by Aristotle
We have already stated that among animals insects do not respire, and the fact is evident 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 [475b1] 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 [5] 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. Their lack [10] of heat does not, however, suffice to keep them finally in life; for many fishes also live in the 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.
[15] 16(10) · 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 [20] 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 [25] 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 turtles, 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. [476a1] 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 [5] from their similarity to wings. But of those with feet one only, so far as observed, has gills. It is called the newt.
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 (πνεύμων) from its function as a receptacle of the [10] breath (πνεῦμα)) 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 [15] two organs one would be purposeless, this is the reason why some animals have gills, others lungs, but none possess both.
17(11) · 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 [20] 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 serve for this purpose.
We shall state further on how it is that these organs have the faculty of [25] 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 [30] 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 [476b1] 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 [5] 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 [10] arranged in a saw-like fashion, for they are debarred from chewing their food.
18(12) · 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 [15] blow-hole. They have no feet, yet possess a lung though admitting the sea-water. The reason for possessing a lung is that which we have just stated; 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 [20] 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 [25] 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 lies before the brain.
It is for the very same reason that molluscs and crustaceans admit water—I [30] mean such animals as crayfish and crabs. 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 expel the water in [477a1] order to prevent its being swallowed simultaneously with the food. Thus crustaceans, like the crayfish and crabs, discharge water through the folds beside their shaggy parts, while cuttle-fish 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 [5] 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 [10] it.
19(13) · 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 [15] part, 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.2 Hence too those with most [20] blood and most warmth in the lung are of greater size, and that 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 part. Hence this as much as any other part must be assigned as a cause of the essence of the animal both in man and in other cases.
[25] 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 [30] amount of water; while winged and terrestrial animals have an excess of air and fire respectively. Each has its station in the appropriate regions.
20(14) · Empedocles is then in error when he says that those animals [477b1] which have the most warmth and fire live in the water to counterbalan
ce 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 [5] 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 [10] 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 [15] 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 [20] 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 dry.
Hence if the fluid and the dry supply the material for all bodies, it is reasonable [25] that things the composition of which is due to the fluid should have liquid for their medium, 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 have migrated there 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 [30] 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 [478a1] of cold, for the region reduces to a mean the excess in the bodily condition. This must be sought in the regions appropriate to each type of matter, and according to the changes of the seasons which are common to all; for, while states of the body can [5] 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. [10]
21(15) · 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 [15] 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 [20] 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. [25]
22(16) · 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. In general, the constitution of animals needs refrigeration because the soul is ignited in the heart. Respiration is the means of effecting refrigeration, of which those animals make use that possess a lung as well [30] 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 to how the heart is situated relatively to the gills we must employ dissections, and for precise details [478b1] we must refer to the History of Animals. 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 in fishes, but the position really is identical, for the apex of the heart is in the [5] 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 [10] 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.
[15] 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 the air is limited in amount and unchanged they are suffocated, for either medium, owing to contact with the blood, rapidly becomes hot, and, being hot, counteracts the refrigeration. And when respiring animals can no longer move the lung or [20] aquatic animals their gills, whether owing to disease or old age, their death ensues.
23(17) · 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 [25] 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 old age. Death and decay pertain to all things that are not imperfectly developed; to the imperfect also they may be [30] 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 sited, 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 [479a1] 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 [5] 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 [10] 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.
[15] 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 pu
ts out. Hence in old age death is painless, for no violent disturbance is [20] required to cause death, and the severance of the soul is entirely imperceptible. 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 [25] of the lung to move far either upwards or downwards. Finally, when motion is no longer possible, the breath is given out and death ensues.
24(18) · 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, [30] 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 [479b1] 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. [5]
This then is our account of generation and life and death, and the reason for their occurrence in animals.
25(19) · It is hence also clear why respiring animals are suffocated in [10] 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.