Aristotle · a new plain-English translation from the original language
1| ON RESPIRATION. About respiration few of the earlier natural philosophers have spoken; and as to what purpose it serves for animals, some have declared nothing at all, while others have spoken, but not well, being more ignorant of the facts than they should be. Further, they say that all animals breathe; but this is not true. So it is necessary to go over these points first, so that we do not seem to be making charges against people who are not there to answer. That all the animals that have lungs breathe is clear. But even among these, those that have a bloodless and spongy lung need respiration less; hence they are able to remain a long time in the water, beyond what the strength of their body would allow. All the egg-laying animals have a spongy lung, for example the frog family. Further, water-tortoises and land-tortoises remain a long time in moist places; for the lung has little heat, since they have little blood in it; so when it is inflated by the motion it cools itself and enables them to remain a long time submerged. If, however, someone forces the issue and holds them under too long, they all suffocate; for none of these creatures takes in water the way fish do. All those that have a lung with blood in it need respiration more, because of the amount of heat they have; and of the rest, whatever has no lung, none of them breathes.
2| Democritus of Abdera and certain others among those who have spoken about respiration have not distinguished anything concerning the other animals, but seem to speak as though all animals breathe; Anaxagoras and Diogenes, however, while asserting that all animals breathe, do explain how fish and shellfish breathe. Anaxagoras says that when fish release the water through their gills, they draw in the air that forms in the mouth and so breathe; for there is no void anywhere. Diogenes says that when they release the water through their gills, they draw the air from the water standing around the mouth into the void that forms in the mouth, on the assumption that air is present in the water. But these accounts are impossible. For, in the first place, they leave out half the matter, because they speak of the common term as applying to only one of its two parts. For the term is called "respiration," and of this one part is exhalation and the other inhalation; and about exhalation they say nothing at all, as to how such animals breathe out. Nor is it possible for them to say; for whenever they breathe in, they must breathe out again by the very same channel, and do this continually in alternation, so that it follows that they take in water at the mouth and breathe out at the same time. And necessarily, when the two meet, each must impede the other. Then again, when they release the water, at that point they breathe out either through the mouth or through the gills, so that it follows that they breathe out and breathe in at the same time; for that, they say, is when the fish breathes in. But to breathe in and breathe out at the same time is impossible. So if it is necessary that things which breathe both breathe out and breathe in, and it is not possible for any of these creatures to breathe out, it is clear that none of them breathes in either.
3| Further, to say that the fish draws air from the mouth, or from the water through the mouth, is impossible; for fish have no windpipe, because they have no lung, but the stomach lies immediately next to the mouth, so that it would have to be the stomach that does the drawing in. And the fish themselves, when out of the water, would plainly do it; but it is evident that they do not do it. Further, in all animals that breathe and draw in breath we see a certain motion occurring in the part that does the drawing, but in the case of fish this does not happen; for they are seen to move nothing around the stomach, but only the gills, both while in the water and when they have been cast out onto dry land and are gasping. Further, when all breathing animals die by suffocation in water, bubbles of breath appear, forced out violently — as happens if one forcibly holds under water tortoises or frogs or any other such creature; but in the case of fish this does not happen, no matter how one tries it, since they have no breath from outside at all. And the manner in which they say respiration occurs for fish could equally well occur for human beings while in water;
4| for if fish draw in from the surrounding water with the mouth, why should not human beings and the other animals do this too? And we would likewise draw the air in from the mouth just as the fish do. So that if the one were possible, the other would be possible too; but since the latter is not so, it is clear that the former is not the case for fish either. Besides this, for what reason do fish die in air and appear to gasp just like creatures that are suffocating, if indeed they breathe? For it is surely not from lack of nourishment that they suffer this. As for the cause Diogenes gives, it is naive; for he says that they draw in too much air when in air, but only a moderate amount when in water, and that this is why they die. But this ought then to be possible in land animals too; yet as it is, no land animal is suffocated by breathing too vigorously. Further, if all animals breathe, it is clear that insects too must breathe; but many of them are observed to live when cut apart, not only into two parts but into several, for example the creatures called centipedes; and how, or by what part, could they possibly breathe? The chief reason they speak wrongly about these matters is that they are inexperienced in the internal parts, and that they do not grasp that nature does everything for the sake of something;
