by Aristotle..............................................................................................................................................1

1..............................................................................................................................................................1

2..............................................................................................................................................................2

3..............................................................................................................................................................3

4..............................................................................................................................................................3

5..............................................................................................................................................................4

6..............................................................................................................................................................4

7..............................................................................................................................................................5

8..............................................................................................................................................................5

9..............................................................................................................................................................6

10............................................................................................................................................................6

11............................................................................................................................................................7

12............................................................................................................................................................7

13............................................................................................................................................................8

14............................................................................................................................................................9

15............................................................................................................................................................9

16..........................................................................................................................................................10

17..........................................................................................................................................................11

18..........................................................................................................................................................11

19..........................................................................................................................................................12

20..........................................................................................................................................................12

21..........................................................................................................................................................13

22..........................................................................................................................................................13

23..........................................................................................................................................................14

24..........................................................................................................................................................14

25..........................................................................................................................................................15

26..........................................................................................................................................................15

27..........................................................................................................................................................15

WE must now treat of youth and old age and life and death. We must probably also at the same time state the

causes of respiration as well, since in some cases living and the reverse depend on this.

We have elsewhere given a precise account of the soul, and while it is clear that its essential reality cannot be

corporeal, yet manifestly it must exist in some bodily part which must be one of those possessing control over

the members. Let us for the present set aside the other divisions or faculties of the soul (whichever of the two

be the correct name). But as to being what is called an animal and a living thing, we find that in all beings

endowed with both characteristics (viz. being an animal and being alive) there must be a single identical part

in virtue of which they live and are called animals; for an animal qua animal cannot avoid being alive. But a

thing need not, though alive, be animal, for plants live without having sensation, and it is by sensation that we

This organ, then, must be numerically one and the same and yet possess multiple and disparate aspects, for

being animal and living are not identical. Since then the organs of special sensation have one common organ

in which the senses when functioning must meet, and this must be situated midway between what is called

before and behind (we call 'before' the direction from which sensation comes, 'behind' the opposite), further,

since in all living things the body is divided into upper and lower (they all have upper and lower parts, so that

this is true of plants as well), clearly the nutritive principle must be situated midway between these regions.

That part where food enters we call upper, considering it by itself and not relatively to the surrounding

universe, while downward is that part by which the primary excrement is discharged.

Plants are the reverse of animals in this respect. To man in particular among the animals, on account of his

erect stature, belongs the characteristic of having his upper parts pointing upwards in the sense in which that

applies to the universe, while in the others these are in an intermediate position. But in plants, owing to their

being stationary and drawing their sustenance from the ground, the upper part must always be down; for there

is a correspondence between the roots in a plant and what is called the mouth in animals, by means of which

they take in their food, whether the source of supply be the earth or each other's bodies.

All perfectly formed animals are to be divided into three parts, one that by which food is taken in, one that by

which excrement is discharged, and the third the region intermediate between them. In the largest animals

this latter is called the chest and in the others something corresponding; in some also it is more distinctly

marked off than in others. All those also that are capable of progression have additional members subservient

to this purpose, by means of which they bear the whole trunk, to wit legs and feet and whatever parts are

possessed of the same powers. Now it is evident both by observation and by inference that the source of the

nutritive soul is in the midst of the three parts. For many animals, when either partthe head or the receptacle

of the foodis cut off, retain life in that member to which the middle remains attached. This can be seen to

occur in many insects, e.g. wasps and bees, and many animals also besides insects can, though divided,

While this member is indeed in actuality single, yet potentially it is multiple, for these animals have a

constitution similar to that of Plants; plants when cut into sections continue to live, and a number of trees can

be derived from one single source. A separate account will be given of the reason why some plants cannot

live when divided, while others can be propagated by the taking of slips. In this respect, however, plants and

It is true that the nutritive soul, in beings possessing it, while actually single must be potentially plural. And it

is too with the principle of sensation, for evidently the divided segments of these animals have sensation.

