring animals and in those that admit water. At the moment of
respiration they do not take in food, for otherwise suffocation
results owing to the food, whether liquid or dry, slipping in
through the windpipe and lying on the lung. The windpipe is situated
before the oesophagus, through which food passes into what is called
the stomach, but in quadrupeds which are sanguineous there is, as it
were, a lid over the windpipe-the epiglottis. In birds and oviparous
quadrupeds this covering is absent, but its office is discharged by
a contraction of the windpipe. The latter class contract the
windpipe when swallowing their food; the former close down the
epiglottis. When the food has passed, the epiglottis is in the one
case raised, and in the other the windpipe is expanded, and the air
enters to effect refrigeration. In animals with gills the water is
first discharged through them and then the food passes in through
the mouth; they have no windpipe and hence can take no harm from
liquid lodging in this organ, only from its entering the stomach.
For these reasons the expulsion of water and the seizing of their food
is rapid, and their teeth are sharp and in almost all cases arranged
in a saw-like fashion, for they are debarred from chewing their food.
18
Among water-animals the cetaceans may give rise to some
perplexity, though they too can be rationally explained.
Examples of such animals are dolphins and whales, and all others
that have a blowhole. They have no feet, yet possess a lung though
admitting the sea-water. The reason for possessing a lung is that
which we have now stated [refrigeration]; the admission of water is
not for the purpose of refrigeration. That is effected by respiration,
for they have a lung. Hence they sleep with their head out of the
water, and dolphins, at any rate, snore. Further, if they are
entangled in nets they soon die of suffocation owing to lack of
respiration, and hence they can be seen to come to the surface owing
to the necessity of breathing. But, since they have to feed in the
water, they must admit it, and it is in order to discharge this that
they all have a blow-hole; after admitting the water they expel it
through the blow-hole as the fishes do through the gills. The position
of the blow-hole is an indication of this, for it leads to none of the
organs which are charged with blood; but it lies before the brain
and thence discharges water.
It is for the very same reason that molluscs and crustaceans admit
water-I mean such animals as Carabi and Carcini. For none of these
is refrigeration a necessity, for in every case they have little
heat and are bloodless, and hence are sufficiently cooled by the
surrounding water. But in feeding they admit water, and hence must
expel it in order to prevent its being swallowed simultaneously with
the food. Thus crustaceans, like the Carcini and Carabi, discharge
water through the folds beside their shaggy parts, while cuttlefish
and the polyps employ for this purpose the hollow above the head.
There is, however, a more precise account of these in the History of
Animals.
Thus it has been explained that the cause of the admission of the
water is refrigeration, and the fact that animals constituted for a
life in water must feed in it.
19
An account must next be given of refrigeration and the manner in
which it occurs in respiring animals and those possessed of gills.
We have already said that all animals with lungs respire. The reason
why some creatures have this organ, and why those having it need
respiration, is that the higher animals have a greater proportion of
heat, for at the same time they must have been assigned a higher
soul and they have a higher nature than plants. Hence too those with
most blood and most warmth in the lung are of greater size, and animal
in which the blood in the lung is purest and most plentiful is the
most erect, namely man; and the reason why he alone has his upper part
directed to the upper part of the universe is that he possesses such a
lung. Hence this organ as much as any other must be assigned to the
essence of the animal both in man and in other cases.
This then is the purpose of refrigeration. As for the constraining
and efficient cause, we must believe that it created animals like
this, just as it created many others also not of this constitution.
For some have a greater proportion of earth in their composition, like
plants, and others, e.g. aquatic animals, contain a larger amount of
water; while winged and terrestrial animals have an excess of air
and fire respectively. It is always in the region proper to the
element preponderating in the scheme of their constitution that things
exist.
20
Empedocles is then in error when he says that those animals which
have the most warmth and fire live in the water to counterbalance
the excess of heat in their constitution, in order that, since they
are deficient in cold and fluid, they may be kept in life by the
contrary character of the region they occupy; for water has less
heat than air. But it is wholly absurd that the water-animals should
in every case originate on dry land, and afterwards change their place
of abode to the water; for they are almost all footless. He,
however, when describing their original structure says that, though
originating on dry land, they have abandoned it and migrated to the
water. But again it is evident that they are not warmer than
land-animals, for in some cases they have no blood at all, in others
little.
The question, however, as to what sorts of animals should be
called warm and what cold, has in each special case received
consideration. Though in one respect there is reason in the
explanation which Empedocles aims at establishing, yet his account
is not correct. Excess in a bodily state is cured by a situation or
season of opposite character, but the constitution is best
maintained by an environment akin to it. There is a difference between
the material of which any animal is constituted and the states and
dispositions of that material. For example, if nature were to
constitute a thing of wax or of ice, she would n
ot preserve it by
putting it in a hot place, for the opposing quality would quickly
destroy it, seeing that heat dissolves that which cold congeals.
Again, a thing composed of salt or nitre would not be taken and placed
in water, for fluid dissolves that of which the consistency is due
to the hot and the dry.