5| for if they had inquired for the sake of what respiration belongs to animals, and had examined this in connection with the parts, for example the gills and the lung, they would have found the cause more quickly. Democritus does say that something results for breathing animals from respiration, asserting that it prevents the soul from being squeezed out; yet he has said nothing to the effect that nature does this for this purpose. For, altogether, like the other natural philosophers, he too touches nowhere on this kind of cause. He says that soul and heat are the same thing, the primary shapes of the spherical bodies. So when these are compressed together by the surrounding air squeezing them, he says respiration comes to their assistance. For in the air there is a great number of the things he calls intellect and soul; so when the animal breathes and the air enters, these enter along with it, and by resisting the compression, they prevent the soul present in the animals from passing out. And this is why, he says, life and death consist in breathing in and breathing out; for whenever the surrounding air prevails, compressing the animal, and the air can no longer enter from outside to resist it, because the animal is unable to breathe, then death occurs for animals; for death, he says, is the departure of such shapes from the body due to the compression exerted by the surrounding air. But as to the cause why it is necessary for all to die, yet not whenever it happens to occur, but by nature in old age, and by force contrary to nature, he has shown nothing at all.
6| And yet he ought to have explained, since this is sometimes seen to happen and sometimes not, whether the cause is external or internal. Nor does he say anything about the origin of breathing — what the cause is, whether it comes from within or from without; for surely it is not the intellect from outside that keeps up the assistance, but the origin of respiration and of the motion arises from within, not as though the surrounding air were forcing it. And it is also absurd that the surrounding air should at once be compressing and, on entering, dilating. What he has said, and how, is roughly this. But if we must hold that what was said earlier is true, and that not all animals breathe, then this cause must be supposed to have been stated not for every kind of death, but only for the death of things that breathe. Yet even for these it is not stated well. This is clear from what actually happens, and from facts of which we all have experience. For in hot weather, being warmed more, we all need respiration more and breathe more rapidly; but when the surrounding air is cold and contracts and congeals the body, it happens that we hold the breath in. And yet on his account it ought to be then that the air entering from outside would prevent the compression. But in fact the opposite happens;
7| for whenever too much heat has been gathered together because we are not breathing out, that is when we need respiration; and having breathed in, we must then breathe out. And in hot weather people often breathe repeatedly, breathing as if for the sake of cooling, at the moment when, as the saying goes, they are adding fire to fire.
8| The account of circulation given in the Timaeus determines nothing about the other animals — by what means the preservation of their heat comes about, whether by the same means or through some other cause. For if breathing belongs only to land animals, one must state the cause of its belonging only to them; but if it belongs to the others too, though in a different way, this too must be determined — that is, if it is even possible for all of them to breathe. Further, the manner of the cause given is contrived. For he says that when the heat goes out through the mouth, the surrounding air, being pushed, moving along, falls into the same place, through the flesh being porous, from where the internal heat went out — since there is nothing empty, as things replace one another in turn; and having been heated, it goes out again by the same place, and pushes the air that falls out hot back in through the mouth; and that they do this continuously, always, breathing in and breathing out. Now it follows for those who think this way that exhalation occurs before inhalation. But it is the opposite. And here is the sign: these occur alternately with each other, but when they come to an end, they exhale — so that the beginning must be inhalation. Further, as to the purpose for which these things belong to animals (I mean breathing in and breathing out), those who speak in this way have said nothing, but declare themselves only as about some incidental occurrence.
9| And yet we see that these are decisive for living and dying: for when creatures are unable to breathe, that is when destruction occurs for those that breathe. Further, it is absurd that the exit and re-entry of heat through the mouth should not escape our notice, while the entry of breath into the chest and its exit again once heated should escape our notice. It is also absurd that inhalation should be an entry of heat. For the opposite appears to be the case: what is exhaled is hot, while what is inhaled is cold. And when it is hot, they breathe panting; for because the incoming breath does not cool sufficiently, it happens that they often gasp for breath. But surely one must not suppose that breathing occurs for the sake of nourishment, as though the internal fire were nourished by the breath, and as though in breathing in, fuel were thrown under the fire, so to speak, and once the fire had been fed, exhalation occurred. For we will say the same things again against this account as against the previous ones: this ought to happen also in the other animals, or something analogous to it, for all of them have vital heat. Further, in what manner one must say that heat comes to be from breath is itself contrived, since we see this coming about rather from nourishment. And it follows on this view that the same process both takes in nourishment and gives off residue — but we do not see this happening in the other cases.