They are unable, however, to preserve their constitution, as plants can, not possessing the organs on which

the continuance of life depends, for some lack the means for seizing, others for receiving their food; or again

Divisible animals are like a number of animals grown together, but animals of superior construction behave

differently because their constitution is a unity of the highest possible kind. Hence some of the organs on

division display slight sensitiveness because they retain some psychical susceptibility; the animals continue

to move after the vitals have been abstracted: tortoises, for example, do so even after the heart has been

The same phenomenon is evident both in plants and in animals, and in plants we note it both in their

propagation by seed and in grafts and cuttings. Genesis from seeds always starts from the middle. All seeds

are bivalvular, and the place of junction is situated at the point of attachment (to the plant), an intermediate

part belonging to both halves. It is from this part that both root and stem of growing things emerge; the

startingpoint is in a central position between them. In the case of grafts and cuttings this is particularly true

of the buds; for the bud is in a way the startingpoint of the branch, but at the same time it is in a central

position. Hence it is either this that is cut off, or into this that the new shoot is inserted, when we wish either a

new branch or a new root to spring from it; which proves that the point of origin in growth is intermediate

Likewise in sanguineous animals the heart is the first organ developed; this is evident from what has been

observed in those cases where observation of their growth is possible. Hence in bloodless animals also what

corresponds to the heart must develop first. We have already asserted in our treatise on The Parts of Animals

that it is from the heart that the veins issue, and that in sanguineous animals the blood is the final nutriment

from which the members are formed. Hence it is clear that there is one function in nutrition which the mouth

has the faculty of performing, and a different one appertaining to the stomach. But it is the heart that has

supreme control, exercising an additional and completing function. Hence in sanguineous animals the source

both of the sensitive and of the nutritive soul must be in the heart, for the functions relative to nutrition

exercised by the other parts are ancillary to the activity of the heart. It is the part of the dominating organ to

achieve the final result, as of the physician's efforts to be directed towards health, and not to be occupied with

Certainly, however, all saguineous animals have the supreme organ of the sensefaculties in the heart, for it is

here that we must look for the common sensorium belonging to all the senseorgans. These in two cases,

taste and touch, can be clearly seen to extend to the heart, and hence the others also must lead to it, for in it

the other organs may possibly initiate changes, whereas with the upper region of the body taste and touch

have no connexion. Apart from these considerations, if the life is always located in this part, evidently the

principle of sensation must be situated there too, for it is qua animal that an animal is said to be a living thing,

and it is called animal because endowed with sensation. Elsewhere in other works we have stated the reasons

why some of the senseorgans are, as is evident, connected with the heart, while others are situated in the

head. (It is this fact that causes some people to think that it is in virtue of the brain that the function of

Thus if, on the one hand, we look to the observed facts, what we have said makes it clear that the source of

the sensitive soul, together with that connected with growth and nutrition, is situated in this organ and in the

central one of the three divisions of the body. But it follows by deduction also; for we see that in every case,

when several results are open to her, Nature always brings to pass the best. Now if both principles are located

in the midst of the substance, the two parts of the body, viz. that which elaborates and that which receives the

nutriment in its final form will best perform their appropriate function; for the soul will then be close to each,

and the central situation which it will, as such, occupy is the position of a dominating power.

Further, that which employs an instrument and the instrument it employs must be distinct (and must be

spatially diverse too, if possible, as in capacity), just as the flute and that which plays itthe handare diverse.

Thus if animal is defined by the possession of sensitive soul, this soul must in the sanguineous animals be in

the heart, and, in the bloodless ones, in the corresponding part of their body. But in animals all the members

and the whole body possess some connate warmth of constitution, and hence when alive they are observed to

be warm, but when dead and deprived of life they are the opposite. Indeed, the source of this warmth must be

in the heart in sanguineous animals, and in the case of bloodless animals in the corresponding organ, for,

though all parts of the body by means of their natural heat elaborate and concoct the nutriment, the governing

organ takes the chief share in this process. Hence, though the other members become cold, life remains; but

when the warmth here is quenched, death always ensues, because the source of heat in all the other members

depends on this, and the soul is, as it were, set aglow with fire in this part, which in sanguineous animals is

the heart and in the bloodless order the analogous member. Hence, of necessity, life must be coincident with

However, it is to be noticed that there are two ways in which fire ceases to exist; it may go out either by

exhaustion or by extinction. That which is selfcaused we call exhaustion, that due to its opposites extinction.

