by Aristotle
And the reason why men especially and horses occasionally have eyes of different colours is the same as the reason why man alone grows grey and the horse is the only other animal whose hairs whiten visibly in old age. For greyness is a [5] weakness of the fluid in the brain and a lack of concoction, and so is blueness of the eyes; excess of thinness or of thickness produces the same effect, according as this liquidity is too little or too much. Whenever then nature cannot make the eyes correspond exactly, either by concocting or by not concocting the liquid in both, but [10] concocts the one and not the other, then the result is differently coloured eyes.
The cause of some animals being keen-sighted and others not so is not simple but double. For things are called keen in two ways (and this is the case in like manner with hearing and smelling). In one sense keen sight means the power of [15] seeing at a distance, in another it means the power of distinguishing as accurately as possible the objects seen. These two faculties do not occur together in the same individual. For the same person, if he shade his eyes with his hand or look through a tube, does not distinguish the differences of colour either more or less in any way, [20] but he will see further; in fact, men in pits or wells sometimes see the stars. Therefore if any animal’s brows project far over the eye, but if the liquid in the pupil is not pure nor suited to the movement coming from external objects and if the skin over the surface is not thin, this animal will not distinguish accurately the [25] differences of the colours but it will be able to see from a long distance (just as it can from a short one)2 better than those in which the liquid and the covering membrane are pure but which have no brows projecting over the eyes. For the cause of seeing keenly in the sense of distinguishing the differences is in the eye itself; as on a clean [30] garment even small stains are visible, so also in a pure sight even small movements are plain and cause sensation. But it is the position of the eyes that is the cause of seeing things far off and of the movements coming to the eyes from distant objects. [35] For animals with prominent eyes do not see well at a distance, whereas those which have their eyes lying deep in the head can see things at a distance because the [781a1] movement is not dispersed in space but comes straight to the eye. For it makes no difference whether we say, as some do, that seeing is caused by the sight going forth from the eye—on that view, if there is nothing projecting over the eyes, the sight must be scattered and so less of it will fall on the objects of vision and things at a [5] distance will not be seen so well—or whether we say that seeing is due to the movement coming from the objects; for the sight also must see, in a manner resembling the movement. Things at a distance, then, would be seen best if there were, so to say, a continuous tube straight from the sight to its object, for the movement from the object would not then be dissipated; but, if that is impossible, [10] still the further the tube extends the more accurately must distant objects be seen.
Let these, then, be given as the causes of the difference in eyes.
2 · It is the same also with hearing and smell; to hear and smell accurately mean in one sense to perceive as precisely as possible all the distinctions of the [15] objects of perception, in another sense to hear and smell far off. As with sight, so here the sense-organ is the cause of distinguishing well the distinctions, if both that [20] organ itself and the membrane round it be pure. For the passages of all the sense-organs, as has been said in the treatise on sensation, run to the heart, or to its analogue in creatures that have no heart. The passage of the hearing, then, since this sense-organ is of air, ends at the place where the innate breath causes in some [25] animals the pulsation and in others respiration;3 and that is why we are able to understand what is said and repeat what we have heard, for as4 was the movement which entered through the sense-organ, such again is the movement which is caused by means of the voice, being as it were of one and the same stamp, so that a man can [30] say what he has heard. And we hear less well during a yawn or expiration than during inspiration, because the starting-point of the sense-organ of hearing is set upon the part concerned with breathing and is shaken and moved as the organ moves the breath, for while setting the breath in motion it is moved itself. The same thing happens in wet weather or a damp climate.5 … And the ears seemed to be [781b1] filled with air because their starting-point is near the region of breathing.6
Accuracy then in judging the differences of sounds and smells depends on the purity of the sense-organ and of the membrane lying upon its surface, for then all [5] the movements become clear in such cases, as in the case of sight.7 Perception and non-perception at a distance also depend on the same things as with sight. For those animals can perceive at a distance which have channels, so to say, running through the parts concerned and projecting far in front of the sense-organs. Therefore all [10] animals whose nostrils are long, as the Laconian hounds, are keen-scented, for the sense-organ being above them, the movements from a distance are not dissipated but go straight to the mark, just as with those who shadow the eyes with the hand.
