by Aristotle
Now as there cannot possibly be an animal without sensation, it follows as a necessary consequence that every animal must have some homogeneous parts; for these alone are capable of sensation, the heterogeneous parts serving for the active functions. Again, as the sensory faculty, the motor faculty, and the nutritive faculty [25] are all lodged in one and the same part of the body, as was stated in a former treatise, it is necessary that the part which is the primary seat of these principles shall on the one hand, in its character of general sensory recipient, be one of the simple parts; and on the other hand shall, in its motor and active character, be one of the heterogeneous parts. For this reason it is the heart which in sanguineous animals [30] constitutes this central part, and in bloodless animals it is that which takes the place of a heart. For the heart, like the other viscera, divides into homogeneous parts; but it is at the same time heterogeneous in virtue of its definite configuration. And the same is true of the other so-called viscera, which are indeed formed from the same material as the heart. For all these viscera have a sanguineous character owing to [647b1] their being situated upon vascular ducts and branches. For just as a stream of water deposits mud, so the various viscera, the heart excepted, are, as it were, deposits from the stream of blood in the vessels. And as to the heart, the very starting-point of the vessels, and the actual seat of the force by which the blood is first fabricated, [5] it is as one would naturally expect, constituted out of the selfsame nutriment which it originates. Such, then, are the reasons why the viscera are of sanguineous aspect; and why in one point of view they are homogeneous, in another heterogeneous.
[10] 2 · Of the homogeneous parts of animals, some are soft and moist, others hard and dry; and of the former some are moist permanently, others only so long as they are in the living body. Such are blood, serum, lard, suet, marrow, semen, bile, milk when present, flesh, and their various analogues. For the parts enumerated are [15] not to be found in all animals, some animals only having parts analogous to them. Of the hard and dry homogeneous parts bone, fish-spine, sinew, blood-vessel, are examples. The last of these points to a sub-division that may be made in the class of homogeneous parts. For in some of them the whole and a portion of the whole in one sense are designated by the same term—as, for example, is the case with blood-vessel and bit of blood-vessel—while in another sense they are not; but a [20] portion of a heterogeneous part, such as face, in no sense has the same designation as the whole.
First, both the moist parts and the dry parts have causes of many kinds. Thus one set of homogeneous parts represent the material; for each separate organ is [25] constructed of bones, sinews, flesh, and the like; which contribute either to its substance or to the proper discharge of its function. A second set are the nutriment of the first, and are moist; for all growth comes from moisture; while a third set are the residue of the second. Such, for instance, are the dregs of the solid nutriment, and—in animals that have a bladder—those of the liquid.
Even the individual homogeneous parts present variations, which are in each [30] case for the sake of the better. The variations of the blood may be selected to illustrate this. For different bloods differ in their degrees of thinness or thickness, of clearness or turbidity, of coldness or heat; and this whether we compare the bloods from different parts of the same individual or the bloods of different animals. For all the differences just enumerated distinguish the blood of the upper and of the lower [648a1] halves of the body; and one section of animals is sanguineous, while the other has no blood, but only something resembling it in its place. The thicker and the hotter blood is, the more conducive is it to strength, while in proportion to its thinness and its coldness is its suitability for sensation and intelligence. A like distinction exists [5] also in the fluid which is analogous to blood. This explains how it is that bees and other similar creatures are of a more intelligent nature than many sanguineous animals; and that, of sanguineous animals, those are the most intelligent whose blood is thin and cold. Best of all are those whose blood is hot, and at the same time [10] thin and clear. For such are suited alike for the development of courage and of intelligence. Accordingly, the upper parts are superior in these respects to the lower, the male superior to the female, and the right side to the left. As with the blood so [15] also with the other parts, homogeneous and heterogeneous alike. For here also such variations as occur must be held either to be related to the substance and the functions of the several animals, or, in other cases, to be matters of better or worse. Two animals, for instance, may have eyes. But in one these eyes may be of fluid consistency, while in the other they are hard; and in one there may be eyelids, in the other no such appendages. In both cases the difference contributes to greater accuracy of vision.
As to why all animals must of necessity have blood or something of a similar [20] character, and what the nature of blood may be, these are matters which can only be considered when we have first discussed hot and cold. For the natural properties of many substances are referable to these two elementary principles; and it is a matter of frequent dispute what animals or what parts of animals are hot and what cold. [25] For some maintain that water animals are hotter than such as live on land, asserting that their natural heat counterbalances the coldness of their medium; and again, that bloodless animals are hotter than those with blood, and females than males. Parmenides, for instance, and some others declare that women are hotter than men, and that it is the warmth and abundance of their blood which causes their [30] menstrual flow, while Empedocles maintains the opposite opinion. Again, comparing the blood and the bile, some speak of the former as hot and of the latter as cold, while others invert the description. If there be this endless disputing about hot and cold, which of all things that affect our senses are the most distinct, what are we to [35] think as to the rest?