[The former is that due to old age, the latter to violence.] But either of these ways in which fire ceases to be

may be brought about by the same cause, for, when there is a deficiency of nutriment and the warmth can

obtain no maintenance, the fire fails; and the reason is that the opposite, checking digestion, prevents the fire

from being fed. But in other cases the result is exhaustion,when the heat accumulates excessively owing to

lack of respiration and of refrigeration. For in this case what happens is that the heat, accumulating in great

quantity, quickly uses up its nutriment and consumes it all before more is sent up by evaporation. Hence not

only is a smaller fire readily put out by a large one, but of itself the candle flame is consumed when inserted

in a large blaze just as is the case with any other combustible. The reason is that the nutriment in the flame is

seized by the larger one before fresh fuel can be added, for fire is ever coming into being and rushing just like

Clearly therefore, if the bodily heat must be conserved (as is necessary if life is to continue), there must be

some way of cooling the heat resident in the source of warmth. Take as an illustration what occurs when

coals are confined in a brazier. If they are kept covered up continuously by the socalled 'choker', they are

quickly extinguished, but, if the lid is in rapid alternation lifted up and put on again they remain glowing for a

long time. Banking up a fire also keeps it in, for the ashes, being porous, do not prevent the passage of air,

and again they enable it to resist extinction by the surrounding air by means of the supply of heat which it

possesses. However, we have stated in The Problems the reasons why these operations, namely banking up

and covering up a fire, have the opposite effects (in the one case the fire goes out, in the other it continues

Everything living has soul, and it, as we have said, cannot exist without the presence of heat in the

constitution. In plants the natural heat is sufficiently well kept alive by the aid which their nutriment and the

surrounding air supply. For the food has a cooling effect [as it enters, just as it has in man] when first it is

taken in, whereas abstinence from food produces heat and thirst. The air, if it be motionless, becomes hot, but

by the entry of food a motion is set up which lasts until digestion is completed and so cools it. If the

surrounding air is excessively cold owing to the time of year, there being severe frost, plants shrivel, or if, in

the extreme heats of summer the moisture drawn from the ground cannot produce its cooling effect, the heat

comes to an end by exhaustion. Trees suffering at such seasons are said to be blighted or starstricken. Hence

the practice of laying beneath the roots stones of certain species or water in pots, for the purpose of cooling

Some animals pass their life in the water, others in the air, and therefore these media furnish the source and

means of refrigeration, water in the one case, air in the other. We must proceedand it will require further

application on our partto give an account of the way and manner in which this refrigeration occurs.

A few of the previous physical philosophers have spoken of respiration. The reason, however, why it exists in

animals they have either not declared or, when they have, their statements are not correct and show a

comparative lack of acquaintance with the facts. Moreover they assert that all animals respirewhich is

untrue. Hence these points must first claim our attention, in order that we may not be thought to make

First then, it is evident that all animals with lungs breathe, but in some cases breathing animals have a

bloodless and spongy lung, and then there is less need for respiration. These animals can remain under water

for a time, which relatively to their bodily strength, is considerable. All oviparous animals, e.g. the

frogtribe, have a spongy lung. Also hemydes and tortoises can remain for a long time immersed in water;

for their lung, containing little blood, has not much heat. Hence, when once it is inflated, it itself, by means of

its motion, produces a cooling effect and enables the animal to remain immersed for a long time. Suffocation,

however, always ensues if the animal is forced to hold its breath for too long a time, for none of this class

take in water in the way fishes do. On the other hand, animals which have the lung charged with blood have

greater need of respiration on account of the amount of their heat, while none at all of the others which do not

Democritus of Abdera and certain others who have treated of respiration, while saying nothing definite about

the lungless animals, nevertheless seem to speak as if all breathed. But Anaxagoras and Diogenes both

maintain that all breathe, and state the manner in which fishes and oysters respire. Anaxagoras says that when

fishes discharge water through their gills, air is formed in the mouth, for there can be no vacuum, and that it

is by drawing in this that they respire. Diogenes' statement is that, when they discharge water through their

gills, they suck the air out of the water surrounding the mouth by means of the vacuum formed in the mouth,