Similar is the case of animals whose ears are long and project far like the eaves [15] of a house, as in some quadrupeds, with the internal spiral passage long; these also catch the movement from afar and pass it on to the sense-organ.
In respect of sense-perception at a distance, man is, one may say, the worst of all animals in proportion to his size, but in respect of judging the differences he is [20] the best of all. The reason is that the sense-organ is pure and least earthy and material, and he is by nature the thinnest-skinned of all animals for his size.
The workmanship of nature is admirable also in the seal, for though a viviparous quadruped it has no ears but only passages for hearing. This is because [25] its life is passed in the water; now the ear is a part added to the passages to preserve the movement of the air at a distance; therefore an ear is no use to it but would even bring about the contrary result by receiving a mass of water into itself.
We have thus spoken of sight, hearing, and smell.
[30] 3 · As for hair, men differ in this themselves at different ages, and also from all other kinds of animals that have hair. These are almost all which are internally viviparous, for even when the covering of such animals is spiny it must be considered as a kind of hair, as in the hedgehog and any other such animal among the vivipara. Hairs differ in respect of hardness and softness, length and shortness, straightness [782a1] and curliness, quantity and scantiness, and in addition to these qualities, in their colours, whiteness and blackness and the intermediate shades. They differ also in [5] some of these respects according to age, as they are young or growing old. This is especially plain in man; the hair gets thicker as time goes on, and some go bald on the front of the head; children indeed do not go bald, nor do women, but men do so [10] by the time their age is advancing. Human beings also go grey on the head as they grow old, but this is not visible in practically any other animal, though more so in the horse than others. Men go bald on the front of the head, but turn grey first [15] on the temples; no one goes bald on these or on the back of the head. Some such affections occur in a corresponding manner also in animals which have not hair but something analogous to it, as the feathers of birds and scales in the class of fish.
For what purpose nature has made hair for animals has been previously stated [20] in the work dealing with the causes of the parts of animals; it is the business of the present inquiry to show under what circumstances and for what necessary causes each particular kind of hair occurs. The principal cause then of thickness and thinness is the skin, for this is thick in some animals and thin in others, rare in some [25] and dense in others. The different quality of the included moisture is also a helping cause, for in some animals this is greasy and in others watery. For generally speaking the skin is of an earthy nature; being on the surface of the body it becomes [30] solid and earthy as the moisture evaporates. Now the hairs on their analogue are not formed out of the flesh but out of the skin, the moisture evaporating and exhaling in them, and therefore thick hairs
arise from a thick skin and thin from a thin. If then the skin is rarer and thicker, the hairs are thick because of the quantity of earthy matter and the size of the pores, but if it is denser they are thin because of the [782b1] narrowness of the pores. Further, if the moisture be watery it dries up quickly and the hairs do not gain in size, but if it be greasy the opposite happens, for the greasy is not easily dried up. Therefore the thicker-skinned animals are as a general rule [5] thicker-haired; however, the thickest-skinned are not more so than other thick-skinned ones, for the causes mentioned, as is shown by the class of swine compared to that of oxen and to the elephant and many others. And for the same reason also the hairs of the head in man are thickest, for this part of his skin is thickest and lies [10] over most moisture and besides is very rare.
The cause of the hairs being long depends on the evaporating moisture not being easily dried. Of this there are two causes, quantity and quality; if the liquid is much it does not dry up easily nor if it is greasy. And for this reason the hairs of the [15] head are longest in man, for the brain, being fluid and cold, supplies great abundance of moisture.
