In the eruptive malady called the white-sickness all the hairs get
grey; and instances have been known where the hair became grey while
the patients were ill of the malady, whereas the grey hairs shed off
and black ones replaced them on their recovery. (Hair is more apt to
turn grey when it is kept covered than when exposed to the action of
the outer air.) In men, the hair over the temples is the first to turn
grey, and the hair in the front grows grey sooner than the hair at the
back; and the hair on the pubes is the last to change colour.

All sanguineous animals, then, have skin; but not all such animals
have hair, save only under the circumstances described above. The hair
changes its colour as animals grow old, and in man it turns white or
grey. With animals, in general, the change takes place, but not very
obviously, or not so obviously as in the case of the horse. Hair turns
grey from the point backwards to the roots. But, in the majority of
cases, grey hairs are white from the beginning; and this is a proof
that greyness of hair does not, as some believe to be the case,
imply withering or decrepitude, for no part is brought into
existence in a withered or decrepit condition.

Of all animals man has the most delicate skin: that is, if we take
into consideration his relative size. In the skin or hide of all
animals there is a mucous liquid, scanty in some animals and plentiful
in others, as, for instance, in the hide of the ox; for men
manufacture glue out of it. (And, by the way, in some cases glue is
manufactured from fishes also.) The skin, when cut, is in itself
devoid of sensation; and this is especially the case with the skin
on the head, owing to there being no flesh between it and the skull.
And wherever the skin is quite by itself, if it be cut asunder, it
does not grow together again, as is seen in the thin part of the
jaw, in the prepuce, and the eyelid. In all animals the skin is one of
the parts that extends continuous and unbroken, and it comes to a stop
only where the natural ducts pour out their contents, and at the mouth
and nails.

Hair is naturally fissile, and in this respect it differs in
degree in diverse animals. In some animals the hair goes on
gradually hardening into bristle until it no longer resembles hair but
spine, as in the case of the hedgehog. And in like manner with the
nails; for in some animals the nail differs as regards solidity in
no way from bone.

The hair differs in the way of thickness and fineness, and of
length, according to the locality of the part in which it is found,
and according to the quality of skin or hide on which it grows. For,
as a general rule, the thicker the hide, the harder and the thicker is
the hair; and the hair is inclined to grow in abundance and to a great
length in localities of the bodies hollow and moist, if the localities
be fitted for the growth of hair at all. The facts are similar in
the case of animals whether coated with scales or with tessellates.
With soft-haired animals the hair gets harder with good feeding, and
with hard-haired or bristly animals it gets softer and scantier from
the same cause. Hair differs in quality also according to the relative
heat or warmth of the locality: just as the hair in man is hard in
warm places and soft in cold ones. Again, straight hair is inclined to
be soft, and curly hair to be bristly.

The following are the properties of hair and of parts analogous to
hair, and of skin or hide. All viviparous animals furnished with
feet have hair; all oviparous animals furnished with feet have
horn-like tessellates; fishes, and fishes only, have scales-that is,
such oviparous fishes as have the crumbling egg or roe. For of the
lanky fishes, the conger has no such egg, nor the muraena, and the eel
has no egg at all.

Of animals furnished with nails-and, by the way, all animals
have nails that have toes, and toes that have feet, except the
elephant; and the elephant has toes undivided and slightly
articulated, but has no nails whatsoever--of animals furnished with
nails, some are straight-nailed, like man; others are crooked
nailed, as the lion among animals that walk, and the eagle among
animals that fly.

Horns in general are hollow at their point of attachment to the
bone which juts out from the head inside the horn, but they have a
solid portion at the tip, and they are simple and undivided in
structure. In the case of the stag alone of all animals the horns
are solid throughout, and ramify into branches (or antlers). And,
whereas no other animal is known to shed its horns, the deer sheds its
horns annually, unless it has been castrated; and with regard to the
effects of castration in animals we shall have much to say
hereafter. Horns attach rather to the skin than to the bone; which
will account for the fact that there are found in Phrygia and
elsewhere cattle that can move their horns as freely as their ears.

And the colours of horns and nails and claw and hoof follow the
colour of the skin and the hair. For according as the skin of an
animal is black, or white, or of medium hue, so are the horns, the
claws, or the hooves, as the case may be, of hue to match. And it is
the same with nails. The teeth, however, follow after the bones.
Thus in black men, such as the Aethiopians and the like, the teeth and
bones are white, but the nails are black, like the whole of the skin.

Furthermore, there are parts of other kinds, neither identical
with, nor altogether diverse from, the parts above enumerated: such as
nails, hooves, claws, and horns; and also, by the way, beaks, such
as birds are furnished with-all in the several animals that are
furnished therewithal. All these parts are flexible and fissile; but
bone is neither flexible nor fissile, but frangible.

Gristle is of the same nature as bone, but differs from it in
the way of relative excess or relative defect. And just like bone,
cartilage also, if cut, does not grow again. In terrestrial viviparous
sanguinea the gristle formations are unperforated, and there is no
marrow in them as there is in bones; in the selachia, however--for, be
it observed, they are gristle-spined--there is found in the case of
the flat space in the region of the backbone, a gristle-like substance
analogous to bone, and in this gristle-like substance there is a
liquid resembling marrow. In viviparous animals furnished with feet,
gristle formations are found in the region of the ears, in the
nostrils, and around certain extremities of the bones.

With oviparous quadrupeds, the skeleton of the
larger ones is more or less osseous; of the smaller ones, more or less
spinous. But all sanguineous animals have a backbone of either one
kind or other: that is, composed either of bone or of spine.

The other portions of the skeleton are found in some animals and
not found in others, but the presence or the absence of this and
that part carries with it, as a matter of course, the presence or
the absence of the bones or the spines corresponding to this or that
part. For animals that are destitute of arms and legs cannot be
furnished with limb-bones: and in like manner with animals that have
the same parts, but yet have them unlike in form; for in these animals
the corresponding bones differ from one another in the way of relative
excess or relative defect, or in the way of analogy taking the place
of identity. So much for the osseous or spinous systems in animals.

Of the other animals supplied with blood, some differ but
little, as is the case with birds; others have systems analogous, as
fishes; for viviparous fishes, such as the cartilaginous species,
are gristle-spined, while the ovipara have a spine which corresponds
to the backbone in quadrupeds. This exceptional property has been
observed in fishes, that in some of them there are found delicate
spines scattered here and there throughout the fleshy parts. The
serpent is similarly constructed to the fish; in other words, his
backbone is spinous.

Now, with all animals that are supplied with blood and furnished
with feet, and are at the same time viviparous, the bones do not
differ greatly one from another, but only in the way of relative
hardness, softness, or magnitude. A further difference, by the way, is
that in one and the same animal certain bones are supplied with
marrow, while others are destitute of it. Some animals might on casual
observation appear to have no marrow whatsoever in their bones: as
is the case with the lion, owing to his having marrow only in small
amount, poor and thin, and in very few bones; for marrow is found in
his thigh and armbones. The bones of the lion are exceptionally
hard; so hard, in fact, that if they are rubbed hard against one
another they emit sparks like flint-stones. The dolphin has bones, and
not fish-spine.