But these theories are untenable. Firstly, they state only what is the common element in both operations and

so leave out the half of the matter. For what goes by the name of respiration consists, on the one hand, of

inhalation, and, on the other, of the exhalation of breath; but, about the latter they say nothing, nor do they

describe how such animals emit their breath. Indeed, explanation is for them impossible for, when the

creatures respire, they must discharge their breath by the same passage as that by which they draw it in, and

this must happen in alternation. Hence, as a result, they must take the water into their mouth at the same time

as they breathe out. But the air and the water must meet and obstruct each other. Further, when they discharge

the water they must emit their breath by the mouth or the gills, and the result will be that they will breathe in

and breathe out at the same time, for it is at that moment that respiration is said to occur. But it is impossible

that they should do both at the same time. Hence, if respiring creatures must both exhale and inhale the air,

and if none of these animals can breathe out, evidently none can respire at all.

Further, the assertion that they draw in air out of the mouth or out of the water by means of the mouth is an

impossibility, for, not having a lung, they have no windpipe; rather the stomach is closely juxtaposed to the

mouth, so that they must do the sucking with the stomach. But in that case the other animals would do so

also, which is not the truth; and the wateranimals also would be seen to do it when out of the water, whereas

quite evidently they do not. Further, in all animals that respire and draw breath there is to be observed a

certain motion in the part of the body which draws in the air, but in the fishes this does not occur. Fishes do

not appear to move any of the parts in the region of the stomach, except the gills alone, and these move both

when they are in the water and when they are thrown on to dry land and gasp. Moreover, always when

respiring animals are killed by being suffocated in water, bubbles are formed of the air which is forcibly

discharged, as happens, e.g. when one forces a tortoise or a frog or any other animal of a similar class to stay

beneath water. But with fishes this result never occurs, in whatsoever way we try to obtain it, since they do

not contain air drawn from an external source. Again, the manner of respiration said to exist in them might

occur in the case of men also when they are under water. For if fishes draw in air out of the surrounding water

by means of their mouth why should not men too and other animals do so also; they should also, in the same

way as fishes, draw in air out of the mouth. If in the former case it were possible, so also should it be in the

latter. But, since in the one it is not so, neither does it occur in the other. Furthermore, why do fishes, if they

respire, die in the air and gasp (as can be seen) as in suffocation? It is not want of food that produces this

effect upon them, and the reason given by Diogenes is foolish, for he says that in air they take in too much air

and hence die, but in the water they take in a moderate amount. But that should be a possible occurrence with

land animals also; as facts are, however, no land animal seems to be suffocated by excessive respiration.

Again, if all animals breathe, insects must do so also. many of them seem to live though divided not merely

into two, but into several parts, e.g. the class called Scolopendra. But how can they, when thus divided,

breathe, and what is the organ they employ? The main reason why these writers have not given a good

account of these facts is that they have no acquaintance with the internal organs, and that they did not accept

the doctrine that there is a final cause for whatever Nature does. If they had asked for what purpose

respiration exists in animals, and had considered this with reference to the organs, e.g. the gills and the lungs,

Democritus, however, does teach that in the breathing animals there is a certain result produced by

respiration; he asserts that it prevents the soul from being extruded from the body. Nevertheless, he by no

means asserts that it is for this purpose that Nature so contrives it, for he, like the other physical philosophers,

altogether fails to attain to any such explanation. His statement is that the soul and the hot element are

identical, being the primary forms among the spherical particles. Hence, when these are being crushed

together by the surrounding atmosphere thrusting them out, respiration, according to his account, comes in to

succour them. For in the air there are many of those particles which he calls mind and soul. Hence, when we

breathe and the air enters, these enter along with it, and by their action cancel the pressure, thus preventing

This explains why life and death are bound up with the taking in and letting out of the breath; for death

occurs when the compression by the surrounding air gains the upper hand, and, the animal being unable to

respire, the air from outside can no longer enter and counteract the compression. Death is the departure of

those forms owing to the expulsive pressure exerted by the surrounding air. Death, however, occurs not by

haphazard but, when natural, owing to old age, and, when unnatural, to violence.