10| Empedocles too speaks about breathing, though he does not say for what purpose, nor does he make clear anything about all the animals, whether they breathe or not. And in speaking about breathing through the nostrils he thinks he is speaking about the principal kind of breathing. But breathing occurs also through the windpipe from the chest, as well as through the nostrils; and the nostrils by themselves, apart from the windpipe, cannot breathe. And animals deprived of the breathing that occurs through the nostrils suffer nothing, but they die when deprived of that which occurs through the windpipe. For nature makes incidental use of breathing through the nostrils, in some animals, for the sake of smell; and this is why nearly all animals share in smelling, though the same sense-organ is not present in all of them. This has been discussed more clearly elsewhere. He says that inhalation and exhalation occur because there are certain vessels in which blood is present, though they are not full of blood, and which have passages to the outside air, smaller than the parts of the body but larger than those of air; so that, since blood is naturally disposed to move up and down, when it moves downward air flows in and inhalation occurs, and when it goes upward it falls out again and exhalation occurs — comparing what happens to water-clocks.
11| This is how all things breathe in and breathe out: all have bloodless channels of flesh stretched over the outer surface of the body, and at their mouths the outermost surface of the nostrils is pierced through and through with close-set channels, so that they hide the blood but leave a passage cut through for the air to pass easily. Then, from there, whenever the soft blood rushes away, the frothing air rushes down with a furious surge, and when it leaps back up, it breathes out again — just as when a child plays with a water-clock of shining bronze: when she places the mouth of the tube against her shapely hand and dips it into the yielding mass of silvery water, no water enters the vessel, but the mass of air pressing from within upon the close-set holes holds it back, until she uncovers the compressed stream; but then, as the air gives way, the water flows in to fill it. And in the same way, when water occupies the depths of the bronze vessel, with the mouth and passage blocked by human flesh, the air outside, straining to get in, holds back the water around the gates of the harsh-sounding narrow opening, gripping its edges, until she releases it with her hand; then again, the reverse of before, as the air rushes in, the water flows out to match. In the same way, when the soft blood, surging through the limbs, rushes back inward, the stream of air at once comes rushing down in a swelling flood, and when it leaps back up, it breathes out again in equal measure.
12| This, then, is what he says about breathing; but, as we have said, the animals that plainly breathe do so through the windpipe, by way of the mouth and the nostrils together. So if he is speaking about this kind of breathing, one must ask how the account of the cause he has given will fit the facts; for the opposite appears to happen. For when they raise that region, as with bellows in a smithy, they breathe in; and it is reasonable that the heat should rise, and that the blood should occupy the place of the heat; and when they contract and compress it, as with the bellows there, they breathe out. Except that there, they do not take in the air and let it out again by the same route, whereas those who breathe do so by the same route. But if he is speaking only about breathing through the nostrils, he is greatly mistaken; for breathing is not peculiar to the nostrils, but occurs by way of the passage around the uvula, where the back of the roof of the mouth ends, since the nostrils are perforated through to it — part of the breath going that way, part through the mouth, entering and leaving alike. The views stated by others about breathing, then, have difficulties of this kind and this many.
13| Since it has been said before that living, and the state of the soul, involve some heat — for not even concoction, through which nourishment comes to be for animals, occurs without soul or without heat, since fire accomplishes everything — it follows that in whatever place of the body, and in whatever part of that place, such a principle must first be found, there too the nutritive soul must first be present. This is the middle place, between that which receives the nourishment and that by which the residue is discharged. In the bloodless animals this part has no name, but in the blooded animals it is the heart. For the nourishment from which the parts of animals already come to be is the nature of blood. And the blood and the blood-vessels must have the same starting point; for the one exists for the sake of the other, as a vessel and its contents. And in the blooded animals the heart is the starting point of the blood-vessels; for they are not suspended through it but from it, all of them. This is clear to us from dissections. Now the other capacities of the soul cannot exist without the nutritive capacity (the reason for this has been stated earlier, in the work on the soul), but this one cannot exist without the natural fire.
