Outline of ‘On Youth, Old Age, Life and Death, and Respiration’
(De juventute et senectute, de vita et morte, de respiratione)
We must now treat of youth and old age and life and death, and probably of respiration as well, since in some cases living and the reverse depend on this. While it is clear that the essential reality of the soul cannot be corporeal, yet it must exist in some bodily part. All perfectly formed animals can be divided into three parts, that by which food is taken, that by which excrement is discharged, and that which is intermediate, called the chest or something equivalent. It is evident both by observation and by inference that the source of the nutritive soul is in the midst of the three parts since many animals, though divided, retain life in that member to which the middle remains attached. 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. In plants genesis from seeds and from grafts and cuttings always starts from the middle. Likewise in animals the heart is the first organ developed, and the blood is the final nutriment from which the members are formed. Hence, in 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. If life is located in this part, then sensation must be too, for it is qua animal that an animal is said to be a living thing, and an animal is an animal because it is sensing. Taste and touch can be clearly seen to extend to the heart, and the other senses must also lead to it. The other senses are situated in the head, which leads some to think that it is by the brain that animals perceive; however, it is the central situation which is the natural position of a dominating power. The source of an animal’s warmth is in the heart, which explains why death occurs when the heart becomes cold, but not when any of the other members do so. Hence, life must be coincident with the maintenance of heat, and death with its destruction. The life fire may be extinguished by a deficiency of nutriment or by excessive accumulation from lack of respiration and of refrigeration. Clearly, therefore, if the bodily heat is to be conserved, there must be some cooling method, just like the coals in a brazier must be ventilated if they are to remain glowing. In plants the natural heat is kept alive by the surrounding air supply and by their nutriment, for food has a cooling effect, and abstinence from food produces heat and thirst. Animals pass their life in air or water, and these media furnish the source and means of their refrigeration. All animals with lungs breathe, even though those with bloodless and spongy lungs have less need for breathing and can therefore remain under water for a longer time. On the other hand, animals with lungs charged with blood have greater need for breathing on account of the amount of their heat. Democritus, Anaxagoras, and Diogenes either imply or maintain that animals without lungs also breathe. Diogenes, for instance, says that when fishes discharge water through the gills, they suck the air out of the water surrounding the mouth by means of the vacuum formed therein, but this and other such theories are untenable. The main reason these thinkers go wrong is that they have no acquaintance with the internal organs, and they do not accept the doctrine that there is a final cause for whatever Nature does. Had they asked why respiration exists in animals, and had they considered this with reference to the gills and lungs, they might have come to the correct conclusions.
Life and the presence of soul involve a certain heat. Not even digestion occurs apart from soul and warmth, for it is to fire that in all cases elaboration is due. For this reason also, the primary nutritive soul must be located in that part of the body which is the immediate vehicle of this principle, namely, that which is intermediate between that where food is taken and that where excrement is discharged. In bloodless animals this organ has no name, but in sanguineous animals it is called the heart. The blood in the heart and in the vessels that originate from it constitutes the nutriment from which the organs are formed. The other psychical faculties cannot exist apart from the nutritive (the reason for this having been given in On the Soul), which itself depends on the natural fire. This natural fire can be destroyed either by extinction through violence or excess of cold, or by exhaustion through excess of heat. In small and bloodless animals, refrigeration by surrounding air or water is sufficient to prevent exhaustion from excess of heat. If these animals are short-lived, it is because they have less scope for deflection towards either extreme. Some insects, though bloodless, are longer-lived as they have a deep indentation beneath the waist where their membrane is thinner and therefore more adapted to the cooling function. The sound made by humming insects such as bees and crickets results from friction against the membrane caused by the rising and falling of air within the middle section. Sanguineous animals with spongy lungs can live a long time without breathing as their lungs, containing little blood, can rise high and thereby produce sufficient refrigeration. Among water animals, those that are bloodless remain alive longer in air than those (such as fishes) that are sanguineous, since they have less heat and the air is for a long time adequate to refrigerate them. Sanguineous animals with lungs produce refrigeration by breathing air in and out, whereas those with gills do so by taking in water. All footless animals have gills with the exception of the tadpole, which has both gills and feet. No animal has yet been observed to have both lungs and gills, as one means of refrigeration is sufficient in every case, and, as I am fond of saying, nature does nothing in vain. Every animal requires nutriment and refrigeration and, except in animals without lungs or with gills, nature employs the mouth for both purposes. Cetaceans such as dolphins and whales seem like a case apart, as they possess a lung and yet admit sea water. However, the admission of sea water is not for the purpose of refrigeration but of feeding, and these animals sleep with their head out of the water (dolphins, at any rate, even snore). After admitting the water they expel it through a blow-hole as fishes do through the gills. The higher animals breathe because they have a higher soul and a greater proportion of heat. Those with most blood and most warmth are of greater size, and man is the most erect as the blood in his lung is the purest and the most plentiful. The natural character of the material of an animal’s body is of the same nature as the region in which it exists: water is found in water, earth on land, and air and fire in air. Thus, whereas the state of an animal’s body can be opposed in character to its environment, not so the material of which it is composed. The source of life is lost when the heat with which it is bound up is no longer tempered by cooling. In time, the lungs or gills get dried up and become hard and earthy and incapable of movement, and then the fire goes out from exhaustion. In old age little heat remains, and a small disturbance easily results in death, whence dying in old age is painless. 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, and the prime of life is the intervening time. There are three separate phenomena related to the heart, palpitation, pulsation, and respiration. Palpitation is the recoil of the heart against the compression due to cold, pulsation the volatilisation of the heated fluid. When the hot substance increases it causes the organ to rise, which causes the outer air to rush in as into a bellows. The air’s chilling influence reduces the excess of the fire, resulting in the contraction and collapse of the organ and the expulsion of the warmed-up air. This completes my account of respiration, of life and death, and of youth and old age.