The hairs become straight or curly on account of the vapour arising in them. If it be smoke-like, it is hot and dry and so makes the hair curly, for it is twisted as [20] being carried with a double motion, the earthy part tending downwards and the hot upwards. Thus, being easily bent, it is twisted owing to its weakness, and this is what is meant by curliness in hair. It is possible then that this is the cause, but it is also possible that, owing to its having but little moisture and much earthy matter in it, it [25] is dried by the surrounding air and so coiled up together. For what is straight becomes bent, if the moisture in it is evaporated, and runs together as a hair does when burning upon the fire; curliness will then be a contraction owing to deficiency of moisture caused by the heat of the environment. A sign of this is the fact that [30] curly hair is harder than straight, for the dry is hard. And animals with much moisture are straight-haired; for in these hairs the moisture advances as a stream, not in drops. For this reason the Scythians on the Black Sea and the Thracians are straight-haired, for both they themselves and the environing air are moist, whereas [783a1] the Aethiopians and men in hot countries are curly-haired, for their brains and the surrounding air are dry.
Some, however, of the thick-skinned animals are fine-haired for the cause previously stated, for the finer the pores are the finer must the hairs be. Hence the [5] class of sheep have such hairs (for wool is a multitude of hairs).
There are some animals whose hair is soft and yet less fine, as is the case with the class of hares compared with that of sheep; in such animals the hair is on the surface of the skin, and so is not long but in much the same state as the scrapings [10] from linen, for these also are not long but are soft and do not admit of weaving.
The condition of sheep in cold climates is opposite to that of man; the hair of the Scythians is soft but that of Sarmatian sheep is hard. The reason for this is the [15] same as it is also in all wild animals. The cold hardens and solidifies them by drying them, for as the heat is pressed out the moisture evaporates, and both hair and skin become earthy and hard. In wild animals then their open-air life is the cause, in the others the nature of their location. A proof of this is also what happens in the [20] sea-urchins which are used as a remedy in stranguries. For these, too, though small themselves, have large and hard spines because the sea in which they live is cold on account of its depth (for they are found in sixty fathoms and even more). The spines [25] are large because the growth of the body is diverted to them, since having little heat in them they do not concoct their nutriment and so have much residual matter and it is from this that spines, hairs, and such things are formed; they are hard and petrified through the congealing effect of the cold. In the same way also plants are [30] found to be harder, more earthy, and stony, if the region in which they grow looks to the north than if it looks to the south, and those in windy places than those in sheltered, for they are all more chilled and their moisture evaporates.
Hardening, then, comes of both heat and cold, for both cause the moisture to [35] evaporate, heat per se and cold per accidens (since the moisture goes out of things along with the heat, there being no moisture without heat), but whereas cold not [783b1] only hardens but also condenses, heat makes a substance rarer.
For the same reason, as animals grow older, the hairs become harder in those which have hairs, and the feathers and scales in the feathered and scaly kinds. For [5] their skins become harder and thicker as they get older, for they are dried up, and old age, as the word implies,8 is earthy because the heat fails and the moisture along with it.
Men go bald visibly more than any other animal, but still such a state is something general, for among plants also some are evergreens while others are [10] deciduous, and birds which hibernate shed their feathers. Similar to this is the condition of baldness in those human beings to whom it is incident. For leaves are shed by all plants, from one part of the plant at a time, and so are feathers and hairs [15] by those animals that have them; it is when they are all shed together that the condition is described by the terms mentioned, for it is called ‘going bald’ and ‘the fall of the leaf’ and ‘moulting’. The cause of the condition is deficiency of hot moisture, such moisture being especially the greasy and hence greasy plants are more evergreen. (However, we must elsewhere state the cause of this—for other [20] causes also contribute to it.) It is in winter that this happens to plants (for the change from summer to winter is more important to them than the time of life), and to those animals which hibernate (for these, too, are by nature less hot and moist than man); in men it is the seasons of life that correspond to summer and winter. [25] Hence no one goes bald before the time of sexual intercourse, and at that time it is in those naturally inclined to such intercourse that baldness appears, for the brain is naturally the coldest part of the body and sexual intercourse makes men cold, being a loss of pure natural heat. Thus we should expect the brain to feel the effect of it [30] first, for a little cause turns the scale where the thing concerned is weak and in poor condition. Thus if we reckon up these points, that the brain itself has but little heat, and further that the skin round it must needs have still less, and again that the hair must have still less than the skin inasmuch as it is the furthest removed from the brain, we should reasonably expect baldness to come about this age upon those who [35] have much semen. And it is for the same reason that the front part of the head alone goes bald in man and that he is the only animal to do so; the front part goes bald [784a1] because the brain is there, and man is the only animal to go bald because his brain is much the largest and the moistest. Women do not go bald because their nature is like that of children, both alike being incapable of producing seminal secretion. [5] Eunuchs do not become bald, because they change into the female condition. And as to the hair that comes later in life, eunuchs either do not grow it at all, or lose it if they happen to have it, with the exception of the pubic hair; for women also grow that though they have not the other, and this mutilation is a change from the male to [10] the female condition.