14| for nature has kindled it in this. Destruction of fire, as has been said before, is quenching and wasting-away. Quenching is destruction by the opposite; and this is why it happens all at once from the coldness of the surrounding medium, and happens faster the more the fire is scattered apart. This kind of destruction, then, is a violent one, and it belongs alike to ensouled things and to soulless things; for animals die when they are cut apart by instruments, and also when they are frozen solid by extreme cold. Wasting-away, by contrast, happens through excess of heat; for if the surrounding heat is too great, or if the fire does not get nourishment, that which is on fire is destroyed, not by being cooled but by wasting away. So it is necessary that cooling occur if the thing is to attain preservation; for this is what helps against this kind of destruction.
15| Since some animals spend their time in water and others on land, for the small ones among these altogether, and for the bloodless ones, the cooling that comes from the surrounding water or air is sufficient to help against this destruction; for having only a little heat, they need only a little help against it. This is why nearly all such creatures are short-lived; for they are close to the tipping point on both sides. But those insects that are longer-lived (for all insects are bloodless) have a split beneath the diaphragm-band, so that they may be cooled through the membrane, which is thinner there; for being hotter, they need more cooling — for example bees (some bees live as long as seven years) and the other creatures that buzz, such as wasps, cockchafers, and cicadas. For they make their sound with breath, as if panting; for within the band itself, as the innate breath rises and subsides, a friction against the membrane results; for they move this region, just as creatures that breathe from outside move it with the lung, and fish with their gills. And something similar happens if one chokes a breathing creature by holding its mouth shut —
16| for this too will produce this same rising motion in the lung. But in these creatures such a motion does not produce sufficient cooling, whereas in the others it does. And by the friction against the membrane they make the buzzing sound, as we say — just as children do through hollowed-out reeds, when they fit a thin membrane over them. This is also why, among cicadas, the ones that sing do sing: they are hotter, and the region beneath the diaphragm-band is split in them, whereas in the ones that do not sing this part is unsplit. And among the blooded creatures that have a lung, some, because they have little blood and a spongy lung, are for this reason able to live for a long time without breathing, because the lung has a great deal of rising-and-falling motion while containing little blood and moisture; for its own proper motion suffices for a long time to provide cooling. It cannot do so forever, though, but the creature is suffocated if it does not breathe, as has also been said before; for the kind of wasting-away that comes from not being cooled is called suffocation, and we say that things destroyed in this way are suffocated. That the insect class of animals does not breathe has also been said before, and it is evident too in the case of small animals, such as flies and bees; for they can be kept swimming in liquids for a long time, provided the liquid is not too hot or too cold.
17| And yet creatures of small power seek to breathe more frequently. But these creatures are destroyed, and are said to be suffocated, when the belly-cavity is filled and the moisture in the diaphragm-band is destroyed. This is why, after lying a while in ashes, they rise up again. And among creatures that live in water, those that are bloodless live longer in air than the blooded ones that take in seawater, such as fish; for because they have little heat, the air suffices to cool them for a long time — for example the soft-shelled creatures and the octopuses. Still, it does not suffice all the way to sustain life, because they have little heat, since indeed most fish live on land too, though motionless, and are found by digging them up. For whatever creatures either have no lung at all, or have a bloodless one, need cooling less often.