The explanation of the difficulty appears to be that things are called hotter in several ways; for each appears to have something to say, although they are at odds [648b1] with one another. There ought, then, to be some clear understanding as to the sense in which natural substances are to be termed hot or cold, dry or moist. For it appears manifest that these are properties on which even life and death are largely dependent, and that they are moreover the causes of sleep and waking, of maturity [5] and old age, of health and disease; while no similar influence belongs to roughness and smoothness, to heaviness and lightness, nor, in short, to any other such properties of matter. That this should be so is but in accordance with rational expectation. For hot and cold, dry and moist, as was stated in a former treatise, are [10] the principles of the natural elements.
Is then the term hot used in one way or in many? To answer this we must ascertain what special effect is attributed to a hotter substance, and if there be several such, how many these may be. A body then is in one sense said to be hotter than another, if it imparts a greater amount of heat to an object in contact with it. In a second sense, that is said to be hotter which causes the keener sensation when [15] touched, and especially if the sensation be attended with pain. This criterion, however, would seem sometimes to be a false one; for occasionally it is the condition of the individual that causes the sensation to be painful. Again, of two things, that is the hotter which the more readily melts a fusible substance, or sets on fire an inflammable one. Again, of two masses of one and the same substance, the larger is said to have more heat than the smaller. Again, of two bodies, that is said to be the [20] hotter which takes the longer time in cooling, as also we call that which is rapidly heated hotter in its nature than that which is long about it—as we call something contrary if it is at a distance, similar if it is nearby. The term hotter is used then in all the various senses that have been mentioned, and perhaps in still more. Now it is impossible for one body to be hotter than another in all these different fashions. [25] Boiling water for instance, though it is more scalding than flame, yet has no power of burning or melting combustible or fusible matter, while flame has. So again this boiling water is
hotter than a small fire, and yet gets cold more rapidly and [30] completely. For in fact fire never becomes cold; whereas water invariably does so. Boiling water, again, is hotter to the touch than oil; yet it gets cold and solid more rapidly than this other fluid. Blood, again, is hotter to the touch than either water or oil, and yet coagulates before them. Iron, again, and stones and other similar bodies [35] are longer in getting heated than water, but when once heated burn other substances with a much greater intensity. Another distinction is this. In some of the [649a1] bodies which are called hot the heat is derived from without, while in others it belongs to the bodies themselves; and it makes a most important difference whether the heat has the former or the latter origin. For one of them comes close to being hot [5] accidentally and not in its own right—as if, finding that some man in a fever was a musician, one were to say that musicians are hotter than healthy men. Of that which is hot per se and that which is hot per accidens, the former is the slower to cool, while not rarely the latter is the hotter to the touch. The former again is the [10] more burning of the two—flame, for instance, as compared with boiling water— while the latter, as the boiling water, which is hot per accidens, is the more heating to the touch. From all this it is clear that it is no simple matter to decide which of two bodies is the hotter. For the first may be the hotter in one sense, the second the [15] hotter in another. Indeed in some of these cases it is impossible to say simply even whether a thing is hot or not. For the actual substratum may not itself be hot, but may be hot when coupled with heat as an attribute, as would be the case if one attached a single name to hot water or hot iron. It is after this manner that blood is hot. In such cases—in those, that is, in which the substratum owes its heat to an [20] external influence—it is plain that cold is not a mere privation, but a fact of nature.
There is no knowing but that even fire may be another of these cases. For the substratum of fire may be smoke or charcoal, and though the former of these is always hot, smoke being an uprising vapour, yet the latter becomes cold when it is extinguished, as also would oil and pinewood under similar circumstances. But even [25] substances that have been burnt nearly all possess some heat, cinders, for example, and ashes, the waste-products of animals, and, among the excretions, bile; because some residue of heat has been left in them after their combustion. It is in another sense that pinewood and fat substances are hot; namely, because they rapidly assume the actuality of fire.
[30] Heat appears to cause both coagulation and melting. Now such things as are formed merely of water are solidified by cold, while such as are formed of nothing but earth are solidified by fire. Hot substances again are solidified by cold, and, when they consist chiefly of earth, the process of solidification is rapid, and the resulting substance is insoluble; but, when their main constituent is water, the solid matter is again soluble. What kinds of substances, however, admit of being solidified, and what are the causes of solidification, are questions that have already been dealt with more precisely in another treatise.
Now what is hot and what sort of thing is hotter are determined in a variety of ways, and those features do not belong to everything in the same way: rather, we [649b1] must specify that this substance is hotter per se, though that other is often hotter per accidens; or again, that this substance is potentially hot, that other actually so; or again, that this substance is hotter in the sense of causing a greater feeling of heat when touched, while that other is hotter in the sense of producing flame and [5] burning. The term hot being used in all these various senses, it plainly follows that the term cold will also be used with like multiplicity.
So much then as to hot and cold, hotter and colder.