But the reason for this and why all must die Democritus has by no means made clear. And yet, since

evidently death occurs at one time of life and not at another, he should have said whether the cause is external

or internal. Neither does he assign the cause of the beginning of respiration, nor say whether it is internal or

external. Indeed, it is not the case that the external mind superintends the reinforcement; rather the origin of

breathing and of the respiratory motion must be within: it is not due to pressure from around. It is absurd also

that what surrounds should compress and at the same time by entering dilate. This then is practically his

But if we must consider that our previous account is true, and that respiration does not occur in every animal,

we must deem that this explains death not universally, but only in respiring animals. Yet neither is it a good

account of these even, as may clearly be seen from the facts and phenomena of which we all have experience.

For in hot weather we grow warmer, and, having more need of respiration, we always breathe faster. But,

when the air around is cold and contracts and solidifies the body, retardation of the breathing results. Yet this

was just the time when the external air should enter and annul the expulsive movement, whereas it is the

opposite that occurs. For when the breath is not let out and the heat accumulates too much then we need to

respire, and to respire we must draw in the breath. When hot, people breathe rapidly, because they must do so

in order to cool themselves, just when the theory of Democritus would make them add fire to fire.

The theory found in the Timaeus, of the passing round of the breath by pushing, by no means determines

how, in the case of the animals other than landanimals, their heat is preserved, and whether it is due to the

same or a different cause. For if respiration occurs only in landanimals we should be told what is the reason

of that. Likewise, if it is found in others also, but in a different form, this form of respiration, if they all can

Further, the method of explaining involves a fiction. It is said that when the hot air issues from the mouth it

pushes the surrounding air, which being carried on enters the very place whence the internal warmth issued,

through the interstices of the porous flesh; and this reciprocal replacement is due to the fact that a vacuum

cannot exist. But when it has become hot the air passes out again by the same route, and pushes back inwards

through the mouth the air that had been discharged in a warm condition. It is said that it is this action which

But according to this way of thinking it will follow that we breathe out before we breathe in. But the opposite

is the case, as evidence shows, for though these two functions go on in alternation, yet the last act when life

comes to a close is the letting out of the breath, and hence its admission must have been the beginning of the

Once more, those who give this kind of explanation by no means state the final cause of the presence in

animals of this function (to wit the admission and emission of the breath), but treat it as though it were a

contingent accompaniment of life. Yet it evidently has control over life and death, for it results synchronously

that when respiring animals are unable to breathe they perish. Again, it is absurd that the passage of the hot

air out through the mouth and back again should be quite perceptible, while we were not able to detect the

thoracic influx and the return outwards once more of the heated breath. It is also nonsense that respiration

should consist in the entrance of heat, for the evidence is to the contrary effect; what is breathed out is hot,

and what is breathed in is cold. When it is hot we pant in breathing, for, because what enters does not

adequately perform its cooling function, we have as a consequence to draw the breath frequently.

It is certain, however, that we must not entertain the notion that it is for purposes of nutrition that respiration

is designed, and believe that the internal fire is fed by the breath; respiration, as it were, adding fuel to the

fire, while the feeding of the flame results in the outward passage of the breath. To combat this doctrine I

shall repeat what I said in opposition to the previous theories. This, or something analogous to it, should

occur in the other animals also (on this theory), for all possess vital heat. Further, how are we to describe this

fictitious process of the generation of heat from the breath? Observation shows rather that it is a product of

the food. A consequence also of this theory is that the nutriment would enter and the refuse be discharged by

Empedocles also gives an account of respiration without, however, making clear what its purpose is, or

whether or not it is universal in animals. Also when dealing with respiration by means of the nostrils he

imagines he is dealing with what is the primary kind of respiration. Even the breath which passes through the

nostrils passes through the windpipe out of the chest as well, and without the latter the nostrils cannot act.

