like the rest of the body gets dry towards old age; but is an excess
of liquidity and so is found more in the younger, for their brain is
more liquid.
The sight of the eye which is intermediate between too much and
too little liquid is the best, for it has neither too little so as
to be disturbed and hinder the movement of the colours, nor too much
so as to cause difficulty of movement.
Not only the above-mentioned facts are causes of seeing keenly or
the reverse, but also the nature of the skin upon what is called the
pupil. This ought to be transparent, and it is necessary that the
transparent should be thin and white and even, thin that the
movement coming from without may pass straight through it, even that
it may not cast a shade the liquid behind it by wrinkling (for this
also is a reason why old men have not keen sight, the skin of the
eye like the rest of the skin wrinkling and becoming thicker in old
age), and white because black is not transparent, for that is just
what is meant by 'black', what is not shone through, and that is why
lanterns cannot give light if they be made of black skin. It is for
these reasons then that the sight is not keen in old age nor in the
diseases in question, but it is because of the small amount of
liquid that the eyes of children appear blue at first.
And the reason why men especially and horses occasionally are
heteroglaucous 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 weakness of the fluid in the brain and an
incapacity to concoct properly, 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 concocts the one and not the
other, then the result is heteroglaucia.
The cause of some animals being keen-sighted and others not so is
not simple but double. For the word 'keen' has pretty much a double
sense (and this is the case in like manner with hearing and
smelling). In one sense keen sight means the power of seeing at a
distance, in another it means the power of distinguishing as
accurately as possible the objects seen. These two faculties are not
necessarily combined in the same individual. For the same person, if
he shades his eyes with his hand or look through a tube, does not
distinguish the differences of colour either more or less in any
way, 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
differences of the colours but it will be able to see from a long
distance (just as it can from a short one) 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 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 in the transparent medium coming to the eyes from
distant objects. A proof of this is that 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
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
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, 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 objects of perception, in another sense to
hear and smell far off. As with sight, so here the sense-organ is
the cause of judging well the distinctions, if both that 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 spiritus causes in some animals the
pulsation of the heart and in others respiration; wherefore also it is
that we are able to understand what is said and repeat what we have
heard, for as 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 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 atmosphere.... And the ears
seemed to be filled with air because their starting-point is near
the region of breathing.
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 the movements become clear in such
cases, as in the case of sight. Perception and non-perception at a
distance also depend on the same things with hearing and smell 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 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 the movements
which cause sight do 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 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 of quality in the objects he is the best of
all. The reason is that the sense-organ in man 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 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.
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 land hedgehog and any other such animal among the vivipara.
Hairs differ in respect of hardness and softness, length and
shortness, straightness 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 some of these
respects according to age, as they are young or growing old. This is
especially plain in man; the hair gets coarser 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 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 on the temples; no one goes bald first on these or on the back
of the head. Some such affections occur in a corresponding manner also
in all 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 in general for animals has
been previously stated 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 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 substratum
of the skin is of an earthy nature; being on the surface of the body
it becomes solid and earthy as the moisture evaporates. Now the
hairs or their analogue are not formed out of the flesh but out of the
skin moisture evaporating and exhaling in them, and therefore thick
hairs arise from a thick skin and thin from 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 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 thicker-haired for the causes mentioned; however, the
thickest-skinned are not more so than other thick-skinned ones, 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 over most moisture and besides is very porous.
The cause of the hairs being long or short 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
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 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 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 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 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 class of sheep have such hairs (for wool
is only 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, not deeply rooted
in it, and so is not long but in much the same state as the
scrapings 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 the Sauromatic
sheep is hard. The reason of this is the same as it is also 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 the exposure to
the cold is the cause of hardness in the hair, in the others the
nature of the climate is the cause. A proof of this is also what
happens in the sea-urchins which are used as a remedy in
stranguries. For these, too, thou
gh 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 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
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 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 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 their skins become harder and thicker
as they get older, for they are dried up, and old age, as the word
denotes, 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 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 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'