3 · In natural sequence we have next to treat of dry and moist. These terms [10] are used in various senses. Sometimes, for instance, they denote things that are potentially, at other times things that are actually, dry or moist. Ice for example, or any other solidified fluid, is spoken of as being actually and accidentally dry while potentially and essentially it is moist. Similarly earth and ashes and the like, when mixed with water, are actually and accidentally moist, but potentially and [15] essentially are dry. Now separate the constituents in such a mixture and you have on the one hand the watery components, which take their shape from their container, and these are both actually and potentially moist, and on the other hand the earthy components, and these are all dry; and it is to bodies of this sort that the term ‘dry’ is most properly and absolutely applicable. So also the opposite term ‘moist’ is strictly and absolutely applicable in an analogous way. The same remark applies also to hot bodies and to cold. [20]
These distinctions, then, being laid down, it is plain that blood is hot in one way;7 for it is spoken of as boiling water would be were it denoted by a single term. But the substratum of blood, that which it is while it is blood is not hot. Blood then in a certain sense is essentially hot, and in another sense is not so. For heat is [25] included in the definition of blood, just as whiteness is included in the definition of a white man; but so far as blood becomes hot from some external influence, it is not hot essentially.
As with hot and cold, so also is it with dry and moist. We can therefore understand how some substances are hot and moist so long as they remain in the living body, but become perceptibly cold and coagulate so soon as they are [30] separated from it; while others are hot and consistent while in the body, but when withdrawn undergo a change to the opposite condition, and become cold and moist. Of the former blood is an example, of the latter bile; for while blood solidifies, yellow bile becomes more moist. We must attribute to such substances the possession of opposite properties in a greater or less degree.
In what sense, then, the blood is hot and in what sense fluid, and how far it [650a1] partakes of the opposite properties, has now been fairly explained. Now since everything that grows must take nourishment, and nutriment in all cases consists of moist and dry substances, and since it is by the force of heat that these are [5] concocted and changed, it follows that all living things, animals and plants alike, must on this account, if on no other, have a natural source of heat; and this, like the working of the food,8 must belong to many parts. For first of all there is the mouth [10] and the parts inside the mouth, on which the first share in the duty clearly devolves, in such animals at least as live on food which requires disintegration. The mouth, however, does not actually concoct the food, but merely facilitates concoction; for the subdivision of the food into small bits facilitates the action of heat upon it. After the mouth come the upper and the lower abdominal cavities, and here it is that [15] concoction is effected by the aid of natural heat. Again, just as there is a channel for the admission of the unconcocted food into the stomach, namely the mouth, and in some animals the so-called oesophagus, which is continuous with the mouth and reaches to the stomach, so must there also be other channels by which the nutriment [20] shall pass out of the stomach and intestines into the body at large, and to which these cavities shall serve as a kind of manger. For plants get their food from the earth by means of their roots; and this food is already elaborated when taken in, which is the reason why plants produce no excrement, the earth and its heat serving them in the place of a stomach. But animals, with scarcely an exception, and conspicuously all such as are capable of locomotion, are provided with a stomachal [25] sac, which is as it were an internal substitute for the earth. They must therefore have some instrument which shall correspond to the roots of plants, with which they may absorb their food from this sac, so that the proper end of the successive stages of concoction may be attained. The mouth then, its duty done, passes over the food to the stomach, and there must necessarily be something to receive it in turn from this. This something is furnished by the blood-vessels, which run throughout the [30] whole extent of the mesentery from its lowest part right up to the stomach. A description of these will be found in the Anatomies and in the Natural History. Now as there is a receptacle for the entire matter taken as food, and also a
receptacle for its excremental residue, and again a third receptacle, namely the vessels, which serve as such for the blood, it is plain that this blood must be the final [35] nutritive material in such animals as have it; while in bloodless animals the same is the case with the analogous stuff. This explains why the blood diminishes in [650b1] quantity when no food is taken, and increases when much is consumed, and also why it becomes healthy and unhealthy according as the food is of the one or the other character. These facts, then, and others of a like kind, make it plain that the purpose of the blood in sanguineous animals is to subserve the nutrition of the body. They also explain why no more sensation is produced by touching the blood than by [5] touching one of the excretions or the food, whereas when the flesh is touched sensation is produced. For the blood is not continuous nor united by growth with the flesh, but simply lies in its receptacle, that is in the heart and vessels. The manner in which the parts grow at the expense of the blood, and indeed the whole question of [10] nutrition, will find a more suitable place for exposition in the treatise on generation, and in other writings. For our present purpose all that need be said is that the blood exists for the sake of nutrition, that is the nutrition of the parts; and with this much let us therefore content ourselves.
4 · What are called fibres are found in the blood of some animals but not of all. There are none, for instance, in the blood of deer and of roes; and for this reason [15] the blood of such animals as these never coagulates. For one part of the blood consists mainly of water and therefore does not coagulate, this process occurring only in the other and earthy constituent, that is to say in the fibres, while the fluid part is evaporating.