Again, when animals are bereft of respiration through the nostrils, no detrimental result ensues, but, when

prevented from breathing through the windpipe, they die. Nature employs respiration through the nostrils as a

secondary function in certain animals in order to enable them to smell. But the reason why it exists in some

only is that though almost all animals are endowed with the sense of smell, the senseorgan is not the same in

A more precise account has been given about this elsewhere. Empedocles, however, explains the passage

inwards and outwards of the breath, by the theory that there are certain bloodvessels, which, while

containing blood, are not filled by it, but have passages leading to the outer air, the calibre of which is fine in

contrast to the size of the solid particles, but large relatively to those in the air. Hence, since it is the nature of

the blood to move upwards and downwards, when it moves down the air rushes in and inspiration occurs;

when the blood rises, the air is forced out and the outward motion of the breath results. He compares this

This then is what he says of respiration. But, as we said, all animals that evidently respire do so by means of

the windpipe, when they breathe either through the mouth or through the nostrils. Hence, if it is of this kind

of respiration that he is talking, we must ask how it tallies with the explanation given. But the facts seem to

be quite opposed. The chest is raised in the manner of a forgebellows when the breath is drawn init is quite

reasonable that it should be heat which raises up and that the blood should occupy the hot regionbut it

collapses and sinks down, like the bellows once more, when the breath is let out. The difference is that in a

bellows it is not by the same channel that the air is taken in and let out, but in breathing it is.

But, if Empedocles is accounting only for respiration through the nostrils, he is much in error, for that does

not involve the nostrils alone, but passes by the channel beside the uvula where the extremity of the roof of

the mouth is, some of the air going this way through the apertures of the nostrils and some through the mouth,

both when it enters and when it passes out. Such then is the nature and magnitude of the difficulties besetting

We have already stated that life and the presence of soul involve a certain heat. Not even the digesting

process to which is due the nutrition of animals occurs apart from soul and warmth, for it is to fire that in all

cases elaboration is due. It is for this reason, precisely, that the primary nutritive soul also must be located in

that part of the body and in that division of this region which is the immediate vehicle of this principle. The

region in question is intermediate between that where food enters and that where excrement is discharged. In

bloodless animals it has no name, but in the sanguineous class this organ is called the heart. The blood

constitutes the nutriment from which the organs of the animal are directly formed. Likewise the bloodvessels

must have the same originating source, since the one exists for the other's behoofas a vessel or receptacle for

it. In sanguineous animals the heart is the startingpoint of the veins; they do not traverse it, but are found to

Now the other psychical faculties cannot exist apart from the power of nutrition (the reason has already been

stated in the treatise On the Soul), and this depends on the natural fire, by the union with which Nature has set

it aglow. But fire, as we have already stated, is destroyed in two ways, either by extinction or by exhaustion.

It suffers extinction from its opposites. Hence it can be extinguished by the surrounding cold both when in

mass and (though more speedily) when scattered. Now this way of perishing is due to violence equally in

living and in lifeless objects, for the division of an animal by instruments and consequent congelation by

excess of cold cause death. But exhaustion is due to excess of heat; if there is too much heat close at hand and

the thing burning does not have a fresh supply of fuel added to it, it goes out by exhaustion, not by the action

of cold. Hence, if it is going to continue it must be cooled, for cold is a preventive against this form of

Some animals occupy the water, others live on land, and, that being so, in the case of those which are very

small and bloodless the refrigeration due to the surrounding water or air is sufficient to prevent destruction

from this cause. Having little heat, they require little cold to combat it. Hence too such animals are almost all

shortlived, for, being small, they have less scope for deflection towards either extreme. But some insects are

longerlived though bloodless, like all the others), and these have a deep indentation beneath the waist, in

order to secure cooling through the membrane, which there is thinner. They are warmer animals and hence

require more refrigeration, and such are bees (some of which live as long as seven years) and all that make a

humming noise, like wasps, cockchafers, and crickets. They make a sound as if of panting by means of air,

for, in the middle section itself, the air which exists internally and is involved in their construction, causing a

rising and falling movement, produces friction against the membrane. The way in which they move this

region is like the motion due to the lungs in animals that breathe the outer air, or to the gills in fishes. What

occurs is comparable to the suffocation of a respiring animal by holding its mouth, for then the lung causes a

heaving motion of this kind. In the case of these animals this internal motion is not sufficient for

refrigeration, but in insects it is. It is by friction against the membrane that they produce the humming sound,

as we said, in the way that children do by blowing through the holes of a reed covered by a fine membrane. It

is thus that the singing crickets too produce their song; they possess greater warmth and are indented at the