18| Concerning the bloodless creatures, then, it has been said that for some the surrounding air, for others the water, helps toward life. As for the blooded creatures that have a heart, all those that have a lung take in air and effect their cooling by breathing in and breathing out. And a lung is possessed by those that bear their young alive within themselves and not only outside (for the cartilaginous fish bear their young alive, but not within themselves), and among the egg-laying creatures, both the feathered ones, such as birds, and the scaly ones, such as tortoises, lizards, and snakes. The former group is blooded; and most of the latter have a spongy lung. This is why they use breathing more sparsely, as has been said before too. And all creatures that spend their lives and pass their time in the waters make use of breathing as well — for instance the class of water-snakes, frogs, crocodiles, freshwater tortoises, both the sea tortoises and the land tortoises, and seals; for all of these and creatures like them give birth on dry land, and sleep either on dry land, or in the water while holding their mouth above it for the sake of breathing. Whatever creatures have gills are all cooled by taking in water —
19| and gills are possessed by the class of the so-called cartilaginous fish and the other footless creatures. And all fish are footless; for even the parts they have that resemble feet, they have in the likeness of little fins. Of the creatures that have feet, only one that has been observed has a single gill, the creature called the tadpole-like one. But nothing has yet been seen having both a lung and gills at the same time. The reason is that the lung exists for the sake of cooling by means of breath (indeed the lung seems to have taken its very name from its reception of breath), whereas gills exist for cooling from water; and one instrument is useful for one purpose, and a single mode of cooling is sufficient for all. So since we see nature doing nothing in vain, and of the two, one would have been in vain, for this reason some creatures have gills and others have a lung, but none has both.
20| Since each of the animals needs nourishment in order to exist, and needs cooling in order to be preserved, nature makes use of the same instrument for both of these purposes — just as in some creatures the tongue serves both for tastes and for speech, so in those that have a lung, the part called the mouth serves both for the working of nourishment and for exhalation and inhalation. In those that do not have a lung and do not breathe, the mouth serves for the working of nourishment, while for those that need cooling, the nature of the gills provides it. How the power of the aforesaid instruments produces the cooling, we shall say later. As for not letting nourishment get in the way, much the same thing happens both in creatures that breathe and in those that take in liquid; for they do not take in nourishment at the same time as they breathe — otherwise it results in choking, when nourishment, whether liquid or dry, slips in beside its path toward the lung by way of the windpipe; for the windpipe lies in front of the gullet, through which the nourishment travels to the part called the stomach. In four-footed and blooded creatures, then, the windpipe has, as it were, a lid, the epiglottis —
21| but in birds, and in those four-footed creatures that lay eggs, this is not present; instead they achieve the same effect by drawing the passage together. For as they take in nourishment, some draw it together, while others place the epiglottis over it. And when the nourishment has passed on, some lift this part again, others open it apart and admit the breath for cooling. Those that have gills, having let the liquid out through these, take in nourishment through the mouth; for they do not have a windpipe, so that in this respect they could not be harmed at all by the liquid falling in beside its path, since it goes into the stomach instead. This is why they make the release and the taking of nourishment quick, and why they have sharp teeth, and nearly all of them are saw-toothed; for it is not possible for them to grind their nourishment smooth.
22| One might raise a difficulty about the cetaceans among the water animals, but their case too conforms to reason — I mean dolphins, whales, and all the others that have what is called the blowhole. These are footless, yet since they have a lung they take in sea water. The cause of this is what has just been stated: they do not take in the liquid for the sake of cooling. That cooling comes to them from breathing, since they have a lung. That is also why they sleep with the mouth held above water, and why dolphins actually snore. Further, if they are caught in nets they are quickly suffocated, because they are not breathing; and such animals are seen floating up to the surface of the sea because of their need to breathe. But since it is necessary for them to take their food in liquid, it is necessary that in taking in the liquid they also let it out again, and this is why they all have the blowhole: having taken in the water, just as fish do at the gills, they draw it up and expel it through the blowhole. A sign of this is also the position of the blowhole: it does not connect to any of the blooded parts, but is positioned in front of the brain, and lets the water out. For this same reason the molluscs and the crustaceans also take in water — I mean, for example, the animals called crayfish and crabs.
23| For none of these happens to need cooling: each of them has little heat and is bloodless, so that it is sufficiently cooled by the surrounding liquid; rather, they take in water for the sake of feeding, so that the liquid does not flow in on them at the same time as they take in food. So the crustaceans, such as crabs and crayfish, let the water out past the hairy parts, through the folds, while cuttlefish and octopuses let it out through the hollow above the part called the head. A more precise account of these has been written in the Researches on Animals. Concerning the taking in of liquid, then, it has been said that it happens both because of cooling and because those animals that are by nature aquatic must take their food from the liquid.
