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On Youth and Old Age, On Life and Death, On Breathing 
By Aristotle 

Translated by G. R. T. Ross 


Part 1 

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 distinguish animal from what is not animal. 

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. 

Part 2 

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 part-the head or the receptacle 
of the food-is 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, continue to live by means of the part connected with nutrition. 

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 insects are alike. 

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 they may be destitute of 
other organs as well. 

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 removed. 

Part 3 

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 starting-point 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 starting-point 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 between 
stem and root. 

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 subordinate offices. 

Certainly, however, all saguineous animals have the supreme organ 

of the sensef acuities in the heart, for it is here that we must look 

for the common sensorium belonging to all the sense-organs. 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 sense-organs 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 perception belongs to animals.) 

Part 4 

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 it-the hand-are 
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 the maintenance of heat, and what we call death is its destruction. 

Part 5 

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 self-caused 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 a 
river, but so speedily as to elude observation. 

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 so-called '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 alive for a 
considerable time) . 

Part 6 

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 star-stricken. Hence the practice of laying 

beneath the roots stones of certain species or water in pots, for 

the purpose of cooling the roots of the plants. 

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 proceed-and it will 
require further application on our part-to give an account of the 
way and manner in which this refrigeration occurs. 

Part 7 

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 respire-which is untrue. Hence these points 
must first claim our attention, in order that we may not be thought 
to make unsubstantiated charges against authors no longer alive. 

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 frog-tribe, 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 possess lungs breathe. 

Part 8 

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, for he believes there is air in 

the water. 

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. 

Part 9 

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 water-animals 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, they would have discovered the reason more speedily. 

Part 10 

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 the expulsion of the soul 

which resides in the animal. 

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 theory, and how he puts it. 

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. 

Part 11 

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 land-animals, their heat is preserved, and whether it is 
due to the same or a different cause. For if respiration occurs only 
in land-animals 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 breathe, must also be described. 

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 goes 
on continuously when the breath is taken in and let out. 

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 process. 

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 . 

Part 12 

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 the same channel, but this does not 
appear to occur in the other instances . 

Part 13 

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 sense-organ is not the same in all. 

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 blood-vessels, 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 process to what occurs in a clepsydra. 

Thus all things outwards breathe and in;- their flesh has tubes 

Bloodless, that stretch towards the body's outmost edge, 

Which, at their mouths, full many frequent channels pierce, 

Cleaving the extreme nostrils through; thus, while the gore Lies hid, 

for air is cut a thoroughfare most plain. And thence, whenever shrinks 

away the tender blood, 

Enters the blustering wind with swelling billow wild. 

But when the blood leaps up, backward it breathes. As when 

With water-clock of polished bronze a maiden sporting, Sets on her 

comely hand the narrow of the tube 

And dips it in the frail-formed water's silvery sheen; Not then the 

flood the vessel enters, but the air, Until she frees the crowded 

stream. But then indeed Upon the escape runs in the water meet. So 

also when within the vessel's deeps the water 

Remains, the opening by the hand of flesh being closed, 

The outer air that entrance craves restrains the flood At the gates 

of the sounding narrow, upon the surface pressing, 

Until the maid withdraws her hand. But then in contrariwise 

Once more the air comes in and water meet flows out. 

Thus to the to the subtle blood, surging throughout the limbs, Whene'er 

it shrinks away into the far recesses Admits a stream of air rushing 

with swelling wave, 

But, when it backward leaps, in like bulk air flows out. 

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 forge-bellows when 
the breath is drawn in-it is quite reasonable that it should be heat 
which raises up and that the blood should occupy the hot region-but 
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 the theories of other writers concerning 
respiration . 


Part 14 

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 behoof-as a vessel or receptacle for it. 

In sanguineous animals the heart is the starting-point of the veins; 

they do not traverse it, but are found to stretch out from it, as 

dissections enable us to see. 

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 extinction. 

Part 15 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 short-lived, 

for, being small, they have less scope for deflection towards either 

extreme. But some insects are longer-lived 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 waist, but the songless variety have no fissure 

there . 

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 respire-as 
we have already said. For of exhaustion that kind which is destruction 
due to lack of refrigeration is called suffocation, and whatsoever 
is thus destroyed is said to be suffocated. 

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. This explains also why they revive after being 
among ashes for a time. 

Again among water-animals those that are bloodless remain alive longer 
in air than those that have blood and admit the sea-water, 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 or have a bloodless one require less refrigeration. 

Part 16 

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 water-snakes and frogs 
and crocodiles and hemydes, both sea- and land-tortoises, and seals. 

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 gills. It is called the tadpole. 

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 . 

Part 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. 

Part 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. 

Part 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. 

Part 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 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 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. 

Part 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. 

Part 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 disease or old age, 
their death ensues. 

Part 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, 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 and death ensues. 

Part 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. 

Part 25 

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 function is removed by a change of environment. 

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, moreover, is the manner of the constitution of the 
organ . 

Part 26 

In connexion with the heart there are three phenomena, which, though 

apparently of the same nature, are really not so, namely palpitation, 
pulsation, and respiration. 

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 extinguished, and the animals die of the fright 
and morbid disturbance. 

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 containing vessel. 

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 older people, for in the young the formation of vapour is 
more abundant. 

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 volatilization of the heated fluid. 

Part 27 

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 full-blooded lung. The numerous 

canal-like ducts in the lung, into which it passes, have each a blood-vessel 

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 the passage of the breath outwards 

and inwards. 

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 disease also claim the attention of the scientist, 
and not merely 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 medical principles.