Hence if the fluid and the dry supply the material for all bodies,
it is reasonable that things the composition of which is due to the
fluid and the cold should have liquid for their medium [and, if they
are cold, they will exist in the cold], while that which is due to the
dry will be found in the dry. Thus trees grow not in water but on
dry land. But the same theory would relegate them to the water, on
account of their excess of dryness, just as it does the things that
are excessively fiery. They would migrate thither not on account of
its cold but owing to its fluidity.
Thus the natural character of the material of objects is of the same
nature as the region in which they exist; the liquid is found in
liquid, the dry on land, the warm in air. With regard, however, to
states of body, a cold situation has, on the other hand, a
beneficial effect on excess of heat, and a warm environment on
excess of cold, for the region reduces to a mean the excess in the
bodily condition. The regions appropriate to each material and the
revolutions of the seasons which all experience supply the means which
must be sought in order to correct such excesses; but, while states of
the body can be opposed in character to the environment, the
material of which it is composed can never be so. This, then, is a
sufficient explanation of why it is not owing to the heat in their
constitution that some animals are aquatic, others terrestrial, as
Empedocles maintains, and of why some possess lungs and others do not.
21
The explanation of the admission of air and respiration in those
animals in which a lung is found, and especially in those in which
it is full of blood, is to be found in the fact that it is of a spongy
nature and full of tubes, and that it is the most fully charged with
blood of all the visceral organs. All animals with a full-blooded lung
require rapid refrigeration because there is little scope for
deviation from the normal amount of their vital fire; the air also
must penetrate all through it on account of the large quantity of
blood and heat it contains. But both these operations can be easily
performed by air, for, being of a subtle nature, it penetrates
everywhere and that rapidly, and so performs its cooling function; but
water has the opposite characteristics.
The reason why animals with a full-blooded lung respire most is
hence manifest; the more heat there is, the greater is the need for
refrigeration, and at the same time breath can easily pass to the
source of heat in the heart.
22
In order to understand the way in which the heart is connected
with the lung by means of passages, we must consult both dissections
and the account in the History of Animals. The universal cause of
the need which the animal has for refrigeration, is the union of the
soul with fire that takes place in the heart. Respiration is the means
of effecting refrigeration, of which those animals make use that
possess a lung as well as a heart. But when they, as for example the
fishes, which on account of their aquatic nature have no lung, possess
the latter organ without the former, the cooling is effected through
the gills by means of water. For ocular evidence as to how the heart
is situated relatively to the gills we must employ dissections, and
for precise details we must refer to Natural History. As a summarizing
statement, however, and for present purposes, the following is the
account of the matter.
It might appear that the heart has not the same position in
terrestrial animals and fishes, but the position really is
identical, for the apex of the heart is in the direction in which they
incline their heads. But it is towards the mouth in fishes that the
apex of the heart points, seeing that they do not incline their
heads in the same direction as land-animals do. Now from the extremity
of the heart a tube of a sinewy, arterial character runs to the centre
where the gills all join. This then is the largest of those ducts, but
on either side of the heart others also issue and run to the extremity
of each gill, and by means of the ceaseless flow of water through
the gills, effect the cooling which passes to the heart.
In similar fashion as the fish move their gills, respiring animals
with rapid action raise and let fall the chest according as the breath
is admitted or expelled. If air is limited in amount and unchanged
they are suffocated, for either medium, owing to contact with the
blood, rapidly becomes hot. The heat of the blood counteracts the
refrigeration and, when respiring animals can no longer move the
lung aquatic animals their gills, whether owing to discase or old age,
their death ensues.
23
To be born and to die are common to all animals, but there are
specifically diverse ways in which these phenomena occur; of
destruction there are different types, though yet something is
common to them all. There is violent death and again natural death,
and the former occurs when the cause of death is external, the
latter when it is internal, and involved from the beginning in the
constitution of the organ, and not an affection derived from a foreign
source. In the case of plants the name given to this is withering,
in animals senility. Death and decay pertain to all things that are
not imperfectly developed; to the imperfect also they may be
ascribed in nearly the same but not an identical sense. Under the
imperfect I class eggs and seeds of plants as they are before the root
appears.
It is always to some lack of heat that death is due, and in
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, t
he 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
and death ensues.
24
Generation is the initial participation, mediated by warm substance,
in the nutritive soul, and life is the maintenance of this
participation. Youth is the period of the growth of the primary
organ of refrigeration, old age of its decay, while the intervening
time is the prime of life.
A violent death or dissolution consists in the extinction or
exhaustion of the vital heat (for either of these may cause
dissolution), while natural death is the exhaustion of the heat
owing to lapse of time, and occurring at the end of life. In plants
this is to wither, in animals to die. Death, in old age, is the
exhaustion due to inability on the part of the organ, owing to old
age, to produce refrigeration. This then is our account of
generation and life and death, and the reason for their occurrence
in animals.
25