24| Next we must speak about cooling — in what way it occurs both in the animals that breathe and in those that have gills. That all animals that have a lung breathe has been stated earlier. As for why some animals have this part, and why those that have it need breathing, the cause of their having it is that the more honorable animals have obtained a greater share of heat; for it is necessary that they have at the same time obtained a more honorable soul as well, since such natures are more honorable than the nature of plants. That is also why the animals that have the most fully blooded and hot a lung are also greater in size, and why man, who among animals has the purest and most abundant blood, stands most upright, and is the only animal whose upper part is oriented toward the upper part of the universe — because of having this part of such a kind. So this must be posited as a cause of the substance both of this animal and of the others, just as any other of the parts is. It has the lung, then, for the sake of this. But one must also think that such animals are constituted by the cause that arises from necessity and from motion, just as many animals not of this kind are also constituted; for some have come to be from a greater share of earth, as the class of plants, others from water, as the class of water animals; and of the winged and land animals, some from air and some from fire. And each has its rank in its own proper region.
25| Empedocles did not put this well when he said that the hottest animals, having the most fire, are aquatic, fleeing the excess of heat in their nature, so that, since they are deficient in the cold and the liquid, they might recover the opposites by their region — for liquid, he says, is less hot than air. Now in general it is strange how it is possible for each of them, having come to be in the dry, to change its region to the liquid; for pretty much all of them are, moreover, footless. And he who says their original constitution came to be says they came to be in the dry, but fled and came into the liquid. Further, they do not appear to be hotter than the land animals: some of them are entirely bloodless, and others have little blood. But what sort of things one ought to call hot and cold has its own inquiry, taken in its own right; as for the cause Empedocles has stated, in one respect it has some plausible account for what is sought, but what he says is not in fact true. For the opposite regions and seasons preserve those that have excesses of state, but nature is preserved most of all in its own proper regions. For the matter of animals, out of which each of them is, is not the same thing as its states and dispositions.
26| I mean, for example, if nature were to constitute something out of wax, she would not have preserved it by placing it in the hot, nor if out of ice; for it would quickly have been destroyed by the opposite, since heat melts what has been constituted by the opposite. Nor if she had constituted something out of salt or soda, would she have carried it and set it down in liquid; for liquid destroys what has been constituted by the hot and the dry. If, then, the liquid and the dry are matter for all bodies, it is reasonable that the things constituted out of liquid and cold are in liquids, and if cold, will be in the cold, and those out of the dry in the dry. This is why trees do not grow in water, but in earth. And yet by the same reasoning they should go into water, on the ground that they are excessively dry, just as he says the excessively fiery animals do — for it was not because of the cold that the animal went into the liquid, but because it is liquid itself. So the natures of matter, in whatever region they are, happen to be of that kind: those in water are liquid, those in earth are dry, those in air are hot. States, however — those that exceed in heat are preserved more by being placed in the cold, and those that exceed in coldness by being placed in the hot —
27| for the region equalizes the excess of the state toward the mean. This, then, one must investigate with reference to the proper regions of each kind of matter, and with reference to the changes of the common season; for it is possible for states to be opposite to their regions, but for matter this is impossible. That it is not, then, because of the heat of their nature that some animals are aquatic and others land-dwelling, as Empedocles says, let this much be said, and also why some do not have a lung while others do. As for why those that have it take in air and breathe, and especially those among them that are blooded, the cause of their breathing is that the lung is spongy and full of passages. And indeed this part is the most fully blooded of the things called viscera. So those animals whose lung is blooded need rapid cooling, because the inclination of the soul's fire is small in them, yet it must penetrate inward throughout because of the quantity of blood and of heat. Both of these air can do easily: because it has a fine nature it both penetrates throughout and, passing through quickly, cools thoroughly; water does the opposite. And that those with a blooded lung breathe most of all is clear from these considerations: what is hotter needs more cooling, and at the same time the breath travels easily toward the source of the heat in the heart.