Animals also which are sanguineous and possess a lung, though that contains little blood and is spongy, can

in some cases, owing to the latter fact, live a long time without breathing; for the lung, containing little blood

or fluid, can rise a long way: its own motion can for a long time produce sufficient refrigeration. But at last it

ceases to suffice, and the animal dies of suffocation if it does not respireas we have already said. For of

exhaustion that kind which is destruction due to lack of refrigeration is called suffocation, and whatsoever is

We have already stated that among animals insects do not respire, and the fact is open to observation in the

case of even small creatures like flies and bees, for they can swim about in a fluid for a long time if it is not

too hot or too cold. Yet animals with little strength tend to breathe more frequently. These, however, die of

what is called suffocation when the stomach becomes filled and the heat in the central segment is destroyed.

Again among wateranimals those that are bloodless remain alive longer in air than those that have blood and

admit the seawater, as, for example, fishes. Since it is a small quantity of heat they possess, the air is for a

long time adequate for the purposes of refrigeration in such animals as the crustacea and the polyps. It does

not however suffice, owing to their want of heat, to keep them finally in life, for most fishes also live though

among earth, yet in a motionless state, and are to be found by digging. For all animals that have no lung at all

Concerning the bloodless animals we have declared that in some cases it is the surrounding air, in others

fluid, that aids the maintenance of life. But in the case of animals possessing blood and heart, all which have

a lung admit the air and produce the cooling effect by breathing in and out. All animals have a lung that are

viviparous and are so internally, not externally merely (the Selachia are viviparous, but not internally), and of

the oviparous class those that have wings, e.g. birds, and those with scales, e.g. tortoises, lizards, and snakes.

The former class have a lung charged with blood, but in the most part of the latter it is spongy. Hence they

employ respiration more sparingly as already said. The function is found also in all that frequent and pass

their life in the water, e.g. the class of watersnakes and frogs and crocodiles and hemydes, both sea and

All these and similar animals both bring forth on land and sleep on shore or, when they do so in the water,

keep the head above the surface in order to respire. But all with gills produce refrigeration by taking in water;

the Selachia and all other footless animals have gills. Fish are footless, and the limbs they have get their name

(pterugion) from their similarity to wings (pterux). But of those with feet one only, so far as observed, has

No animal yet has been seen to possess both lungs and gills, and the reason for this is that the lung is

designed for the purpose of refrigeration by means of the air (it seems to have derived its name (pneumon)

from its function as a receptacle of the breath (pneuma)), while gills are relevant to refrigeration by water.

Now for one purpose one organ is adapted and one single means of refrigeration is sufficient in every case.

Hence, since we see that Nature does nothing in vain, and if there were two organs one would be purposeless,

this is the reason why some animals have gills, others lungs, but none possess both.

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 nonrespiring 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 windpipethe 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 sawlike fashion,

Among wateranimals the cetaceans may give rise to some perplexity, though they too can be rationally

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 seawater. 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 blowhole; after

admitting the water they expel it through the blowhole as the fishes do through the gills. The position of the

blowhole is an indication of this, for it leads to none of the organs which are charged with blood; but it lies

It is for the very same reason that molluscs and crustaceans admit waterI 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

Thus it has been explained that the cause of the admission of the water is refrigeration, and the fact that

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

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.