The reason why the hair does not grow again in cases of baldness, although both hibernating animals recover their feathers or hair and trees that have shed their leaves grow leaves again, is this. The seasons of the year are the turning-points of their lives so that when these seasons change they change with them by growing [15] and losing feathers, hairs, or leaves respectively. But the winter and summer, spring and autumn of man are defined by his age, so that, since his ages do not return, neither do the conditions caused by them return, although the cause is similar. [20]
We have now spoken pretty much of all the other conditions of hair.
4 · But as to their colour, it is the nature of the skin that is the cause of this in other animals (and also of their being unicoloured or varicoloured); but in man it is not the cause, except of the hair going grey through disease (not through old age), [25] fo
r in what is called leprosy the hairs become white; on the contrary, if the hairs are white because of old age, the whiteness does not derive from the skin. The reason is that the hairs grow out of skin; if, then, the skin is diseased and white the hair [30] becomes diseased with it, and the disease of hair is greyness. But the greyness of hair which is due to age results from weakness and deficiency of heat. For as the body declines in vigour we tend to be cold at every time of life, and especially in old age,9 this age being cold and dry. We must remember that the nutriment coming to each part of the body is concocted by the heat appropriate to the part; if the heat is [784b1] inadequate the part perishes, and deformity or disease results. (We shall speak more in detail of causes in the treatise on growth and nutrition.) Whenever, then, the hair in man has naturally little heat and too much moisture enters it, its own [5] proper heat is unable to concoct the moisture and so it is decayed by the heat in the environing air. All decay is caused by heat—not the innate heat, as has been stated elsewhere. And as there is a decay of water, of earth, and all such material bodies, so [10] there is also of the earthy vapour, for instance what is called mould (for mould is a decay of earthy vapour). Thus also the liquid nutriment in the hair decays because it is not concocted, and what is called greyness results. It is white because mould also, practically alone among decayed things, is white. The reason for this is that it has [15] much air in it, all earthy vapour being equivalent to thick air. For mould is, as it were, the antithesis of hoar-frost; if the ascending vapour be frozen it becomes hoar-frost, if it be decayed, mould. Hence both are on the surface of things, for vapour is superficial. And so the comic poets make a good metaphor in jest when [20] they call grey hairs ‘mould of old age’ and ‘hoar-frost’. For the one is generically the same as greyness, the other specifically; hoar-frost generically (for both are a vapour), mould specifically (for both are a form of decay). A proof that this is so is this: grey hairs have often grown on men in consequence of disease, and later on [25] dark hairs instead of them after restoration to health. The reason is that in sickness the whole body is deficient in natural heat and so the parts besides, even the very small ones, participate in this weakness; and again, much residual matter is formed in the body and all its parts, so that the incapacity in the flesh to concoct the [30] nutriment causes the grey hairs. But when men have recovered health and strength again they change, becoming as it were young again instead of old; in consequence the states change also. Indeed, we may rightly call disease an acquired old age, old age a natural disease; at any rate, some diseases produce the same effects as old age.