28| The way the heart has its passage toward the lung must be studied both from dissections and from the researches written on animals. Now the nature of animals in general needs cooling, on account of the fire of the soul that burns in the heart. Those animals produce this cooling by means of respiration which have not only a heart but also a lung. Those that have a heart but no lung, as fish do not, because their nature is aquatic, produce their cooling by water, through the gills. How the position of the heart stands relative to the gills must be studied, for the visible fact, from dissections, and for precision, from the researches; but to state it now in summary, it stands as follows. It might seem that the heart does not have the same position in land animals as in fish, but it does have the same position. For in whatever direction the animals incline their heads, there the heart has its point. But since the heads of land animals and of fish do not incline the same way, the heart has its point toward the mouth.
29| From the tip of the heart a channel, like a vein and sinew together, stretches to the middle, where all the gills join with one another. This is the largest such channel; but on either side of the heart other channels also stretch to the tip of each of the gills, through which the cooling reaches the heart, as the water continually runs its course through the gills. In the same way, in animals that breathe, the chest moves up and down repeatedly as they take in and let out breath, just as the gills do for fish. And breathing animals suffocate in a small and unchanging quantity of air; for in each case the medium quickly grows hot, since the touch of the blood heats it in both cases. The blood, being hot, prevents the cooling; and when breathing animals are unable to move the lung, or aquatic animals the gills, because of disease or old age, then death occurs.
30| Now coming-to-be and death are common to all animals, but the manners of it differ in kind; for destruction is not undifferentiated, though it does have something common to it. Death is either violent or natural: violent whenever the principle is from outside, natural whenever it is within the thing itself. And natural death is when the constitution of the part is such from the start, and not some acquired affection. In plants this is called withering, in animals it is called old age. Death and destruction occur alike for all things that are not incomplete; for incomplete things, in a similar way but by a different manner. By incomplete I mean, for example, eggs, and the seeds of plants, those that have not yet taken root. For all things, then, destruction occurs through the failing of some heat, but for complete things, through the failing of the heat in that part where the principle of the substance resides. This, as has been said before, is the point where the upper and lower parts join — in plants, the middle between shoot and root; among animals, in the blooded ones the heart, in the bloodless ones the analogous part. Some of these have many such principles potentially, though not in actuality.
31| That is why some insects go on living when cut apart, and why, among blooded animals, those that are not overly full of life live for a long time even with the heart removed — tortoises, for example, which still move with their feet while the shell is still on, because their nature is not well put together, in a way similar to insects. The principle of life fails, in those that possess it, whenever the heat that shares in it is no longer cooled. For, as has often been said, this heat consumes itself by itself. So whenever the lung in the one case, or the gills in the other, become hardened — the gills or the lung drying out over a long stretch of time and becoming earthy — these parts can no longer move, nor rise and draw together. And in the end, as the strain increases, the fire gradually dies away. That is why, when small afflictions befall them in old age, they die quickly; for because the heat is small, most of it having been breathed away over the course of a long life, whatever strain then occurs in the part quickly extinguishes it — just as when a tiny, faint flame is present in something, it is put out by the smallest disturbance. That is why death in old age is painless: for they die with no violent affection befalling them, but the release of the soul comes about wholly without being perceived.
32| And among diseases, those that make the lung hard — whether by growths, or by residues, or by an excess of morbid heat, as in fevers — make the breath rapid, because the lung is unable to rise high and settle far down. And in the end, when they can no longer move it at all, they die, breathing their last. Coming-to-be, then, is the first participation of the nutritive soul in the hot element, and life is the persistence of this participation. Youth is the growth of the primary cooling part, old age is its decline, and the prime of life is the mean between the two. A violent ending and destruction is the quenching and withering of the heat; for it could be destroyed through either of these two causes, while the natural death is the withering of this same heat coming about through length of time and completeness. In plants this is called withering, in animals it is called death. Of this, death in old age is the withering of the part through an incapacity to produce cooling, caused by old age. What, then, coming-to-be and life and death are, and through what causes they belong to animals, has been stated.