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 wateranimals 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 landanimals, for in some cases

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 not 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

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

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

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

fullblooded 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

The reason why animals with a fullblooded 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

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

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 landanimals 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

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

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

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

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

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

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

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

It is hence also clear why respiring animals are suffocated in water and fishes in air. For it is by water in the

latter class, by air in the former that refrigeration is effected, and either of these means of performing the

There is also to be explained in either case the cause of the cause of the motion of the gills and of the lungs,

the rise and fall of which effects the admission and expulsion of the breath or of water. The following,

In connexion with the heart there are three phenomena, which, though apparently of the same nature, are

Palpitation is the rushing together of the hot substance in the heart owing to the chilling influence of residual

or waste products. It occurs, for example, in the ailment known as 'spasms' and in other diseases. It occurs

also in fear, for when one is afraid the upper parts become cold, and the hot substance, fleeing away, by its

concentration in the heart produces palpitation. It is crushed into so small a space that sometimes life is

The beating of the heart, which, as can be seen, goes on continuously, is similar to the throbbing of an

abscess. That, however, is accompanied by pain, because the change produced in the blood is unnatural, and

it goes on until the matter formed by concoction is discharged. There is a similarity between this phenomenon

and that of boiling; for boiling is due to the volatilization of fluid by heat and the expansion consequent on

increase of bulk. But in an abscess, if there is no evaporation through the walls, the process terminates in

suppuration due to the thickening of the liquid, while in boiling it ends in the escape of the fluid out of the

In the heart the beating is produced by the heat expanding the fluid, of which the food furnishes a constant

supply. It occurs when the fluid rises to the outer wall of the heart, and it goes on continuously; for there is a

constant flow of the fluid that goes to constitute the blood, it being in the heart that the blood receives its

primary elaboration. That this is so we can perceive in the initial stages of generation, for the heart can be

seen to contain blood before the veins become distinct. This explains why pulsation in youth exceeds that in

All the veins pulse, and do so simultaneously with each other, owing to their connexion with the heart. The

heart always beats, and hence they also beat continuously and simultaneously with each other and with it.

Palpitation, then, is the recoil of the heart against the compression due to cold; and pulsation is the

Respiration takes place when the hot substance which is the seat of the nutritive principle increases. For it,

like the rest of the body, requires nutrition, and more so than the members, for it is through it that they are

nourished. But when it increases it necessarily causes the organ to rise. This organ we must to be constructed

like the bellows in a smithy, for both heart and lungs conform pretty well to this shape. Such a structure must

be double, for the nutritive principle must be situated in the centre of the natural force.

Thus on increase of bulk expansion results, which necessarily causes the surrounding parts to rise. Now this

can be seen to occur when people respire; they raise their chest because the motive principle of the organ

described resident within the chest causes an identical expansion of this organ. When it dilates the outer air

must rush in as into a bellows, and, being cold, by its chilling influence reduces by extinction the excess of

the fire. But, as the increase of bulk causes the organ to dilate, so diminution causes contraction, and when it

collapses the air which entered must pass out again. When it enters the air is cold, but on issuing it is warm

owing to its contact with the heat resident in this organ, and this is specially the case in those animals that

possess a fullblooded lung. The numerous canallike ducts in the lung, into which it passes, have each a

bloodvessel lying alongside, so that the whole lung is thought to be full of blood. The inward passage of the

air is called respiration, the outward expiration, and this double movement goes on continuously just so long

as the animal lives and keeps this organ in continuous motion; it is for this reason that life is bound up with

It is in the same way that the motion of the gills in fishes takes place. When the hot substance in the blood

throughout the members rises, the gills rise too, and let the water pass through, but when it is chilled and

retreats through its channels to the heart, they contract and eject the water. Continually as the heat in the heart

rises, continually on being chilled it returns thither again. Hence, as in respiring animals life and death are

bound up with respiration, so in the other animals class they depend on the admission of water.

Our discussion of life and death and kindred topics is now practically complete. But health and discase also

claim the attention of the scientist, and not mercly of the physician, in so far as an account of their causes is

concerned. The extent to which these two differ and investigate diverse provinces must not escape us, since

facts show that their inquiries are, to a certain extent, at least conterminous. For physicians of culture and

refinement make some mention of natural science, and claim to derive their principles from it, while the most

accomplished investigators into nature generally push their studies so far as to conclude with an account of