33| From this it is also clear why it happens that breathing animals suffocate in liquid, while fish suffocate in air: for in the one case cooling comes about through water, in the other through air, and each is deprived of its proper medium when the two change places. The cause of the motion — of the gills in the one case, of the lung in the other, by whose rising and settling the one group breathes out and in while the other takes in the liquid and expels it — and further the constitution of the organ itself, stands as follows.
34| There are three things that occur with respect to the heart which seem to have the same nature but do not: throbbing, pulsation, and respiration. Throbbing, then, is a contraction of the heat within the heart caused by a cooling that is either excremental or dissolving, as in the disease called palpitation, and in other diseases too, and also in states of fear; for those who are afraid grow cold in the upper parts, and the heat, fleeing inward and drawing together, produces the throbbing, being pushed together into so small a space that animals sometimes are extinguished and die from fear and from this morbid affection. But the pulsation that occurs in the heart, which it evidently keeps producing continuously, is like that of abscesses — the motion they produce with pain, because the change taking place in the blood is contrary to nature. This continues until the matter is concocted into pus. This affection is like boiling; for boiling occurs when the liquid is turned into vapor by the heat, and it rises because its bulk becomes greater. The cessation of this, in the case of abscesses, if the matter does not disperse by breathing out, is putrefaction, as the liquid grows thicker; but in the case of boiling, the end comes by spilling over the vessel's rim.
35| In the heart, the swelling produced by heat in the moisture that continually arrives from nutriment causes the pulse, as it rises toward the outermost membrane of the heart. And this happens continuously and without interruption; for the moisture from which the substance of blood arises continually flows in without interruption — for blood is first produced in the heart. This is clear from its generation, from the beginning: for even before the veins are yet distinctly formed, the heart is visibly seen to contain blood. And because of this the pulse beats more strongly in the young than in the old; for the exhalation is greater in the young. And all the veins pulse, and pulse together with one another, because they are suspended from the heart. And the heart is always in motion; so those veins too are always moving, and moving together with one another, whenever it moves. The rebound, then, is the resistance that occurs against the contraction caused by the cold, while the pulse-beat is the aeration of the moisture as it is heated.
36| Respiration occurs as the heat increases, the heat in which the nutritive principle resides. For just as everything else needs nourishment, so too does this heat, and more than the rest; for it is the cause of nourishment for the rest. So, as it becomes greater, it necessarily raises the organ. One should suppose that the structure of this organ is similar to the bellows used in smithies; for neither the lung nor the heart is far from admitting such a shape; and this structure is double, for the nutritive part of the natural power must lie in the middle. So it rises as it becomes greater, and as it rises the part surrounding it must necessarily rise as well. This is plainly what those who breathe do: they raise the chest, because the principle residing within it makes that part do this same thing. For as it rises, just as with bellows, it necessarily draws in the air from outside, and this air, being cold and cooling, quenches the excess of the fire. And just as this part rose as the heat increased, so, as the heat diminishes, the part must necessarily sink down, and as it sinks the air that entered must go out again — entering cold but leaving hot, on account of its contact with the heat present in this part, and most of all in those creatures whose lung contains blood;
37| for the air falls into the passages in the lung, which are like many channels, and alongside each of these veins are stretched out, so that the whole lung seems to be full of blood. The entrance of the air is called inhalation, and its exit exhalation. And this happens continuously without interruption, for as long as the animal lives and this part continues to move without interruption. And because of this, life consists in breathing in and breathing out. And in the same way, the movement of the gills occurs in fish. For as the heat in the blood rises through the parts, the gills also rise, and let the water pass through; and as it descends toward the heart through the passages and is cooled, the gills sink down, and release the water. And since the heat in the heart is always rising, the fish always takes in water again as it cools. For this reason, for land animals life and death find their end in breathing, and for fish in taking in moisture. Concerning life and death, then, and the topics related to this inquiry, nearly everything has been said. But concerning health and disease, it belongs not only to the doctor but also to the natural philosopher, up to a point, to state the causes.
38| But it should not escape notice in what way they differ, and in what respect they study different things, since what actually happens bears witness that the two inquiries border on one another up to a point: for among doctors, those who are refined or meticulous say something about nature and think it right to take their principles from there, and among those who have written treatises on nature, the most refined pretty much end up at medical principles.